Journal of Molecular Structure 976 (2010) 377–391

Contents lists available at ScienceDirect

Journal of Molecular Structure

journal homepage: www.elsevier .com/ locate /molst ruc

Deuterium isotope effects on 13C and 15N chemical shifts of intramolecularlyhydrogen-bonded enaminocarbonyl derivatives of Meldrum’s and Tetronic acid

Saif Ullah, Wei Zhang, Poul Erik Hansen *

Department of Science, Systems and Models, Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmark

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 February 2010Received in revised form 19 March 2010Accepted 20 March 2010Available online 25 March 2010

In honour of Professor Austin Barnes on his65th birthday.

Keywords:Deuterium isotope effectsEnaminocarbonylMeldrum’s acidTetronic acidsDFT calculationsRAHB

0022-2860/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.molstruc.2010.03.059

* Corresponding author. Tel.: +45 46 742432; fax: +E-mail address: poulerik@ruc.dk (P.E. Hansen).

Secondary deuterium isotope effects on 13C and 15N nuclear shieldings in a series of cyclic enamino-dies-ters and enamino-esters and acyclic enaminones and enamino-esters have been examined and analysedusing NMR and DFT (B3LYP/6-31G(d,p)) methods. One-dimensional and two-dimensional NMR spectra ofenaminocarbonyl and their deuterated analogues were recorded in CDCl3 and CD2Cl2 at variable temper-atures and assigned. 1JNH coupling constants for the derivatives of Meldrum’s and tetronic acids revealthat they exist at the NH-form.

It was demonstrated that deuterium isotope effects, for the hydrogen bonded compounds, due to thedeuterium substitution at the nitrogen nucleus lead to large one-bond isotope effects at nitrogen,1D15N(D), and two-bond isotope effects on carbon nuclei, 2DC(ND), respectively. A linear correlationsexist between 2DC(ND) and 1D15N(D) whereas the correlation with dNH is divided into two. A goodagreement between the experimentally observed 2DC(ND) and calculated dr13C/dRNH was obtained. Avery good correlation between calculated NH bond lengths and observed NH chemical shifts is found.The observed isotope effects are shown to depend strongly on Resonance Assisted Hydrogen bonding.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction �0.8 ppm (asymmetric). Hydrogen bond can be further subdivided

A key feature of molecular structure is hydrogen bonds whichhave been studied both theoretically and experimentally over theyears. Characterization of hydrogen-bonded systems is an impor-tant feature in both organic and biological molecules and it istherefore of importance to explore the strength and geometry ofhydrogen bonding [1–5]. Looking at deuterium isotope effects onchemical shifts, intramolecular hydrogen bonded compounds canbe divided into compounds with localized hydrogen bond andnon-localized hydrogen bond (equilibrium cases or part of a tauto-meric system). A localized hydrogen bond and non-localizedhydrogen bond can be differentiated by measuring deuterium iso-tope effects on the carbon atom directly attached to the proton do-nor functional group. For the localized hydrogen bond cases the2DC(D) is not influenced by the temperature, in contrast to non-localized hydrogen bond which indicates temperature sensitivedeuterium isotope effect values [8]. The 2DC(D) values reportedfor localized hydrogen bond lies only in the positive range�0.1 ppm to �0.5 ppm, while for non-localized hydrogen bondcase high positive values observed, in the range �0.5 ppm to�0.7 ppm (symmetric double-minimum proton potential), highpositive values 0.7–2.3 ppm and negative values �0.3 to

ll rights reserved.

45 46 743011.

into those with resonance assisted hydrogen bond (RAHB)(Scheme 1) and those with non-resonance assisted (non-RAHB)hydrogen bonds [1,2,6,7]. RAHB systems can be recognized usingPCA analysis [8]. Non-RAHB systems are found, e.g. in protonatedDMAN’s [9] and in Mannich bases [10] but these show usuallysmaller values for deuterium isotope effects [9]. Deuterium isotopeeffects on chemical shifts can be primary, pD or secondary sD. Theprimary deuterium isotope effect is defined as pD1H(D) =d(H) � d(D) and the secondary isotope effects on 13C chemicalshifts as follows: nD13C(OD) = d13C (OH) � d13C(OD), where n isthe number of bonds between the site of deuteration and the ob-served nuclei, while on 15N it is 1D15N(D) =d15N(H) � d15N(D) [7]. Various structural factors such as conjuga-tion, torsional angle, hybridization, and resonance might affectthe sign and magnitude of isotope effects [8,9]. In order to investi-gate the strength and geometry of hydrogen bonds, isotopic substi-tution is potentially very useful. Secondary deuterium isotopeeffect (DIE) on 13C chemical shifts, nD13C (OD), have been studiedextensively in intramolecular hydrogen-bonded systems and as-cribed qualitatively to the hydrogen bond strength [4,8–25]. Deu-terium isotope effects on 1D15N(D) chemical shifts have not beenstudied as frequently, since indirect detection via long-range cou-plings is often necessary due to the very long relation times ofthe deuteriated nitrogen, but one advantage to be considered forthe observation of 15N nucleus is its large chemical shift range

Scheme 1. RAHB.

378 S. Ullah et al. / Journal of Molecular Structure 976 (2010) 377–391

[4,18,22,26–36]. The important parameters which directly tellabout the strength and structure of hydrogen bond are donor–acceptor distance, bond order, 2DC(ND) or 2DC(OD) and 1D15N(D)[37].

With the development of fast, efficient computers and commer-cial software in recent years, theoretical calculations have showngreat promise to calculate structures of hydrogen-bonded systems[20,23,38–40], chemical shifts calculations (nuclear shieldings)[38,41–48] and deuterium isotope effects on nuclear shieldings[38,45,48]. The two-bond deuterium isotope effects on 13C and15N chemical shifts at the neighbouring atom can be approximatedusing the Jameson formula [49]:

r� r� ¼Xðdr=drXHÞe DrXHh i � DrXDh i½ � where X ¼ C;N ð1Þ

Scheme 2. Possible tautomers of enaminocarbonyl (derivatives of tetronic acids). Two sesignals were seen by ‘internal’ equilibria (a–b and c–d) because the equilibrium is too fcyclic enaminocarbonyl (derivatives of Meldrum’s acid).

‘‘Enaminocarbonyl” are a group of organic compounds bearingan amino entity linked through a C@C to a keto group or keto-estergroup forming the conjugated system ANAC@CACOA orANAC@CACO2A, having the ambident nucleophilicity of enamino-carbonyl and ambident electrophilicity of enones, belong to a classof versatile intermediates for the synthesis of various pharmaceu-tical and bioactive heterocyclic compounds, natural products andnumerous products of medicinal interest [50–52]. Accordingly, anumber of important medicinal agents contain enaminocarbonylas common pharmacophores and have demonstrated a wide rangeof potential therapeutic utility including anti-inflammatory [53],anticonvulsant [54,55], antibacterial [53,56] and antitumouragents [53,57].

In spite of their biological importance, the synthesis of enami-nocarbonyl, especially cyclic ones, (Scheme 2) has gained littleattention so far. In the present work we have synthesized a numberof enaminocarbonyl derived from Meldrum’s acid and Tetronicacid and they are investigated by means of deuterium isotope ef-fects on 13C and 15N chemical shifts combined with theoretical(DFT type) of structures and NMR properties.

The main objective of the present study is to evaluate the sen-sitivity of the NAH� � �O@C type of intramolecular hydrogen bond-ing by measuring deuterium isotope effects on chemical shifts,

ts of signals were detected by the ‘external’ equilibrium (a, b to c, d) but no separateast on the NMR time scale at 298 K or non-existing and structure of six-membered

S. Ullah et al. / Journal of Molecular Structure 976 (2010) 377–391 379

especially for 1D15N(D) which is a sensitive gauge to hydrogenbonding, as it has been shown that compounds lacking NAH pro-tons could be deprived of their activity [58]. A further aim is to

Scheme 3. a and b Experimentally observed deuterium isotope effects in CD2Cl2 on 13C cb220 K.

investigate deuterium isotope effect on 13C and 15N chemical shiftsand to show how 2DC(ND) and 1D15N(D) relate to structuralparameters. In addition, DFT calculations are used to obtain struc-

hemical shifts. 1H NH chemical shifts are given in italics. Superscript letters: a298 K;

Scheme 3 (continued)

380 S. Ullah et al. / Journal of Molecular Structure 976 (2010) 377–391

Scheme 4. General structure for the enaminocarbonyl 13_Z–19_Z taken from Ref.[4]. Only the Z forms are shown, but compounds 16 and 18 are also found insolution as E isomers.

Fig. 2. Chemical structure, atom numbering scheme and long range 1H–13Cinteractions observed for 2.

S. Ullah et al. / Journal of Molecular Structure 976 (2010) 377–391 381

tural parameters, to calculate nuclear shieldings and isotope ef-fects. All compounds are presented in Schemes 3 and 4.

2. Results

The 1H, 13C and 15N signal assignments were performed by thecombined use of one-dimensional and two-dimensional heteronu-clear correlation experiments, chemical shifts and coupling con-stants to 15N. The 1H, 13C and 15N NMR assignments are collectedin the experimental section.

2.1. NMR assignments

2.1.1. 2,2-Dimethyl-5-(1-methylamino-ethylidene)-1,3-dioxane-4,6-dione (1_15N)

The main problem is the assignment of C-4, C-6 and C-9 as theyhave rather similar 13C chemical shifts (see experimental). C-9 canbe assigned based on the one-bond CAN coupling constant of14.8 Hz (see Fig. 1). The two carbonyl carbons C-4 and C-6 canbe distinguished based on a three-bond CAN coupling of 1.8 Hz.This is to C-6 as the coupling geometry in this case is trans.

The main difference between compound 1 and compounds 2–7is an extra ester functional group in the latter (see Scheme 3). C-4and C-6 can be assigned in analogy with 1, whereas C-24 is as-signed based on the HMBC correlation to CH2-16 and CH2-25(see Fig. 2). The same approach was adopted to assign compounds9–12 (see Fig. 4).

2.1.2. 3-(1-Methylamino-ethylidene)-furan-2,4-dione (8_15N)The 1H, 13C and 15N NMR assignments including chemical shifts

and coupling constants for approximately 70% deuteriated sample

Fig. 1. Chemical structure, atom numbering scheme and long range 1H–15N and13C–15N interactions observed for 1_15N.

of compound 8_15N was completed on the basis of J-values, col-lected in the experimental section and depicted in Fig. 3. The 1HNMR spectra show two sets of resonance signals including theNH proton. The 1H chemical shifts of the NH proton were assignedon the basis of the nature of the acceptor group, as the C@O groupis a stronger acceptor group than COOR [4], the NH proton of tau-tomer-b having a keto acceptor group will be at a higher frequency.Therefore, the downfield shifted NH resonance signal at 11.04 ppmwas attributed to tautomer-b and the upfield resonance signal at10.09 ppm to tautomer-d, both of the NH resonances are ratherbroad. The proton NMR spectrum recorded in CDCl3 at 298 K wasanalysed by taking into account the maximum number of peakpairs using the line shape analysis algorithm (Levenberg–Marqu-ardt fit) available in Mestre-C [59] and the determined average ra-tio b/d was found to be 1.45.

Again, the main difficulty lies in the assignment of the C-9 andthe two carbonyl groups. The C-6 carbonyl groups can due to theirhigh chemical shifts be distinguished from C-4 and C-9. C-9 can beassigned due to the one-bond coupling constant to 15N. C-4 is thenassigned by default. To assign the C-9 pair one can use the 3J(C,N)coupling to assign the one belonging to the b-tautomer and for thetwo C-4 carbons the one belonging to the d-tautomer shows a3J(C,N) coupling.

For derivatives like 11 the situation is like that described abovefor 1 and 2.

2.2. Structural assignments

Compounds like 1 and 2 have previously in a single case beenreported [60]. However, in this case they were described as tauto-meric systems between an enaminone as seen in Scheme 2 and anenolimine. This is not the case as demonstrated for 1_15N as the1J(N,H) coupling constant is 90.6 Hz. This clearly demonstratesonly a single form, the enaminone one.

For compounds 8–12 two tautomers are found (Scheme 3). Thiscorresponds to the parent tetronic acids [24]. From NMR spectrathe b/d ratios could be determined as: 1.45; 1.33; 1.92; 1.46 and2.18, respectively.

2.3. Deuterium Isotope Effects

Deuterium isotope effects on 13C chemical shifts were deter-mined for compounds with deuterium substitution of the NH pro-ton involved in an intramolecular hydrogen bond. The solvent used

Fig. 3. 8_15N Chemical structure, atom numbering scheme and long range 1H–15N and 13C–15N interactions observed for tautomers b and tautomer-d.

Fig. 4. Compound 11. Chemical structure, atom numbering scheme and long range 1H–13C interactions observed for tautomers b and tautomer-d.

Fig. 5. Plot of experimentally observed 2DC(ND)Obs vs. dNHObs for acyclic and cyclicenaminocarbonyl. Cyclic enaminocarbonyl from this work; acyclic from Ref. [4].

382 S. Ullah et al. / Journal of Molecular Structure 976 (2010) 377–391

is CD2Cl2 and the data at two different temperatures are given inScheme 3. The isotope effects for enamino-diesters and enamino-esters (derivatives of Meldrum’s acid and tetronic acid) could beobserved at ambient temperature. The values of 1D15N(D),2DC(ND) and dNH for compounds 13_Z to 19_Z are taken fromRef. [4] and the compounds are presented in Scheme 4.

2.3.1. 2DC-9(ND)The 2DC(ND) isotope effects on 13C chemical shielding are larger

for ketone acceptors (C@O) as compared to esters (COOR). Thegraph in Fig. 5 shows a displacement between the two types ofenaminocarbonyl (acyclic and cyclic) is possibly caused by the factthat the cyclic compounds have two mesomerically electron with-drawing substituents. There are no clear-cut temperature varia-tions observed for all the compounds given in Scheme 3 as thecyclic enamino-diesters 1, 2, 5 and cyclic enamino-esters 9_b to12_b show a decreasing 2DC-9(ND) isotope effect with decreasingtemperature, whereas compounds 3–4, 6–7, 8_b and 8_d to 12_dshow a slightly increasing tendency.

2.3.2. nDC(ND)The 4DC(ND) deuterium isotope effects observed at the car-

bonyl carbon (C@O) in the studied enamino-esters are larger thanenaminones, but vary in an irregular way (Scheme 3).

The 4DC-4(ND) isotope effects at the C@O carbon increases withdecreasing temperature for all the cyclic enamino-diesters except1, whereas the isotope effect for all the cyclic enaminones motif(4DC-6(ND)) and enamino-esters motif (4DC-4(ND)) decrease withdecreasing temperature. Deuteriation at the NH position also led tolong range isotope effect at 13C chemical shifts up to six bonds on

S. Ullah et al. / Journal of Molecular Structure 976 (2010) 377–391 383

both side of nitrogen nucleus ranging from 0.005 ppm to0.019 ppm as also found earlier for Meldrum’s and barbituric acids[61].

A steric effect (twist) was found for the structure of 4 containingtert-butyl moiety leading to different isotope effects.

2.3.3. 1D15N(D)Deuterium isotope effects on 15N chemical shifts were mea-

sured from a one-tube experiment for 1_15N and 8_15N (both15N-enriched) in CDCl3 solution at 298 K, and summarized in Figs. 6and 7. A linear dependence was observed between 1D15N(D)Obs vs.dNHObs and d15NObs vs.1JNH coupling constants (plot not shown).

One-bond deuterium isotope effect on 15N chemical shifts canbe measured from the HMBC spectra of partially deuteriation atthe NAH hydrogen site: 1D15N(D) = d15N(H) � d15N(D). A value of1.16 ppm for the deuterium isotope effect of 1_15N was measuredfrom the centre of the NH resonance to the centre of ND resonancein the F1 dimension, using the correlations between H-16 to NAHand NAD, respectively or to H-10. It can be seen from Fig. 7 that2DC(ND) and 1D15N(D) correlate to a reasonable degree.

3. Theoretical work

3.1. Calculated dr13C/dRNH

The two-bond deuterium isotope effects on 13C nuclear shield-ing, dr13C/dRNH, was calculated using the above mentioned geom-etries simply by shortening the NAH bond 0.01 Å, recalculating thenuclear shieldings and finally, subtracted the optimized nuclearshieldings on 13C nucleus from the nuclear shielding of reducedNAH bond lengths. The plot of calculated dr13C/dRNH vs. experi-

Fig. 6. Plot of experimentally observed 1D15N(D)Obs vs. dNHObs for cyclic enamino-carbonyl from this work; acyclic enaminocarbonyl from Ref. [4].

Fig. 7. Plot of 2DC(ND)Obs vs. 1D15N(D)Obs.

mentally observed 2DC(ND)Obs for both acyclic and cyclic enamino-carbonyl shows a good correlation (Fig. 8).

3.2. Structures

For derivatives of Meldrum’s acid only the NH-form is pre-dicted. In all but 4 a hydrogen bonded form is found. The NAHbond lengths are given in Table 1 and so are the N� � �O distances.For 4 a twisted forms are arrived at as seen in Fig. 9a and b. Thetwisted form in Fig. 9b is at higher energy (6.96 kcal/mol) as com-pared to the structure in Fig. 9a.

3.3. Calculated RNAH

The signals of the NH� � �O protons (dNH) involved in intramolec-ular hydrogen bond appear in the range 6.33–11.78 ppm. A plot ofcalculated RNAH bond lengths in Å vs. experimentally observeddNH in ppm gives a linear relationship and is shown in Fig. 10. Aplot of 1D15N(D)Obs vs. calculated RNAH bond lengths again showsa split in correlations (Fig. 11) (see also Figs. 5 and 6). In the plotsare included data (acyclic enaminones and enamino-esters) fromRef. [4].

3.4. Calculated 1H, 13C and 15N chemical shifts

The Density Functional Theory (DFT) calculations of all the com-pounds in Scheme 3 were performed to determine the 1H, 13C and15N chemical shifts. Then, a simple averaging is done for equivalentprotons. A linear regression fit of all the predicted 1H and 13C nu-clear shieldings vs. experimental values are given in the followingplots (Figs. 12–14).

4. Coupling constants

As already mentioned 1JNH spin–spin coupling constants can beused to determine the presence of either a single form or a tauto-meric equilibrium. The values of the NH coupling constant areknown to be dependent on several factors like hybridization,charge density, hydrogen bonding and polarity of the NAH bond[62,63]. 1JNH for a hydrogen bonded situation varies from 87 upto 95 Hz [63,64]. In this work, we have synthesized two 15N-en-riched model compounds (Figs. 1 and 3), 1_15N (enamino-diester)and 8_15N (enamino-ester). Other coupling constants obtainedfrom the 15N enriched derivatives are seen in Fig. 15. Especiallythe three-bond 3JCN coupling turned out to be useful as they de-pend on geometry (see previously).

Fig. 8. Plot of experimentally observed two-bond deuterium isotope effect on 13Cchemical shifts vs. calculated change in nuclear shieldings for acyclic and cyclicenaminocarbonyl.

Table 1Deuterium isotope effects in various enaminocarbonyl (measured at 298 K in CD2Cl2 unless otherwise stated), d13CObs, 2D13CObs, calculated dr13C/dRNH, 1D15NObs, dNHObs,calculated bond lengths (Å), RNAH, calculated distances RO� � �N (Å) and energies (kcal/mol).

Comp. d13CObs (ppm) 2D13CObs (ppm) dr13C/dRNHd (ppm) 1D15NObs (ppm) dNHObs (ppm) Calc. RN–H

d (Å) Calc. RO� � �Nd (Å) Energiesd (kcal/mol)

1 174.8 0.168 0.167 1.16a 11.32 1.0278 2.6053 �443330.062 174 0.159 0.173 n.m.b 11.7 1.03 2.6105 �610988.663 178.3 0.162 0.144 n.m. 11.68 1.0311 2.6027 �635659.344 166.7 0.079 0.062 n.m. 6.33 1.0142 2.86 �684997.715 170.8 0.149 0.142 n.m. 11.43 1.0296 2.639 �859628.836 173.4 0.174 0.165 n.m. 11.78 1.0299 2.5921 �780649.237 173.7 0.168 0.147 n.m. 11.72 1.0288 2.6026 �803176.478_b 172.1 0.175 0.185 1.02a 11.05 1.0265 2.7297 �346774.318_d 171.2 0.158 0.139 0.91a 10.09 1.0249 2.7342 �346774.449_b 175.7 0.168 0.174 n.m. 11.32 1.0293 2.7697 �539103.829_d 175.1 0.135 0.137 n.m. 10.38 1.0276 2.7125 �539103.0410_b 175.8 0.166 0.171 n.m. 11.33 1.0293 2.7691 �563779.6810_d 175.3 0.135 0.148 n.m. 10.44 1.0272 2.7085 �563779.0411_b 176 0.167 0.169 n.m. 11.3 1.0293 2.7671 �588454.7111_d 175.5 0.137 0.149 n.m. 10.49 1.0276 2.7057 �588454.2012_b 176.1 0.16 0.161 n.m. 11.25 1.029 2.7684 �684097.0912_d 175.6 0.134 0.144 n.m. 10.46 1.0267 2.7132 �684096.2313_Zc n.m. 0.241 0.269 1.25 10.15 1.0233 2.6876 �278545.0014_Zc 148.3 0.235 0.274 1.3 10.2 1.024 2.6781 �327887.1815_Zc n.m. 0.244 0.238 1.18 9.7 1.0235 2.6656 �278540.7816_Zc 151.5 0.272 0.293 1.33 9.7 1.0253 2.6263 �278542.7416_Ec 147.3 0.104 0.121 0.84 4.45 1.0095 – �278534.0417_Zc 161.5 0.259 0.292 1.44 10.8 1.0297 2.6515 �278548.3118_Zc 152.2 0.183 0.176 0.93 7.3 1.0187 2.6946 �301081.1418_Ec 148.5 0.095 0.113 0.69 4.6 1.0073 – �301072.7319_Zc 163 0.165 0.142 0.97 8.2 1.0194 2.7074 �301083.47

a Obtained for 15N enriched sample.b Not measured.c 2D13CObs, 1D15NObs and dNHObs for 13_Z to 19_Z are taken from Ref. [4].d Geometries for all the enaminocarbonyl were optimized at the B3LYP level of theory and using a 6-31G(d,p) basis set.

Fig. 9. (a) Calculated structure of 4 (twisted) and (b) calculated structure of 4 (twisted).

Fig. 10. Calculated RNAH bond length values plotted against the experimentallyobserved dNHObs.

Fig. 11. Calculated RNAH bond length values plotted against the experimentallyobserved 1D15N(D)Obs.

384 S. Ullah et al. / Journal of Molecular Structure 976 (2010) 377–391

Fig. 12. Relationships between the experimental and B3LYP/6-31G(d,p) 1H nuclearshieldings of all the compounds given in Scheme 3. All NAH protons are at the top,aromatic protons in the middle and non-aromatic protons at the bottom.

Fig. 13. The linear regression between the experimental and B3LYP/6-31G(d,p) 13Cnuclear shieldings of all the compounds given in Scheme 3.

Fig. 14. Plot of calculated 15N nuclear shieldings vs. observed 15N chemical shifts.

S. Ullah et al. / Journal of Molecular Structure 976 (2010) 377–391 385

A value of 13.8 Hz was obtained for 1J(N, D) coupling constantfor 1_15N. For 8_15N both couplings were 14.1 Hz. In both casesbroad line widths prevented a very accurate determination.

5. Discussion

The DFT calculations give access to NAH bond lengths and Mul-liken charges in addition to NMR parameters. The plot of calculatedNAH bond lengths vs. observed NH chemical shifts show as seen inFig. 10 a very good correlation. For intermolecular hydrogen-bonded systems Limbach et al. have also observed a correlation be-tween NH chemical shifts but in that case to a function of valencebond order and limiting chemical shifts [65].

A correlation of dNHObs vs. Mulliken charges is seen in Fig. 16.One outliner is for the twisted compound 4. The correlation indi-cates that the NH chemical shift to a large extent is governed bythe charge. A similar finding was made for OH chemical shifts of

OH groups intramolecularly hydrogen bonded to carbonyl groupsof nitro substituted o-hydroxy acylaromatics [66].

5.1. Coupling constants

d15N for the cyclic compounds correlate with 1JNH (plot nowshown). d15N increases with decreasing 1JNH coupling constant. Adecrease in 1JNH can either be ascribed to more positive chargeon nitrogen [33] or to a decrease of the NAH bond order. In thepresent case a partial positive charge will build up as a functionof conjugation with both of the C@OX groups. From the very lim-ited data set it seems like the C@OX group involved in hydrogenbonding is the determining one as seen from the finding that1JNH for 8_d is larger than for 8_b as the conjugation is to an esterin 8_b rather than to a carbonyl group (the Mulliken charges con-firm the less positive charge on 8_d).

5.2. Isotope effect on 13C chemical shifts

The secondary isotope effects on 13C chemical shifts due to thedeuterium substitution at the NAH site clearly reflect the strengthof the hydrogen bond [22,67]. Inspection of Table 1 gives a compar-ison for selected enaminocarbonyl. The 2DC(ND) of the enamino-diesters are larger than for enamino-esters, an indication of stron-ger hydrogen bond in the diester compounds. A plot of RN� � �O vs.2DC(ND) (Fig. 17) shows that hydrogen bonding to a ketone moietygives a larger 2DC(ND) for the same N� � �O distance.

NH chemical shifts are also expected to reflect the hydrogenbond strength [7]. Nevertheless, a plot of 2DC(ND) vs. dNHObs re-sults in separate lines for the cyclic and the linear compounds.The former having an extra carbonyl or ester group in conjugationwith the NH group and thereby leading to a larger positive charge.This extra positive charge is apparently influence the NH chemicalshift more than it influences 2DC(ND).

5.3. Isotope effect on 15N chemical shifts

The 15N chemical shifts and one-bond deuterium isotope effectson nitrogen nuclear shieldings are known to be very useful tools inthe investigation of NH� � �O (N� � �HO) hydrogen-bonded systems[67–69].

Simple amines and amides in general show 1D15N(D) isotope ef-fects of the order of 0.65 ppm [27]. An interesting finding is that forintermolecular hydrogen-bonded systems as seen, e.g. in proteins1D15N(D) decrease with shortening of the N� � �O@C distance [67]whereas for enaminones and esters 1D15N(D) increases withincreasing hydrogen bond strength [4]. The present study wasundertaken to investigate this phenomenon further. As seen inFig. 6 1D15N(D) correlates with dNHObs but on separate correlationlines for cyclic and non-cyclic compounds. 1D15N(D) is like2DC(ND) a gauge for hydrogen bond strength. 1D15N(D)Obs also cor-relates to some degree to d15NObs (Fig. 18).

Like 1JNH1D15N(D) depends on the charge. The less positive

charge on nitrogen the larger the isotope effect [70]. In addition,the potential will be more asymmetric for the stronger hydrogenbonded compounds, thereby explaining the larger one-bond iso-tope effects.

5.4. 1H, 13C and 15N chemical shifts

Experimental and predicted chemical shifts for both 1H, 13C and15N nuclei are presented in Figs. 12–14. All the figures reveal goodcorrelation coefficients. Data points for 4 are less good possiblyindicating that the single conformation shown in Fig. 9a is notthe only populated one.

Fig. 15. Coupling constants.

Fig. 16. Plot of Mulliken charges on NH vs. dNHObs.

Fig. 17. 2DC(ND) plotted vs. N� � �O distance.

386 S. Ullah et al. / Journal of Molecular Structure 976 (2010) 377–391

5.5. Calculations

The DFT calculations provide geometries, nuclear shieldings andchanges in nuclear shieldings. The two latter properties could beproportional [49]. This is clearly not the case for 13C or for 15N asseen from Figs. 18 and 19. According to Eq. (1) the 2DC(ND) canbe calculated by knowing dr13C/dRNH as well as [hDrXHi � hDrXDi].A plot of dr13C/dRNH vs. d13CObs shows a very good correlation(Fig. 8). This is rather unusual as for o-hydroxyacylaromatics itwas found that dr13C/dROH was rather invariant, whereas[hDrXHi � hDrXDi] was the determining factor [38].

For calculated r15N a correlation is found to the NAH bondlength (Fig. 20).

It can be summarized that 15N chemical shifts can be calculatedwell in RAHB systems.

Fig. 18. Plot of 1D15N(D) vs. d15NObs.

Fig. 19. Plot of 2DC(ND) vs. observed carbon chemical shifts (C-9 or C-1).

Fig. 20. Plot of calculated NAH bond lengths vs. calculated 15N nuclear shieldings.

Fig. 21. Plot of observed NH chemical shifts vs. calculated ones.

S. Ullah et al. / Journal of Molecular Structure 976 (2010) 377–391 387

A plot of the observed NH chemical shift vs. the calculated onesshows as seen in Fig. 21 a rather good correlation for the Z-typeenaminocarbonyl, whereas the E-type fall off the line. This couldindicate that solvent effects are important for the experimentalNH chemical shifts.

5.6. Resonance Assisted Hydrogen Bonding

It has recently been claimed that RAHB plays very little role forthe NMR properties [71,72]. 1D15N(D) are markedly larger for theZ-derivative than for the E-derivative (Table 1). In addition1D15N(D) is larger for ketones as acceptors than for esters as accep-tors (Table 1). This supports that Resonance Assisted hydrogenbonding is important in the Z-isomer. In the present study we alsofind that the NH chemical shift is proportional to the NAH bondlength both for E- and Z-derivatives (Fig. 10) illustrating the impor-

tance of conjugation and resonance. The importance of resonanceis also supported by the correlation between Mulliken chargesand observed NH chemical shifts (Fig. 16). Such a trend was seenvery clearly in o-hydroxynitroacyl aromatics [66]. The N� � �O dis-tance is not very important but again seems to depend more onthe type of acceptor than the N� � �O distance (Fig. 17). The acceptoris important for the resonance. Finally, if the acceptor was not act-ing in a RAHB way it would give rise to a decrease in 1D15N(D) withdecreasing N� � �O distance as seen in proteins [67] and ammoniumions [34].

6. Conclusions

Intramolecular hydrogen bonded and non-hydrogen bondedacyclic and cyclic enaminocarbonyl can be characterized by meansof secondary deuterium isotope effects on 13C as well as on 15Nchemical shifts. The strength of intramolecular hydrogen bondcan be estimated by 2DC(ND), 1D15N(D) and dNH. However, thetwo former correlate but not in a simple way with dNH. All thestructures of enaminocarbonyl including dr13C/dRNH, RNAH bondlengths and chemical shifts are calculated theoretically to a goodaccuracy using DFT methods. A very good correlation is found be-tween calculated NAH bond lengths and NH chemical shifts. Reso-nance Assisted Hydrogen Bonding is found to be important forexplaining the isotope effects.

For sterically hindered compounds like 4, a twist in the bondangle around the six-membered hydrogen bond system isobserved.

7. Experimental

7.1. Chemicals

Compounds Glycine ethyl ester hydrochloride, L-Phenylalanineethyl ester hydrochloride, L-Tyrosine methyl ester hydrochloride,Methylmagnesium bromide solution 3.0 M in diethyl ether,Methylamine hydrochloride, Methylamine–15N hydrochloride 98atom% 15N were purchased from Aldrich Chemicals, Weinheim,Germany, Tetrahydrofuran–2,4-dione from TCI Europe nv, Bel-gium. All organic solvents were of analytical grade and used asreceived.

7.2. Synthesis of Enaminocarbonyl (cyclic-esters)

The synthesis of acyl Meldrum’s acid [73], acyltetronic acids[24] and 5-(bis(methylthio)methylene)-2,2-dimethyl-1,3-diox-ane-4,6-dione and 3-(bis(methylthio)methylene)furan-2,4-dione[74] was accomplished according to the literature methods. Thesynthesis of all cyclic enaminocarbonyl is done by following therepresentative procedures below.

7.2.1. Representative procedure (1)The title compounds were prepared according to the literature

method [75].To a well stirred solution of 5-(bis(methylthio)methylene)-2,2-

dimethyl-1,3-dioxane-4,6-dione (2.5 mmol) in dry tetrahydrofuran(5 ml) or 3-(bis(methylthio)methylene)furan-2,4-dione (5 mmol)in dry tetrahydrofuran (5 ml) at �4 �C was added a solution ofGrignard reagent (7.5 mmol) in diethyl ether drop wise over a per-iod of 10 min under nitrogen environment and the resulting mix-ture was stirred at room temperature for further one hour.Amine salts (4%, 10 ml) were added to the reaction mixture andthe solution was stirred for additional 15 min. The organic layerwas separated and the aqueous layer was extracted with CH2Cl2

(3 � 10 ml). The combined organic layer and extracts were washed

388 S. Ullah et al. / Journal of Molecular Structure 976 (2010) 377–391

with water (3 � 30 ml), dried over MgSO4, filtered and solvent wasevaporated under reduced pressure. The residue of 2a, 6 and 7) waschromatographed on silica gel (Sigma–Aldrich, 200–400 mesh,60A) using chloroform–acetone (90:10 v/v) as eluent. The rawproduct was recrystallised from tetrahydrofuran–petroleum etherto yield pure product.

7.2.2. Representative procedure (2)To a stirred solution of aminoester hydrochloride (5 mmol) in

dry toluene (15 ml) at 0 �C under nitrogen supply, triethylamine(5 mmol) was added drop wise over a period of 15 min. Then Mel-drum’s acid derivatives (5 mmol) were added to the solution andthe mixture was stirred at reflux for 3 h. The cooled solution wasfiltered and evaporated under reduced pressure. The residue waschromatographed on silica gel (Sigma–Aldrich, 200–400 mesh,60A) using chloroform–acetone (90:10 v/v) as eluent. The solventwas evaporated from the pooled fractions and recrystallised fromchloroform–petroleum ether to afford pure product.

7.2.3. Representative procedure (3)To a stirred solution of aminoester hydrochloride (4 mmol) in

dry toluene (15 ml) at 0 �C under nitrogen supply, triethylamine(4 mmol) was added drop wise over a period of 15 min. Thentetronic acid derivatives (4 mmol) were added to the solutionand the mixture was stirred at reflux for 3 h. The cooled solutionwas filtered and hexane was added to force crystallisation. The sol-vent was filtered and residue was recrystallised from diethylether–dichloromethane–n-hexane to afford pure product.

7.2.3.1. 2,2-dimethyl-5-(1-methylamino-ethylidene)-1,3-dioxane-4,6-dione (1). The compound was prepared by following the represen-tative procedure 1. Yield 0.46 g (93%), white crystals; mp 116–117 �C. 1H NMR (CDCl3, 300 MHz) d (ppm): 11.32 (s, NH), 3.14 (d,J = 5.1 Hz, H16), 2.63 (s, H10), 1.69 (s, H7-8); 13C NMR (CDCl3,75 MHz) d (ppm) 174.8 (C9), 167.4 (C4), 163.2 (C6), 102.5 (C2),84.6 (C5), 30.6 (C16), 26.4 (C7-8), 17.5 (C10); IR (KBr) mmax 3224,3155, 3099, 2993, 2945, 1709, 1659, 1578, 1266, 1067 cm�1;MS(ESI+) for C9H13NO4 (MNa+) 221.9. Elemental analysis: calc. forC9H13NO4: C, 54.26; H, 6.58; N, 7.03. Found: C, 54.03; H, 6.53; N,7.04.

7.2.3.2. 2,2-dimethyl-5-(1-methylamino-ethylidene)-1,3-dioxane-4,6-dione (1_15N). The title compound (15N labelled) was prepared byfollowing the representative procedure 1. Yield 0.46 g (93%), whitecrystals; 1H NMR (CDCl3, 300 MHz) d (ppm): 11.32 (dq, 1J = 90.6 Hz,NH), 3.14 (dd, 3J = 5.1 Hz, H16), 2.63 (d, 3J = 2.7 Hz, H10), 1.69 (s,H7-8); 13C NMR (CDCl3, 75 MHz) d (ppm) 174.872, 174.678(CAN, 1J = 14.8 Hz) (C9), 167.398 (C4), 163.204, 163.180 (CAN,3J = 1.8 Hz) (C6), 102.435 (C2), 84.599, 84.607 (CAN, 2J = 0.5 Hz)(C5), 30.653, 30.508 (CAN, 1J = 10.9 Hz) (C16), 26.366 (C7-8),17.525 (C10). 15N NMR (60 MHz; CDCl3) d (ppm) �245.5 (N1AH),�246.66 (N1AD).

7.2.3.3. Ethyl 2-(1-(2,2-dimethyl-4,6-dioxo-1,3-dioxan-5-ylidene)eth-ylamino)acetate (2). The title compound was prepared by followingthe representative procedure 2 and 1. Yield 0.54 g (40%) with pro-cedure 2 and 0.62 g (91%) with procedure 1, white crystals; mp104–105 �C, 1H NMR (CDCl3, 300 MHz) d (ppm): 11.70 (s, NH),4.30 (q, J = 7.2 Hz, H25), 4.22 (d, J = 5.4 Hz, H16), 2.60 (s, H10),1.69 (s, H7-8), 1.33 (t, J = 7.2 Hz, H26); 13C NMR (CDCl3, 75 MHz)d (ppm) 174.0 (C9), 167.3 (C24), 167.1 (C4), 163.0 (C6), 102.6(C2), 85.74 (C5), 62.4 (C25), 45.3 (C16), 26.5 (C7-8), 18.2 (C26),14.1 (C10); IR (KBr) mmax 3239, 3185, 3103, 2997, 2939, 2909,2873, 1740, 1713, 1656, 1611, 1228, 1019 cm�1; MS (ESI+) forC12H17NO6 (MNa+) 293.8. Elemental analysis: calc. for C12H17NO6:C, 53.13; H, 6.32; N, 5.16. Found: C, 53.09; H, 6.35; N, 5.10.

7.2.3.4. Ethyl 2-(1-(2,2-dimethyl-4,6-dioxo-1,3-dioxan-5-ylidene)pro-pylamino)acetate (3). The title compound was prepared by follow-ing the representative procedure 2. Yield 0.53 g (37%), whitecrystals; mp 104–105 �C, 1H NMR (CDCl3, 300 MHz) d (ppm):11.68 (s, NH), 4.30 (q, J = 7.2 Hz, H25), 4.24 (d, J = 5.4 Hz, H16),3.03 (q, J = 7.5 Hz, H10), 1.69 (s, H7-8), 1.33 (t, J = 7.2 Hz, H26),1.25 (t, J = 7.5 Hz, H11); 13C NMR (CDCl3, 75 MHz) d (ppm) 178.3(C9), 167.6 (C24), 167.5 (C4), 162.5 (C6), 102.6 (C2), 84.8 (C5),62.5 (C25), 44.8 (C16), 26.6 (C7-8), 23.9 (C10), 14.2 (C26), 11.6(C11); IR (KBr) mmax 3172, 2991, 2944, 2924, 2883, 1746, 1703,1665, 1583, 1225, 1015 cm�1; MS (ESI+) for C13H19NO6 (MNa+)309.9. Elemental analysis: calc. for C13H19NO6: C, 54.73; H, 6.71;N, 4.91. Found: C, 54.47; H, 6.73; N, 4.91.

7.2.3.5. Ethyl 2-(1-(2,2-dimethyl-4,6-dioxo-1,3-dioxan-5-ylidene)-2,2-dimethylpropylamino)acetate (4). The title compound was pre-pared by following the representative procedure 2. Yield 0.47 g(30%), yellow oil; 1H NMR (CDCl3, 300 MHz) d (ppm): 6.33 (s,NH), 4.22 (q, J = 7.2 Hz, H25), 4.02 (d, J = 5.1 Hz, H16), 1.29 (t,J = 7.2 Hz, H26), 1.24 (s, H11-12-13), 1.23 (s, H7-8); 13C NMR(CDCl3, 75 MHz) d (ppm) 184.2 (C6), 178.9 (C9), 170.4 (C24),169.6 (C4), 101.5 (C2), 78.0 (C5), 61.5 (C25), 41.6 (C16), 38.7(C10) 27.4 (C11-12-13), 26.5 (C7-8), 14.1 (C26); IR (KBr) mmax

2928, 2855, 1740, 1663, 1602, 1515, 1466, 1377, 1353 cm�1; MS(ESI+) for C15H23NO6 (MNa+) 336.30.

7.2.3.6. Ethyl 2-((2,2-dimethyl-4,6-dioxo-1,3-dioxan-5-ylidene)(4-nitrophenyl)methylamino)acetate (5). The title compound was pre-pared by following the representative procedure 2. Yield 0.66 g(35%), white crystals; mp 134–135 �C 1H NMR (CDCl3, 300 MHz)d (ppm): 11.43 (s, NH), 8.37 (d, J = 9.0 Hz, ArH), 7.42 (d, J = 9.0 Hz,ArH), 4.20 (q, J = 7.2 Hz, H25), 3.82 (d, J = 6.0 Hz, H16), 1.74 (s,H7-8), 1.25 (t, J = 7.2 Hz, H26); 13C NMR (CDCl3, 75 MHz) d (ppm)170.8 (C9), 167.3 (C4), 166.7 (C24), 161.4 (C6), 148.6 (C10), 139.1(CAr), 127.7 (CAr), 124.3 (CAr), 103.7 (C2), 86.8 (C5), 62.5 (C25),46.2 (C16), 26.8 (C7-8) 14.0 (C26); IR (KBr) mmax 3163, 3088,2987, 2943, 2856, 1750, 1721, 1660, 1603, 1249, 1022 cm�1; MS(ESI+) for C17H18N2O8 (MNa+) 400.9; found 309.9. Elemental analy-sis: calc. for C17H18N2O8: C, 53.97; H, 4.80; N, 7.40. Found: C, 54.62;H, 5.01; N, 7.17.

7.2.3.7. Ethyl 2-(1-(2,2-dimethyl-4,6-dioxo-1,3-dioxan-5-ylidene)eth-ylamino)-3-phenylpropanoate (6). The title compound was pre-pared by following the representative procedure 2 and 1. Yield0.61 g (34%) with procedure 2 and 0.77 g (85%) with procedure 1,yellow viscous oil; 1H NMR (CDCl3, 300 MHz) d (ppm): 11.78 (d,J = 8.4 Hz NH), 7.18–7.34 (ArH, 5H), 4.57 (ddd, J = 4.8 Hz, H16),4.25 (q, J = 7.2 Hz, H25), 3.30 (dd, J = 13.8 Hz, H17), 3.10 (dd,J = 13.8 Hz, H17), 2.28 (s, H10), 1.67 (s, H7-8), 1.28 (t, J = 7.2 Hz,3H); 13C NMR (CDCl3, 75 MHz) d (ppm) 173.4 (C9), 169.3 (C4),167.1 (C25), 163.0 (C6), 134.8 (CAr), 129.4 (CAr), 129.0 (CAr),127.8 (CAr), 102.5 (C2), 85.6 (C5), 62.4 (C25), 58.5 (C16), 39.5(C17), 26.5 (C7-8) 17.6 (C10), 14.1 (C26); IR (KBr) mmax 2927,2855, 1741, 1710, 1660, 1588, 1270, 1022 cm�1; MS (ESI+) forC19H23NO6 (MNa+) 383.8.

7.2.3.8. Methyl 2-(1-(2,2-dimethyl-4,6-dioxo-1,3-dioxan-5-yli-dene)ethylamino)-3-(4-hydroxyphenyl)propanoate (7). The titlecompound was prepared by following the representative proce-dure 2 and 1. Yield 0.60 g (33%) with procedure 2 and 0.73 g(80%) with procedure 1, yellow viscous oil; 1H NMR (CDCl3,300 MHz) d (ppm): 11.72 (d, J = 8.7 Hz, NH), 7.02 (d, J = 8.4 Hz,ArH), 6.78 (d, J = 8.7 Hz, ArH), 6.47 (s, OH), 4.57 (dt, J = 4.5 Hz,H16), 3.81 (s, H25), 3.24 (dd, J = 14.1 Hz, H17), 3.03 (dd,J = 14.1 Hz, H17), 2.29 (s, H10), 1.65 (s, H7–8); 13C NMR (CDCl3,75 MHz) d (ppm) 173.7 (C9), 169.9 (C4), 167.3 (C24), 163.4 (C6),

S. Ullah et al. / Journal of Molecular Structure 976 (2010) 377–391 389

156.0 (CAr), 130.5 (CAr), 125.8 (CAr), 116.0 (CAr), 102.8 (C2), 85.4(C5), 58.6 (C16), 53.1 (C25), 38.6 (C17), 26.4 (C7-8) 17.8 (C10); IR(KBr) mmax 3620, 2927, 2855, 1748, 1709, 1660, 1589, 1518, 1271,1018 cm�1; MS (ESI+) for C18H21NO7 (MNa+) 385.8.

7.2.3.9. 3-(1-methylamino)ethylidene)furan-2,4(5H)-dione (8). Thetitle compound was prepared by following the representative pro-cedure 1. Yield 0.35 g (45%), yellow crystals; mp 106–107 �C. 1HNMR (CDCl3, 300 MHz) d (ppm): 11.04 (s, 0.66NH_b), 10.09 (s,0.34NH_d), 4.42 (s, H2_b), 4.41 (s, H2_d), 3.16 (d, J = 5.4 Hz,H16_b), 3.14 (d, J = 5.1 Hz, H16_d), 2.61 (s, H10_d), 2.56 (s,H10_b); 13C NMR (CDCl3, 75 MHz) d (ppm) 197.2 (C6b), 193.5(C6d), 176.4 (C4d), 172.4 (C4b), 172.1 (C9b), 171.2 (C9d), 92.1(C5b), 90.6 (C5d), 72.1 (C2d), 70.0 (C2b), 30.1 (C16b), 29.8(C16d), 14.2 (C10d), 13.9 (C10b); IR (KBr) mmax 3216, 2980, 2945,2926, 1727, 1651, 1504, 1254, 1228, 1022 cm�1; MS (ESI+) forC7H9NO3 (MNa+) 178.0. Elemental analysis: calc. for C7H9NO3: C,54.19; H, 5.85; N, 9.03. Found: C, 53.70; H, 5.80; N, 8.67.

7.2.3.10. 3-(1-methylamino)ethylidene)furan-2,4(5H)-dione(8_15N). The title compound (15N labelled) was prepared by fol-lowing the representative procedure 1. Yield 0.35 g (45%), 1HNMR (CDCl3, 300 MHz) d (ppm): 11.04 (dq, 1J = 91.8 Hz,0.66NH_b), 10.09 (dq, 1J = 92.4 Hz, 0.34NH_d), 4.42 (s, H2_b),4.41 (s, H2_d), 3.16 (dd, 3J = 4.5 Hz, H16_b), 3.14 (dd, 3J = 4.2 Hz,H16_d), 2.61 (d, 3J = 2.7 Hz, H10_d), 2.56 (d, 3J = 2.7 Hz, H10_b);13C NMR (CDCl3, 75 MHz) d (ppm) 197.197 (C6b), 193.553,193.533 (CAN, 3J = 1.5 Hz) (C6d), 176.408 (C4d), 172.448,172.411 (CAN, 3J = 2.8 Hz) (C4b), 172.175, 171.978 (CAN,1J = 14.8 Hz) (C9b), 171.268, 171.064 (CAN, 1J = 15.3 Hz) (C9d),92.056 (C5b), 90.624 (C5d), 72.129 (C2d), 69.980 (C2b), 30.180,30.035 (CAN, 1J = 10.9 Hz) (C16b), 29.904, 29.761(CAN,1J = 10.7 Hz) (C16d), 14.231 (C10d), 13.853 (C10b). 15N NMR(60 MHz; CDCl3) d (ppm) �239.8 (N10AH, tautomer-d), �240.71(N10AD, tautomer-d), �241.2 (N1AH, tautomer-b), �242.22(N1AD, tautomer-b).

7.2.3.11. Ethyl 2-(1-(2,4-dioxo-dihydrofuran(5H)-3-ylidene)propyla-mino)acetate (9). The title compound was prepared by followingthe representative procedure 3. Yield 0.87 g (90%), white crystals;mp 120–121 �C, 1H NMR (CDCl3, 300 MHz) d (ppm): 11.32 (s,0.77NH_b), 10.38 (s, 0.23NH_d), 4.45 (s, H2_b), 4.42 (s, H2_d),4.31 (q, J = 7.2 Hz, H25_b), 4.30 (q, J = 7.2 Hz, H25_d), 4.27 (d,J = 5.7 Hz, H16_b), 4.26 (d, J = 6.0 Hz, H16_d), 2.98 (q, J = 7.8 Hz,H10_b), 2.94 (q, J = 7.5 Hz, H10_d), 1.34 (t, J = 7.2 Hz, H26_b),1.33 (t, J = 7.2 Hz, H26_d), 1.25 (t, J = 7.5 Hz, H11_b), 1.23 (t,J = 7.5 Hz, H11_d); 13C NMR (CDCl3, 75 MHz) d (ppm) 197.8(C6b), 193.0 (C6d), 176.1 (C9b), 175.7 (C9d), 175.1 (C4d), 171.5(C4b), 167.2 (C24d), 167.1 (C24b), 91.6 (C5b), 90.4 (C5d), 72.1(C2d), 70.2 (C2b), 62.6 (C25b), 62.5 (C25d), 44.3 (C16b), 44.0(C16d), 21.4 (C10d), 21.1 (C10b), 14.1 (C26d), 14.1 (C26b), 11.4(C11b), 11.3 (C11d); IR (KBr) mmax 3453, 3355, 3196, 3130, 2988,2939, 1732, 1710, 1673, 1667, 1599, 1227, 1200, 1014 cm�1; MS(ESI+) for C11H15NO5 (MNa+) 264.1. Elemental analysis: calc. forC11H15NO5: C, 54.76; H, 6.27; N, 5.81. Found: C, 54.65; H, 6.28; N,5.80.

7.2.3.12. Ethyl 2-(1-(5-methyl-2,4-dioxo-dihydrofuran(5H)-3-yli-dene)propylamino)acetate (10). The title compound was preparedby following the representative procedure 3. Yield 0.89 g (87%),white crystals; mp 117–118 �C. 1H NMR (CDCl3, 300 MHz) d(ppm): 11.32 (s, 0.67NH_b), 10.43 (s, 0.33NH_d), 4.58 (q,J = 6.9 Hz, 2H_b), 4.43 (q, J = 6.9 Hz, H2_d), 4.32 (q, J = 7.2 Hz,H25_b), 4.30 (q, J = 7.2 Hz, H25_d), 4.25 (d, J = 5.7 Hz, H16_b),4.24 (d, J = 6.0 Hz, H16_d), 2.96 (q, J = 7.8 Hz, H10_b), 2.92 (q,J = 7.5 Hz, H10_d), 1.45 (d, J = 6.9 Hz, H7_b), 1.44 (d, J = 6.9 Hz,

H7_d), 1.34 (t, J = 7.2 Hz, H26_b), 1.33 (t, J = 7.2 Hz, H26_d), 1.25(t, J = 7.5 Hz, H11_b), 1.22 (t, J = 7.5 Hz, H11_d); 13C NMR (CDCl3,75 MHz) d (ppm) 200.6 (C6b), 195.9 (C6d), 175.8 (C9b), 175.3(C9d), 175.2 (C4d), 170.8 (C4b), 167.2 (C24d), 167.1 (C24b), 91.0(C5b), 90.0 (C5d), 79.4 (C2d), 77.3 (C2b), 62.6 (C25b), 62.5(C25d), 44.3 (C16b), 44.0 (C16d), 21.3 (C10d), 21.1 (C10b), 17.1(C7b), 17.1 (C7d), 14.1 (C26b), 14.1 (C26d), 11.4 (C11b), 11.3(C11d); IR (KBr) mmax 3190, 3051, 2978, 2934, 2872, 1728, 1651,1603, 1513, 1248, 1240, 1027 cm�1; MS (ESI+) for C12H17NO5

(MNa+) 278.2. Anal. Calc. for C12H17NO5: C, 56.46; H, 6.71; N,5.49. Found: C, 56.97; H, 6.90; N, 5.35.

7.2.3.13. Ethyl 2-(1-(5,5-dimethyl-2,4-dioxo-dihydrofuran(5H)-3-yli-dene)propylamino)acetate (11). The title compound was preparedby following the representative procedure 3. Yield 1.08 g (85%),white crystals; mp 66 �C, 1H NMR (CDCl3, 300 MHz) d (ppm):11.30 (s, 0.72NH_b), 10.49 (s, 0.28NH_d), 4.32 (q, J = 6.9 Hz,H25_b), 4.30 (q, J = 6.9 Hz, H25_d), 4.27 (d, J = 5.7 Hz, H16_b),4.26 (d, J = 6.0 Hz, H16_d), 2.97 (q, J = 7.8 Hz, H10_b), 2.96 (q,J = 7.5 Hz, H10_d), 1.43 (s, H7-8_b), 1.41 (s, H7-8_d), 1.34 (t,J = 7.2 Hz, H26_b), 1.33 (t, J = 7.2 Hz, H26_d), 1.26 (t, J = 7.5 Hz,H11_b), 1.22 (t, J = 7.5 Hz, H11_d); 13C NMR (CDCl3, 75 MHz) d(ppm) 202.7 (C6b), 198.1 (C6d), 176.0 (C9b), 175.5 (C9d), 174.4(C4d), 170.0 (C4b), 167.3 (C24d), 167.2 (C24b), 89.9 (C5b), 89.8(C5d), 85.7 (C2d), 83.4 (C2b), 62.6 (C25b), 62.5 (C25d), 44.3(C16b), 44.0 (C16d), 23.9 (C7-8b), 23.8 (C7-8d), 21.2 (C10b), 21.1(C10d), 14.1 (C26b), 14.1 (C26d), 11.5 (C11b), 11.4 (C11d); IR(KBr) mmax 3211, 3144, 3108, 2983, 2941, 2909, 2878, 1747, 1744,1740, 1737, 1671, 1643, 1614, 1611, 1607, 1215, 1203,1010 cm�1; MS (ESI+) for C13H19NO5 (MNa+) 292.1. Elemental anal-ysis: calc. for C13H19NO5: C, 57.98; H, 7.11; N, 5.20. Found: C, 58.01;H, 7.11; N, 5.27.

7.2.3.14. Ethyl 2-(1-(2,4-dioxo-5-phenyl-dihydrofuran(5H)-3-yli-dene)propylamino)acetate (12). The title compound was preparedby following the representative procedure 3. Yield 0.64 g (80%),white crystals; mp 66 �C, 1H NMR (CDCl3, 300 MHz) d (ppm):11.25 (s, 0.61NH_b), 10.46 (s, 0.39NH_d), 7.28–7.44 (m, ArH),5.46 (s, H2_b), 5.40 (s, H2_d), 4.28 (q, J = 6.9 Hz, H25_b), 4.25 (q,J = 6.9 Hz, H25_d), 4.21 (d, J = 5.7 Hz, H16_b), 4.20 (d, J = 6.0 Hz,H16_d), 2.96 (q, J = 7.8 Hz, H10_b), 2.94 (q, J = 7.5 Hz, H10_d),1.32 (t, J = 7.2 Hz, H26_b), 1.30 (t, J = 7.2 Hz, H26_d), 1.25 (t,J = 7.5 Hz, H11_b), 1.17 (t, J = 7.5 Hz, H11_d); 13C NMR (CDCl3,75 MHz) d (ppm) 197.4 (C6b), 192.7 (C6d), 176.1 (C9b), 175.7(C9d), 175.4 (C4d), 170.8 (C4b), 167.1 (C24d), 167.0 (C24b), 135.0(CAr), 128.6 (CAr), 128.5 (CAr), 125.8 (CAr), 90.1 (C5b), 90.0(C5d), 83.3 (C2d), 81.5 (C2b), 62.6 (C25), 62.6 (C25), 44.3 (C16b),44.1 (C16d), 21.4 (C10d), 21.3 (C10b), 14.0 (C26), 14.0 (C26), 11.4(C11b), 11.3 (C11d); IR (KBr) mmax 3204, 3062, 3037, 2983, 2946,1732, 1643, 1603, 1512, 1252, 1233, 1019 cm�1; MS (ESI+) forC17H19NO5 (MNa+) 340.1. Elemental analysis: calc. for C13H17NO5:

C, 64.34; H, 6.04; N, 4.41. Found: C, 64.31; H, 6.11; N, 4.44.

7.3. Instrumentation

7.3.1. NMRNMR spectra were recorded on a Varian Mercury 300 or an

Unity-Inova 600 spectrometer with operating frequency at 300 or600 MHz (5 mm triple-resonance gradient probe) for 1H, 75 or150 MHz for 13C and 60 MHz (1H 600 MHz) for 15N. 2D experi-ments were run by using the standard Varian software. TMS wasused as an internal standard both for 1H and 13C spectra whilefor 15N the data were referenced to formamide as external stan-dard. These values are converted to nitromethane as reference sub-tracting 269.7 ppm. The spectra for a range of samples wererecorded between 223 and 298 K. Deuterium Isotope Effects on

390 S. Ullah et al. / Journal of Molecular Structure 976 (2010) 377–391

13C and 15N chemical shifts were measured in one-tube experi-ments. Mestrec software was used in order to process and analysedNMR data [59]. The compounds enaminocarbonyl were dissolvedin CDCl3, while deuteriation for low temperature work was doneby dissolution of the compounds in CD2Cl2 and D2O (50:50), stirredfor 5–10 min, separating the organic layer and this was dried overanhydrous sodium sulphate overnight. Samples of 60 mg of 2,60 mg of 11, 52 mg of 1_15N (15N labelled) and 22 mg of 8_15N(15N labelled) were dissolved in 650 ll of CDCl3. 1D 1H and 2D1H–13C HSQC, HMBC; 2D 1H–15N HMQC, HMBC spectra were re-corded at 25 �C. For 1H–13C HSQC and HMBC, an acquisition timeof 0.4 s, 8 K data points with a spectral window of 10,000 Hz inthe F2 (1H) dimension and 256 data points with a spectral windowof 35,000 Hz in the F1 (13C) dimension, a 2.0 s relaxation delay and288 transient per increment were recorded. The tau delay in HMBCwas 0.063 s. For 1H–15N HMQC and HMBC, an acquisition time of0.2 s, 4 K data points with a spectral window of 10,000 Hz in theF2 (1H) dimension and 512 data points with a spectral windowof 1200 Hz in the F1 (15N) dimension, a 2.6 s relaxation delay and32 transient per increment were recorded. The tau delay in HMBCwas 0.1 s.

7.3.2. IRInfrared spectra were recorded on a Perkin Elmer FTIR 2000

spectrometer in KBr or CHCl3, respectively.

7.3.3. LC–MSLC–MS measurements were performed on HPLC instrument (a

TSP Spectra system and equipped with an AS3000 auto sampler,P4000 gradient pump, and a UV 6000 LP diode array detector)and mass detector (LCQ-Deca ion trap instrument from Thermo-Finnigan equipped with an electrospray ionization interface (ESI)run in the positive mode).

7.3.4. Melting pointsMelting points were determined on a Buchi-510 equipped with

Mettler TM-15 counter and are uncorrected.

7.4. Computational work

All the theoretical calculations were carried out using theGaussian 03 code [41] and the molecular geometries were fullyoptimized using the B3LYP variant of the Density Functional The-ory (DFT) [76,77] with the 6-31G(d,p) basis set. The NMR nuclearshieldings were calculated with the same level of theory and basisset using the GIAO method [42,78]. The obtained nuclear shiel-dings are converted in chemical shifts by comparison with calcu-lated values of tetramethylsilane.

Acknowledgements

The authors express their thanks to R. Buch for recording NMRspectra. Thanks are also due to Associate Professor Fritz Duus andDr. Fadhil S. Kamounah for their helpful discussions and to Jan Phil-lipp Hofmann for providing tetronic acids and for initialcalculations.

References

[1] G. Gilli, F. Bellucci, V. Ferretti, V. Bertolasi, J. Am. Chem. Soc. 111 (1989) 1023.[2] G. Gilli, V. Bertolasi, in: Z. Rappoport (Ed.), The Chemistry of Enols, vol. 13,

Wiley, Chichester, 1990, p. 713 (Chapter 13).[3] P. Gilli, V. Ferretti, V. Bertolasi, G. Gilli, Adv. Mol. Struct. Res. 2 (1996) 67.[4] P.E. Hansen, R. Kawecki, A. Krowczynski, L. Kozerski, Acta Chem. Scand. 44

(1990) 826.[5] S. Scheiner, Hydrogen Bonding: A Theoretical Perspective, Oxford, New York,

1997.

[6] D.K. Zheglova, D.G. Genov, S. Bolvig, P.E. Hansen, Acta Chem. Scand. 51 (1997)1016.

[7] T. Dziembowska, P.E. Hansen, Z. Rozwadowski, Prog. Nucl. Magn. Reson.Spectrosc. 45 (2004) 1.

[8] P.M. Dominiak, A. Filarowski, P.E. Hansen, K. Wozniak, Chem. Eur. J. 11 (2005)4758–4766.

[9] E. Grech, J. Klimkiewicz, J. Nowicka-Scheibe, M. Pietrzak, W. Schilf, A.F.Pozharski, V.A. Ozeryanskii, S. Bolvig, J. Abildgaard, P.E. Hansen, J. Mol. Struct.615 (2002) 121.

[10] M. Rospenk, L. Sobczyk, P. Schah-Mohammedi, H. Limbach, N.S. Golubev, S.M.Melikova, Magn. Reson. Chem. 39 (2001) S81–S90.

[11] P.E. Hansen, Prog. Nucl. Magn. Reson. Spectrosc. 20 (1988) 207.[12] J. Reuben, J. Am. Chem. Soc. 108 (1986) 1735.[13] P.E. Hansen, Magn. Reson. Chem. 24 (1986) 903.[14] E. Wenkert, H.T.A. Cheung, H.E. Gottlieb, M.C. Koch, A. Rabaron, M.M. Plat, J.

Org. Chem. 43 (1978) 1099.[15] J. Reuben, J. Am. Chem. Soc. 109 (1987) 316.[16] P.E. Hansen, Magn. Reson. Chem. 31 (1993) 23.[17] A. Filarowski, A. Koll, M. Rospenk, I. Krol-Starzomska, P.E. Hansen, J. Phys.

Chem. A. 109 (2005) 4464.[18] J. Bordner, P.D. Hammen, E.B. Whipple, J. Am. Chem. Soc. 111 (1989) 6572.[19] P.E. Hansen, in: S. Patai (Ed.), The Chemistry of Double Bonded Functional

Groups, Wiley, New York, 1989, p. 83.[20] P.E. Hansen, in: A. Kohen, H.-H. Limbach (Eds.), Isotope Effects in Chemistry

and Biology, vol. 2, CRC Press, Berlin, 2006, p. 253 (Chapter 9).[21] P.E. Hansen, Magn. Reson. Chem. 46 (2008) 726.[22] P.E. Hansen, S. Bolvig, F. Duus, M.V. Petrova, R. Kawecki, R. Krajewski, L.

Kozerski, Magn. Reson. Chem. 33 (1995) 621.[23] P.E. Hansen, F.S. Kamounah, S. Ullah, J. Mol. Struct. 844–845 (2007) 300.[24] J.P. Hofmann, F. Duus, A.D. Bond, P.E. Hansen, J. Mol. Struct. 790 (2006)

80.[25] S. Ng, H.-H. Lee, G.J. Bennett, Magn. Reson. Chem. 28 (1990) 337.[26] J. Abildgaard, P.E. Hansen, A.E. Hansen, J. Cell. Biochem. 21B (1995) 68.[27] P.E. Hansen, Isotope Effects on Chemical Shifts as a Tool in Structural Studies,

Roskilde University Press, Frederiksberg, Denmark, 1996.[28] P.E. Hansen, Magn. Reson. Chem. 38 (2000) 1.[29] P.E. Hansen, A. Lycka, Acta Chem. Scand. 43 (1989) 222.[30] V.A. Jaravine, F. Cordier, S. Grzesiek, J. Biomol. NMR 29 (2004) 309.[31] S.A. Khatipov, N.N. Shapetko, Y.S. Bogachev, Y.S. Andreichikov, Zh. Fiz. Khim.

59 (1985) 2095.[32] L. Kozerski, W. von Philipsborn, Helv. Chim. Acta 65 (1982) 2077.[33] A. Lycka, P.E. Hansen, Magn. Reson. Chem. 23 (1985) 973.[34] M. Munch, A.E. Hansen, P.E. Hansen, T.D. Bouman, Acta Chem. Scand. 46 (1992)

1065.[35] C.L. Perrin, J.B. Nielson, J. Am. Chem. Soc. 119 (1997) 12734.[36] C.L. Perrin, B.K. Ohta, J. Am. Chem. Soc. 123 (2001) 6520.[37] P.E. Hansen, J. Label, Compd. Radiopharm. 50 (2007) 967.[38] J. Abildgaard, S. Bolvig, P.E. Hansen, J. Am. Chem. Soc. 120 (1998) 9063.[39] S. Bolvig, P.E. Hansen, Curr. Org. Chem. 4 (2000) 19.[40] E. Grech, Z. Malarski, P. Milart, W. Sawka-Dobrowolska, L. Sobczyk, J. Mol.

Struct. 81 (1997) 436.[41] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,

J.A. Montgomery, T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J.Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson,H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian,J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O.Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K.Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.Martin, D.J. Fox, T. Keith, A. Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe,P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian03, Revision C.02, Inc. Gaussian: Wallingford, CT, 2004.

[42] K. Wolinski, J.F. Hinton, P. Pulay, J. Am. Chem. Soc. 112 (1990) 8251.[43] M. Barfield, P. Fagerness, J. Am. Chem. Soc. 119 (1997) 8699.[44] D.A. Forsyth, A.B. Sebag, J. Am. Chem. Soc. 119 (1997) 9483.[45] J. Abildgaard, P.E. Hansen, Wiadomosci Chem. 54 (2000) 846.[46] A.C. de Dios, J. Prog. NMR 29 (1996) 229.[47] A.C. de Dios, E. Oldfield, Solid State Nucl. Magn. Reson. 6 (1996) 101.[48] P.E. Hansen, M. Langgard, S. Bolvig, Pol. J. Chem. 72 (1998) 269.[49] C.J. Jameson, Isotopes in the Physical and Biomedical Sciences Isotopic

Application in NMR Studies, Elsevier, Amsterdam, The Netherlands, 1991.[50] N.D. Eddington, D.S. Cox, R.R. Roberts, J.P. Stables, C.B. Powell, K.R. Scott, Curr.

Med. Chem. 7 (2000) 417.[51] Z. Rappoport, The Chemistry of Enamines, John Wiley & Sons, Chichester–New

York–Brisbane–Toronto–Singapore, 1994.[52] Y.-Y. Ke, Y.-J. Li, J.-H. Jia, W.-J. Sheng, L. Han, J.-R. Gao, Tetrahedron Lett. 50

(2009) 1389.[53] J.P. Michael, C.B. de Koning, G.D. Hosken, T.V. Stanbury, Tetrahedron 57 (2001)

9635.[54] I.O. Edafiogho, M.S. Alexander, J.A. Moore, V.A. Farrar, K.R. Scott, Curr. Med.

Chem. 1 (1994) 159.[55] I.O. Edafiogho, J.A. Moore, M.S. Alexander, K.R. Scott, J. Pharm. Sci. 83 (1994)

1155.

S. Ullah et al. / Journal of Molecular Structure 976 (2010) 377–391 391

[56] I.O. Edafiogho, O.A. Phillips, E.E. Udo, S. Samuel, B. Rethish, Eur. J. Med. Chem.44 (2009) 967.

[57] D.L. Boger, T. Ishizaki, R.J. Wysocki, S.A. Munk, P.A. Kitos, O. Suntornwat, J. Am.Chem. Soc. 111 (1989) 6461.

[58] K.R. Scott, I.O. Edafiogho, E.L. Richardson, V.A. Farrar, J.A. Moore, E.I. Tietz, C.N.Hinko, H.J. Chang, A. Elassadi, J.M. Nicholson, J. Med. Chem. 36 (1993) 1947.

[59] <http://mestrelab.com/>.[60] N. Tilting, Synthesis monomerer und dimerer 3-acyl-tetramsäuren und 3-acyl-

4-hydroxy-2-pyrimidon as potentielle chelatoren für Mg2+ ionen, Ph.D. Thesis,Göttingen University, 1983.

[61] S. Bolvig, F. Duus, P.E. Hansen, Magn. Reson. Chem. 36 (1998) 315.[62] G. Dudek, E.P. Dudek, J. Chem. Soc. B – Phys. Org. (1971) 1356.[63] G.O. Dudek, E.P. Dudek, J. Am. Chem. Soc. 86 (1964) 4283.[64] L.N. Kurkovskaya, R.N. Nurmukhametov, D.N. Shigorin, Zh. Strukt. Khim 21

(1980) 61.[65] S.N. Smirnov, H. Benedict, N.S. Golubev, G.S. Denisov, M.M. Kreevoy, R.L.

Schowen, H.H. Limbach, Can. J. Chem. – Revue Canadienne De Chimie 77(1999) 943.

[66] M. West-Nielsen, P.M. Dominiak, K. Wozniak, P.E. Hansen, J. Mol. Struct. 789(2006) 81.

[67] J. Abildgaard, P.E. Hansen, M.N. Manalo, A. LiWang, J. Biomol. NMR 44 (2009)119.

[68] P.E. Hansen, Z. Rozwadowski, T. Dziembowska, Curr. Org. Chem. 13 (2009) 194.[69] T. Dziembowska, Z. Rozwadowski, A. Filarowski, P.E. Hansen, Magn. Reson.

Chem. 39 (2001) S67.[70] R.E. Wasylishen, J.O. Friedrich, Can. J. Chem. 65 (1987) 2238.[71] I. Alkorta, J. Elguero, O. Mo, M. Yanez, J.E. Del Bene, Mol. Phys. 102 (2004) 2563.[72] I. Alkorta, J. Elguero, O. Mo, M. Yanez, J.E. Del Bene, Chem. Phys. Lett. 411

(2005) 411.[73] Y. Oikawa, K. Sugano, O. Yonemitsu, J. Org. Chem. 43 (1978) 2087.[74] G.A. Hunter, H. McNab, J. Chem. Soc., Perkin Trans. 1 (1995) 1209.[75] X. Huang, B.-C. Chen, Synthesis (1987) 480.[76] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.[77] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.[78] R. Ditchfield, Mol. Phys. 27 (1974) 789.