Tetrahedron Letters. Vo1.32, No.2, pp 255.258. 1991

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A NEW CONVENIENT APPROACH TO PEPTIDE SYYTHESIS

USING A DISELENIDE AXD A PHOSPHINE

Usha Singh, Sunil K. Ghosh, Mohindra S. Chadha and Vasant R. MamdaplirC Bio-Organic Division, Bhabha Atomic Research Centre, Bombay 400 085, INDI.4.

Abstract: A new a"d highly efficient lwthod based on d redox reaction usirlg diphenyl diselenide snd tributylphosphine for the general synthesis of peptides is described. The side reaction due to selenophenol formation is overcome by using chalcoile as a scavenger or by elective oxidation to diselenide.

Several methods for the synthesis of peptides have been developed' over the past

few decades. Nevertheless, interest in the development of newer and more eEficacious

methods for peptide synthesis continue3 unabated. I" particular, the problex of

racemizatio" 2

and undesired side reactions 3

still remains. To circumvent these, Mukaiyama

et. al. 4

used different chemical pr0toc01s to establish conditions for a

reduction-oxidation condensation for peptide synthesis using disul.fides and p!lospl:i"es.

The course of this redox reaction goes via initial Eormation of a pent‘3covalent

acyloxyphosphonium intermediate 4

which in turn reacts with the amine colnponeut to afford

a peptide.

I" our endeavours, we have formulated a novel selenium mediated 5

approach to pcptide

synthesis. Due to the same periodicity as sulfur, we presumed that the seleniua-analo&ue

should follow similar course of reaction. I" addition, the softness of seleniun1 coupled

with the weakness of P-Se bond should drive the reaction under milder conditions thereby

preserving the optical integrity of the product. This communication describes ozr results

from the newly developed method using diphenyl diselenide and tributylphosphine as the

oxidant and reductant respectively.

Treatment of a mixture of N-protected amino acid, amino acid ester and Ph2Se2 with

BugP at room temperature led to the formation of the peptide albeit in moderate yield.

This was attributed to the formation of a complex between the amine ‘olnponcllt dnd the

selenophenol generated & situ. However, this was overcome by using chalcone as d

scavenger which forms a neutral complex with the 6

selenophenol as described

in Procedure A7.

Here also, we envisaged the involvement of d pentacoordinated

intermediate 4'8'9 (SCHEME 1)

acylowyphosphonium

- -* which in turn can generate the peptide via two routes:

(a) direct nucleophiLic attack of the amine component on it, (b) intramolecular PhSe

transfer to generate selenol ester and its subsequent conversionlO to the peptide. In

contrast to the sulfur-intermediate, the second pathway seems to be more plausible I1 here,

in view of the exceptional properties of selenium as mentioned above and due tc its hi::h

nucleophilicity 12

and weak basicity.

25s

256

S C 3 E ME I

Bu3 P+Ph,Se2 y [Bu&Ph Phje] _“pcRs”,+ E”,i B”T SePh

RCONH R’+ PhSeH

f

R’N Hz

:. R-C-SePh M

PhSe transfer

Pseudorotation

PhSe C=O

The dibOVe procedure has afforded a non-racemising and simple route to peptide

synthesis. liowever, the possible formation of by-products necessitates use of

chromatogrdphic separation of the desired peptides. We, therefore felt that one of the

essrlrtial features to optimize this method requires the removal of the selenophenol and

synchrouous ~elleratioil of free a m i n e component to avoid its side reactions with

selenoptlenol. This was achieved by usin;: aaine saltI and oxidising the selenophenol

with N~met'nylnlorpho~ine H-oxide. 'I'h e rasultinj N-methylmorpholine 14

can in turn

effectively deprotonate the amine salt to furnish free amine for subsequent peptide

condensation15 and release of selenophenol. This in turu reacts again with

N-mrthylmotpholine !i-oxide constituting thereby d self propagated peptide synthesis a~ 16

described in Procedure B .

The extent of racemization during coupling was evaluated by diftrrent racemization

tests 17-19 . The results are summarized in Table 1, which illustrates a low level of

racemiLation, demonstrating thereby the significance of this imethod.

This methodology has been effectively applied with different amine and carboxyl

components (Table 2) leading to several peptides without racemization of which valine

enkeyhalin and decapcptide 20

(entry 16). 3 linear precursor of antamanide 21

are worth

cf note. Furthermore, the reaction conditions (Procedure B) are ideally suited for the

synthesis of cyclic peptides 1,22

where activstion of linear peptides need to be done under

bimolecular reaction conditions followed by unimoleculdr cycliLation.

Frouz the analysis of the reatilts, it can be concluded that highly efficient procedures

for peptide synthesis usin;; a novel redox systeu hdve been developed. The method alsc

obviates the need of flrotection of hydroxyl group (phenolic and alcoholic) and use of

rigorously dried solvents. It is noteriortiy that Procedure B is also applicable tc

tTyptUphd,l and >uethionine containing peptide; without ariy side rraction. Furthermore,

it iilvolver use of amine components as their salts and in majority of cases products dc

not require chromdtogrdphic purification. III addition, the valuable diselenide may be

recuvrred dnd recycled.

257

TABLE 1: Degree of Racemization During, Peptide Bond Formation

Entry Peptide Solvent XII or D-L in product with coupling agent

1. i. 3. 4. 5. 6.

BzmLewGly-OEt 17

, t Bz-Pile-Val-OMe

18

1 , A-Phe-Ala-OPle

19

I 1

Entry Peptide

1.

i. 3. 4. 5. 6. i. 8. 9.

10. Il. Z-Phe-Leu-oqe

Z-Gly-PhelAla-OBz 1 ,

B 90 110 -17.8 A 71 85 -38.7 B 87 120 -19.6 B 80 94 -19.6 B 85 116 -12.3 A 72 101 -14.0 A 74 107 t6.1 B 85 107 +6.1 A 73 Liq. -35.4 B 80 " -36.2 A 76 111 -24.3

(2.0 (1.0

;:*: (1:o (0.9

g*: (1:o (1.0

(1.0 12.

77 102 -22.1 13. 82 101 -22.4 :;*: 14. Z-TyrlGly2-Phe-Val-OB"' A 60 103 -16.0 (0:3

15. Z-Pro2-GlylPro2-Gly-OEt 13 80 -168.5 16. Z-Phe-Pro2-Phe2lVal-Pr~2-Ala-Phe-OMe R iz 125-6 -131.1 ):*I: 17. Cycle (G~Y)~ 36 276(dec.) - (1 18. Cycle (Pro 2-GlylPro2-Gly)

B;'; ;b,c ;:, 211-3 -140.7 (0.5

19. Cycle (Gig-LOU-GlylGly-LOU-Gly) 257(dec.) -8.7 (0.8

Z-Phe-Gly-OEt Z-Ala-Val-OMe Z-Tyr-Gly-OEt Z-Met-Gly-OEt Boc-Trp-Gly-OBt Z-Ala--Phc-OMe Z-Thr-Phe-OMe

I , Z-Pro-Phe-OMe , I

Ph2Se2-Bu3P

Procedure A Procedure B DCCI DCCI-HOBT

15 3 48 15 7 12 12

14 42 12 29 293 32 5 13 28 <3 7 4 35 5

TABLE 2: List of Peptides!

Procedure Yield m.p. L4D23 (C Solvent)

EtOH) MeOH) EtOH) f I

1 I ; MeOH) AcOH)

" ) MeOH)

" ) ,1

!A11 ainino acids used are of L configuration. Peptides obtained are identified by comparison of m.p., Ictl,, IK, and WR spectra reported in the literature or with authentic samples. The"vertical line indicates the point of coupling. a) Reaction is c<3rried out by treating peptide hydrochloride (leq,) with Bu3P (2.5eq.) and Ph$e:, (1.2eq.) in WI1 at r.t. over 4h., then diluted with N-methylluvrpholine N-oxide monohydrate (2.5~1.)~ solution in Cl12C12-DIF (1:l) and standing over 24h. b) Reaction is carried out by treating peptide hydrochloride (leq.) with Bu3P (2.5eq.) and Ph2Se2 (1.5eq.) in CHC13,-DMF (1:l) at 60°C over 3h., then diluted with N-;ilethylmurpholiriE H-owidc monohydrate (2.5eq.) solution in DMF and kept at 55OC over 48h. c) Peptide is 'xll-er soluble, therefore aqueous part frq the workup _(Procedure B) which contains peptide was isolated after successive elution through cation (II ) and anion (OH ) exchange resins.

Ac!knowledymcnr : Xe are thadcful to Dr. K. Nagardjan, Scarle R. R D. Centre, Bombay, for optical rotations. The authors

(U.S. and 9.S.C) gratefully acknowledge the sugpurt uf DST, New Delhi.

1. Y. Bodansrky, "Principles of Peptide Synthesis" Springer-Verlag, Berlin, Heidelberg 1984; M. Bodanszky, w. J. Pept. Prot. Res., 1985, 25, 449; "I. Bodanszky, "Peptide Chemistry, A Practical Text Book", Springer-VerlaA, Berlin, 1988.

2. For recent reviews on racemisation in peptide synthesis, see: D. S. Kemp, "The Peptides", vol. 1, E. Grz;:;, .I. ?!leiwhufer, eds., pp. 315, !!w York, icademic Press 1979.

3. For rece,it reviews on side reactions in peptide synthesis, see: M. Hodanszky, 3. Martinez, "The Peptides", vol. 5. E. Gross, J. Meienhofer, eds., pp. 111, New York, Academic Press 1983.

d . T. Mukaiyao3, 1,:. !Jeki, II. Slaruyama and R. tiatsueda, J. Amer. Chem. SIX., 1968, 90, 4490; idem., ibid.,

1969, 21, 1554. Fur recent reviews on reduction-oxidation condensation, see: T. Mukaiyama, Angew. Cilf, 1976, 8& 111; T. E!ukaiyamd, R. Matsueda, M. Ueki, Peptides, 1980, 2, 383.

258

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

For direct preparation of amide using selenium reagents, see: K. C. Nicolaou, N. A. Petasis and II .A. Claremon, Tetrahedron, 1985, 41, 4835. For recent reviews on organoselenium chemistry, see: C. Paulmier "Scleuium Reagents and Intermediates in Organic Synthesis" Pergamon Press, New York, 1986; D. Liocta "Organoselenium Chemistry", Wiley, New York, 1987.

Ft. Gilman and L. F. Cason, J. Amer. Chem. Sot., 1951, 73, 1074.

Procedure A: In a typical experiment Bu3P (1.5eq.) is added dropwise under argon/nitrogen to a stirred solution of N-protected amino acid or peptide (leq.), amino acid or peptide ester (leq.), diphenyl diselenide (leq.) and chalcone (Zeq.) in CH2C12/DMF at room temperature. The reaction mixture was

stirred for 3-Sh. Ilsual workup gave the product which was purified by silica gel chromatography.

R. Ramage, "Organophosphorus Reagents in the Synthesis of Peptides" in Organophosphorus Reageutu in Organic Synthesis, 1979, 511, Cadogan, John Ivan George, Academic, London; J. Zheng, Y. Zhao, youji HLiaxlle R.

1989, 8, 398. For reaction mechanism involving pentacoordinated phosphorus species, see: R. H0lmes "Pentacoordinated Phosphorus-Reaction Mechanism", ACS Monograph 176, American Chemical

Society, Washington DC, 1980, vol. II, Chapter 2.

The acyloxyphosphonium intermediate which is likely to be more ionic in character in this case is detected by low temperature 31P NMR (-60 ppm) which vanishes with the formation of tributylphosphine oxide (-50 ppm); cf. B. E. Maryanoff and A. B. Reitz, Chem. Rev., 1989, B, 863.

For use of selenol esters in auide and peptide synthesis, see: H. D. Jakubke, Z. Chem., 1963, 3, 65;

H. D. Jakubke, Chem. Ber., 1964, 97, 2816; K.D. Jakubke, Justus Liebigs Ann. Chem., 1965, 682, 244.

The selenol ester formation and its subsequent conversion to peptide has been confirmed by low temperature exprrimerlts.

6. Guanti, C. Dell'Erba and D. Syinelli, Gazz. Chim. Ital., 1970, 100, 184.

The preseijce of amine s&c also accelerater the selenol ester formation.

In terms of preservation of optical integrity !&methylmorpholine is better than other tertiary amines; see; G. W. Anderson, J. E. Zimmerman, F. ?l. Callahan, J. Aner. Chem. SOL., and M. Itoh, Bull. Chem. Sac. Jpn., 1967, 40, 646.

1966, 88, 1338; S. Sakakibara

Although overall reaction goes via selenol ester intermediate, selenol ester formation and peptide condensatiou steps are well separated.

Procedure B: In a typical experiment Bu3P (leq.) i; added dropwise under argon/nitrogen to a mixture

of N-yrotected amino acid or peptide (leq.), amino acid or peptide ester hydrochloride or p-toluenesulfonate in CH2C1, or CH Cl -DMF yellow color disappears Indicating % 2

at room temperature and stirred over 0.5-2h. The initial he completion of selenol ester formation. To this stirred mixture

-lO"C, !&methylmorpholine N-oxide monohydrate (leq.) was added over lo-15 min. and the mixture was stirred at room temperature over 12h. The solution turned yellow-orange due to regeneration of diphenyl drselenide. Usual workup and triturdtion of the crude product with chloroform-pet. ether gives peptides which are separated by filtration. The filtrate contains tributylphosphine oxide and dipheayl diseleaide.

R. Knot-r, A. Trzeciak, W. Bannwarth and D. Gillessen, Tetrahedron Lett., 1989, 30, 1927.

B. Halpern, L. F. Chew and B. Weinstein. J. Aver. Chem. Sot., ail11 A. E. Pritchard, J. Chew. Sot. Perk. I, 1972, 1015,

1967, 89, 5051; see, also: B. Weinsteiu

Synthesis of decapeptide from pentapeptide units by this method gave product of optical rotation practically equal to that obtained by stepwise DCCI-HOBT method.

T. !?ieland, G. Luben, H. Ottenheyw and H. Schiefer, Liebigs Ann. Chem., 1969, 722, 173.

For review 011 cyclic peptides, see: Y. Imanishi, Adv. Polym. SC., 1976, 0, 1; U. Schmidt, Pure Appl. G, 1986, s, 295.

(Receivedin UK5 September 1990)