2

Synopsis

Title of the Thesis: Synthesis of Triazolylated Nucleosides, Coumarins and

Coumarinyldihydropyrimidinones and Selective Biocatalytic Acylation

& Antifungal Activity Studies on Coumarins.

The thesis is divided into three chapters, i.e. Chapter I, Chapter II and Chapter III. Chapter II

is further divided into two Sections, i.e. Section-A and Section-B.

Chapter I: Synthesis of 3'-substituted Triazolylnucleosides

Chapter II: Synthesis and Antifungal Activity of Novel Azido and 1,2,3-Triazole containing

Coumarins & their Enzymatic Stereoselective Acylation Studies

This Chapter is divided into two Sections:

Section A: Synthesis and Antifungal Activity of Novel Azido and 1,2,3-Triazole containing

Coumarins

Section B: Enzymatic Stereoselective Acylation Studies on Novel Azido and 1,2,3-Triazole

containing Coumarins

Chapter III: Synthesis of Novel Coumarinyldihydropyrimidinones and their N-Acylates

A brief account of each Chapter is given below:

Chapter I

Synthesis of 3'-substituted Triazolylnucleosides

The copper (I)-catalyzed Huisgen-Sharpless-Meldal click reaction has gained significant

importance because of its wide range of applications in the synthesis of drugs and drug like

molecules, bioconjugates and useful materials. Further, triazolyl nucleosides, which can be

3

generated using click reaction, are of special interest because of their pronounced biological

activities.

A very promising area for the preparation of new bioconjugates is the synthesis of

azidonucleosides. In general, azido analogs have been used mostly as intermediates in the

preparation of aminonucleosides. But discovery of 3'-azido-3'-deoxythymidine (AZT) as an

inhibitor of HIV reverse transcriptase triggered explosive developments in the synthetic

chemistry of azidonucleosides. In order to discover new derivatives potentially endowed with

biological activity, the copper-catalyzed azide/alkyne 1,3-dipolar cycloaddition reaction has

also been applied to the functionalization of sugar and base moieties of nucleosides. A

number of reports have demonstrated the potency of triazole linked nucleosides. Triazole

moiety can be linked on nucleoside at various positions such as 2', 3', 5' and also at anomeric

position of sugar part as well as base. Based on these compounds, the most common

application of the Cu-catalyzed azide–alkyne 1,3-cycloaddition reaction has been the

condensation of azido sugar moiety with various alkynes in order to form modified

nucleosides bearing a substituted 1,2,3-triazole. In this context, it has to be mentioned that the

potency of of all azido derivatives of nucleosides in such cycloaddition reactions has been

explored (Figure 1) except of 3'-azido-3'-deoxy-5-methyluridine.

Figure 1

Coumarin derivatives are widely used as fluorescent probes, labels and pigments,

laser dyes and signalling units in sensors. They are also attractive molecules due to their

extended spectral range, high emission quantum yields and photo stability.

In the present work, we have achieved the synthesis of a series of coumarin, aryl and

alkyl conjugated triazolylnucleosides by using click chemistry. One of the precursor moieties

4

3'-Azido-3'-deoxy-5-methyluridine 8 required for the synthesis of the targeted compound was

prepared in seven steps from readily available D-xylose (Scheme 1). D-xylose (1) was

selectively protected as a monoketal, 1,2-O-isopropylidene-α-D-xylofuranose 2 in a two step

reaction in 90 % yield. The primary hydroxyl group of compound 2 was selectively protected

with benzoyl chloride and pyridine to produce the benzoyl derivative 1,2-O-isopropylidene-

5-O-benzoyl-α-D-xylofuranose 3 in 85% yields. In order to introduce a 3-azido group,

compound 3 was converted into 1,2-O-isopropylidene-5-O-benzoyl-3-O-

trifluoromethanesulfonyl-α-D-xylofuranose 4 by reaction with trifluoromethanesulfonic

anhydride in 85 % yield, which was subsequently converted into a 3-azido-1,2-O-

isopropylidene-5-O-benzoyl-3-deoxy-α-D-xylofuranose derivative 5 in approximately 45%

yield with sodium azide in DMF at 60 oC. The azide 5 was converted into 3-azido-1,2-di-O-

acetyl-5-O-benzoyl-3-deoxy-β-D-ribofuranose 6 in 75% yield as an epimeric mixture with

acetic acid, acetic anhydride in pyridine. The 3-deoxy-3-azidoribofuranoside 6 was then

coupled with thymine as base by using trimethylsilyl triflate as lewis acid (Vorbruggen

coupling) to afford the corresponding azido nucleoside 3'-azido-2'-O-acetyl-5'-O-benzoyl-3'-

deoxy-5-methyluridine 7 in 83 % yield. Removal of acetate and benzoate esters of the azido

nucleoside gave the key intermediate 3'-azido-3'-deoxy-5-methyluridine 8 in 79 % yield.

The mild reaction conditions and high fidelity of Cu (I)-catalysed process allowed the

1,3-dipolar cycloaddition of 3'-azido-3'-deoxy-5-methyluridine 8 with commercially available

alkynes phenylacetylene (9a), propargyl alcohol (9b) and 5-Chloro-1-pentyne (9c) by using

0.15 molar equiv. of CuI in mixture of THF: H2O:EtOH (1:1:1) solution at 60 oC to afford the

conjugates 3'-deoxy-3'-(4-phenyl-1,2,3-triazol-1-yl)-5-methyluridine (10a), 3'-deoxy-3'-(4-

hydroxymethyl-1,2,3-triazol-1-yl)-5-methyluridine (10b) and 3'-deoxy-3'-{4-(3-chloroproyl)-

1,2,3-triazol-1-yl]-5-methyluridine (10c) in yields ranging from 80 to 92 % (Schemes 2).

5

Scheme 1: Synthesis of 3'-azido-3'-deoxy-5-methyluridine 8.

Scheme 2: Synthesis of 3'-triazole bridged nucleoside conjugates via Cu (I) catalysed 1,3-

dipolar cycloaddition reaction.

The other key substrates, propargyloxycoumarins and propargyloxynaphthalenes 13a-

f (Scheme 3) were prepared using an established method. Reaction of 7-hydroxycoumarins

(11a), 7-hydroxy-4-methylcoumarin (11b), 3-ethyl-7-hydroxy-4-methylcoumarin (11c), 4-

6

hydroxycoumarin (11d) with propargylbromide (12) afforded 7-propargyloxycoumarin (13a),

4-methyl-7-propargyloxycoumarin (13b), 3-ethyl-4-methyl-7-propargyloxycoumarin (13c),

4-propargyloxycoumarin (13d) in 81 to 95 % yields. Similarly, Reaction of β-naphthol (11e)

and α-naphthol (11f) with propargylbromide (12) afforded 2-propargyloxynaphthalene (13e)

and 1-propargyloxynaphthalene (13f) in 91 to 93 % yields, respectively.

Further, Cu (I)-catalysed 1,3-dipolar cycloaddition reaction between 3'-

azidonucleoside 8 and alkyne derivatives 13 a–f using 0.15 molar equiv. of CuI in a mixture

of THF: H2O: EtOH (1:1:1) solution at 60 oC afforded the coumarin / naphthylene conjugates

of nucleosides, i. e. 3'-deoxy-3'-[4-(coumarin-7-yloxymethylene)-1,2,3-triazol-1-yl]-5-

methyluridine (14a), 3'-deoxy-3'-[4-(4-methylcoumarin-7-yloxymethylene)-1,2,3-triazol-1-

yl]-5-methyluridine (14b), 3'-deoxy-3'-[4-(3-ethyl-4-methylcoumarin-7-yloxymethylene)-

1,2,3-triazol-1-yl]-5-methyluridine (14c), 3'-deoxy-3'-[4-(coumarin-4-yloxymethylene)-1,2,3-

triazol-1-yl]-5-methyluridine (14d), 3'-deoxy-3'-[4-(naphthyl-2-yloxymethylene)-1,2,3-

triazol-1-yl]-5-methyluridine (14e) and 3'-deoxy-3'-[4-(naphthyl-1-yloxymethylene)-1,2,3-

triazol-1-yl]-5-methyluridine (14f) in 76 to 85 % yields (Schemes 4). The structures of

synthesized compounds were unambiguously established on the basis analysis of their

spectral data (1H-,

13C NMR, IR spectroscopy and HRMS).

Scheme 3: Synthesis of coumarin/naphthyl derivatives with terminal alkyne functionality.

7

Scheme 4: Synthesis of 3'-triazole bridged coumarin/naphthyl conjugated nucleoside via Cu

(I) catalysed Huisgen 1,3-dipolar cycloaddition.

Chapter II

This Chapter is divided into two Sections, i.e. Section-A and Section-B:

Section-A

Synthesis and Antifungal Activity of Novel Azido and 1,2,3-Triazole containing

Coumarins

Harmful infections caused by various fungal species, such as Aspergillus and Candida,

have been increasing in prevalence throughout the world. Due to the increase in the

number of immune-compromised patients, such as cancer and HIV patients, primary and

opportunistic fungal infections are also mounting rapidly. Furthermore, resistance to the

existing antifungal agents is a growing problem. Almost all major classes of commercial

antibiotics have encountered resistance in clinical applications, even though the

pharmaceutical industry has produced a number of new antibiotics. Therefore, the need for

novel therapeutic agents with potency, a wide therapeutic window, and broad-spectrum

activity is critical. The first generation of azoles antifungal inhibitors of CYP51, have

revolutionized treatment of some serious fungal infections. Triazoles have been the

leading agents for the control of fungal diseases of humans and animals for over last 20

8

years. According to this, azole derivatives are currently the most widely studied class of

antifungal agents.

The 1,2,3-triazole scaffold, a small molecular heterocyclic sub-structure, is very

important in the field of medicinal chemistry and has received much attention in the last

few decades due to its chemotherapeutic value. As well as coumarins (Benzopyran-2-ones)

form an elite class of compounds, which occupy a special role in nature and have been

isolated both from synthetic and natural sources. It was found to be crucial for a variety of

pharmacological effects such as inhibition of platelet aggregation, anti-inflammatory, anti-

convulsant, anti-viral, anti-HIV, anti-coagulant, anti-oxidant, anti-bacterial, anti-

tubercular, anti-carcinogenic and anti-fungal.

Encouraged by these results, we have synthesized a new series of triazolyl derivatives

containing a coumarin backbone with diversity at the C-3 and C-7 positions via Cu(I)-

catalyzed click chemistry and have investigated their antifungal potential using Aspergilus

as model pathogens. These compounds contain different alkyl chain at the C-3 position of

the coumarin ring and different aryl ethers at the C-4 position of 1,2,3-triazole ring. The

synthesis of desired product was achieved starting from C-3 alkyl coumarins 18 and 19,

which were synthesized via. pechman condensation of 2-alkylated ethylacetoacetate 17a

and 17b with resorcinol (Scheme 6). The 2-alkylated ethyl acetoacetate in turn were

prepared by alkylation of ethyl acetoacetate (15) using alkyl bromide 16a and 16b in THF

in the presence of sodium hydride (Scheme 5). The C-3 alkylated substituted coumarins

were condensed with epichlorohydrin using 0.6 M NaOH solution in methanol to yield the

corresponding epoxides, 20 and 21.

Scheme 5: Alkylation of ethyl acetoacetate

9

Further, the opening of the epoxy ring compounds 20 and 21 using sodium azide in

the presence of ammonium chloride in a methanol-water (4:1 v/v) solution afforded the 7-

(3-azido-2-hydroxypropoyloxy)-3-alkyl-4-methylcoumarins 22 and 23 in quantitative

yields (Scheme 6).

Scheme 6: Synthesis of 7-(3-azido-2-hydroxypropyloxy)-3-alkyl-4-methylcoumarin

The mild reaction conditions and high fidelity of Cu (I)-catalyzed process allowed the

1,3-dipolar cycloaddition of 7-(3-azido-2-hydroxypropyloxy)-3-alkyl-4-methylcoumarins

22 and 23 with commercially available alkynes 24 a-d in presence of catalytic amount of

copper sulphate and sodium ascorbate in t-BuOH/H2O/THF at 50 °C to afford 7-(3-(4-

alkyl-1,2,3-triazol-1-yl)-2-hydroxypropyloxy)-3-alkyl-4-methylcoumarins 25 a-d and 26

a-d in 80 to 92 % yield (Schemes 7).

10

Scheme 7: Synthesis of 7-(3-(4-alkyl-1,2,3-triazol-1-yl)-2-hydroxypropyloxy)-3-alkyl-4-

methylcoumarins

Similarly, coumarins 22 and 23 were condensed with propargyl aryl ethers 28a-u to

afford triazole containing coumarins 29a-u and 30a-u in 90 to 95 % yields (Schemes 9).

Propargyl aryl ethers 28a-u were prepared in excellent yield by heating substituted phenols

27 a-u with propargyl bromide and K2CO3 in acetone at 60 °C for 12 hr (Scheme 8).

Scheme 8: Synthesis of Propargyl aryl ethers

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Scheme 9: Synthesis of 7-(2-hydroxy-3-(4-substituted-1,2,3-triazol-1-yl)propyloxy)-3-

alkyl-4-methylcoumarins.

Thus, a series of fifty triazole containing coumarins 25a-d, 26a-d, 29a-u and 30a-u

have been synthesized together with the coumarin precursors 18, 19, 20, 21, 22 and 23. The

antifungal activity of fifty two compounds, i. e. coumarins 22, 23, 25a-d, 26a-d, 29a-u and

30a-u were evaluated against A. fumigatus, A. niger and A. flavus. The structures of

synthesized compounds 17a-b, 18, 19, 20, 21, 22, 23, 25a-d, 26a-d, 29a-u and 30a-u were

unambiguously established by analysis of their spectral data (1H NMR,

13C NMR, IR, HRMS

spectra).

12

Antifungal Activity

The synthesized coumarin derivatives 22, 23, 25a-d, 26a-d, 29a-u and 30a-u were evaluated

for the antifungal inhibition. The detailed results of Antifungal activity of these compounds

are shown in (Table 1).

Table 1: Antifungal inhibitory activity of 22, 23, 25a-d, 26a-d, 29a-u and 30a-u

Comp. A. fumigatus

MIC

A. niger

MIC

A. flavus

MIC

μg/ml

(MDA)

μg/disc

(DDA)

μg/ml

(MDA)

μg/disc

(DDA)

μg/ml

(MDA)

μg/disc

(DDA)

22 15.62 3.91 62.50 15.62 125.00 31.25

23 62.50 15.62 62.50 31.25 250.00 62.50

25a 500.00 125.00 500.00 250.00 - -

25b 62.50 31.25 125.00 31.25 - -

25c 62.50 15.62 62.50 31.25 125.00 31.25

25d 125.00 31.25 125.00 31.25 250.00 62.50

26a 500.00 125.00 - - - -

26b 125.00 31.25 - - - -

26c 250.00 62.50 - - 500.00 125.00

26d 31.25 7.81 31.25 15.62 125.00 31.25

29a 500.00 125.00 250.00 62.50 500.00 125.00

29b 500.00 250.00 500.00 125.00 - -

29c 250.00 62.50 125.00 62.50 250.00 62.50

29d 125.00 31.25 125.00 31.25 250.00 125.00

29e - - - - - -

29f 500.00 125.00 500.00 250.00 - -

29g 500.00 125.00 500.00 250.00 - -

29h 500.00 125.00 250.00 125.00 - -

29i 62.50 15.62 125.00 31.25 125.00 31.25

29j - - - - - -

13

29k - - - - - -

29l - - 250.00 62.50 - -

29m - - - - - -

29n - - - - - -

29o - - - - - -

29p 500.00 250.00 500.00 125.00 - -

29q 500.00 125.00 - - - -

29r 62.50 15.62 62.50 31.25 250.00 62.50

29s - - - - - -

29t - - - - - -

29u - - - - - -

30a - - - - - -

30b - - - - - -

30c 500.00 250.00 500.00 125.00 - -

30d 500.00 125.00 250.00 62.50 - -

30e - - - - - -

30f 125.00 31.25 62.50 31.25 - -

30g - - - - - -

30h - - - - - -

30i 125.00 62.50 125.00 31.25 - -

30j - - - - - -

30k - - - - - -

30l - - - - - -

30m - - - - - -

30n - - - - - -

30o - - - - - -

30p 500.00 125.00 - - 500.00 125.00

30q 500.00 250.00 500.00 250.00 - -

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30r 250.00 125.00 500.00 250.00 500.00 250.00

30s - - - - - -

30t - - - - - -

30u - - - - - -

‘-’ These compounds did not show any activity even at the highest tested concentration.

These results clearly show that among all the tested azido-coumarin and 1,2,3-triazole

containing coumarins, 7-(3-azido-2-hydroxypropyloxy)-3-ethyl-4-methylcoumarin (22)

inhibited A. fumigatus at lowest concentration i. e. 3.91 μg/disc in disc diffusion (DDA) and

15.62 μg/ml in microbroth dilution assays (DDA). In case of A. niger it showed activity at

15.62 μg/disc in DDA and 62.50 μg/ml in MDA. While in case of A. flavus it inhibited at

31.25 μg/disc in DDA and 125.00 μg/ml in MDA, 7-(2-hydroxy-3-(4-hydroxymethyl-1,2,3-

triazol-1-yl)propyloxy)-3-hexyl-4-methylcoumarin (26d) inhibited at low concentration 7.81

μg/disc in DDA and 31.25 μg/ml in MDA in case of A. fumigatus. In case of A. niger it

showed activity at 15.62 μg/disc in DDA and 31.25 μg/ml in MDA. While in case of A. flavus

it inhibited at 31.25 μg/disc in DDA and 125.00 μg/ml in MDA. Compounds 23, 25b, 25c,

25d, 26b, 29d, 29i, 29r, 30f and 30i exhibited moderate to good antifungal activity whereas

remaining compounds i. e. 25a, 26a, 26c, 29a, 29b, 29c, 29e, 29f, 29g, 29h, 29l, 29p, 29q,

29c, 30d, 30p, 30q and 30r did not show any appreciable activity. And the rest are inactive as

evident from the data presented in the (Table 1).

Section B

Enzymatic Stereoselective Acylation Studies on Novel Azido/1,2,3-Triazole containing

Coumarins

Since several years, enzymes are being recognized as efficient catalyst for many of the

stereo-specific and regio-selective reactions. The potential of enzymes is well recognized for

selective acylation /deacylation of different functional groups of similar reactivity present in

the molecule. Some of the lipases have been found selective for acylation/deacylation of

hydroxyl group(s). This stimulating background and our own interest in the lipase-mediated

chemical transformations prompted us to explore the possibility of lipase-mediated selective

acylation studies on coumarin derivatives. Some of the synthesized racemic azidocoumarins

and 1,2,3-triazole containing coumarins in Chapter II Section A have been found to be potent

15

antifungal agents. Thus, in this chapter we have studied enzymatic resolution of biologically

active azidocoumarins 22-23 and triazole containing coumarins 25d and 26d by enzymatic

acylation reaction using CRL (Candida rugosa lipase) as biocatalyst. Different lipases i. e.

Candida antarctica lipase-B (CAL-B), Theremomyces lanuginosous lipase immobilized on

silica (Lipozyme TL IM) and porcine pancreatic lipase (PPL) were screened for

enantioselective acylation of racemic azidocoumarins 22-23 and triazole containing

coumarins 25d and 26d in six sets of organic solvents, i.e. diisopropylether (DIPE), toluene,

tetrahydrofuran (THF), dioxane, acetonitrile (CH3CN) and acetone using vinyl acetate as

acylating agent for acylation at 50 oC and at 200 rpm in an incubator shaker. Initial screening

of reactions was done at small scale and reactions were monitored using TLC. Reaction using

PPL in any of the solvents didn’t progress at all even at higher temperature upto 60 oC. The

acylation reaction using Novozyme-435 was not found to be selective because it led to almost

complete acylation of racemic azidocoumarins and triazolylated coumarins compound in all

the selected solvents. The Lipozyme catalyzed acylation reaction in THF, acetonitrile and

acetone didn’t yield any product. Reactions using CRL in toluene and DIPE were better

among the others and didn’t go to completion at all even after long time period. It was

observed that lipase CRL in toluene at 50 oC selectively and most efficiently acylates the

azidocoumarins 22-23 and triazolylated coumarins 25d and 26d. No reaction was observed in

the absence of enzyme.

After initial screening of the acylation reaction with different lipases and solvents,

CRL catalyzed acylation reaction of 7-(3-azido-2-hydroxypropyloxy)-3-ethyl-4-

methylcoumarin 22 in toluene at 50 oC was monitored using reverse phase HPLC and it was

observed that after nearly 50 % completion of the reaction, reaction progress was not

significant (Figure 1). So, it was a good idea to stop the acylation reaction at approximately

50 % completion to achieve better enantioselectivity.

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Figure 1: Progress of enzymatic acylation reaction of 7-(3-azido-2-hydroxypropyloxy)-3-ethyl-4-

methylcoumarin (22) as monitored on HPLC technique using reverse phase column.

After selecting the better condition for the enzymatic acylation reaction, both the

azidocoumarins 22-23 and triazolylated coumarin derivatives 25d and 26d were selectively

acylated in toluene at 50 oC in the presence of CRL. In a typical reaction, synthesized racemic

7-(3-azido-2-hydroxypropyloxy)-3-ethyl/hexyl-4-methylcoumarins (22 / 23) or 7-(2-

hydroxy-3-(4-phenyl-1,2,3-triazol-1-yl)propyloxy)-3-ethyl/hexyl-4-methylcoumarin-2H-1-

benzopyran-2-one (25d / 26d), (1 mmol) in dry toluene (15 ml) was incubated with CRL (50

% by weight w.r.t. reactant in the presence of vinyl acetate (3 mmol) at 50 oC and at 200 rpm

in the incubator shaker. The progress of the reaction was monitored periodically by reverse

phase HPLC. After approximately 50 % conversion of the starting material into the product,

the reaction was quenched by filtering off the enzyme and solvent was removed from the

reaction-mixture to get the gummy solid which was subjected to column chromatography to

afford optically enriched enzymatically acylated (+)-7-(3-azido-2-acyloxypropyloxy)-3-

ethyl/hexyl-4-methylcoumarin (31 / 32) or (+)-7-(2-acyloxy-3-(4-phenyl-1,2,3-triazol-1-

yl)propyloxy)-3-ethyl/hexyl-4-methylcoumarin (33 / 34) and the unreacted (-)-7-(3-azido-2-

hydroxypropyloxy)-3-ethyl/hexyl-4-methylcoumarin (22 / 23) or (-)-7-(2-hydroxy-3-(4-

phenyl-1,2,3-triazol-1-yl)propyloxy)-3-ethyl/hexyl-4-methylcoumarin (25d / 26d)

respectively. All the enzymatically acylated substrates, i.e. (+)-7-(3-azido-2-

acyloxypropyloxy)-3-ethyl/hexyl-4-methylcoumarin (31 / 32) or (+)-7-(2-acyloxy-3-(4-

phenyl-1,2,3-triazol-1-yl)propyloxy)-3-ethyl/hexyl-4-methylcoumarin (33 / 34) were

17

hydrolyzed with potassium carbonate in dry methanol to afford (+)-7-(3-azido-2-

hydroxypropyloxy)-3-ethyl/hexyl-4-methylcoumarin (22 / 23) or (+)-7-(2-hydroxy-3-(4-

phenyl-1,2,3-triazol-1-yl)propyloxy)-3-ethyl/hexyl-4-methylcoumarin (25d / 26d) in 80-84 %

yield (Scheme 10 and 11). Optical rotations of unreacted laevorotatory substrates, i.e. (-)-22,

(-) 23, (-) 25d and (-) 26d and the corresponding dextrorotatory hydroxy azidocoumarins and

triazolylated coumarins (+)-22, (+)-23, (+)-25d and (+)-26d obtained by chemical hydrolysis

of enzymatically acylated dextrorotatory azidocoumarins and 1,2,3-triazolylated coumarin

compounds (+)-31, (+)-32, (+)-33 and (+)-34 substrates were found to be comparable (Table

2). This shows the selectivity of CRL for the acylation of particularly one enantiomer in the

racemic azidocoumarins and 1,2,3-triazolylated coumarins 22, 23, 25d and 26d. Optical

rotation of compounds (-)-22 and (+)-22 were found to be comparable (Table 2). The

structures of compounds (-)-22-23, (-)-25d, (-)-26d, (+)-31-34 were unambiguously

identified by comparison of their spectral data (IR, 1H NMR,

13C NMR spectra and HRMS)

with the corresponding racemic compounds, i.e. (±)-22-23, (±)-25d and (±)-26d.

Scheme 10: Acylation of racemic azidocoumarins, (±)-22 and (±)-23 in the presence of CRL.

18

Scheme 11: Acylation of racemic triazolylated coumarins (±)-25d and (±)-26d in the

presence of CRL.

Table 4 Specific rotation values ([α]D, c (0.1, MeOH)

Substrate Recovered, unreacted

hydroxy

azido/triazolylated

coumarin (-)-22, (-

)23, (-)-25d and (-)-

26d

Acyloxy

azido/triazolylated

coumarin (+)-31-34

obtained by

enzymatic acetylation

of (±)-22-23, (±)-25d

and (±)-26d

Hydroxy

azido/triazolylated

coumarin (+)-22-23,

(+)-25d and (+)-26d

obtained by chemical

deacylation of

enzymatically acylated

(+)-31-34

22 (-)-22: -5.99 (+)-31: +16.57 (+)-22: +4.67

23 (-)-23: -4.68 (+)-32:+15.01 (+)-23: +3.80

25d (-)-25d: -6.01 (+)-33:+18.22 (+)-25d: +5.12

26d (-)-26d: -5.73 (+)-34:+15.87 (+)-26d: +4.89

19

Chapter III

Synthesis of Novel Coumarinyldihydropyridinones and their N-Acylates

In the design of new drugs, the development of hybrid molecules through the combination of

different pharmacophores may lead to the compounds with interesting biological profiles.

Heterocyclic compounds, viz. coumarin constitute an important class of natural/synthetic

polyphenolic compounds that shows diverse biological properties like anticoagulant,

antifungal, antibiotics, antimicrobial, antiviral, antioxidant, anticancer, anti-inflammatory,

etc. These pharmacological properties of coumarins attracted attention of organic chemists to

synthesize libraries of several new compounds featuring different heterocyclic rings attached

to the coumarin moiety with an aim to obtain more potent pharmacological active

compounds. In addition, 3,4-dihydropyrimidinone (DHPM) class of compounds are excellent

starting synthons which show many interesting properties including calcium channel

modulators, α1a-adrenergic receptor antagonists, mitotic kinesin inhibitors and antiplatelate

activity. Encouraged by these results shown by coumarin and dihydropyrimidinones, we have

synthesized coumarinyldihydropyrimidinones (CDHPMs) and their N-acylates which have

both coumarin (isomer of chromone) and dihydropyrimidinone moieties in the same

molecule. These hybrid molecules are expected to give a synergistic effect.

The synthesis of compounds 51 (a-s) have been achieved starting from the synthesis

of 4-methylcoumarins 39-41, following the well known Pechmann condensation reaction.

Pyrogallol (35), resorcinol (36) and phloroglucinol (37) were condensed with ethyl

aetoacetate (38) in the presence of sulphuric acid to give 4-methylcoumarins 39-41 in 77 to

80 % yields. The coumarins so obtained had free hydroxyl which was then methylated using

dimethylsulphate (42) in acetone in the presence of potassium carbonate to yield 7,8-

dimethoxy-4-methylcoumarin 43, 7-methoxy-4-methylcoumarin 44 and 5,7-

dimethoxycoumarin 45. Selenium dioxide (46) was then used to convert the active methyl

group present at C-4 position in coumarins 43, 44 and 45 to coumarin aldehydes 47, 48 and

49 (Scheme 12).

20

Scheme 12: Synthesis of 4-formylcoumarins 47-49

The well known Biginelli reaction was followed to synthesise

coumarinyldihydropyrimidinones using condensation of 7,8-dimethoxy-4-formylcoumarin

47, 7-methoxy-4-formylcoumarin 48 and 5,7-dimetoxy-4-formylcoumarin 49 with urea and

appropriate β-keto ester 50 a-g in absolute ethanol in the presence of conc. sulphuric acid as

catalyst to afforded compounds 51 a-s in 50-55% yields (Scheme 13). It is worthy to mention

here that the Biginelli cyclocondensation reaction remained incomplete even after 40 hrs of

refluxing in ethanol when 1 molar equivalent of β-keto ester and urea with respect to

coumarinyl aldehyde were used. The same reaction was completed within 24 hrs of refluxing

in ethanol with 50-55 % yield when molar equivalents of β-keto ester and urea were

increased upto 3 equivalents with respect to coumarinyl aldehyde.

21

Scheme 13: Synthesis of CoumarinylDHPMs

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The synthesis of CDHPMs having variation at the C-6 and at the ester linkage of the

DHPM ring and C-5, C-7 and C-8 positions of coumarin has been achieved (Scheme 13).

The 7,8-dimethoxy coumarin DHPM were acylated at N-3 position with acid anhydride

(acetic-, propanoic-, butanoic-, pentanoic-, hexanoic and benzoic anhydride) (2 molar equiv.)

in DCM at room temperature using 4-N,N-dimethylaminopyridine (DMAP) (0.5 equiv.) as a

catalyst to afford N-acytaed derivatives, i.e. 53a-f, 54a-f and 55a-f in 61-70 % yield

(Scheme-14).

Scheme 14: Synthesis of N-acylated CoumarinylatedDHPMs

Hence, a series of thirty seven different coumarinyldihydropyrimidinones and their N-

acylates were synthesized having different ester chain and acyloxy chain. The structures of all

synthesized thirty seven compounds 51a-s, 53a-f, 54a-f and 55a-f were unambiguously

established on the basis of their spectral data (1H,

13C NMR, IR and HRMS) analysis.