Upload
others
View
10
Download
0
Embed Size (px)
Citation preview
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
11
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 - -
14
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.
16
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
22
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.