estelar - inflibnetshodhganga.inflibnet.ac.in/bitstream/10603/48801/3...product. the crude product...

51

Chapter 3

Synthesis and antibacterial activity of

phenolic monoterpene derivatives

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3.1. Introduction

Natural products have been the mainstay in providing novel chemical

scaffolds for many drugs1 as well as leads that were chemically modified and

developed as antibacterial agents. Herbs and higher plants containing terpenoids and

their oxygenated derivatives have been used as fragrance and flavours for centuries.

Terpenes have been drawn increasing commercial attention because of their role in

prevention and therapy of several diseases including natural insecticides and

antimicrobial agents. The large terpene family has provided numerous examples of

antibacterial compounds including monoterpenoids, sesquiterpenoids, diterpenoids

and triterpenoids.2-5 Monoterpenes are abundant natural C-10 compounds that meet

the non-toxicity and ecological role for new drug candidates. Antiparasitic activities

have already been reported for pure monoterpenes.6

The phenolic monoterpenes are well represented in the family Asteraceae,7

particularly within the genus Senecio, Eupatorium and Inula.8,9 Monoterpenoid

phenol derivatives, like zingerone and vanillin are potent vanilloid receptor agonists.

Guaiacol is used as an expectorant.10 Eugenol, the principal chemical component of

clove oil from Eugenia aromatica and Cinnamon11 has been reported to inhibit the

growth of several microbes viz. Listeria monocytogenes, Bacillus cereus,

Campylobacter jejuni, Escherichia coli, Salmonella enterica and fungi viz.

Microsporum gypseum and Aspergillus sp. by interaction with the cell membrane.12,13

The monoterpenes thymol, carvacrol, menthol and carveol exhibited marked potency

in the snail vector of schistosomiasis and showed molluscicidal activity.14 Thymol and

carvacrol are biologically active monoterpene phenols that have been isolated from

Thymus vulgaris, Origanum vulgare, Satureja thymbra and Thymbra capitata15-18

and

are widely used as general antiseptic in medicine, cosmetics and food industry due to

their potent fungicide, bactericide and antioxidants properties.19-21

As component of

volatile oils in many plants they have been proved to possess antimicrobial and

antioxidant activities.22

The essential oils of Satureja cuneifolia, Origanum dictamnus and Thymus

caramanicus having thymol and carvacrol as major constituents showed significant

antimicrobial activity against Staphylococcus aureus, Bacillus subtilis, Salmonella

enterica, Escherichia coli and Campylobacter jijuni and human pathogenic fungus.23-

26 Several hydroxyl derivatives of thymol, isolated from Centipida minima and 10-

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isobutyryloxy-8,9-epoxythymol isobutyrate from Inula helenium root showed good to

moderate antimicrobial activity against Gram-positive bacteria.27,28

Semi-synthetic

studies of natural products have been a reliable method for the generation of more

active and less toxic derivatives. Chemical modification of natural monoterpenoids to

various ether and ester derivatives have been reported to result in modification of

biological activity.29,30

Keeping the diverse therapeutic activities of phenolic monoterpenes in view, it

was contemplated to synthesize a novel series of thymol and carvacrol derivatives to

improve the antibacterial activity of title compounds. Literature search revealed

limited reports on the ester and ether derivatives of thymol and carvacrol. The

synthesis and antibacterial evaluation of fourteen esters and six ether analogues of

thymol and carvacrol have been given here.

3.2. Experimental

3.2.1. Scheme for the synthesis of ester derivatives

OH

O

OHR O R

O+

i. SOCl2

ii. TEA, DCM

O

OHR+

i. SOCl2

ii. TEA, DCM

OH

Th 1 - Th 7thymol acid derivatives

Ca 1 - Ca 7carvacrol acid derivatives

CH3 CH2CH3 CH

CH3

CH3

H2C CH

CH3

CH3

HCCH

CH3 H2C

where R =

Scheme 3.1. Reagents and conditions: i. SOCl2, reflux, 2 h; ii. TEA, anhydrous DCM,

0 oC, 10 h

O R

O

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3.2.2. Scheme for the synthesis of ether derivatives

OH

Br

O

R

O

O

R

+

i

Br

O

R+

i

thymol p-substituted

phenacyl bromide derivativesTh 8 - Th 10

OH O

O

R

carvacrol p-substituted

phenacyl bromide derivativesCa 8 - Ca 10

where R =Br CH3 OCH3

Scheme 3.2. Reagent and condition: i. K2CO3, MeCN, 0 oC - rt, 8 -10 h

3.2.3. General experimental procedure for the synthesis of ester derivatives

(Th 1 to Th 7) and (Ca 1 to Ca 7)

To the acid derivatives (1.0 mequiv) was added thionyl chloride (3.0 mequiv)

drop wise at 0 oC then the resulting reaction mixture was refluxed for 2 h with

constant stirring. After completion of reaction, excess of thionyl chloride was

evaporated under reduced pressure to obtain acid chloride derivatives. To a solution

of thymol or carvacrol (1.0 equiv) and triethylamine (1.1 equiv) in anhydrous

dichloromethane (20 ml) at 0 oC were added corresponding acid chlorides (1.1 equiv).

The reaction mixture was stirred at 0 oC for about 1 h and the stirring was continued

at room temperature for about 10 h (progress of reaction was monitored by TLC and

GC). After completion of reaction, reaction mixture was diluted with 50 ml of

distilled water and extracted with DCM (3×50 ml). The combined organic layer was

washed with distilled water (3×50 ml), brine solution (3×50 ml) and dried over

anhydrous sodium sulphate, concentrated it under reduced pressure to get crude

product. The crude product was purified by column chromatography using silica gel

(60-120 mesh) column with 3-4% diethyl ether: hexane as an eluting solvent to get

pure corresponding ester derivatives (Th 1 to Th 7) and (Ca 1 to Ca 7) with 75 - 85%

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yield. All the ester derivatives were characterized with the help of their IR, MS, and

NMR (1H &

13C NMR) spectroscopic data.

3.2.4. General experimental procedure for the synthesis of ether

derivatives (Th 8 to Th 10) and (Ca 8 to Ca 10)

To a solution of thymol or carvacrol (1.0 equiv) in dry acetonitrile, were added

powdered potassium carbonate (5.0 equiv) and corresponding p-substituted phenacyl

bromide derivatives (1.1 equiv) at 0 oC. The reaction mixture was stirred at 0

oC for

about 15 min and the stirring was continued at room temperature for about 8-10 h

(progress of reaction was monitored by TLC). After completion of reaction, the

reaction mixture was quenched with distilled water and extracted with

dichloromethane (3×50 ml), combined organic layer was washed with aqueous

solution of sodium bicarbonate (3×50 ml), distilled water (3×50 ml), brine solution

(3×50 ml) and dried over anhydrous sodium sulphate. After removal of solvent in

vacuo, the residue obtained was purified by column chromatography using silica gel

(60-120 mesh) column with Hexane/Et2O (1:20 to 2:20) as eluent to afford the pure

ether derivatives (Th 8 to Th 10) and (Ca 8 to Ca 10) in 61– 78% yields. All the ether

derivatives were characterized with the help of their IR, MS, and NMR (1H &

13C

NMR) spectroscopic data.

3.3. Synthesis of derivatives

3.3.1. Synthesis of ester derivatives of Thymol (Th 1 to Th 7)

3.3.1.1. Synthesis of 2-isopropyl-5-methylphenyl acetate (Th 1)

The general synthetic method described earlier, afforded compound Th 1 from

thymol (1.0 g, 6.6 mmol, 1.0 mequiv) with acetyl chloride (0.5 g, 7.3 mmol. 1.1

mequiv) and triethyl amine (0.7 g, 7.3 mmol, 1.1 mequiv) in anhydrous DCM (20 ml)

as reaction solvent.

O

O

2-isopropyl-5-methylphenyl acetate

Molecular Formula: C12H16O2 (M.W. 192.0); Physical state: colourless liquid;

Yield: 80%; Purity: 97%; GC-MS (EI, 70 eV): m/z (%)= 192 ([M+], 5), 150 (36),

135 (100), 115 (6), 107 (10), 91 (8); IR υmax: 3028, 2962, 2872, 1760, 1622, 1368,

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1207, 1149, 1089, 899, 671 cm-1;

1H NMR (CDCl3, 500 MHz): δ 7.18 (d, J = 8.0 Hz,

1H, Ar-H), 7.01 (d, J = 7.5 Hz, 1H, Ar-H), 6.80 (s, 1H, Ar-H), 2.93-3.00 (m, 1H, CH),

2.30 (s, 6H, CH3), 1.18 (d, J = 7.0 Hz, 6H, CH3); 13

C NMR (CDCl3, 125 MHz): δ

169.7 (C=O), 147.8 (CAr), 136.9 (CAr), 136.3 (CAr), 127.0 (CHAr), 126.4 (CHAr), 122.6

(CHAr), 27.0 (CH), 25.4 (COCH3), 22.9 (2 × CH3), 20.8 (CH3).

3.3.1.2. Synthesis of 2-isopropyl-5-methylphenyl propionate (Th 2)


thymol (1.0 g, 6.6 mmol, 1.0 mequiv) with propionyl chloride (0.6 g, 7.3 mmol. 1.1



O

O

2-isopropyl-5-methylphenyl propionate



135 (100), 115 (4), 107 (6), 91 (8); IR υmax: 3028, 2964, 2872, 1759, 1621, 1461,

1344, 1194, 1151, 897 cm-1;

1H NMR (CDCl3, 500 MHz): δ 7.17 (d, J = 8.0 Hz, 1H,

Ar-H), 6.99 (d, J = 7.5 Hz, 1H, Ar-H), 6.80 (s, 1H, Ar-H), 2.95-2.97 (m, 1H, CH),

2.58 (q, J = 7.5 Hz, 2H, CH2), 2.29 (s, 3H, CH3), 1.27 (t, J = 7.5 Hz, 3H, CH3), 1.18

(d, J = 7.0 Hz, 6H, CH3); 13

C NMR: (CDCl3, 125 MHz): δ 172.5 (C=O), 147.8 (CAr),

136.8 (CAr), 136.3 (CAr), 126.9 (CHAr), 124.0 (CHAr), 122.6 (CHAr), 34.9 (CH2), 27.0

(CH), 22.8 (2 × CH3), 20.6 (CH3), 12.0 (CH3).

3.3.1.3. Synthesis of 2-isopropyl-5-methylphenyl isobutyrate (Th 3)


thymol (1.0 g, 6.6 mmol, 1.0 mequiv) with isobutyryl chloride (0.7 g, 7.3 mmol. 1.1



O

O

2-isopropyl-5-methylphenyl isobutyrate

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135 (100), 107 (6), 91 (8); IR υmax: 3029, 2964, 2871, 1756, 1618, 1461, 1344, 1224,

1149, 878 cm-1;

1H NMR (CDCl3, 500 MHz): δ 7.20 (d, J = 8.0 Hz, 1H, Ar-H), 7.04

(d, J = 8.0 Hz, 1H, Ar-H), 6.78 (s, 1H, Ar-H), 3.24 (hept, J = 7.0 Hz, 1H, CH), 2.83

(hept, J = 7.0 Hz, 1H, CH), 2.30 (s, 3H, CH3), 1.32 (d, J = 6.5 Hz, 6H, CH3), 1.18 (d,

J = 6.5 Hz, 6H, CH3); 13

C NMR (CDCl3, 125 MHz): δ 175.3 (C=O), 148.1 (CAr),

137.1 (CAr), 136.1 (CAr), 126.5 (CHAr), 126.2 (CHAr), 122.5 (CHAr), 33.4 (CH), 26.9

(CH), 22.6 (2 × CH3), 20.5 (CH3), 18.7 (2 × CH3).

3.3.1.4. Synthesis of 2-isopropyl-5-methylphenyl 3-methylbutanoate (Th 4)

The general synthetic method described earlier, afforded compound Th 4 from thymol

(1.0 g, 6.6 mmol, 1.0 mequiv) with isovaleryl chloride (0.8 g, 7.3 mmol. 1.1 mequiv)

and triethyl amine (0.7 g, 7.3 mmol, 1.1 mequiv) in anhydrous DCM (20 ml) as

reaction solvent.

O

O

2-isopropyl-5-methylphenyl 3-methylbutanoate



135 (100), 107 (4), 91 (8); IR υmax: 3028, 2931, 2872, 1757, 1621, 1463, 1364, 1238,

1150, 816 cm-1;


(d, J = 8.0 Hz, 1H, Ar-H), 6.79 (s, 1H, Ar-H), 2.96-2.99 (m, 1H, CH), 2.46 (d, J = 7.0

Hz, 2H, CH2), 2.31 (s, 3H, CH3), 2.25-2.29 (m, 1H, CH), 1.18 (d, J = 7.0 Hz, 6H,

CH3), 1.07 (d, J = 7.0 Hz, 6H, CH3); 13

C NMR (CDCl3, 125 MHz): δ 171.6 (C=O),

147.7 (CAr), 136.8 (CAr), 136.3 (CAr), 126.8 (CHAr), 126.2 (CHAr), 122.5 (CHAr), 43.2

(CH2), 26.8 (CH), 25.6 (CH), 22.9 (2 × CH3), 22.3 (2 × CH3), 20.7 (CH3).

3.3.1.5. Synthesis of (E)-2-isopropyl-5-methylphenyl but-2-enoate (Th 5)


thymol (1.0 g, 6.6 mmol, 1.0 mequiv) with crotonyl chloride (0.7 g, 7.3 mmol, 1.1

mequiv) and triethyl amine (0.7 g, 7.33 mmol, 1.1 mequiv) in anhydrous DCM (20

ml) as reaction solvent.

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O

O

(E)-2-isopropyl-5-methylphenyl but-2-enoate



135 (100), 107 (6), 91 (8); IR υmax: 3025, 2927, 2871, 1761, 1657, 1621, 1458, 1363,

1231, 1154, 816 cm-1;

1H NMR (CDCl3, 500 MHz,): δ 7.16 (d, J = 8.0 Hz, 1H, Ar-

H), 7.00 (d, J = 8.0 Hz, 1H, Ar-H), 6.80 (s, 1H, Ar-H), 6.00-6.08 (m, 1H, CH), 5.25

(d, J = 11.0 Hz, 1H, CH-), 2.88-3.01 (m, 1H, CH), 2.29 (s, 3H, CH3), 1.94 (d, J = 6.0

Hz, 3H, CH3), 1.17 (d, J = 7.0 Hz, 6H, CH3); 13

C NMR: (CDCl3, 125 MHz): δ 169.0

(C=O), 146.8 (CAr), 145.5 (CH), 135.8 (CAr), 135.4 (CAr), 126.0 (CHAr), 125.2 (CHAr),

121.7 (CHAr), 118.0 (CH), 25.9 (CH), 21.8 (2 × CH3), 19.7 (CH3), 17.0 (CH3).

3.3.1.6. Synthesis of 2-isopropyl-5-methylphenyl benzoate (Th 6)


thymol (1.0 g, 6.6 mmol, 1.0 mequiv) with benzoyl chloride (1.0 g, 7.3 mmol. 1.1

mequiv) and triethyl amine (7.4 g, 7.3 mmol, 1.1 mequiv.) in anhydrous DCM (20 ml)


O

O

2-isopropyl-5-methylphenyl benzoate



149 (82), 105 (100), 91 (4), 77 (48), 51 (8); IR υmax: 3032, 2926, 2870, 1736, 1621,

1451, 1363, 1236, 1150, 816 cm-1;

1H NMR (CDCl3, 500 MHz): δ 8.27 (d, J = 7.5

Hz, 2H, Ar-H), 7.69 (t, J = 7.5 Hz, 1H, Ar-H), 7.57 (t, J = 8.0 Hz, 2H, Ar-H), 7.29 (d,

J = 8.0 Hz, 1H, Ar-H), 7.12 (d, J = 7.5 Hz, 1H, Ar-H), 6.99 (s, 1H, Ar-H), 3.07-3.15

(m, 1H, CH), 2.39 (s, 3H, CH3), 1.26 (d, J = 6.5 Hz, 6H, CH3); 13

C NMR (CDCl3, 125

MHz): δ 165.4(C=O), 148.1 (CAr), 137.2 (CAr), 136.7 (CAr), 133.5 (CAr), 130.1 (2 ×

CHAr), 129.6 (CHAr), 128.6 (2 × CHAr), 127.2 (CHAr), 126.5 (CHAr), 122.9 (CHAr),

27.3 (CH), 23.1 (2 × CH3), 20.9 (CH3).

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3.3.1.7. Synthesis of 2-isopropyl-5-methylphenyl-2-phenylacetate (Th 7)


thymol (1.0 g, 6.6 mmol, 1.0 mequiv) with 2-phenyl acetyl chloride (1.1 g, 7.3 mmol.

1.1 mequiv) and triethyl amine (7.4 g, 7.3 mmol, 1.1 mequiv.) in anhydrous DCM (20


O

O

2-isopropyl-5-methylphenyl-2-phenylacetate



135 (100), 118 (26), 91 (34), 65 (6); IR υmax: 3031, 2927, 2870, 1753, 1621, 1454,

1363, 1230, 1149, 817 cm-1;


Ar-H), 7.30 (t, J = 7.5 Hz, 2H, Ar-H), 7.24 (t, J = 7.5 Hz, 1H, Ar-H), 7.10 (d, J = 8.0

Hz, 1H, Ar-H), 6.94 (d, J = 7.5 Hz, 1H, Ar-H), 6.74 (s, 1H, Ar-H), 3.80 (s, 2H, CH2),

2.69-2.75 (m, 1H, CH), 2.23 (s, 3H, CH3), 1.02 (d, J = 8.0 Hz, 6H, CH3); 13

C NMR:

(CDCl3, 125 MHz): δ 170.2 (C=O), 148.0 (CAr), 137.1 (CAr), 136.5 (CAr), 133.7 (CAr),

129.4 (2 × CHAr), 128.8 (2 × CHAr), 127.4 (CHAr), 127.2 (CHAr), 126.4 (CHAr), 122.7

(CHAr), 41.7 (CH2), 27.0 (CH), 23.0 (2 × CH3), 19.7 (CH3).

3.3.2. Synthesis of ester derivatives of Carvacrol (Ca 1 to Ca 7)

3.3.2.1. Synthesis of 5-isopropyl-2-methylphenyl acetate (Ca 1)

The general synthetic method described earlier, afforded compound Ca 1 from

carvacrol (1.0 g, 6.66 mmol, 1.0 mequiv) with acetyl chloride (0.5g, 7.3 mmol. 1.1



O

O

5-isopropyl-2-methylphenyl acetate



135 (100), 107 (12), 91 (10); IR υmax: 3025, 2961, 2928, 2871, 1766, 1623, 1460,

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1369, 1215, 1169, 819 cm-1;


Ar-H), 7.00 (dd, J = 1.5 Hz & 7.5 Hz, 1H, Ar-H), 6.85 (d, J = 1.5 Hz, 1H, Ar-H),

2.85-2.87 (m, 1H, CH), 2.29 (s, 3H, CH3), 2.12 (s, 3H, CH3), 1.21 (d, J = 7.0 Hz, 6H,

CH3); 13

C NMR (125 MHz, CDCl3): δ 167.8 (C=O), 147.8 (CAr), 146.6 (CAr), 129.4

(CAr), 125.7 (CHAr), 122.7 (CHAr), 118.3 (CHAr), 32.1 (CH), 22.4 (2 × CH3), 19.3

(CH3),14.3 (CH3).

3.3.2.2. Synthesis of 5-isopropyl-2-methylphenyl propionate (Ca 2)


carvacrol (1.0 g, 6.6 mmol, 1.0 mequiv) with propionyl chloride (0.6 g, 7.3 mmol. 1.1



O

O

5-isopropyl-2-methylphenyl propionate



135 (100), 107 (8), 91 (6); IR υmax: 3028, 2961, 2871, 1761, 1622, 1460, 1351, 1236,

1148, 898 cm-1;


(d, J = 7.5 Hz, 1H, Ar-H), 6.86 (s, 1H, Ar-H), 2.84-2.88 (m, 1H, CH), 2.60 (q, J = 7.5

Hz, 2H, CH2), 2.12 (s, 3H, CH3), 1.28 (t, J = 7.5 Hz, 3H, CH3), 1.22 (d, J = 7.0 Hz,

6H, CH3); 13

C NMR (CDCl3, 125 MHz): δ 172.5 (C=O), 149.1 (CAr), 147.9 (CAr),

130.7 (CAr), 127.0 (CHAr), 123.9 (CHAr), 119.6 (CHAr), 33.4 (CH), 27.5 (CH2), 23.8 (2

× CH3), 15.6 (CH3), 9.1 (CH3).

3.3.2.3. Synthesis of 5-isopropyl-2-methylphenyl isobutyrate (Ca 3)


carvacrol (1.0 g, 6.66 mmol, 1.0 mequiv) with isobutyryl chloride (0.7 g, 7.3 mmol.

1.1 mequiv) and triethyl amine (0.7 g, 7.3 mmol, 1.1 mequiv) in anhydrous DCM (20


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O

O

5-isopropyl-2-methylphenyl isobutyrate



135 (100), 107 (8), 91 (8); IR υmax: 3024, 2963, 2931, 2874, 1757, 1622, 1469, 1343,

1232, 1132, 818 cm-1;

1H NMR (CDCl3, 500 MHz): δ 7.13 (d, J = 7.0 Hz, 1H, Ar-H),

7.01 (d, J = 5.0 Hz, 1H, Ar-H), 6.85 (s, 1H, Ar-H), 2.81-2.88 (m, 2H, CH), 2.13 (s,

3H, CH3), 1.35 (d, J = 6.5Hz, 6H, CH3), 1.23 (d, J = 7.0 Hz, 6H, CH3); 13

C NMR

(CDCl3, 125 MHz): δ 175.0 (C=O), 149.1 (CAr), 147.8 (CAr), 130.6 (CAr), 126.9

(CHAr), 123.7 (CHAr), 119.5 (CHAr), 34.0 (CH), 33.0 (CH), 23.7 (2 × CH3), 18.9 (2 ×

CH3), 15.5 (CH3).

3.3.2.4. Synthesis of 5-isopropyl-2-methylphenyl 3-methylbutanoate (Ca 4)


carvacrol (1.0 g, 6.6 mmol, 1.0 mequiv) with isovaleryl chloride (0.8 g, 7.3 mmol. 1.1



O

O

5-isopropyl-2-methylphenyl 3-methylbutanoate



135 (100), 107 (4), 91 (8); IR υmax: 3024, 2961, 2930, 2872, 1759, 1622, 1462, 1364,

1233, 1115, 818 cm-1;

1H NMR (CDCl3, 500 MHz): δ 7.14 (d, J = 8.0 Hz, 1H, Ar-H),

7.00 (dd, J = 1.5 Hz & 6.5 Hz, 1H, Ar-H), 6.84 (d, J = 1.5 Hz, 1H, Ar-H), 2.84-2.91

(m, 1H, CH), 2.46 (d, J = 7.00 Hz, 2H, CH2), 2.22-2.30 (m, 1H, CH), 2.13 (s, 3H,

CH3), 1.22 (d, J = 7.0 Hz, 6H, CH3), 1.07 (d, J = 6.5 Hz, 6H, CH3); 13

C NMR

(CDCl3, 125 MHz): δ 171.1 (C=O), 149.0 (CAr), 147.8 (CAr), 130.6 (CAr), 126.9

(CHAr), 123.7 (CHAr), 119.6 (CHAr), 43.0 (CH2), 33.3 (CH), 25.6 (CH), 23.7 (2 ×

CH3), 22.3 (2 × CH3), 15.6 (CH3).

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3.3.2.5. Synthesis of (E)-5-isopropyl-2-methylphenyl but-2-enoate (Ca 5)


carvacrol (1.0 g, 6.6 mmol, 1.0 mequiv) with crotonyl chloride (0.7 g, 7.3 mmol, 1.1



O

O

(E)-5-isopropyl-2-methylphenyl but-2-enoate



135 (52), 107 (5), 91 (10), 69 (25); IR υmax: 3024, 2961, 2928, 2871, 1738, 1657,

1622, 1459, 1363, 1232, 1157, 819 cm-1;

1H NMR (CDCl3, 500 MHz): δ 7.16-7.19

(m, 1H, CH), 7.13 (d, J = 8.0 Hz, 1H, Ar-H), 7.00 (dd, J = 1.5 Hz & 6.5 Hz, 1H, Ar-

H), 6.88 (s, 1H, Ar-H), 6.06 (dd, J = 1.5Hz & 14 Hz, 1H, CH), 2.86 (m, 1H, CH), 2.12

(s, 3H, CH3), 1.94 (dd, J = 1.5 Hz & 7.0Hz, 3H, CH3), 1.22 (d, J = 7.0 Hz, 6H, CH3);

13C NMR (CDCl3, 125 MHz): δ 164.5 (C=O), 149.1 (CAr), 147.8 (CH), 146.5 (CAr),

130.7 (CAr), 127.2 (CHAr), 123.8 (CHAr), 121.9 (CH), 119.7 (CHAr), 33.4 (CH), 23.7

(2 × CH3), 18.0 (CH3), 14.0 (CH3).

3.3.2.6. Synthesis of 5-isopropyl-2-methylphenyl benzoate (Ca 6)


carvacrol (1.0 g, 6.6 mmol, 1.0 mequiv) with benzoyl chloride (1.0 g, 7.3 mmol. 1.1

mequiv) and triethyl amine (7.4 g, 7.3 mmol, 1.1 mequiv.) in anhydrous DCM (20 ml)


O

O

5-isopropyl-2-methylphenyl benzoate



77 (26), 51 (4); IR υmax: 3062, 2960, 2927, 2870, 1737, 1621, 1451, 1341, 1238,

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1116, 819 cm-1;


(t, J = 7.5 Hz, 1H, Ar-H), 7.52 (t, J = 7.5 Hz, 2H, Ar-H), 7.20 (d, J = 8.0 Hz, 1H, Ar-

H), 7.06 (d, J = 8.0 Hz, 1H, Ar-H), 7.00 (s, 1H, Ar-H), 2.89-2.92 (m, 1H, CH), 2.19

(s, 3H, CH3), 1.25 (d, J = 7.0 Hz, 6H, CH3); 13

C NMR (CDCl3, 125 MHz): δ 164.7

(C=O), 149.2 (CAr), 148.0 (CAr), 133.3 (CHAr), 130.7 (CHAr), 130.0 (2 × CHAr), 129.4

(CAr), 128.4 (2 × CHAr), 127.2 (CAr), 124.0 (CHAr), 119.7 (CHAr), 33.4 (CH), 23.8 (2 ×

CH3), 15.7 (CH3).

3.3.2.7. Synthesis of 5-isopropyl-2-methylphenyl 2-phenylacetate (Ca 7)


carvacrol (1.0 g, 6.6 mmol, 1.0 mequiv) with 2-phenyl acetyl chloride (1.1 g, 7.3

mmol. 1.1 mequiv) and triethyl amine (7.4 g, 7.3 mmol, 1.1 mequiv.) in anhydrous

DCM (20 ml) as reaction solvent.

O

O

5-isopropyl-2-methylphenyl 2-phenylacetate



135 (96), 118 (35), 91 (42), 65 (8); IR υmax: 3088, 3064, 3031, 2960, 2926, 2870,

1747, 1622, 1454, 1234, 1119, 820 cm-1;

1H NMR (CDCl3, 500 MHz): δ 7.41 (d, J =

7.0 Hz, 2H, Ar-H), 7.37 (t, J = 7.0 Hz, 1H, Ar-H), 7.30 (t, J = 6.0 Hz, 2H, Ar-H), 7.09

(d, J = 8.0 Hz, 1H, Ar-H), 6.99 (dd, J = 1.5 Hz & 7.5 Hz, 1H, Ar-H), 6.83 (d, J = 1.5

Hz, 1H, Ar-H), 3.87 (s, 2H, CH2), 2.83-2.86 (m, 1H, CH), 1.97 (s, 3H, CH3), 1. 20 (d,

J = 6.5 Hz, 6H, CH3); 13

C NMR (CDCl3, 125 MHz): δ 169.7 (C=O), 149.24 (CAr),

148.0 (CAr), 133.6 (CAr), 130.9 (CHAr), 129.4 (2 × CHAr), 128.7 (2 × CHAr), 127.3

(CAr), 127.1 (CHAr), 124.1 (CHAr), 119.7 (CHAr), 41.5 (CH2), 33.6 (CH), 23.9 (2 ×

CH3), 15.6 (CH3).

Estelar

64

Figure 3.1. 1H NMR spectrum of Th 1

Figure 3.2. 13

C NMR spectrum of Th 1

O

O

O

O

Estelar

65


Figure 3.4. 13


O

O

O

O

Estelar

66


Figure 3.6. 13


O

O

O

O

Estelar

67


Figure 3.8. 13


O

O

O

O

Este

lar

68


Figure 3.10. 13


O

O

O

O

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69


Figure 3.12. 13


O

O

O

O

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70


Figure 3.14. 13


O

O

O

O

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71

Figure 3.15. 1H NMR spectrum of Ca 1

Figure 3.16. 13

C NMR spectrum of Ca 1

O

O

O

O

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72


Figure 3.18. 13


O

O

O

O

Estelar

73


Figure 3.20. 13


O

O

O

O

Estelar

74


Figure 3.22. 13


O

O

O

O

Estelar

75


Figure 3.24. 13


O

O

O

O

Estelar

76


Figure 3.26. 13


O

O

O

O

Estelar

77


Figure 3.28. 13


O

O

O

O

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78

3.3.3. Synthesis of ether derivatives of thymol (Th 8 to Th 10) and

carvacrol (Ca 8 to Ca 10)

3.3.3.1. Synthesis of 2-(2-isopropyl-5-methylphenoxy)-1-(4-bromophenyl)

ethanone (Th 8)

The general synthetic method described earlier, afforded compounds 15 from

thymol (1.0 g, 6.6 mmol, 1.0 mequiv) with 4-bromophenacyl bromide (2.0 g, 7.3

mmol, 1.1 mequiv) and potassium carbonate (4.6 g, 33.3 mmol, 5.0 mequiv.) in

acetonitrile (20 ml) as reaction solvent.

O

O

Br

2-(2-isopropyl-5-methylphenoxy)-1-(4-bromophenyl)ethanone

Molecular Formula: C18H19BrO2 (M.W. 346.0); Physical state: off white solid;

Yield: 64.6%; Purity: 97%; mp: 54-56 oC; MS: [M

+1]: m/z 347; IR υmax: 3075,

3060, 2942, 2924, 2865, 1690, 1609, 1356, 1177, 1145, 806 cm-1;

1H NMR (CDCl3,

500 MHz): δ 7.88 (d, 2H, J = 6.5 Hz, Ar-H), 7.63 (d, 2H, J = 7.0 Hz, Ar-H), 7.12 (d,

1H, J = 8.0 Hz, Ar-H), 6.79 (d, 1H, J = 8.0 Hz, Ar-H), 6.57 (s, 1H, Ar-H), 5.16 (s, 2H,

CH2), 3.29- 3.36 (m, 1H, CH), 2.29 (s, 3H, CH3), 1.20 (d, 6H, J = 7.0 Hz, CH3); 13

C

NMR (CDCl3, 125 MHz): δ 194.1 (C=O), 154.7 (CAr), 136.2 (CAr), 134.3 (CAr),

133.3 (CAr), 131.8 (2 × CHAr), 129.7 (2 × CHAr), 128.8 (CAr), 126.1 (CHAr), 122.2

(CHAr), 112.2 (CHAr), 71.1 (CH2), 26.2 (CH), 22.7 (2 × CH3), 21.1 (CH).

3.3.3.2. Synthesis of 2-(2-isopropyl-5-methylphenoxy)-1-p-tolylethanone

(Th 9)

The general synthetic method described earlier, afforded compounds Th 9 from

thymol (1.0 g, 6.6 mmol, 1.0 mequiv) with 4-methylphenacyl bromide (1.5 g, 7.3

mmol, 1.1 mequiv) and potassium carbonate (4.6 g, 33.3 mmol, 5.0 mequiv.) in


O

O

CH3

2-(2-isopropyl-5-methylphenoxy)-1-p-tolylethanone

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79

Molecular Formula: C19H22O2 (M.W. 282.0); Physical state: off white solid; Yield:

69.9%; Purity: 98%; mp: 56- 58 oC; MS: [M

+1], m/z 283; IR υmax: 3070, 3027, 2940,

2904, 2864, 1695, 1606, 1453, 1356, 1146, 807 cm-1;

1H NMR (CDCl3, 500 MHz): δ

7.92 (d, 2H, J = 6.5 Hz, Ar-H), 7.26 (d, 2H, J = 7.5 Hz, Ar-H), 7.12 (d, 1H, J = 7.5

Hz, Ar-H), 6.78 (d, 1H, J = 7.5 Hz, Ar-H), 6.58 (s, 1H, Ar-H), 5.21 (s, 2H, CH2),

3.35- 3.40 (m, 1H, CH), 2.42 (s, 3H, CH3), 2.28 (s, 3H, CH3), 1.20 (d, 6H, J = 7.0 Hz,

CH3); 13

C NMR (CDCl3, 125 MHz): δ 194.4 (C=O), 155.0 (CAr), 144.5 (CAr), 136.2

(CAr), 134.4 (CAr), 132.1 (CAr), 129.2 (2 × CHAr), 128.2 (2 × CHAr), 126.1 (CHAr),

122.0 (CHAr), 112.4 (CHAr), 71.1 (CH2), 26.3 (CH), 22.7 (2 × CH3), 21.6 (CH3), 21.1

(CH3).

3.3.3.3. Synthesis of 2-(2-isopropyl-5-methylphenoxy)-1-(4-methoxyphenyl)

ethanone (Th 10)

The general synthetic method described earlier, afforded compounds Th 10

from thymol (1.0 g, 6.6 mmol, 1.0 mequiv) with 4-methoxyphenacyl bromide (1.6 g,

7.3 mmol, 1.1 mequiv) and potassium carbonate (4.6 g, 33.3 mmol, 5.0 mequiv) in


O

O

OCH3

2-(2-isopropyl-5-methylphenoxy)-1-(4-methoxyphenyl)ethanone


61.2%; Purity: 98%; mp: 88- 90 oC; MS: [M

+1], m/z 299; IR υmax: 3054, 3020, 2960,

2904, 2866, 1689, 1603, 1364, 1179, 1114, 807 cm-1;

1H NMR (CDCl3, 500 MHz): δ

8.01 (dd, 2H, J = 2.0 Hz & 7.0 Hz, Ar-H), 6.95 (dd, 2H, J = 2.0 & 7.0 Hz, Ar-H), 7.11

(d, 1H, J = 8.0 Hz, Ar-H), 6.77 (d, 1H, J = 8.0 Hz, Ar-H), 6.59 (s, 1H, Ar-H), 5.17 (s,

2H, CH2), 3.87 (s, 3H, OCH3), 3.34- 3.40 (m, 1H, CH), 2.28 (s, 3H, CH3), 1.21 (d,

6H, J = 7.0 Hz, CH3); 13

C NMR (CDCl3, 125 MHz): δ 193.3 (C-12), 163.7 (CAr),

155.0 (CAr), 136.1 (CAr), 134.3 (CAr), 130.5 (2 × CHAr), 127.7 (CAr), 126.0 (CHAr),

121.9 (CHAr), 113.7 (2 × CHAr), 112.3 (CHAr), 71.0 (CH2), 55.3 (OCH3), 26.2 (CH),

22.7 (2 × CH3), 21.1 (CH3).

3.3.3.4. Synthesis of 2-(5-isopropyl-2-methylphenoxy)-1-(4-bromophenyl)

ethanone (Ca 8)

Estelar

80

The general synthetic method described earlier, afforded compounds Ca 8

from carvacrol (1.0 g, 6.6 mmol, 1.0 mequiv) with 4-bromophenacyl bromide (2.0 g,

7.3 mmol, 1.1 mequiv) and potassium carbonate (4.6 g, 33.3 mmol, 5.0 mequiv.) in


O

O

Br

2-(5-isopropyl-2-methylphenoxy)-1-(4-bromophenyl) ethanone

Molecular Formula: C18H19BrO2 (M.W. 346.0); Physical state: off white solid;

Yield: 77.3%; Purity: 97%; mp: 70- 72 oC; MS: [M

+1], m/z 347; IR υmax: 3053,

2956, 2926, 2866, 1690, 1585, 1360, 1174, 1132, 822 cm-1;

1H NMR (CDCl3, 500

MHz): δ 7.90 (d, 2H, J = 6.5 Hz, Ar-H), 7.64 (d, 2H, J = 6.5 Hz, Ar-H), 7.07 (d, 1H,

J = 7.5 Hz, Ar-H), 6.78 (dd, 1H, J = 1.5 Hz & 7.5 Hz, Ar-H), 6.62 (d, 1H, J = 1.5 Hz,

Ar-H), 5.16 (s, 2H, CH2), 2.81- 2.86 (m, 1H, CH), 2.22 (s, 3H, CH3), 1.21 (d, 6H, J =

7.0 Hz, CH3); 13

C NMR (CDCl3, 125 MHz): δ 194.5 (C=O), 155.7 (CAr), 147.9

(CAr), 133.3 (CAr), 131.8 (2 × CHAr), 130.7 (CHAr), 129.8 (2 × CHAr), 128.8 (CAr),

124.2 (CAr), 119.1 (CHAr), 109.7 (CHAr), 71.2 (CH2), 33.8 (CH), 23.9 (2 × CH3), 15.7

(CH3).

3.3.3.5. Synthesis of 2-(5-isopropyl-2-methylphenoxy)-1-p-tolylethanone

(Ca 9)


from carvacrol (1.0 g, 6.6 mmol, 1.0 mequiv) with 4-methylphenacyl bromide (1.5 g,

7.3 mmol, 1.1 mequiv) and potassium carbonate (4.6 g, 33.3 mmol, 5.0 mequiv.) in


O

O

CH3

2-(5-isopropyl-2-methylphenoxy)-1-p-tolylethanone


71.6%; Purity: 96%; mp: 45- 48 oC; MS: [M

+1], m/z 283; IR υmax: 3065, 3030, 2960,

2925, 2868, 1703, 1606, 1341, 1180, 819 cm-1;

1H NMR (CDCl3, 500 MHz): δ 7.92

(d, 2H, J = 8.0 Hz, Ar-H), 7.27 (d, 2H, J = 7.5 Hz, Ar-H), 7.06 (d, 1H, J = 7.5 Hz, Ar-

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81

H), 6.76 (dd, 1H, J = 1.5 & 7.5 Hz, Ar-H), 6.63 (s, 1H, Ar-H), 5.19 (s, 2H, CH2),

2.79- 2.86 (m, 1H, CH), 2.45 (s, 3H, CH3), 2.24 (s, 3H, CH3), 1.20 (d, 6H, J = 7.0 Hz,

CH3); 13


(CAr), 132.3 (CAr), 130.8 (CHAr), 129.4 (2 × CH3), 128.4 (2 × CH3), 124.5 (CAr), 119.0

(CHAr), 110.0 (CHAr), 71.3 (CH2), 34.0 (CH), 24.1 (2 × CH3), 21.7 (CH3), 15.9 (CH3).

3.3.3.6. Synthesis of 2-(5-isopropyl-2-methylphenoxy)-1-(4-methoxyphenyl)

ethanone (Ca 10)


from carvacrol (1.0 g, 6.6 mmol, 1.0 mequiv) with 4-methoxyphenacyl bromide (1.6

g, 7.3 mmol, 1.1 mequiv) and potassium carbonate (4.6 g, 33.3 mmol, 5.0 mequiv) in


O

O

OCH3

2-(5-isopropyl-2-methylphenoxy)-1-(4-methoxyphenyl)ethanone


62.3%; Purity: 95%; mp: 50-52 oC; MS: [M

+1], m/z 299; IR υmax: 3058, 3033, 2963,

2905, 2865, 2844, 1694, 1603, 1177, 813 cm-1;

1H NMR (CDCl3, 500 MHz): δ 8.02

(d, 2H, J = 7.0 Hz, Ar-H), 6.95 (d, 2H, J = 6.5 Hz, Ar-H), 6.87 (d, 1H, J = 7.5 Hz, Ar-

H), 6.76 (dd, 1H, J = 1.5 & 8.0 Hz, Ar-H), 6.64 (s, 1H, Ar-H), 5.16 (s, 2H, CH2), 3.88

(s, 3H, OCH3), 2.80-2.85 (m, 1H, CH), 2.24 (s, 3H, CH3), 1.21 (d, 6H, J = 6.5 Hz,

CH3); 13


(CAr), 130.6 (3 × CHAr), 127.7 (CAr), 124.2 (CAr), 118.8 (CHAr), 113.7 (2 × CHAr),

109.8 (CHAr), 71.2 (CH2), 55.3 (OCH3), 33.8 (CH), 23.9 (2 × CH3), 15.8 (CH3).

Estelar

82


Figure 3.30. 13


O

O

Br

O

O

Br

Estelar

83


Figure 3.32. 13


O

O

CH3

O

O

CH3

Estelar

84


Figure 3.34. 13


O

O

OCH3

O

O

OCH3

Estelar

85


Figure 3.36. 13


O

O

Br

O

O

Br

Estelar

86


Figure 3.38. 13


O

O

CH3

O

O

CH3

Estelar

87


Figure 3.40. 13


O

O

OCH3

O

O

OCH3

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88

3.4. Results and discussion

The esters of thymol (Th 1 to Th 7) and carvacrol (Ca 1 to Ca 7) have been

prepared by the conventional esterification reaction with corresponding acid chloride

in the presence of triethyl amine. The condensation of 1.0 mequiv of thymol or

carvacrol with the 1.1 mequiv of acetyl chloride, propionyl chloride, isobutyryl

chloride, 3-methylbutanoyl chloride, (E)-but-2-enoyl chloride, benzoyl chloride, 2-

phenylacetyl chloride and 1.1 mequiv of triethylamine in dry DCM at 250 C for 10 h

gave thymyl acetate (2-isopropyl-5-methylphenyl acetate) Th 1, thymyl propionate

(2-isopropyl-5-methylphenylpropio-nate) Th 2, thymyl isobutyrate (2-isopropyl-5-

methylphenylisobutyrate) Th 3, thymyl isovalerate (2-isopropyl-5-methylphenyl-3-

methylbutanoate) Th 4, thymyl crotonate ((E)-2-isopropyl-5-methylphenyl-but-2-

enoate) Th 5, thymyl benzoate (2-isopropyl-5-methylphenylbenzoate) Th 6, thymyl

phenylacetate (2-isopropyl-5-methylphenyl-2-phenylacetate) Th 7, carvacryl acetate

(5-isopropyl-2-methylphenylacetate) Ca 1, carvacryl propionate (5-isopropyl-2-

methylphenylpropionate) Ca 2, carvacryl isobutyrate(5-isopropyl-2-methylphenyl-

isobutyrate) Ca 3, carvacryl isovalerate (5-isoprop-yl-2-methylphenyl-3-methyl-

butanoate) Ca 4, carvacryl crotonate ((E)-5-isopropyl-2-methylphenylbut-2-enoate)

Ca 5, carvacryl benzoate (5-isopropyl-2-methyl phenylbenzoate) Ca 6, carvacryl

phenyl acetate (5-isopropyl-2-methylphenyl-2-phenylacetate) Ca 7 respectively in 75-

85 % yields as colourless liquids. The reaction conditions and physicochemical

properties of ester analogues of thymol and carvacrol have been summarized in Table

3.1.

The ether derivatives of thymol (Th 8 to Th 10) and carvacrol (Ca 8 to Ca

10) have been synthesized by the nucleophilic substitution of thymol and carvacrol

with different p-substituted phenacyl bromide derivatives.31-33

The nucleophilic

substitution reaction of 1.0 mequiv of thymol or carvacrol with 1.1 mequiv of 2-

bromo-1-(4-bromophenyl)ethanone, 2-bromo-1-p-tolylethanone, 2-bromo-1-(4-

methoxyphenyl)ethanone and 5.0 mequiv of potassium carbonate in acetonitrile at 0

oC to 25

oC for 8-10 h gave 2-(2-isopropyl-5-methylphenoxy)-1-(4-bromophenyl)-

ethanone Th 8, 2-(2-isopropyl-5-methylphenoxy)-1-p-tolylethanone Th 9, 2-(2-

isopropyl-5-methylphenoxy)-1-(4-methoxyphenyl)ethanone Th 10, 2-(5-isopropyl-2-

methylphenoxy)-1-(4-bromophenyl)ethanone Ca 8, 2-(5-isopropyl-2-methylphen-

oxy)-1-p-tolylethanone Ca 9, 2-(5-isopropyl-2-methylphenoxy)-1-(4-methoxyphenyl)

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89

ethanone Ca 10 respectively in 61-77% yields as off white solids. The reaction

conditions and physicochemical properties of ether analogues of thymol and carvacrol

have been summarized in Table 3.2.

Table 3.1. Reaction conditions and physicochemical properties of

ester analogues

Comp. Structures Reaction conditions Yield

(%)

Physical state Purity

(%)

Th 1

O

O

i. SOCl2, 2h, ∆.

ii. TEA, dry. DCM,

16h, rt.

80%

colourless liquid

97%

Th 2

O

O

i. SOCl2, 2h, ∆.

ii. TEA, dry. DCM,

16h, rt

80%

colourless liquid

96%

Th 3

O

O

i. SOCl2, 2h, ∆.

ii. TEA, dry. DCM,

16h, rt

76%

colourless liquid

98%

Th 4

O

O

i. SOCl2, 2h, ∆.

ii. TEA, dry. DCM,

16h, rt

80%

colourless liquid

92%

Th 5

O

O

i. SOCl2, 2h, ∆.

ii. TEA, dry. DCM,

16h, rt

84%

colourless liquid

87%

Th 6

O

O

i. SOCl2, 2h, ∆.

ii. TEA, dry. DCM,

16h, rt

85%

colourless liquid

98%

Th 7

O

O

i. SOCl2, 2h, ∆.

ii. TEA, dry. DCM,

16h, rt

85%

colourless liquid

98%

Ca 1 O

O

i. SOCl2, 2h, ∆.

ii. TEA, dry. DCM,

16h, rt

78%

colourless liquid

97%

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90

Ca 2 O

O

i. SOCl2, 2h, ∆.

ii. TEA, dry. DCM,

16h, rt

75%

colourless liquid

98%

Ca 3 O

O

i. SOCl2, 2h, ∆.

ii. TEA, dry. DCM,

16h, rt

85%

colourless liquid

98%

Ca 4 O

O

i. SOCl2, 2h, ∆.

ii. TEA, dry. DCM,

16h, rt

75%

colourless liquid

97%

Ca 5 O

O

i. SOCl2, 2h, ∆.

ii. phenol, TEA, dry.

DCM, 16h, rt

78%

colourless liquid

91%

Ca 6

O

O

i. SOCl2, 2h, ∆.

ii. TEA, dry. DCM,

16h, rt

82%

colourless liquid

97%

Ca 7 O

O

i. SOCl2, 2h, ∆.

ii. TEA, dry. DCM,

16h, rt

78%

colourless liquid

88%

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91

Table 3.2. Reaction conditions and physicochemical properties of

ether analogues

Comp. Structures Reaction conditions Yield

(%)

Physical state M.p. Purity

(%)

Th 8

O

O

Br

K2CO3, CH3CN, rt,

15h

64.6%

off white solid

54-56oC

97%

Th 9

O

O

CH3

K2CO3, CH3CN, rt,

15h

69.9%

off white solid

56-58 oC

98%

Th 10

O

O

OCH3

K2CO3, CH3CN, rt,

15h

61.2%

off white solid

88-90 oC

98%

Ca 8 O

O

Br

K2CO3, CH3CN, rt,

15h

77.3%

off white solid

70-72 oC

97%

Ca 9 O

O

CH3

K2CO3, CH3CN, rt,

15h

71.6%

off white solid

45-48 oC

96%

Ca 10 O

O

OCH3

K2CO3, CH3CN, rt,

15h

70%

off white solid

50-52 oC

95%

The EI-MS of compounds Th 1-Th 7 and Ca 1-Ca 7 showed their

corresponding m/z values related to their molecular weight. Formation of ester

derivatives were confirmed by their IR spectral data which revealed the disappearance

of –OH str. and appearance of >C=O str. at 1736- 1766 cm-1, which was further

supported by the 1H &

13C NMR data. Presence of doublet at δ 7.0 (d, 1H, J = 7.0

Hz), 7.13 (d, 1H, J = 8.0 Hz), 1.22 (d, 6H, J = 7.0 Hz) and singlet at δ 6.85 (s, 1H) &

2.12 (s, 3H), showed the presence of thymol and carvacrol skeleton in all the ester and

ether derivatives, which was further confirmed by 13C-NMR data. The

1H NMR

spectra of compound Th 1 and Ca 1 showed a sharp singlet at δ 2.30 & 2.29 (3H for

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92

each) revealed the presence of acetyl group. A quartet at δ 2.58 & 2.60 (2H for each, J

= 7.5 Hz) and triplet at δ 1.27 & 1.28 (3H for each, J = 7.5 Hz) in the 1H NMR of

compound Th 2 & Ca 2 showed the presence of –CH2CH3 group in parent skeleton.

The presence of –CH(CH3)2 group was confirmed by the presence of a doublet at δ

1.32 & 1.35 (6H for each, J = 6.5 Hz) and multiplet at δ 2.81-2.99 & 2.82- 2.88 (1H

for each) in the 1H NMR of compound Th 3 & Ca 3 respectively, approximately

similar pattern was noticed for compounds Th 4 & Ca 4 except a doublet at δ 2.46 for

CH2 group. The presence of doublet at δ 5.25 and δ 6.06 with coupling constant (J)

11.0 Hz and 14.0 Hz showed the trans- arrangement in compounds Th 5 & Ca 5

respectively. Compounds Th 6, Th 7, Ca 6 and Ca 7 have similar pattern with phenyl

and benzyl group in compounds Th 6, Ca 6 and Th 7, Ca 7, respectively.

Formation of ether derivatives Th 8 to Th 10 and Ca 8 to Ca 10 was

confirmed by presence of C-O-C stretching vibrational absorption at 1150- 1180 cm-1

in IR spectra of ethers derivatives. The mass spectra of compounds Th 8 - Th 10 and

Ca 8 - Ca 10 showed their corresponding [M+] values related to their molecular

formula. All the ether analogues where characterized by their NMR (1H &

13C NMR)

data.

3.5. Antibacterial activity of phenolic monoterpene derivatives

Antibacterial activity of all the synthesized derivatives were performed against

four Gram positive bacterial strains viz. Streptococcus mutans (MTCC- 890),

Staphylococcus aureus (MTCC- 96), Bacillus subtilis (MTCC- 121), Staphylococcus

epidermidis (MTCC- 435) and one Gram negative bacterial strain Escherichia coli

(MTCC- 723) using disc diffusion34 and microbroth dilution methods.

35 Ampicillin

was used as a standard drug. The zone of inhibition (in mm) and minimum inhibitory

concentration (in µg/ml) of tested compounds are shown in Table 3.3 and Table 3.4.

The antibacterial activity of the synthesized analogues were lower in

comparison with the standard antibiotic ampicillin but some of the synthetic

analogues showed better activity than the parent compound. Most of the compounds

showed significant to moderate activity excepting Ca 6 to Ca 8 and Th 8 to Th 10

which were found to be inactive against all the tested strains. Among the thymyl ester

derivatives (Th 1 to Th 7) and carvacryl ester derivatives (Ca 1 to Ca 7), Th 1 to

Th 3, Ca 1, Ca 2, Ca 4 and Ca 5 showed significant activity against all the tested

Gram-positive bacterial strains. Compounds Th 1 to Th 3, Ca 4 and Ca 5 showed

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93

lower activity against E. coli whereas Ca 1 and Ca 2 were found to be inactive. The

thymyl isovalerate and thymyl benzoate esters showed moderate activity against

B. subtilis and S. epidermidis. The most notable enhancement in the activity was

noticed for thymyl ester derivatives Th 1 to Th 3 for Gram-positive bacterial strains.

Thymyl acetate Th 1 and thymyl isobutyrate Th 3 were found to be more effective

than thymol (IZ= 17mm; MIC= 125µg/ml) and all other esters against S. mutans (IZ=

30mm and 18mm; MIC= 11.7 and 93.7µg/ml respectively), B. subtilis (IZ= 30mm

and 21mm; MIC= 11.7 and 46.8µg/ml respectively) and S. epidermidis (IZ= 32mm

and 20mm; MIC= 11.7 and 46.8µg/ml respectively) whereas Th 2 was found to be

more active for B. subtilis (IZ= 25mm; MIC= 46.8µg/ml) and S. epidermidis (IZ=

28mm; MIC= 46.8µg/ml) as compared to thymol. All other derivatives possessed

much less activity as compared to thymol itself and generally showed decrease in

their activity with increase in the size of substituent (R). Carvacrol was much more

active than its isomer thymol against all the test bacteria even at lower concentration.

Nikumbh et al., (2003), have also synthesized carvacryl esters namely carvacryl

acetate and carvacryl phenyl acetate and evaluated their antibacterial activity against

B. japonicum, B. megaterium, B. substilits, and B. polymyx.36 In the present study,

carvacryl phenyl acetate was not active against the tested strains. All the thymyl ether

derivatives (Th 8 to Th 10) were found to be inactive against all the bacterial strains

whereas carvacrol ether derivatives (Ca 8 to Ca 10) showed moderate activity against

Gram positive bacterial strains.

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94

Tab

le 3

.3. A

nti

bacte

rial acti

vit

y o

f th

ym

ol der

ivati

ves

(Th

1 t

o T

h 1

0)

In-vitro activity – zone of inhibition (in mm)a and MIC (in µg ml-1)

Streptococcus mutans

Staphylococcus aureus

Bacillus subtilis

Staphylococcus epidermidis

Escherichia coli

Compounds

zone of

inhibition

MIC

zone of

inhibition

MIC

zone of

inhibition

MIC

zone of

inhibition

MIC

zone of

inhibition

MIC

Thym

ol

17 ± 0.79

125

25 ± 0.98

62.5

15 ± 0.68

125

14 ± 0.57

125

13 ± 0.51

250

Th 1

30 ±

1.3

1

11.7

18 ± 0.81

93.7

30 ±

1.2

8

11.7

32 ±

1.3

7

11.7

12 ± 0.53

375

Th 2

12 ± 0.53

187.5

10 ± 0.41

187.5

25 ±

1.0

7

46.8

28 ±

1.2

1

46.8

7 ± 0.27

>1000

Th 3

18 ±

0.8

6

93.7

17 ± 0.78

93.7

21 ±

0.9

6

46.8

20 ±

0.8

7

46.8

7 ± 0.26

>1000

Th 4

NI

ND

NI

ND

8 ± 0.29

ND

7 ± 0.27

ND

NI

ND

Th 5

12 ± 0.53

750

9 ± 0.37

750

10 ± 0.46

750

9 ± 0.35

ND

7 ± 0.28

>1000

Th 6

NI

ND

NI

ND

7 ± 0.29

ND

10 ± 0.39

ND

NI

ND

Th 7

8 ± 0.31

187.5

15 ± 0.66

93.7

8 ± 0.33

750

12 ± 0.49

187.5

7 ± 0.28

>1000

Th 8

NI

ND

NI

ND

NI

ND

NI

ND

NI

ND

Th 9

NI

ND

NI

ND

NI

ND

NI

ND

NI

ND

Th 1

0

NI

ND

NI

ND

NI

ND

NI

ND

NI

ND

Am

pic

illin

27 ± 1.21

4

22 ± 0.94

8

25 ± 1.12

4

25 ± 1.07

2

12 ± 0.49

12

a values are mean of three determinations, the ranges of which are less than 5% of the mean in all cases.

ampicillin (20µg/disc) was used as positive reference; compounds (100µg/disc) were used for experiments.

NI = no inhibition. ND = not determined. Este

lar

95

Tab

le 3

.4. A

nti

bact

eria

l act

ivit

y o

f carvacro

l d

eriv

ati

ves

(C

a 1

to C

a 1

0)

In-vitro activity – zone of inhibition (in mm)a and MIC (in µg ml-1)

Streptococcus mutans

Staphylococcus aureus

Bacillus subtilis

Staphylococcus epidermidis

Escherichia coli

Compounds

zone of

inhibition

MIC

zone of

inhibition

MIC

zone of

inhibition

MIC

zone of

inhibition

MIC

zone of

inhibition

MIC

Carvacr

ol

30 ± 1.29

23.4

25 ± 1.07

23.4

35 ± 1.41

11.7

32 ± 1.39

11.7

35 ± 1.49

11.7

Ca 1

17 ± 0.78

93.7

15 ± 0.66

93.7

25 ± 0.98

46.8

20 ± 0.89

93.7

NI

ND

Ca 2

12 ± 0.46

187.5

15 ± 0.74

187.5

20 ± 0.87

46.8

18 ± 0.72

93.7

NI

ND

Ca 3

7 ± 0.29

>1000

9 ± 0.37

375

9 ± 0.38

375

12 ± 0.56

187.5

NI

ND

Ca 4

22 ± 1.03

46.8

21 ± 0.91

46.8

25 ± 1.12

23.4

21 ± 0.96

46.8

10 ± 0.42

375

Ca 5

17 ± 0.82

187.5

20 ± 0.89

93.7

20 ± 0.88

93.7

18 ± 0.86

93.7

10 ± 0.46

375

Ca 6

NI

ND

NI

ND

NI

ND

NI

ND

NI

ND

Ca 7

NI

ND

NI

ND

NI

ND

NI

ND

NI

ND

Ca 8

NI

ND

NI

ND

NI

ND

NI

ND

NI

ND

Ca 9

22 ± 1.03

46.8

NI

ND

15 ± 0.74

187.5

11 ± 0.46

375

10 ± 0.42

375

Ca 1

0

10 ± 0.46

375

9 ± 0.37

750

12 ± 0.56

375

11 ± 0.46

375

NI

ND

Am

pic

illi

n

27 ± 1.21

4

22 ± 0.94

8

25 ± 1.12

4

25 ± 1.07

2

12 ± 0.49

12

a values are mean of three determinations, the ranges of which are less than 5% of the mean in all cases.

ampicillin (20µg/disc) was used as positive reference; compounds (100µg/disc) were used for experiments.

NI = no inhibition. ND = not determined. Este

lar

96

3.6. Structures of active compounds

O

O

Th 1

O

O

Th 2

O

O

Th 3

O

O

O

O

Ca 2 Ca 3

3.7. Conclusion

In conclusion, fourteen ester and six ether derivatives of thymol and carvacrol

have been synthesized and evaluated as antibacterial agents. These modifications

resulted in change in the antibacterial activity of thymol and carvacrol analogues. The

enhancement in activity was noticed in the thymyl ester derivatives Th 1 to Th 3

against S. mutans, B. subtilis and S. epidermidis whereas it diminished in case of

carvacryl esters derivatives. Based on the present results, compounds Th 1, Th 2 and

Th 3 possess potential for developing as antibacterial agents.

Estelar

97

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