ISSN 1070-4280, Russian Journal of Organic Chemistry, 2008, Vol. 44, No. 6, pp. 823–829. © Pleiades Publishing, Ltd., 2008. Published in Russian in Zhurnal Organicheskoi Khimii, 2008, Vol. 44, No. 6, pp. 834–841. Oxidation Reactions of Some Natural Volatile Aromatic Compounds: Anethole and Eugenol* E. M. Elgendya and S. A. Khayyatb a Faculty of Specific Education, Mansoura University, Mansoura, Egypt e-mail: [email protected] b Faculty of Educational’s Girls, Ministry of Education, Jeddah, Saudi Arabia Received September 30, 2006 Abstract—trans-Anethole [1-methoxy-4-(trans-prop-1-en-1-yl)benzene] was isolated from anise seed oil (Pimpinella anisum). Its photochemical oxidation with hydrogen peroxide gave the corresponding epoxy derivative together with 4-methoxybenzaldehyde. The thermal oxidation of trans-anethole with 3-chloroper- oxybenzoic acid at room temperature resulted in the formation of dimeric epoxide, 2,5-bis(4-methoxyphenyl)- 3,6-dimethyl-1,4-dioxane, as the only product. Photochemical oxygenation of trans-anethole in the presence of tetraphenylporphyrin, Rose Bengal, or chlorophyll as sensitizer led to a mixture of 1-(4-methoxyphenyl)prop-2- en-1-yl hydroperoxide and 4-methoxybenzaldehyde. Eugenol was isolated from clove oil [Eugenia caryo- phyllus (Spreng.)]. It was converted into 2-methoxy-4-(prop-2-en-1-yl)phenyl hydroperoxide by oxidation with hydrogen peroxide under irradiation. Thermal oxidation of eugenol with 3-chloroperoxypenzoic acid at room temperature produced 2-methoxy-4-(oxiran-2-ylmethyl)phenol, while sensitized photochemical oxygenation (in the presence of Rose Bengal or chlorophyll) gave 4-hydroperoxy-2-methoxy-4-(prop-2-en-1-yl)cyclohexa- 2,5-dien-1-one. DOI: 10.1134/S1070428008060079 Many naturally occurring alkenylbenzene deriva- Taking into account important activities of plant tives, usually relatively simple allyl- or propenylben- phenylpropenoides and contradictory published data zenes having methoxy and/or methylenedioxy substit- on epoxidation of anethole (I), in the present work we uents in the benzene ring, have been identified as com- studied in detail oxidation reactions of trans-anethole ponents of numerous plants or their essential oils [1, 2] (I) and eugenol (II) under different conditions (thermal and used as natural flavoring and fragrance chemicals. and photochemical). Microbial metabolism of phenylpropenoids involves trans-Anethole [1-methoxy-4-(trans-prop-1-en-1- oxidation of the side chain to carboxylic acid prior to yl)benzene, I] is the major component of several hydroxylation and cleavage of the benzene ring. For essential oils, including Chinese Star Anise (Illicium example, eugenol is oxidized to vanillic acid [3]. On verum), Anise seed oil (Pimpinella anisum), and sweet the other hand, plant phenylpropenoides undergo Fennel (Foeniculum vulgare Mill. var. dulce) [8]. The oxidation on exposure to air. The oxidation process is chemical structure of I was confirmed by spectral enhanced by heat, irradiation [4], or in the presence of measurements. The 1H NMR spectrum of I showed catalysts [5]. a doublet at δ 1.86 from protons in the methyl group, Mohan and Whalen [6] reported that oxidation of and side-chain olefinic protons on C2′ and C1′ resonat- anethole (I) with 3-chloroperoxybenzoic acid gives ed, respectively, as a doublet of quartets at δ 6.08 ppm epoxy derivative Ib. Waumans et. al. [7] found that and a doublet at δ 6.34 ppm. In the 13C NMR spectrum 2′ 1′ analogous reaction with hydrogen peroxide in presence of I, the C and C signals were located at δC 123.5 of formic or acetic acid on heating leads to 3,5-bis(4- and 130.5 ppm, respectively. methoxyphenyl)-2,4-dimethyltelrahydrofuran and that Photochemical epoxidation of trans-anethole (I) intermediate epoxide Ib could not be isolated. with hydrogen peroxide (H2O2, 30% by volume) in ethanolic medium under irradiation with sodium light * The text was submitted by the authors in English. (irradiation time 55 h) to give 65% of 4-methoxybenz- 823 824 ELGENDY, KHAYYAT Scheme 1. O Me CHO 1' 2' H2O2, hν + MeO MeO Ia Ib Me Me 3-ClC6H4CO3H O CHCl3, 0°C MeO OMe MeO O Me I Ic OOH 3' 1 1' O2, TPP CH2 hν, –5°C Ia + 2' MeO Id aldehyde (Ia) and 35% of monoepoxy derivative, (C2′) and 71.4 ppm (C1′). The mass spectrum of Ib 2-(4-methoxyphenyl)-3-methyloxirane (Ib) (Scheme 1). contained the molecular ion peak at m/z 164. The structure of epoxidation products Ia and Ib Thermal oxidation of trans-anethole (I) with was established by spectral measurements. The IR m-choroperoxybenzoic acid in chloroform at room spectrum of Ia contained an absorption band at temperature gave 2,5-bis(4-methoxyphenyl)-3,6-di- 1699 cm–1 due to the aldehyde carbonyl group, and the methyl-1,4-dioxane (Ic) as the only product in almost 1 CHO proton signal appeared in the 1H NMR spectrum quantitative yield (Scheme 1). The H NMR spectrum as a singlet at δ 9.86 ppm; no signals assignable to of Ic contained a six-proton doublet at δ 1.19 ppm protons at the anethole side-chain double CH=CH from the methyl protons, a doublet of quartets at bond were present. The aldehyde carbonyl carbon atom δ 4.27 ppm from protons in positions 3 and 6 of the 13 resonated in the C NMR spectrum at δC 190.7 ppm, dioxane ring, and a doublet at δ 5.82 ppm from protons and the molecular ion peak in the mass spectrum of Ia in positions 2 and 5. Signals at δC 70.1 and 81.5 ppm in 13 3 6 had an m/z value of 136. Compound Ib displayed in the C NMR spectrum of Ic were assigned to C /C 2 5 the 1H NMR spectrum a doublet at δ 0.96 ppm from and C /C in the 1,4-dioxane ring. The molecular ion the CH3 group in the oxirane ring, a doublet of quartets of Ic had an m/z value of 328. at δ 3.36 ppm from 2′-H, and a doublet at δ 3.90 ppm Interestingly, the photoinduced oxygenation of from 1′-H. In the 13C NMR spectrum of Ib, signals compound I in the presence of tetraphenylporphyrin from the oxirane carbon atoms were present at δC 64.1 (TPP), Rose Bengal (RB), or chlorophyll (CP) as Scheme 2. H O H O O O Me Me CHO –H2 –MeCHO MeO MeO MeO A Ia H2O2, hν I H OH O O Me Me –H2O MeO MeO B Ib RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 6 2008 OXIDATION REACTIONS OF SOME NATURAL VOLATILE AROMATIC COMPOUNDS: 825 Scheme 3. 3-ClC6H4 O O H O O 3-ClC6H4CO3H Me Me CHCl3 I –3-ClC6H4COOH MeO MeO Ib Me Me OMe O O O MeO OMe MeO O Me Me Ic Scheme 4. O O H OOH CH2 MeO MeO 1 O2, hν Id –5°C I O O Me CHO –MeCHO MeO MeO A Ia singlet oxygen sensitizer led to the formation of benzoic acid at room temperature according to [6], 1-(4-methoxyphenyl)-prop-2-en-l-yl hydroperoxide were unsuccessful. In all cases, the only isolated prod- (Id) together with aldehyde Ia (Scheme 1). Compound uct was 1,4-dioxane derivative Ic which could be Id showed in the 1H NMR spectrum a doublet at formed via dimerization of epoxide Ib (Scheme 3). δ 3.90 ppm from the side-chain allylic proton at C1′, Scheme 4 illustrates a probable mechanism of photo- a two-proton doublet of doublets at δ 5.35 from the C3′ sensitized oxygenation of trans-anethole (I) in the H2 group, a complex multiplet at δ 6.05 ppm from presence of tetraphenylporphyrin (TPP), Rose Bengal 2′-H, and a singlet at δ 8.60 ppm due to proton in the (RB), or chlorophyll (CP), leading to the formation of hydroperoxide group. hydroperoxide Id and aldehyde Ia. Presumably, the A probable mechanism for the formation of alde- process involves peroxirane and dioxetane interme- hyde Ia and epoxy derivatives Ib in the photochemical diates, respectively. oxidation of trans-anethole (I) with hydrogen peroxide Eugenol (4-allyl-2-methoxyphenol, II) is the major is shown in Scheme 2. Attack by hydrogen peroxide on component of the essential oil extracted from Eugenia the side-chain double bond in molecule I gives diox- Caryophyllus (Myrtaceae) [9]. The chemical structure etane intermediate A which decomposes to form acet- of II was confirmed by spectral measurements. The aldehyde and 4-methoxybenzaldehyde (Ia). Alterna- 1H NMR spectrum of II contains a doublet from the 1′ tively, the reaction of I with H2O2 could involve C H2 group at δ 3.31 ppm, a doublet of doublets at oxirane intermediate B, and elimination of water mole- δ 5.05 ppm from two methylene protons in position 3′, cule from the latter yields oxirane Ib. Numerous at- and a multiplet at δ 5.94 ppm which is characteristic of tempts to obtain epoxide Ib under thermal conditions, the side-chain 2′-H proton. In the mass spectrum of II in particular by reaction of I with m-chloroperoxy- the molecular ion peak with m/z 164 was observed. RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 6 2008 826 ELGENDY, KHAYYAT Scheme 5. MeO CH2 H2O2, hν HOO IIa 3-ClC H CO H MeO CH2 6 4 3 MeO CHCl3 O HO HO II IIb 1 OOH OOH O2, RB MeO MeO hν, –5°C CH2 O O OOH IIc IId Photochemical epoxidation of eugenol (II) with tained a doublet of doublets at δ 2.54 from one proton 3′ hydrogen peroxide (H2O2, 30 % by volume) in etha- of the methylene group on C , a complex multiplet at nolic medium using sodium lamp gave 4-allyl-2-meth- δ 2.78 ppm from the other proton on C3′, a doublet at 1′ oxyphenyl hydroperoxide (IIa) in ~45% yield, while δ 2.80 ppm from the C H2 group, and a complex no other products were detected (Scheme 5).