Oxidation studies on some natural monoterpenes: Citral, pulegone, and camphene

Citral extracted from Cymbopogon citratus (Gramineae) was subjected to photochemical epoxidation with hydrogen peroxide to obtain a mixture of epoxy derivatives at the C²=C³ and C⁶=C⁷ double bonds. The thermal oxidation of citral with m-chloroperoxybenzoic acid at room temperature gave only the corresponding 6,7-epoxy derivative as a mixture of E and Z isomers with respect to the C²=C³ double bond. Photosensitized oxygenation of citral in the presence of tetraphenylporphyrin, Rose Bengal, or chlorophyll lead to a mixture of two isomeric hydroperoxides, (2E)-6-hydroperoxy-3,7-dimethylocta-2,7-dienal and (2E,5E)-7-hydroperoxy-3,7-dimethylocta-2,5-dienal. Epoxidation of pulegone isolated from Penny royal oil (Mentha pulegium, Lamiaceae) with hydrogen peroxide under irradiation with a sodium lamp lead to a mixture of cis- and trans-isomeric 2,2,6-trimethyl-1-oxaspiro2.5]octan-4-ones, whereas under conditions of photosensitized oxygenation two hydroperoxide derivatives, 2-(1-hydroperoxy-1-methylethyl)-5-methylcyclohex-2-en-1-one and 2-hydroperoxy-5-methyl-2-(1-methylethenyl)cyclohexan-1-one, were also formed. Camphene reacted with hydrogen peroxide under irradiation to give a mixture of the corresponding endo- and exo-epoxy derivatives and camphor, while its thermal oxidation with m-chloroperoxybenzoic produced only the two former. Published in Russian in Zhurnal Organicheskoi Khimii, 2008, Vol. 44, No. 6, pp. 825–833. The text was submitted by the authors in English.