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

Abstract

Citral extracted from Cymbopogon citratus ( Gramineae ) was subjected to photochemical epoxidation with hydrogen peroxide to obtain a mixture of epoxy derivatives at the C 2 =C 3 and C 6 =C 7 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 C 2 =C 3 double bond. Photosensitized oxygenation of citral in the presence of tetraphenylporphyrin, Rose Bengal, or chlorophyll lead to a mixture of two isomeric hydroperoxides, (2 E )-6-hydroperoxy-3,7-dimethylocta-2,7-dienal and (2 E ,5 E )-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-oxaspiro[2.5]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.