j. Soc. Cosmet. Chem. 22 231-248 (1971) ¸ 1971 Society of Cosmetic Chemists of Great Britain The biogenesis of terpenoid essential oils G. P. MOSS* Presented on 20th April 1970 at the Symposium on "Perfumery", organised by the British Society of Perfumers and the Society of Cosmetic Chemists of Great Britain, at Eastbourne, Sussex. Synopsis--The BIOSYNTHESIS of mono- and SESQUITERPENOIDS is reviewed with especial reference to the ESSENTIAL OILS. INTRODUCTION Although there has been considerable speculation (1-3) on the detailed routes by which plants produce terpenoid essential oils there has been relatively little direct study of this problem. This arises in part from some of the technical difficulties inherent in this research. With the present knowledge concerning the biosynthesis of selected terpenoids it is possible to rationalise most terpenoid structures in biosynthetic terms. However, it cannot be emphasised too strongly that there is no substitute for direct experimental proof. Although the biogenesis of a particular terpenoid usually appears obvious, the plant may have evolved a less direct, or even totally different approach to the synthesis of the compound. Terpenoid biosynthesis may be conveniently considered in two stages - the formation of the universal acyclic precursors and related compounds and the formation of cyclic terpenoids. The acyclic precursors have been studied in considerable detail (4). Geranyl pyrophosphate and farnesyl pyrophosphate are then the precursors of all the numerous mono- and *Department of Chemistry, Queen Mary College, London, E. 1. 231