THE BIOGENESIS OF TERPENOID ESSENTIAL OILS 233 corresponding acyclic precursor, squalene, from mevalonic acid showed the complete stereospecificity of every step in the biosynthesis. Their conclu- sions are summarised in Figs. 1 and oe. 0 . "' 0 similarly & Triterpenes Farn½syl pyrophosphate s•milarly Direrpenes & • • ,, ©©o T½ tr• t ½ rpcncs Gcranyl gcraniol pyrophosphate Gutta Pcrcha Figure The initial stages (Fig. 1) involve phosphorylation of the primary alcohol group of mevalonic acid (MVA), followed by an eliminative de- carboxylation to isopentenyl pyrophosphate (IPP) This compound is then isomerised to dimethylallyl pyrophosphate (DMAPP). Successive units of isopentenyl pyrophosphate are then added to dimethylallyl pyrophosphate to give (Fig. oe) a series of prenyl pyrophosphates, of which geranyl pyro- phosphate and farnesyl pyrophosphate are the key precursors of the mono- and sesquiterpenoids respectively. The latter precursor is also used in the biosynthesis of the hydrocarbon squalene, from which all triterpenoids and steroids are derived. It is of note that at each stage where a prochiral (7) methylene group is involved the enzyme is completely stereospecific with respect to the proton utilised.

234 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS A cyclic terpenoids Although geranyl pyrophosphate and farnesyl pyrophosphate are key intermediates they have rarely been detected in nature. However the corresponding free alcohols are more frequently encountered, not only in the all trans form, but also the cis isomers, nerol and 2-cis-farnesol and the rearranged isomers linalool and nerolidol. It seems probable that the corresponding isomcric pyrophosphates are readily interconverted by intramolecular allylic rearrangements (Fig. $). The enzymic formation of nerolidol from farnesyl pyrophosphate was demonstrated by Popj?tk (8). Figure 3 Recent work with rose petals by Francis (9) showed that although the free alcohols are formed from [2-14Cl mevalonic acid, most of the radioactivity is incorporated into the corresponding monoterpenoid glucosides. Many terpenoids possess an acyclic carbon skeleton which is clearly derived by simple elimination, oxidation and/or reduction of the precursor (Fig. 4). Myrcene biosynthesis (10) in Santolina, and citral and citronellal in Eucalyptus (11), as well as ants (12), has been briefly studied. The ses- •o citral dendrolasin Figure ••V• o citronellal quiterpenoids dendrolasin (13) and ipomeamarone (14) were studied in ants. and sweet potatoes infected with Ceratocystis fimbriata, respectively.