24O JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Elimination of a proton from the 11-membered ring "ion" (Fig. 9) will give all trans-humulene present in several essential oils. Alternatively further cyclisation will give caryophyllene. This sesquiterpenoid has been briefly studied (48). Humulene is presumably the precursor of the tricyclic fungal metabolites illudol (49), illudin (50) and marasmic acid (51). The 10-membered rinõ "carbonium ion" from trans-farnesyl pyrophos- phate on elimination of a proton would give germacratriene. Although a y b• cbo ½n½ k4½0 0 '•a•illin •'-curcurncnc0 caro•ol 0 % ø helico basid•n l:igure 10 cyclodecadiene compound is clearly the precursor of a large group of sesquiterpenoids, santonin is the only example studied (52). This compound is the only sesquiterpenoid where details of the intermediate sesquiter- penoids have also been investigated. Probably germacratriene (or germa- cradienol formed by hydration of the "carbonium ion") is the precursor of all the eudesmane (e.g. santonin), and guaiane (e.g. bulnesol) sesquiter-

THE BIOGENESIS OF TERPENOID ESSENTIAL OILS 241 penolds, and their related rearrangement products. Several germacrane derived sesquiterpenoids have a cyclopropane ring which may be derived from either "carbonium ion" by loss of the adjacent allylic proton (Fig. 9). Whenever a sesquiterpenoid contains a cis-double bond derived from the acyclic precursor it is most probably derived from 2-cis-farnesyl pyrophosphate. Although some biogenetic schemes postulate the involve- ment of 6-cis-farnesyl pyrophosphate there is no evidence for its occurrence ongifolcn½ o Figure 11 •n nature. Initial cyclisation of 2-cis-farnesyl pyrophosphate will generate a 6- or 7-membered ring "carbonium ion" (Fig. 10). Few examples derived from the 7-membered ring system are known. One of these is carotol (53) which is probably derived by further cyclisation followed by a hydride shift and hydration at the ring junction. A key intermediate postulated in the biosynthesis of many sesqui-