206 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS •-Farnesene Nerolidol ,8-Farnesene Farnesol Figure 2. Garnere (6) found 13-farnesene to be one of the most important constituents of lavender oil. The a-isomer has not been reported to occur in nature but has been synthesized by dehydration of nerolidol or famesol (7, 8). The isomeric alcohols nerolidol and famesol are important perfumery chemicals. Nerolidol was first isolated from neroli oil (9) and since has been found to occur in several other essential oils, e.g. jasmine, citronella, pepper and cabreuva oil (10-12). According to Arctander (13) it has a mild and woody-floral, slightly green odour with excellent tenacity, and good blending and fixative properties. With the availability of synthetic nerolidol, a drastic reduction in its price has been observed. The synthesis which is based on cheap raw materials such as acetylene and acetone, makes nero- lidol a feasible entry into the sesquiterpene field from a commercial view- point (Fig. 3). Several CHECH +CH3COCH:3 steps o *•Na C--CH • Nero Figure 3. The chemical possibilities of converting nerolidol into various sesquiter- penes are enormous. Some work has already been carried out on the iso- merization to famesol (15) and cyclization to an interesting range of cyclci terpenes (16). However, unlike nature, the product selectivity in these cyclizations is low, but further research in this area should prove to be extremely useful.

SESQUITERPENES IN THE PERFUMERY INDUSTRY 207 Farnesol occurs in several essential oils, e.g. oils of ambrette seed, neroli, rose, cyclamen, jasmine etc. (17, 18). It has a delicate sweet oily odour developing into a floral fresh-green note and finds uses in floral, oriental and chypre fragrances (13). Biogenetically speaking, this alcohol occupies a very significant position in sesquiterpene chemistry since it has been established that farnesyl pyrophosphate is the in vivo precursor of a variety of sesquiterpenes. The acid catalysed cyclization of farnesol, in a manner similar to nerolidol, gives a complex mixture of cyclic products among which bisabolane and cadinane structures are the most prominent (16). Several syntheses of farnesol have been reported in the literature but none of these is commercially attractive. Isomerization of nerolidol to farnesol in acceptable yields should have a significant effect on the usage of this alcohol which is presently restricted because of its high price. Recently, Stevens, Ludin and Teranishi (19a), isolated and characterized two isomeric aldehydes a- and 13-sinesal belonging to the farnesane group which occur in chinese orange oil (19b, c and d). Both aldehydes exist in the all trans- form and have attracted the attention of flavourists and perfumers and several commercial syntheses have been accomplished. a-Sinesal /•- Sinesal Figure 4. Several members of the bisabolane group of sesquiterpenes occur in many commercially important essential oils. The parent hydrocarbon bisabolene can have several isomers with regard to the position of double bonds, but the ?-bisabolene is the most important one and occurs in oil of bisabol myrrh, lemon, lime, bergamot, cardamom, sandalwood, etc. It is also obtained by dehydration of nerolidol during the cyclization (16). It is interesting perfumery chemical with a pleasant, warm, sweet-spicy-balsamic odour which makes it invaluable in the reconstitution of essential oils and as a perfumery material in its own right. It is a vital part of the oriental, opopanax, chypre and novelty fragrances (13). The hydrocarbons zingiberene and ar-curcumene constitute the major part of the sesquiterpene fraction of the oil of Zingiber officinale (ginger oil).