J. Soc. Cosmet. Chern. 22 249-284 (1971} ¸ 1971 $octety of Cosmetic Chemists of Great Britain ß ' rig ' A remew of some nves atons the chemistry of carene of W. COCKER* Presented on 21st 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 HYDROGENATION, HYDROBORONATION, PYROLYSIS, AMIN- ATION, OXIDATION, and the PHOTOCHEMICAL ISOMERISATION of (+)-CAR-3- ENE, a potential raw material for the fine chemical industry, have been performed, and are here reviewed. CARANOLS, CARANONES, CARANAMINES and their rearrangement products have been SYNTHESISED. The bicyclic terpene, carene, exists in nature as (q-)-car-2-ene (formula 1) and {+)-car-3-ene {formula 2). Both are present in Indian and in Baltic {1) oils of turpentine, (q-)-car-3-ene {formula 2) being the more abundant. Both are however found in sufficient quantity to make them potentially valuable as starting points for the synthesis of terpenoids. Although a considerable amount of investigation was carried out in the early nineteen twenties, particularly by the late Sir John Simonsen and his conaborators (2), the advent of the techniques of chromatography and spectroscopy has made a re-examination of the chemistry of this bicyclic monoterpene a profitable exercise, with a view to its economic use, as well as the uncovering of some interesting chemistry. We started on this work some five years ago. There is a considerable body of evidence based upon conformational analysis and n.m.r. spectra (3) that the configuration and preferred con- formation of (+)-car-3-ene is the boat form (formula 3) rather than the upturned boat form {formula 4). *Chemi'cal Laboratory, Trinity College, Dublin 2, Ireland. 249

250 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS IO 2 4 5 (/) (2) H 2 H ($) (4) This is particularly revealed by the 8- methyl signal (x 9.24) which shows diamagnetic shielding by the double bond. (-+-)-Car-2-ene (formula 1) similarly takes up a boat form. Thus the a- side of the double bond seemed ripe for elaboration. HYDROGENATION (4) Catalytic hydrogenation of (q-)-car-3-ene (formula 2) was expected to take place preferentially from the a- side, giving (--)-cis-carane (formula 5). Little (+)-trans-carane (formula 6) was expected, and none was found in any catalytic hydrogenation experiments. In the event it was found that the products varied widely according to reaction conditions. Using 5% platinised charcoal, ethanol as solvent, hydrogen at 10.1 MN m-2 pressure, and at room temperature, (--)-cis-carane (formula 5) was obtained almost quantitatively in at least 98% purity (glc on a 3 m, 20% squalene-Chromo- sorb W column). However at 101 KN m-2 pressure, using 5% palladised charcoal and ethanol as solvent, (--)-cis-carane (formula 5) represented only about 50% of the product. The other product was 1,1,4-trimethyl- cycloheptane (formula 7). The proportions of these two products varied widely with reaction conditions. Thus using 5% palladised charcoal, 101 KN m-2 of hydrogen, and propionic acid as solvent, 1,1,4-trimethyl- cycloheptane was always the maj or product, and the yield of this increased with increasing temperature, until at 65 ø it was the only product. The 1,1,4-trimethylcycloheptane (formula 7) was identified (a) by its lack of optical activity, and (b) by its nmr spectrum which showed signals at • 9.12 (singlet, 6H, gem-dimethyl protons)and 9.12 (doublet,