Studies of some 7-substituted pinane monoterpenes (1974)
AuthorsCant, P. A. E.show all
A number of pinane derivatives, oxygenated at the 7-position, have been pyrolysed and the resulting product mixtures analysed. Nowhere in the extensive literature concerned with the pyrolysis of pinane derivatives has there been a report of the cyclobutane ring opening initially by way of either 1,7- or 5;7- cleavage, although there are examples of both 1,6- and 5,6- bond cleavage. The present study has shown that it is possible to induce cleavage of the 1,7- and 5,7- bonds by inserting suitable substituents at C7. Cleavage of the 1,6- and 1,7- bonds occurs to a similar extent in the pyrolysis of cis-chrysanthenol (38) (28% and 25% respectively), whereas in that of 7-methylchrysanthenol (54), 1, 7- cleavage becomes the more favoured mode - 43% compared to 35%. When the hydroxyl group of cis-chrysanthenol was acetylated, the resulting compount proved very unstable under the pyrolysis conditions, giving a large number of products, only two of which (29% in all) were present in identifiable quantities. The pyrolyses of the three 7-oxygenated derivatives of α -pinene displayed one feature in common. When 1,6- cleavage occurred and monocyclic products were formed, the major product in each case was formed by migration of a hydrogen from the iso-propyl radical to C1. This may imply that the allylic radical is affected by and perhaps polarised towards the adjacent oxygen function since transfer to C3 appears sterically more favourable. [Diagrams] Neither cis-chrysanthanol (48) nor cis-chrysanthanyl acetate (49) gave any cyclic products on pyrolysis. Both compounds rearranged to give the aldehydes citronellal and 5,7-dimethyl-6-octenal but, in addition, cis-chrysanthanyl acetate gave the cis- (3%) and trans- (28%) enol acetates of citronellal. [Diagrams] Verbenone (5) was pyrolysed at 425° and four compounds not mentioned in previous reports were identified as 2,6,6-trimethylbicyolo [3.2.0] hepten-7-one (45), 1-formyl-2,6,6-trimethylcyclohexa-1,3-cliene (57), and the trienals 3,7-dimethyl-2-cis-, 4-cis-,6-octatrienal (60) and 3,7-dimethyl-2-cis-, 4-trans-6-octatrienal (59). It was also observed that the unsaturated pinene derivatives underwent thermal rearrangement at a much lower temperature (425°) than did the saturated analogues (600°).