Elimination reactions. Part II. Pyrolytic and base-promoted decompositions of thujyl compounds

Abstract

Dehydrations of (–)-thujol, (–)-neothujol, (+)-isothujol, and (+)-neoisothujol by pyridine–phosphorus oxychloride, and also pyrolyses of the derived dimethylamine oxides and S-methyl xanthates, gave, singly or in mixture, (–)-α-thujene, (+)-β-thujene, and the hitherto uncharactised (+)-β-isothujene. In view of the known stereospecificity of these reactions, the results confirm recently assigned configurations for the alcohols. Hofmann degradation of the four thujyltrimethylammonium hydroxides gave products attributable to concurrent anti- and syn-eliminations, and tracer studies showed that part of the latter process from the isothujyl ‘onium compound involved the α′-β mechanism. Pyrolysis of thujyl acetates and borates, or treatment of the alcohols with iodine or zinc chloride, resulted exclusively in ring-opened products. A brief study of additions to (+)-sabinene and (–)-α-thujene suggests that generation of the 4-thujyl carbonium ion is favoured over that of the 3-thujyl ion, despite the potentialities of the latter for formation of a non-classical species.