Studies in Vitamin A 22 . OPPENAUER OXIDATION OF VITAMIN

The merit of the Oppenauer oxidation (Oppenauer, 1937; reviewed by Djerassi, 1951) using aluminium alkoxides and a hydrogen acceptor, is that unsaturated alcohols can be converted to aldehydes and ketones without affecting the ethylenic linkages. Applied to vitamin A alcohol it ought to yield retinene, (vitamin A1 aldehyde) as the primary product. Hawkins & Hunter (1944a), using aluminium isopropoxide and acetaldehyde as hydrogen acceptor, have in fact obtained retinenel. There is, however, a marked tendency for secondary reactions to occur and so to diminish the yield of the initial dehydrogenated product. Haworth, Heilbron Jones, Morrison & Polya (1939) varied the conditions, and in particular used aluminium tert.butoxide and diethyl ketone as hydrogen acceptor. They did not obtain retinene, but found instead that a crystalline oxime (isolated in low yield from the reaction mixture) corresponded with an 'aldehyde' C20H260. At that time there was no suspicion that the retinenes were the aldehydes of vitamins Al and A2 and for a time the observation attracted no special interest. The tentative structure