A stereospecific synthesis of vitamin A from 2,2,6-trimethyl-cyclohexanone.

An efficient synthesis of all-(E) vitamin A acetate from 2,2,6-trimethyl-cyclohexanone has been achieved via the intermediacy of 1-(9-acetoxy-3, 7-dimethyl-nona-3,5,7-trien-1-ynyl)-2,2,6-trimethyl-cyclohexanol ( 25 ), readily prepared in high yield by allylic rearrangement of tertiary propenols with glacial acetic acid. The key step in the synthesis is the transformation of 25 to the unsaturated ketone 27 (9-acetoxy-3,7-dimethyl-1-(2,6,6-trimethyl-cyclohex-1-enyl)-nona-3,5,7-trien- 2-one) using a novel vanadium(V)-catalysed rearrangement reaction. The carbonyl in 27 affords the means for the essential isomerization of the adjacent double bond to the (E) isomer and the product is readily transformed into the polyene by reduction and elimination. An overall yield of 18–31% of vitamin A acetate from 2,2,6-trimethyl-cyclohexanone has been realized.