The [2,3]-sigmatropic
rearrangement of a series of acetonitrile-derived allylic ammonium ylides [RCH2N+
(C4H8)-CHCN; R = 4'-t- butylcyclohexylidenemethyl
(3), 4'-t-butylcyclohex-1-enyl (10), 5'-t- butyl-2'-methylcyclohex-1-enyl (14),
4'-t-butyl-2'-methylcyclohex-1- enyl (20)], followed
by acid-catalysed hydrolysis in each case, furnished an excellent yield of a
diastereomeric pair of β,γ-olefinic
aldehydes [(5)/(6), (12)/(13), (15)/(16) and (21)/(22)]. With cyclohexylidene
ylide (4) addition of the carbanion to the more exposed (equatorial vector)
face is strongly preferred. In the cyclohexenyl examples there is a clear
stereoelectronic requirement in opposition to steric forces which leads mainly
to axial carbon-carbon bond formation.