Studies on intramolecular alkylation. XII. Stereochemical aspects of the preparation of β,γ-unsaturated aldehydes from the [2,3]-sigmatropic rearrangement of ammonium ylides

Abstract

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.