Conformation of 1-Oxy Cyclohexenes Deriving from Diels-Alder Cycloaddition: Spectroscopic and X-Ray Crystal Structure Analysis

For cycloadduct (6), derived by reaction of the (Z)-oxy diene (7) with the dienophile (8), the newly formed cyclohexene ring is shown by spectroscopic and X-ray crystal structure analysis to be based on the half-chair conformation (9). This contrasts with analogous diastereomeric adducts from (E)-oxy dienes, which are based on the ring-flipped conformation (4). The determinant for this conformational difference is suggested to be the stereoelectronic preference of the allylic oxy substituent, in (4) and (9), for a pseudoaxial orientation to the double bond of the cyclohexene system.