Substituent interactions geometric consequences

Johnson rearrangement of allylic alccdiols (274) and (277) led to the enantiomeric 7,8-unsaturated esters (276) and (279), respectively (Scheme 48). Both transition states (275) and (278) favor a pseudo-equatorial position of the benzyloxymethylene substituent the newly formed chiral center is obtained in very hi optical purity throu the chirality transfer process. As a consequence of this geometrical preference, secondary allylic alcohols invariably provide predominantly ( )-configured double bonds upon thermal Gaisen rearrangement The ( -selectivity usually increases with the steric bulk of the C-2 substituent, an effect which was rationalized by a pseudo-1,3-diaxial interaction in the transition state leading to the (Z)-alkene (280 Figure S). ... 

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