Stereochemical aspects of some Claisen rearrangements with cyclic orthoesters

Abstract

For some representative reactant pairs, chemical determinations have been made of the relative stereochemistry at the two new asymmetric centres which are set up when an allylic alcohol reacts with a cyclic orthoester. The results give directly the fractions of the total reaction which proceed through chair- and boat-type transition states; interpretations are provided in terms of steric interactions in these states. For some of the reactions there is evidence that the transition state (chair or boat) is not a symmetrical cyclohexane-like entity, the incipient new C–C bond being considerably longer than the C–O bond which is undergoing cleavage.