α'-benzoyloxy and α'-methoxymethoxy lithium enolates giving opposite diastereofacial selectivities in aldol reactions. Use of (probable) extended chelation for reversal of stereoselectivities

Article abstract of DOI:10.1016/S0040-4039(00)77578-2

The Li enolates of α-benzoyloxy and α-methoxymethoxy ketones 1a and 1c afford nonchelation and chelation aldol products, respectively, both with usefully high diastereofacial selectivities. Evidence suggests that transition state chelation of the benzoyl C = O is responsible for the observed 'nonchelation' stereoselectivity of 1a.