Reagent control of geometric selectivity and enantiotopic group preference in asymmetric Horner-Wadsworth-Emmons reactions with meso-dialdehydes

Results from asymmetric Horner-Wadsworth-Emmons reactions between chiral phosphsnate reagents 3a-d, which contain (1R,2S,5R)-8-phenylmenthol as a chiral auxiliary, and meso-dialdehydes 6 and 14 are presented. It was found that both the geometric selectivities and the levels of asymmetric induction depended on the structure of the phosphonate (i.e., the alkyl group R-1 in the phosphoryl unit) and to a certain extent also on the reaction conditions. Furthermore, the nature of the protecting group used on the a-oxygen substituent in dialdehydes 14 influenced the outcome somewhat. By an appropriate choice of reagent and conditions, either (E)- or (Z)-monoaddition products could be obtained geometrically pure and with good to excellent diastereoselectivities, in synthetically useful yields. Analyses of the absolute configurations of the products showed that the (E)-selective reagents (3a-c) and the (Z)-selective phosphonate 3d reacted at opposite enantiotopic carbonyl groups in the substrates. A mechanistic model which accounts for the products formed is presented.