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 chir al phosphsnate reagents 3a-d, which contain (1R,2S,5R)-8-phenylmenthol as a chiral auxiliary, and meso-dialdehydes 6 and 14 are presented. I t was found that both the geometric selectivities and the levels of as ymmetric induction depended on the structure of the phosphonate (i.e., the alkyl group R-1 in the phosphoryl unit) and to a certain extent a lso on the reaction conditions. Furthermore, the nature of the protect ing group used on the a-oxygen substituent in dialdehydes 14 influence d the outcome somewhat. By an appropriate choice of reagent and condit ions, either (E)- or (Z)-monoaddition products could be obtained geome trically pure and with good to excellent diastereoselectivities, in sy nthetically 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 gr oups in the substrates. A mechanistic model which accounts for the pro ducts formed is presented.