The stereochemistry of the reduction with four aluminum hydride reagents of five α-ketols and four α-diones has been studied. The results, together with those of a previous study, representing a total of seven substitutional types, are considered in terms of current models of asymmetric induction in α-ketols. The stereoselectivity observed may be correlated by Cram's cyclic model for asymmetric induction in chiral ketones only with a monomeric reagent, triisobutylaluminum. A modification of Cram's model, wherein the intermediate alanate complex exists in a cyclopentanoid half-chair conformation, allows explication of several observed anomalies. A dependence of the stereoselectivity of the reduction upon the bulk of the reagent is accommodated by this model. Two side reactions, Meerwein-Ponndorf-Verley reduction and hydride-catalyzed keto-hydroxyl isomerization, are considered as alternative reaction paths which might affect observed stereoselectivity. While both reactions are noted under specified conditions, the degree to which each occurs (and in the former case, kinetic considerations) suggests that their effect is minimal.
ASJC Scopus subject areas
- Organic Chemistry