Stereoselective Aldol Condensations via Boron Enolates. The Syntheses of (+)-Prelog-Djerassi Lactone and (+)-Tylonolide

The use of boron enolates of chiral N-acyl oxazolidinones i and ii in highly stereoselective aldol condensations is described. Mild base hydrolysis of the products affords optically pure erythro β-hydroxy acids and the chiral auxiliary, which can be reconverted into i or ii in one step. The utility and scope of these reactions is demonstrated by the total syntheses of (+)-Prelog-Djerassi lactone and (+)-Tylonolide, cyclic 5,20-hemiacetal . [Chemical structures included in scanned thesis' abstract, p. v.]