Control of the stereochemistry of C14 hydroxyl during the total synthesis of withanolide E and physachenolide C

Abstract

The epoxide configuration in oxidation of C14–C15 alkenes is determined by the configuration of the C17 substituent.

Structural verification of petromyzestrosterol by total syntheses of both C14-epimers of its 3-O-methyl derivative

“…Reductive cleavage of 17a with LiAlH 4 furnished 3- O -methyl-14α-petromyzestrosterol ( 1b ) in 83% yield, the stereochemistry of which was determined by comparison of its 1 H and 13 C NMR spectra with those of 14β-isomer 2b . The stereochemistry of epoxide 17a was inferred based on the stereochemistry of 1b ; the α-facial selectivity in the epoxidation is attributed to the directing effect of the pseudo-axial C7 hydroxy group 10 as well as the steric repulsion of the angular methyl group 11 (Fig. 6).…”