Asymmetric Synthesis of trans-2,3-Diaryloxiranes. Benzylidene Transfer from Chiral Arsonium Ylides

Abstract

Potassium diphenylarsenide reacts with (-)-menthyl chloride in boiling tetrahydrofuran to produce (+)-diphenylneomenthylarsine. Quaternization of this compound with benzyl bromide affords (+)-benzyldiphenylmenthylarsonium bromide, which upon reduction with lithium aluminium hydride gives the epimeric (-)-diphenylmenthylarsine. Equilibrium concentrations of the semistabilized benzylidene ylide derived from the menthylarsonium salt, and from a variety of other optically active arsonium salts containing asymmetric arsenic atoms, react with prochiral aromatic aldehydes to produce good to excellent yields of trans-2,3-diaryloxiranes having optical purities of up to 41%. The degree of asymmetric induction depends upon the nature of substituents within the ylide and on the substrate molecule and upon reaction conditions. The stereochemistry of the products has been rationalized in terms of the conformational differences between the intermediate erythro-betaines. Show more