Optically active arsines stereoselective synthesis

Chiral arsonium ylides containing the benzylidene group react with aromatic aldehydes to produce trans-2,3-diaryloxiranes with optical purities of up to 41%. The degree of asymmetric induction depends upon the nature of the substituents on the ylide, the substrate, and upon the reaction conditions. For the variety of arsonium ylides investigated, however, the yields of recovered optically active arsines were almost quantitative with complete retention of configuration at arsenic in each case. Thus, apart from the value of stereoselective benzylidene transfer for the asymmetric synthesis of trans-2,3-diaryl oxiranes, the reaction provides the cleanest route to the recovery of optically active arsines from resolved benzylarsonium salts (see Section III.B.5). The mechanism of the reaction, which must take into account the exclusive formation of trans-diaryloxiranes, is believed to occur via the stereoselective decomposition of a pair of (R, S )-betaines (Scheme 16). 

The discoverj( of the stereoselective biomethylation of a prochiral arsinic acid by a microorganism opens up an exdting new route to optically active tertiary arsines. Numerous microorganisms reductively methylate arsenic(V) compounds . The biological synthesis of (+)-126 by Scopulariopsis brevicaulis was described in 1936. At that time, however, it was not recognized that simple tertiary arsines chiral at arsenic were configurationally stable and amenable to optical resolution. 

---