STEREOSELECTIVITY OF ANKEN IN WITTIG REACTION

Abstract

As general rule, in Wittig reaction the use of a nonstabilized ylide (with H or alkyl substituents) and salt-free conditions in a nonprotic, polar solvents favors the formation of (Z)-alkene isomer in reactions with an aldehydes. A stabilized ylide (with strongly conjugated substituents such as an aromatic system) forms predominantly the (E)-alkene. Two Wittig reactions are mentioned in this paper. In the first reaction a nonstabilized ylide formed from 12-hydroxydodecyltriphenylphosphonium bromide reacts with octadecanal to form predominantly (Z)-12-triaconten-1-ol, and in the second reaction a stabilized ylide formed from 1,6,8-trimethoxy-9,10-dioxoanthryl-3-methyltriphenylphosphoniumbromide reacts with octadecanal to yield almost (E)-3-(nonadec-1-enyl)-1,6,8-trimethoxyanthraquinon. The configuration of the two alkenes could be deduced from their IR and NMR spectroscopic data.