Stereochemical fate of an asymmetric migrating group in the pinacol rearrangement

Abstract

The optically active pinacol, (4S)-2,3,4-trimethylhexane-2,3-diol (I) was synthesized from (–)-(S)-2-methyl-butan-1-ol (III) and rearranged to (+)-3,3,4-trimethylhexan-2-one (II), which was degraded to (–)-2,2,3-tri-methylpentanoic acid (XI). The acid (XI) obtained by this method has a specific rotation and circular dichroism experimentally equal to that of (–)-(S)-2,2,3-trimethylpentanoic acid synthesized through dialkylation of (+)-(S)-1-cyano-2-methylbutane (XIII), indicating that in the rearrangement of (I) to (II), the asymmetric migrating group [(S)-s-butyl] underwent migration with complete retention of configuration.