Enantioselective synthesis of phospholenes via asymmetric organocatalytic alkene isomerization

An asymmetric synthesis of the 2,5-diphenylphosphol-2-ene fragment (≥95% ee) has been realized via enantioselective Cinchona-alkaloid catalyzed double bond isomerization of a meso-2,5-diphenylphosphol-3-ene amide to a 2,5-diphenylphosphol-2-ene amide (up to 83% ee), followed by enantiomeric enrichment to ≥95% ee by crystallization. The 2,5-diphenylphosphol-2-ene amide (a cyclic phosphinic acid amide) was hydrolyzed to the 2,5-diphenylphosphol-2-ene acid (a cyclic phosphinic acid) with retention of configuration at C-5.