(−)-Antirhine 2 was synthesized in an efficient and stereocontrolled fashion from the readily available (1R-2S)-cyclohexene dimethanol monoacetate 6. A key step was the regio- and stereoselective Pictet Spengler cyclization of the masked dialdehyde 12 to the indoloquinolizidinone 13.
Total synthesis of (-)-antirhine (5) has been achieved from a potentially useful chiral synthon, (3R)-[3-hydroxy-(E)-prop-1-enyl]cyclopentanone (2) [derived from (R)-1, 2-isopropylideneglyceraldehyde (1)], via the β, γ-unsaturated aldehyde (12), which was obtained by mild hydrolysis of the α-cyano-N, N-dimethylaminocyclopentanone (11).