Synthesis and Desymmetrization of meso ‐2,3‐Diphenylpiperazine for Application in Asymmetric Transformations

The meso ‐2,3‐diphenylpiperazine was prepared in 86 % yield with 99:1 dr from the readily accessible 5,6‐diphenyl‐2,3‐dihydropyrazine by NaBH 4 /MeOH reduction. The corresponding diastereomeric amides prepared using ( R )‐(‐)‐menthyl chloroformate, after separation, benzylation and hydrolysis gave both isomers of optically active N ‐benzyl‐2,3‐diphenylpiperazine samples in up to 94 % ee which after further enrichment using the simple achiral oxalic acid afforded samples of both enatiomers in 99 % ee . Reaction of the chiral optically active N ‐benzyl‐2,3‐diphenylpiperazine with terminal alkynes and aldehydes in the presence of ZnCl 2 gave the corresponding propargylamines with very high diasterioselectivities (up to 99:1 dr ) from which both enantiomers of chiral allenes were obtained in up to 82–86 % ee by reaction with ZnBr 2 in toluene at 120 °C.