Enantioselective nitrile anion cyclization to substituted pyrrolidines.: A highly efficient synthesis of (3S,4R)-N-tert-butyl-4-arylpyrrolidine-3-carboxylic acid

A practical asymmetric synthesis of N-tert-butyl disubstituted pyrrolidines via a nitrile anion cyclization strategy is described. The five-step chromatography-free synthesis of (3S,4R)-l-tertbutyl-4-(2,4-difluorophenyl)pyrrolidine-3-carboxylic acid (2) from 2-chloro-l-(2,4-difluorophenyl)ethanone achieved a 71% overall yield. The cyclization substrate was prepared via a catalytic CBS asymmetric reduction, t-butylamine displacement of the chlorohydrin, and a conjugate addition of the hindered secondary amine to acrylonitrile. The key nitrile anion 5-exo-tet cyclization concomitantly formed the pyrrolidine ring with clean inversion of the C-4 center to afford 1,3,4trisubstituted chiral pyrrolidine in > 95% yield and 94-99% ee. Diethyl chlorophosphate and lithium hexamethy1disilazide were shown to be the respective optimum activating group and base in this cyclization. The trans-cis mixture of the pyrrolidine nitrile undergoes a kinetically controlled epimerization/ saponification to afford the pure trans-pyrrolidine carboxylic acid target compound in > 99.9% chemical and optical purity. This chemistry was also shown to be applicable to both electronically neutral and rich substituted phenyl substrates.