Asymmetric Synthesis of γ-Amino-Functionalised Vinyl Sulfones: De Novo Preparation of Cysteine Protease Inhibitors

The enantioselective azo-based α-amination of an aldehyde followed by a Horner.Wadsworth.Emmons-based vinyl sulfone formation is reported. The thus obtained optically active N,N'-diprotected trans-(phenylsulfonyl)vinyl hydrazine products were then converted into the corresponding N-functionalised trans-(phenylsulfonyl)vinyl amines. Specifically, reaction of 4-phenylbutanal with di-tert-butyl azodicarboxylate (DBAD) in the presence of L-or D-proline, followed by addition of diethyl [(phenylsulfonyl)methyl]phosphonate, gave either enantiomer of di-tert-butyl trans-1-[5-phenyl-1-(phenylsulfonyl)pent-1-en-3-yl]hydrazine-1,2-dicarboxylate. The enantiomeric excesses of the (+)-and (-)-enantiomers prepared in this manner were in the range 86.89%. The conversion of these γ-hydrazino vinyl sulfones into the corresponding γ-amino-substituted compounds was achieved following a Boc deprotection, Zn reduction, N-functionalisation sequence. This three-step sequence was reasonably efficient (approx. 50%) and no erosion of enantiopurity was found to have taken place. The compounds accessed via this process include both enantiomers of tert-butyl trans-[5-phenyl-1-(phenylsulfonyl)pent-1-en-3-yl]carbamate and epimeric dipeptide mimetics including 4-methyl-N-{(S)-1-oxo-3-phenyl-1-[((S,E)-5-phenyl-1-(phenylsulfonyl)pent-1-en-3-yl)amino]propan-2-yl}piperazine-1-carboxamide (also known as K777).