Evaluation of sparteine-like chiral diamines in the enantioselective lithiation–electrophilic trapping of an O-alkyl carbamate

Abstract

Seven (+)-sparteine-like diamines and (−)-sparteine were evaluated in the diamine-mediated asymmetric lithiation–trapping of an O-alkyl carbamate. The (+)-sparteine-like diamines (≥98 : 2 er by chiral shift NMR spectroscopy) were prepared from (−)-cytisine (>99 : 1 er by chiral HPLC of N-benzyl cytisine) and two new (+)-sparteine-like diamines containing N-CD₃ substituents were included as part of this study. The following results from the ligand evaluation study were obtained: (−)-sparteine is unrivalled in its ability to induce near-perfect enantioselectivity (99 : 1 er); N-methyl diamine and the two N-CD₃-substituted diamines are the optimal (+)-sparteine surrogates (up to 96 : 4 er); sterically more hindered N-alkyl substituents gave reduced enantioselectivity (N-iso-propyl: 86 : 14 er; N-CH₂^tBu: 54 : 46 er). From a synthetic point of view, these results show that either enantiomer of α-substituted O-alkyl carbamates can be obtained by enantioselective lithiation–trapping using (−)-sparteine and the N-methyl (+)-sparteine surrogate.