Journal article

Sparteine-mediated a-lithiation of N-BOC-N-methylbenzylamine: Rapid racemization and subsequent deracemization

Upon a-lithiation of N-BOC-N-methylbenzylamine (tert-Bu N-benzyl-N-methylcarbamate) with sec-BuLi in the presence of (-)-sparteine, the pro-(S) proton is preferentially abstracted (kinetic resoln. of the racemic a-deuterated deriv. up to 88% enantiomeric excess). The enantiomerically enriched organometallic intermediate thus generated instantaneously racemizes. Enantioselectivity, however, is restored in the course of 15-45 min., apparently due to adduct formation between the (S)-aminobenzyllithium species and sparteine. Subsequent reaction with typical electrophiles (CO2, CS2, MeI, Me2SO4, w-deuteriophenylacetylene) occurs with retention (51-90% enantiomeric excess), in hexane or Et2O, and with inversion (68-85%), in THF. [on SciFinder (R)]


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