Is the A-ring of sparteine essential for high enantioselectivity in the asymmetric lithiation-substitution of N-Boc-pyrrolidine?

The simplest chiral portion of sparteine, N,N'-dimethyl-2-endo-methylbispidine, was prepared and evaluated in the asymmetric lithiation-substitution of N-Boc-pyrrolidine. The results indicate that the complete A-ring of sparteine is essential for high levels of asymmetric induction. DFT-QSSR analyses of the diamine/Li(+) complexes and DFT calculations of the pertinent i-PrLi/diamine/N-Boc-pyrrolidine complexes are predictive and provide complementary pictures of the stereochemical features critical to this transformation.