Asymmetric Induction in C-Alkylation of Tropane-Derived Enamines: Congruence Between Computation and Experiment.

Quantum chemical studies of C-ethylation of 1-methyl- and 1,4,4-trimethyl-tropane-derived enamines predict good (89:11 er, B3LYP) and high (98:2 er, B3LYP) levels, respectively, of asymmetric induction in the resulting α-alkylated aldehydes. The nonracemic tropanes were synthesized using Mannich cyclization strategies (Robinson-Schöpf and by way of a Davis-type N-sulfinyl amino bisketal, respectively), and ethylation of the derived enamines was found to support the predicted sense and magnitude of asymmetric induction (81:19 er and 95:5 er, respectively). A comparison of several computational methods highlights the robustness of predicted trends in enantioselectivity, enabling theory to guide synthesis.