Enantioselective desymmetrization of tropinone derivatives by hydroboration

N-Protected tropenone derivs. I (R = CO2Me, CBZ, Ts) prepd. from the resp. pyrroles and tetrabromoacetone, were used as starting materials for desymmetrization by hydroboration of the C-C double bond. Hydroboration of I with (-)-(Ipc)2BH followed by oxidn., however, gave the desired 6-hydroxylated product only in low yield due to byproduct formation. After acetalization of the carbonyl group in I, the corresponding acetals were desymmetrized with (Ipc)2BH and oxidative workup to chiral alcs. II in good yields with excellent enantiomeric excesses in most cases.