C-Alkylation of Chiral Tropane- and Homotropane-Derived Enamines

The synthesis and alkylation of chiral, non-racemic tropane- and homotropane-derived enamines is examined as an approach to enantioenriched alpha-alkylated aldehydes. The two bicyclic N auxiliaries, which differ by a single methylene group, give opposite senses of asymmetric induction on alkylation with EtI and provide modestly enantioenriched 2-ethylhexanal (following hydrolysis of the alkylated iminium). The observed stereoselectivity is supported by density functional studies of ethylation for both enamines.