Lewis acid assisted vinylogous Mannich and Mukaiyama aldol reactions: A route to densely hydroxylated indolizidine alkaloid analogues

The hydroxymethyl-substituted indolizidines 6 and 7, representative members of a ring-B-expanded alexine- australine subclass, are readily accessible by starting with furan-based silyloxydiene 12 and hydroxymethyl hemiaminal 11, through a synthesis sequence involving a scantily exploited vinylogous version of the Mannich reaction. The key iminium electrophilic acceptor 11 is, in turn, available through a vinylogous intermolecular Mukaiyama aldolization process between pyrrole-based silyloxydiene 8 and (S)-glyceraldehyde 9.