Synthesis of cyclohexapyrrolizidines: C-methylation of vinylogous amides

A synthesis of the cyclohexapyrrolizidines 32 and 33 is described; the structure of the latter was determined by X-ray crystallography. It is anticipated that this synthesis will serve as a model for the construction of a similar pyrrolizidine system during a projected synthesis of thelepogine 2 .A convenient method for the C-methylation of cyclic vinylogous amides is also described. This procedure, which is similar to published procedures, affords high yields of methylated product and is regiospecific.