Enantioselective Approach to the Hetisine Alkaloids. Synthesis of the 3-Methyl-1-aza-tricyclo[5.2.1.0³,⁸]decane Core via Intramolecular Dipolar Cycloaddition

[structure: see text]. An efficient, enantioselective approach to the hetisine class of the C(20)-diterpenoid alkaloids is described. The strategy involves an intramolecular oxidopyridinium dipolar cycloaddition as the key transformation, in which simultaneous formation of the C5-C6 and C10-C20 bonds in the 3-methyl-1-aza-tricyclo[5.2.1.0(3,8)]decane core of the hetisine alkaloids is effected.