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

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.