Approach to the Homoerythrina Alkaloids Using a Tandem N-Alkylation/Azomethine Ylide Cycloaddition

Synthetic efforts toward the homoerythrina alkaloids 1−3 are described. Two separate model systems
guided the pivotal [3 + 2] azomethine ylide cycloaddition cascade to form the A−C rings of these alkaloids.
The cycloaddition precursors 63 and 68, prepared in nine and ten steps, respectively, from alkyne 47,
each contain an enolizable ketone, a tethered electrophile, and an electron-poor dipolarophile. Heating
63 and 68 with the stannyl amine 17 generated demethoxyschelhammeridine 65 and demethoxyschelhammericine 70, the products of intramolecular azomethine ylide cycloadditions. Subsequent attempts
to install the C-3 methoxy group of 1−3 are also described.