Stereoselective syntheses of α-isosparteine

Abstract

Stereoselective syntheses of α-isosparteine were accomplished by means of 1,3-dipolar cycloadditions. The cycloaddition of nitrone (4) to cyclopentadiene proceeded stereoselectively to afford 2:1-adduct (7a), which in turn led to the construction of the tetracyclic quinolizidine framework. The synthesis using 4H-pyran as a dipolarophile was more convenient, for the cycloaddition proceeded with high regio- and stereo-selectively to afford 2:1-adduct (16a), which was converted into α-isosparteine by catalytic hydrogenation in high yield.