20-ISO-20-Deethylaspidospermidine

The two-step process of hydrogenation of l-alkyl-3-acylpyridinium salts and cyclization of the resultant 1, 4, 5, 6-tetrahydropyridines has been the foundation of a general scheme of alkaloid synthesis.¹ Its application in the indole alkaloid field has yielded ready access to tetrahydro-β-carboline systems, e.g. 1a → 2 → eburnamonine.² Since acid treatment of N_b -acyl derivatives of substances related to la has been shown to lead to products of indole β-cyclization³, it became of interest to test the cyclization behavior of the vinylogous imide lb, prepared by the dicyclohexylcarbodiimide-induced acylation of 3-acetyl-1, 4, 5, 6-tetrahydropyridine⁴ with indoleacetic acid. Treatment of lb with boron trifluoride gave a 62 yield of ketolactam 3a. Thus a two-step entry into the pentacyclic Aspidosperma alkaloid skeleton is on hand.