SYNTHESIS OF THE ABCD RING SYSTEM OF VINCA ALKALOIDS USING TANDEM INTRAMOLECULAR [2+2]-PHOTOCYCLOADDITION-RETRO-MANNICH FRAGMENTATION

Irradiation of 3-alkyl indole 20 gave spiropyrroline 22 via [2+2]-photocycloaddition and subsequent in situ retro-Mannich fragmentation of fused cyclobutane 21. N-Alkylation of 22 followed by treatment of the resulting pyrrolinium salt with sodium hydride and lithium diisopropylamide generated a dienolate dianion, e.g. 24, which underwent cyclization to afford tetracyclic products 25, 27 and 30. The configuration of 25 was proven by a series of NMR experiments which established that ring C in the major stereoisomer resides in a boat conformation. Tetracycles 25, 27, and 30 contain structural features including the ABCD ring system and substituents found in certain alkaloids of the Vinca family.