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Communication | Special issue | Vol 88, No. 2, 2014, pp.899-910
Published online, 11th September, 2013
DOI: 10.3987/COM-13-S(S)52
Synthesis of the ABCD Ring System of Vinca Alkaloids Using Tandem Intramolecular [2+2]Photocycloaddition-Retro-Mannich Fragmentation

James D. White* and Yang Li

*Department of Chemistry, Oregon State University, Gilbert Hall 153, Corvallis, Oregon 97331, U.S.A.

Abstract

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.