Biosynthesis. Part 29.1 Colchicine: studies on the ring expansion step focusing on the fate of the hydrogens at C-3 of autumnaline

Abstract

The dienone ring of the intermediate 2 undergoes expansion to form the tropolone nucleus as N-formyldemecolcine 3 and colchicine 4 are biosynthesised. The additional carbon atom needed to form the 7-membered ring is provided by C-12 of the dienone 2 whilst C-13 becomes the N-formyl group of 3. It is shown by incorporation experiments using Colchicum plants with precursors (as 1) stereospecifically ³H-labelled at the centre corresponding to C-13 of 2 that H_S is entirely lost whereas H_R is fully retained as N-formyldemecolcine 3 is formed. The syntheses of the labelled precursors (as 1) are described.