BIOSYNTHESIS - PART 29 - COLCHICINE - STUDIES ON THE RING EXPANSION STEP FOCUSING ON THE FATE OF THE HYDROGENS AT C-3 OF AUTUMNALINE

The dienone ring of the intermediate 2 undergoes expansion to form the tropolone nucleus as N-formyldemecolcine 3 and colchicine 4 are biosy nthesised. The additional carbon atom needed to form the 7-membered ri ng is provided by C-12 of the dienone 2 whilst C-13 becomes the N-form yl group of 3. It is shown by incorporation experiments using Colchicu m plants with precursors (as 1) stereospecifically H-3-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.