Biosynthesis. Part 28.1,2 Colchicine: definition of intermediates between O-methylandrocymbine and colchicine and studies on speciosine

Abstract

Labelled samples are prepared of demecolcine 3, colchicine 4, N-formyl-N-deacetylcolchicine 6 and N-deacetylcolchicine 7, the last depending on a new method for its preparation from colchicine. Incorporation experiments with these compounds and with specifically labelled autumnaline 1 support the pathway 2 → 5 → 3 → 6 → 7 → 4 as the terminal sequence for the biosynthesis of colchicine.

The key intermediate O-methylandrocymbine 2 is isolated from Colchicum autumnale plants together with speciosine 14 and its O-acetyl derivative 15; all three are first isolations from this plant. Speciosine 14 and N-methyldemecolcine 8 are shown to be formed in vivo largely from demecolcine 3 whereas N-formyldemecolcine 5 is the precursor of demecolcine and its N-formyl group is derived from C-3 of autumnaline. This discovery of a tropolone alkaloid which retains both carbons of the ethanamine bridge of 2 is important for future stereochemical work on the ring-expansion process.