Biosynthesis. Part 27.1,2 Colchicine: studies of the phenolic oxidative coupling and ring-expansion processes based on incorporation of multiply labelled 1-phenethylisoquinolines
Earlier research from our group had proved that the biosynthesis of colchicine and its tropolonoid relatives involves the oxidative ring-closure of a 1-phenethyltetrahydroisoquinoline, autumnaline, followed by extensive modification of the cyclised product. These steps pose many mechanistic and stereochemical questions which are set out in the Introduction. The sequel then provides the answers based on the results from a series of incorporation experiments on Colchicum plants involving multiply labelled forms of autumnaline and its biological precursors. These multiply labelled samples required the synthesis of eleven differently labelled tetrahydroisoquinolines; the methods used to introduce the labels are described. Autumnaline is shown to be present in Colchicumautumnale plants and labelled forms of some precursors of it are synthesised and studied. Taken together, the findings allow further definition of a substantial part of the biosynthetic pathway to colchicine and its relatives.