Biosynthesis of the furoquinoline alkaloids, skimmianine, evoxine, and choisyine: mechanism of formation of the furan ring. The timing of aromatic hydroxylation and of methylation

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Tracer feeding experiments with Choisya ternata using doubly labelled precursors confirm that dictamnine (2a) is converted into skimmianine (2b), and show that the 4-methoxy-group of a 3-prenylquinolone precursor (1b) is retained in skimmianine; in the formation of the furan ring from platydesmine (3), a carbonyl derivative is not an intermediate; a similar biosynthetic route applies to the 7,8-dioxygenated quinoline, evoxine (4) and to the 6,7-derivative, choisyine (5).