Alkaloid biosynthesis. Part XV. Partial synthesis and isolation of vincoside and isovincoside: biosynthesis of the three major classes of indole alkaloids from vincoside

Tryptamine acts as a precursor of the indole alkaloids in Vinca rosea. Secologanin (1) condenses with tryptamine to yield vincoside (2) and isovincoside (5), the absolute configuration of these products is determined and their chemistry is explored. It is demonstrated by single- and double-labelling experiments that vincoside is the precursor of representative indole alkaloids from the Corynanthe, Aspidosperma, and Iboga families in Vinca rosea; isovincoside is not involved in these biosyntheses. Vinca rosea plants are shown to contain vincoside, isovincoside, and N-acetylvincoside by a combination of radiochemical dilution analysis and direct isolation. [5-3H]-Loganin fed to Vinca rosea is incorporated efficiently into vincoside and isovincoside.The steps are discussed by which vincoside is converted into the three types of indole alkaloids and it is proved that the side-chain unsaturation of vincoside is essential for operation of the later biosynthetic stages. The main Part OF This work has been briefly reported.2,3