Carboxy terpenoid indole alkaloids

In principle there appears no reason why a condensation product of secologanin with tryptophan instead of tryptamine should not occur in nature to give a new series of carboxy terpenoid indole alkaloids parallel to the known tryptamine derivatives. A few representatives have been isolated in recent years, and probably more will follow with the adoption of isolation techniques suited to amino acids. 

The biosynthesis of terpenoid indole alkaloids from tryptophan has long been known.1,2 In new experiments with DL-[2-14C,2, 3 - 3C2]tryptophan [as (94)], this has been confirmed for vindoline (106).35 Incorporation measured by assay for radioactivity and by n.m.r. spectroscopy was the same, thus indicating an intact incorporation (with loss, of course, of the carboxy-group) (for discussion of the particular application of 13C labelling used here, see pp. 1-2). 

The isolation of cordifoline and related bases led to the prediction of the existence of a common precursor derived by condensation between tryptophan and secologanin. This tetrahydrodeoxycordifoline (TDC) could be (a) a biogenetic cul-de-sac (b) an alternative intermediate to vincoside for some terpenoid indole alkaloids, or (c) the progenitor of a novel range of acidic terpenoid indole alkaloids in which the carboxy group of tryptophan was retained. The last possibility stimulated a search for such alkaloids which was amply rewarded by the discovery in A. rubescens of adirubine (7), 5a-carboxytetrahydroalstonine (102), and 5a-carboxycorynanthine (103), shown to have the tryptophan-based Corynanthe type structures 189,190, and 191, respectively. Surely, with the adoption of modern techniques for... 

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