The Vincadifformine-Tabersonine Group

Apodine (93) has been obtained from Peschiera van heurckii (Muell.-Arg.) L. Allorge (131) and P. buchtieni (Tabernaemontana buchtieni Mgf.) (34). Deoxapodine (modestanine, 94) occurs in Hazunta modesta var. mod-esta subvar. montana (103) and Ervatamia corymbosa Roxb. ex Wall. (132). Vandrikine (95) occurs in E. corymbosa (132), and hedrantherine (96) in P. van heurckii (131). 

The structure of 98 was deduced from the proton NMR spectrum of its methylation product, 11-methoxyvincadifformine, and confirmed by comparison of 98 with the hydrogenation product of 11-hydroxytabersonine (86), which occurs in the same plant (114). 

The situation with regard to ISB-hydroxyvincadifformine (100) is rather different. Its structure was deduced (133) from its spectroscopic properties, mainly its proton and - C NMR spectra some critical chemical shifts, such as H-15 and C-15, were compared with those reported (134) for 15a-hydroxyvincadifformine and 15j8-hydroxyvincadifformine (100), which had earlier been prepared in racemic form as intermediates in a synthesis of 

A dihydroxy derivative of vincadifformine, formulated as 14,15-dihydroxyvincadifformine (101), has been isolated from Hazunta modesta var. modesta subvar. montana (103). Unfortunately, the paucity of alkaloid obtained precluded determination of its stereochemistry. 

Echitoveniline (102), 11-methoxyechitoveniline (103), and 11-methoxyechitovenidine (104) are three new alkaloids from the fruits of Alstonia venenata R. Br. (135,136), although the leaves are a better source of 103. In consonance with these structures, ester exchange with sodium methoxide affords (-)-19R-minovincinine (80) from 102, and (—)-19/ -ll-methoxyminovincinine, a minor alkaloid of Vinca minor (137), from 103 and 104. 

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The vincadifformine-tabersonine group of alkaloids provide a rich field for the study of oxidative rearrangements. New work reported recently101" includes a study of the oxidation of 3-oxovincadifformine (189) with m-chloroper oxy benzoic acid (Scheme 27). The products were 3-oxovincamine (190), 3-oxo-16-ep/-vincamine (191), and a dilactam (192), which is simply the result of oxidative fission of the 2,16 bond. Under carefully controlled conditions, the hydroxy-indolenine (193) was isolated, and it was subsequently shown to be an intermediate (as expected) in the formation of (190)—(192). Photochemical oxidative rearrangement gave similar results. 

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