Search

19S-Hydroxytacamine (3) and 19R-hydroxytacamine (4) were prepared from cis hydroxyesters 7 and 8 via corresponding O-TMS esters 9 and 22. Comparison of the spectral data of isomers 3 and 4 with those of 19-hydroxytacamine from Tabernaemontana eglandulosa confirms that isomer 3 represents the naturally occurring 19-hydroxytacamine.

Total syntheses are described for seven tacamine-type indole alkaloids (1–7) found in Tabernaemontana eglandulosa. (±)-Tacamine (1), (±)-16-epitacamine (2), and (±)-apotacamine (3) were prepared from pentacyclic intermediates 18. Apotacamine (3) was also obtained from aldehyde 12 via epimerization of 20-epiapotacamine (19) by the Polonovski-Potier reaction. Homologation of ester 13 led to 20-epitacamonine (23), which was similarly converted to (±)-tacamonine (4). Reduction of 4 gave (±)-descarbomethoxytacamines 5 and 6. Aldehyde 11 was reacted with trimethylsilyl cyanide (TMSCN) to yield the two (±)-17-hydroxytacamonines (7), of which the isomer with an axial hydroxy group (17β-OH) was found to be identical with the natural compound. Finally, an attempt to prepare (±)-19S-hydroxytacamine (8) is described. As an intermediated for this, a new synthesis of the pyridine alkaloid methyl 5-(1′-hydroxyethyl)nicotinate (34) was achieved.

From the leaves and twigs of Tabernaemontana eglandulosa 22 alkaloids, of which 12 are new, have been isolated. Besides tacamine (pseudo-vincamine) 7 other alkaloids of this new class of indole alkaloids were identified. A further three alkaloids of the cleavamine type, namely the known ( + )20R-15, 20-dihydro-cleavamine and (-)20S-15, 20-dihydro-cleavamine and one new alkaloid, which was tentatively identified as 14S, 20R-velbanamine, two new alkaloids of the pseudo-aspidospermidine type, namely 20R-l,2-dehydro-pseudo-aspidospermidine and 20S-hydroxy-l,2-dehydro-pseudoaspidospermidine and an alkaloid of a novel iboga type were isolated. More common alkaloids isolated were coronaridine, 11-hydroxycoronaridine, voaphylline, tubotaiwine, ibogamine, norfluorocurarine and probably a mixture of 20R- and 20S-pseudo-vincadifformine. Only quantitative differences in the alkaloid content of the leaves, twigs and stembark were observed.

From the leaves of Tabernaemontana eglandulosa a new alkaloid was isolated, which was assigned structure 1 on the basis of speetroscopic evidence.