Alkaloids of Tabernaemontana crassa. Crassanine, a new oxindole alkaloid

Abstract

As part of an extended chemotaxonomic study of the genus Tabernaemontana (Apocynaceae), we now describe the results of a study of the alkaloids of the African species, Tabernaemontana crassa Benth. The major alkaloid of T. ctassa proved to be the known ibogamme-type base, conopha.ryngine (1) an amorphous base, which we were unable to complete purify by chromatography, was the second most. abundant alkaloid. This latter base, which was assigned the structure of 20-hydroxyconopharyngine (2) was purified instead by a new procedure which should be widely applicable for the isolation of related bases. Thus, the reaction of impure 2 with benzyl chlorofor­mate in pyridine afforded the crystalline carboben­zoxy ester 3, from which pure 2 was readily regenerated by hydrogenolysis in the presence of palladium .
The assigned structure of 2 fully agreed with its spec­tral properties. Its ultraviolet absorption spectrum was that of a typical 5,6-dimethoxyindole; indeed, it was essentially identical with that of conopharyngine (1) In addition, the nmr spectrum of 2 was similar to that of its unmethoxylated analog, heyneanine (4) except that the spectrum of 2 clearly showed the pres­ence of the 5,6-dimethoxyindole system in the form of methoxyls at o 3.78 and 3.86 and a pair of unsplit aro­matic protons at 6.71and6.83. Furthermore, the mass spectrum of 2 was exactly analogous to the published spectrum of heyneanine (4) except that the fragments from 2 containing the indole nucleus were 60 mass units heavier than those from 4 because of two methoxy sub­stituents on the aromatic ring.

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