The hydroxyindolenine derivative of voacangine, a new indole alkaloid from Voacanga africana.

A new alkaloid from Voacanga africana was identified as the hydtoxyindolenine derivative (6) of voacangine, and its structure was proven hy synthesis. A rearrangement product of 6, the pseudoindoxyl9, was also encountered and characterized. Two additional new alkaloids, the hydroxyindolenine (7) and pseudoindoxyl (10) derivatives of voacristine were synthesized. Mass spectra of all compounds are presented and interpreted. ELEVEN indole alkaloids have previously been isolated from the tree bark of Voacanga africana, and structures have been assigned to many of these.’ A re-examination of this species, using the very sensitive methods of gas chromatography and mass spectrometry, was considered likely to reveal the presence of many minor constituents which had as yet remained undetected. Thus a complete investigation was undertaken, leading to the isolation of twelve additional compounds.’ One of the new alkaloids (compound A) was isolated initially in small quantity , by.gas chromatography. Its mass spectrum (Fig. 5) indicated a mol wt of 384, and ’ the presence of a OH and a carbomethoxyl group was shown by relatively intense peaks at M 17 and M 59. Additional mass spectral peaks at M 15 and M 29 were characteristic3 of the Et side chain found in all the iboga alkaloids, the class to which many of the Voacanga alkaloids belong. For a more detailed characterization, additional amounts of this new compound were obtained by repeated column chromatography over alumina. According to TLC and its mass spectrum, the material obtained by this method was a single compound, but when subjected to gas chromatography, two major peaks were always observed. The first of these proved to be compound A. The .&ccind substance (B) also had a mol wt of 384, but its mass spectrum (Fig. 2) had little resemblance to that of A. Especially revealing was the absence of an M 17 peak, which was the major fragment peak in the spectrum of A. As described belob, the identification of B as the pseudoindoxyl derivative (9) of voacangine (2) provided a key to the structure of A, subsequently proven to be the hydroxyindolenine derivative (6) of voacangine. A similarity between B and the known alkaloid iboluteine (8),4 originally apparent from the bright yellow color and intense fluorescence, was further demonstrated by a comparison of their UV spectra (B : I,,,,, McoH 228 rnp (log c 4.44), shoulder 255 (3.89), 410 (351); iboluteine: ,I:::” 226 mp (log E 4.44), shoulder 255 (3.90), 418 (3.48).) The difference in mol wts of these, two compounds suggested that B was a carbomethoxyiboluteine (9), i.e. the pseudoindoxyl derivative of voacangine (2). Additional inforkntinn which nnintd tn ctrllrtllre 0 WDC the oct.at ~t\rnrn+;nn n+ 173n p--t :-