The methanolysis of the epimeric 7-chloro-7H-yohimbine derivatives 2 and 3 was reinvestigated. In case of the 7a-epimer 2, the reaction was uneventful and conformed with earlier observations, i.e., under sufficiently mild conditions, only the imino ether 4 ( = imino ether A) was produced. Under the same conditions, the less reactive p-isomer 3 furnished a mixture of both imino ethers 4 and 5, accompanied by the elimination product 11, and by equal amounts of yohimbine (1) and 3,4,5,6-tetradehydroyohimbine (12), which are believed to arise through a disproportionation process of the putative intermediate 5,6-didehydroyohimbine (23). The nature of this divergent reactivity and of the ready equilibration of 4 and 5 was investigated by means of extensive force-field and semi-empirical calculations (AM1 and PM3) of various conformers of the compounds 2-5 and of some possible reaction intermediates.

Contrary to earlier reports, the base-induced rearrangement of the 7-hydroxy-7H-indolenines derived from ajmalicine (l), yohimbine (5), corynanthine (6), methyl reserpate (16), and methyl isoreserpate (17) in each case furnished not just one, but two epimeric spiro-pseudoindoxyl derivatives which have opposite configuration at the spiro centre C(2). In all cases, the major component was shown by NOE experiments to be the A-type isomer (carbonyl group located below the plane defined by rings C and D). The thermodynamically less stable B-type pseudoindoxyl epimers 4,10, 12, and 22 were isolated and characterized for the first time.

The oxidative transformation of synthetic (+)-aristoteline ((+)-6) into other metabolites which had been isolated from Aristoteiia species was investigated. Thus, treatment of (+)-6 with I, as the single oxidant furnished the naturally occurring indole alkaloids (+)-makonine ((+)-9), (+)-aristotelinone ((+)-1 I), or (+)-I 1,12-didehydroaristoteline ((+)-7) in good yields, the selectivity of the oxidation process depending on the chosen reaction conditions.

Dedicated to Professor Dieter Seebach on the occasion of his 65th birthday Two independent total syntheses of the Aristotelia alkaloid (À)-serratenone ((À)-1) are disclosed, one starting with (À)-a-pinene, the other one with (S)-a-terpineol. These correlations led to a revision of the originally proposed absolute configuration of the natural product. In the course of systematic investigations of the behavior of the indole alkaloids ()-makomakine (()-18) and (À)-hobartine ((À)-22) towards oxidizing reagents, it was found that treatment with I 2 leads to no less than five different products. Depending on the exact reaction conditions, each of them can be obtained as the major component in yields between 40 and 60%. One of these compounds was shown to be identical with the natural product ()-11,12-didehydromakonin-10-one (()-28).

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