Vobasine synthesis

The absolute configuration of koumine has also been confirmed by a biomimetic synthesis of koumine from vobasine (50) (43). The configuration was correlated with L(—)-tryptophan, since vobasine was reduced to drega-mine (51) and the latter had been synthesized from L(—)-tryptophan with conservation of the (S) configuration of the amino acid, which corresponds to C-5 of koumine (44). 

Another biomimetic partial synthesis of koumine (29) was reported in 1986 by Liu et al (Fig. 25) (89, 1). Vobasine (139) on reduction with LiAlH4 gave vobasinediol which was dehydrated with aqueous sulfuric acid to afford anhydrovobasinediol (16). Allylic oxidation of 16 with Se02/H202 gave koumine (29) in a modest (25%) yield. Application of this procedure to 19(Z)-anhydrovobasinediol (6) led to the formation of a trace amount of koumine (29) detectable by HPLC analysis (90). 

StiU with a tetrahydopyridine derivative alcohol 354, a high-yielding orthoester rearrangement was used in a synthesis of antitumor alkaloid ellipticine 353 [82] (Scheme 6.60). The same kind of reaction was applied to a synthesis of descarbo-methoxy vobasine 356b (Scheme 6.60) [83]. This type of rearrangement was also used by Kametani [84] in an approach to ervitsine 359 (Scheme 6.60). 

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