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Synthesis of Aristotelia-type alkaloids. Part IX. Synthesis of (±)-alloaristoteline

✍ Scribed by Rolf Güller; Hans-Jürg Borschberg

Publisher
John Wiley and Sons
Year
1991
Tongue
German
Weight
613 KB
Volume
74
Category
Article
ISSN
0018-019X

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✦ Synopsis

Abstract

The probably most straightforward plan to synthesize the indole alkaloid alloaristoteline (5) failed, because– in marked contrast to the regular Aristotelia series‐electrophilic reagents attack with preference C(3) of the indole moiety in the key intermediate allohobartine ((−)‐12), instead of C(18). The only product that could be isolated when (−)‐12 was treated with mineral acid was isomer (+)‐15 of 5 (Scheme 2). As a consequence, the crucial electrophilic site at C(17) was created by taking recourse to the preparation of the stabilized allylic cation VI. Gratifyingly, this alleged intermediate, obtained from precursor (±)‐18, cyclized smoothly to protected (±)‐18,19‐didehydroalloaristoteline (±)‐17, which was transformed in two high‐yield steps into the racemic form of the target molecule 5 (Scheme 4). This successful alternative provides unambiguous evidence that the recently revised structure of 5 is indeed correct.

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