Bridged-Ring Indole Alkaloids

Prenylations and ferf-prenylations of indole most frequently occur at positions Nl, C2 and C3 (Sects. 2-4), with fewer indole alkaloids prenylated in the benzene section (Sect. 5). There are no indole derivatives with prenylated or ferf-prenylated bridge head positions C3a or C7a. Regioselective introduction of prenyl or tert-prenyl groups at the indole ring can be challenging, depending on the position. Certain cases are simple, such as N-prenylation, others are still not reported, such as... 

The importance of indoloazocine scaffold in indole alkaloids has been supported by several methodologies and total syntheses published through the years. One example is the total synthesis of ( )-apparicine by the Re.nne.sar group (Scheme 4.1). Indeed, the indoloazocine scaffold was synthesized via an indole-template ring-closing metathesis reaction. Another key step was the Heck cycli-zation to finally assemble the bridged skeleton [27]. 

The structure of the piperidine moiety and the presence of an unsubstituted tryptamine bridge in alkaloids of the mavacurine-pleiocarpamine class can be proved by a catalytic reduction to the corresponding indolines, which show very different spectra because now the main fragments contain ring D,los (212)- -(216)- -[2I7]-> [218]->[219]. The reduction of indole to indoline alkaloids seems an attractive route for the structure determination of those alkaloids.25... 

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