Bischler-Napieralski ring closure 2,3-dihydro

A second direct route to an extended 3,4-dihydro-j8-carbolinium system (120) using the Bischler-Napieralski ring closure is based on the cyclization of intermediates of general structure 118. Three approaches to intermediates of this type have been developed in connection with stereospecific syntheses in the indole alkaloid field. The first approach, introduced independently by Stork and Hill and by van Tamelen and co-workers and often used... 

An analogous acylation reaction has been described in another 3,4-dihydro-j8-carboline derivative. The amide 315, on Bischler-Napieralski ring closure with phosphorus oxychloride, yields an... 

Methods of synthesis of bisbenzylisoquinoline alkaloids have been reviewed83 and the use of electrochemical methods for the reduction of the dihydro-isoquinolinium salts that result from Bischler-Napieralsky ring-closures in the synthesis of, for example, espinidine has been studied.84... 

Acylation of 3-substituted indoles is more difficult, however 2-acetylation can be effected with the aid of boron trifluoride catalysis." " Indoles, with a carboxyl-containing side-chain acid at C-3, undergo intramolecular acylation forming cyclic 2-acylindoles." Intramolecular Vilsmeier processes, using tryptamine amides, have been used extensively for the synthesis of 3,4-dihydro-p-carbolines, a sub-structure found in many indole alkaloids (P-carboline is the widely used, trivial name for pyrido[3,4-fc]indole). Note that it is the imine, rather than a ketone, that is the final product the cyclic nature of the imine favours its retention rather than hydrolysis to amine plus ketone as in the standard Vilsmeier sequence " this ring closure is analogous to the Bischler-Napieralski synthesis of 3,4-dihydro-isoquinolines (9.15.1.7). 

---