The “tert‐amino effect” in heterocyclic chemistry: Synthesis of tetra‐ and pentacyclic compounds

A new and convenient route for the synthesis of several heterotetra‐ and pentacyclic compounds is presented. Heating the 2‐vinyl‐N,N‐dialkylanilines 3a‐h, synthesized from the aldehydes 2a‐f,h or the acetophenone 2g and malononitrile or ethyl cyanoacetate, in refluxing 1‐butanol yields selectively the tetra‐ and pentacyclic compounds 4a‐h. This cyclization proceeds via a 1,5 hydrogen shift, followed by an addition of the carbanion to the iminium double bond. The benzo[f]thieno[3,2‐a]quinolizine 4c and the benzo[f]thieno[2,3‐a]quinolizine 4d represent new classes of heterocycles. The first example of formation of a six‐membered ring via carboncarbon bond formation, starting from 2‐vinyl‐substituted enamines, was the conversion of 5a and 5b into the benzo[h]pyrrolo[1,2‐a]‐quinoline 6a and the naphtho[2,1‐c]quinolizine 6b, respectively. Most probably, the cyclization of 5a and 5b proceeds in the same way as does the cyclization of the 2‐vinyl N,N‐disubstituted aniline derivatives. Heating the tricyano compound 5c in refluxing 1‐butanol gave both the benzo[g]pyrrolo[1,2‐a]indole 7 and malononitrile. This cyclization proceeds probably via a 1,6 hydrogen shift, followed by a 1,5 electrocyclization, and a subsequent fast elimination of malononitrile.