A review of the synthesis of 1,2-dihydroisoquinoline, [2,1-a] isoquinoline and [5,1-a] isoquinoline since 2006

The synthesis of 1,2-dihydroisoquinoline, [2,1-a] isoquinoline and [5,1-a] isoquinoline is very important because of their biological and medicinal activities as well as their conversion to corresponding isoquinoline. Various nucleophiles and pronucleophiles could be added to Ortho-alkynylbenzaldehydes, ortho-alkynylaryl aldimines and ortho-alkynylbenzaldoximes in the presence of different Lewis acids and generate derivatives of 1,2-dihydroisoquinoline in tandem route. Also, 1,4-dipolar intermediate which synthesized from the reaction of isoquinoline and electron-deficient compounds reacted with various dipolarophiles for the formation of five- and six-membered heterocycles including [2,1-a] isoquinoline and [5,1-a] isoquinoline. In recent years, these interesting zwitterions have been used in multicomponent reactions to produce a considerable number of heterocyclic compounds. In this review, I have classified a variety of methods since 2006 until now, for the generation of 1,2-dihydroisoquinoline, [2,1-a] isoquinoline and [5,1-a] isoquinoline via the above routes.