Cyclisation of N-(prop-2-ynyl)benzylamines. Part II. Synthesis of 1,2-dihydro-3-phenylisoquinolines and an isopavine derivative

Abstract

N-(1-Phenylprop-2-ynyl)- and N-methyl-N-(1-phenylprop-2-ynyl)-benzylamine cyclised in polyphosphoric acid to give good yields of 1,2-dihydro-4-methyl-3-phenylisoquinolines (2). The initial products of cyclisation were unstable and underwent atmospheric oxidation. The tertiary base (2; R = Me) gave an isoquinolin-1(2H)-one (7) and the secondary base (2; R = H) an unusual 1,4-dihydroisoquinolin-4-ol (8), which was readily dehydrated to the isoquinoline. Polyphosphoric acid treatment of N-(prop-2-ynyl)-1,2-diphenylethylamine (3) caused double cyclisation to give 10,11-dihydro-10,5-(iminomethano)-5-methyl-5H-dibenzo[a,d]cycloheptene (4)(an isopavine derivative).