Convenient synthesis of 2, 2-dimethyl-3, 4-dihydro-2H-parano [2,3- b]quinolines

Abstract

A convenient general synthesis of 2,2-dimethyl-3,4-dihydro-2H-pyrano[2,3-b]quinolines using the Wittig reaction is described.The o-nitrobenzaldehydes (1a-d) on reaction with phosphorane 2 provided (E)-ethyl-alpha-(2,2-dimethylprop-2-ene)-2-nitrocinnamates (3a-d) in excellent yields, which on cyclization with polyphosphoric acid followed by reductive cyclization using Fe/HCl afforded dihydropyranoquinolines (5a-d). Alternatively, the pyranoquinolines 5a-d were also synthesised from esters 3a-d by employing domino reductive cyclization in a single step.