Convenient Synthesis of 2,2-Dimethyl-3,4-dihydro-2H-pyrano[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-α-(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.

Keywords:

• Domino reaction
• lactones
• pyranoquinoline
• reductive cyclization
• Wittig reaction

ACKNOWLEDGMENTS

The authors thank the University Grants Commission, New Delhi, for financial assistance, the National Institute of Oceanography, Goa, for spectral analysis, and the Department of Organic Chemistry, IISC, Bangalore, for HRMS.