Chemoselective Azidation of o-Alkynylaldehydes over [3 + 2] Cycloaddition and Subsequent Staudinger Reaction: Access to Benzonaphthyridines/Naphthyridines.

An efficient tandem approach for the chemoselective synthesis of functionalized azido-pyranoquinolines and azido-iodo-pyranoquinolines via electrophilic cyclization of o-alkynylaldehydes in the presence of sodium azide under mild reaction conditions is described. Mechanistic studies confirm the formation of azido-pyranoquinolines through nucleophilic attack of azide on pyrilium intermediate over [3 + 2] cycloaddition of the azide on the alkyne. The synthesized azido-pyranoquinolines were transformed into benzonaphthyridines via Staudinger reaction. The mechanistic pathway was supported by deuterium labeling experiment and X-ray crystallographic studies.