Metal- and Protection-Free [4 + 2] Cycloadditions of Alkynes with Azadienes: Assembly of Functionalized Quinolines

A base
promoted, protection-free, and regioselective synthesis
of highly functionalized quinolines via [4 + 2] cycloaddition of azadienes
(generated in situ from o-aminobenzyl alcohol) with
internal alkynes has been discovered. The reaction tolerates a wide
variety of functional groups which has been successfully extended
with diynes, (2-aminopyridin-3-yl)­methanol, and 1,4-bis­(phenylethynyl)­benzene
to afford (Z)-phenyl-2-styrylquinolines, phenylnaphthyridine,
and alkyne-substituted quinolines, respectively. The proposed mechanism
and significant role of the solvent were well supported by isolating
the azadiene intermediate and deuterium-labeling studies.