Reductive (3 + 2) Annulation of Benzils with Pyrylium Salts: Stereoselective Access to Furyl Analogues of cis-Chalcones.

An unprecedented reductive (3 + 2) annulation of both symmetrical and unsymmetrical benzils with pyrylium salts mediated by P(NMe2)3 is described, leading to facile and stereoselective access to the challenging cis-chalcones decorated by various substituted furyl rings under mild conditions. Rather than the extensively studied C1 synthons, the Kukhtin-Ramirez adducts derived from benzils serve as the underexplored C3 synthons in this (3 + 2) annulation with the 2,3-double bond of the 2,6-disubstituted pyrylium ions.