Three-Component Condensation of Pyridinium Ylides, β-Ketonitriles, and Aldehydes with Divergent Regioselectivity: Synthesis of 4,5-Dihydrofuran-3- and 2H-Pyran-5-carbonitriles.

A library of trans-4,5-dihydrofuran-3-carbonitriles was synthesized in a diastereoselective manner in good yields by the three-component reaction of β-ketonitriles, carbonyl- and semistabilized pyridinium ylide precursors, and aldehydes in the presence of piperidine. This one-pot transformation generates two C-C and one C-O bond and proceeds through a cascade Knoevenagel condensation, a Michael addition, and intramolecular SN2 cyclization. Formation of cyclopropanecarbonitrile derivatives, which in some cases were obtained as major products, was found to be a competing reaction. The use of arylglyoxals changes regioselectivity and leads to 2-hydroxy-2H-pyran-5-carbonitriles.