A Domino Knoevenagel/1,6-Heteroelectrocyclization Sequence to Access Phosphono-2H-thiopyrans

Knoevenagel-type condensation reactions between phosphonodithioacetate 1 and α,β-unsaturated aldehydes 2 directly afford 5-phosphono-substituted 2H-thiopyrans 4. Use of heteroaromatic aldehydes allows the consecutive heteroelectrocyclization of the triene intermediate 3, provided that the aromatic character of the C5–C6 double bond is sufficiently decreased, which is the case for electron-deficient indolenuclei. This Knoevenagel/1,6-heteroelectrocyclization sequence represents a new domino process.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)