In this review, we discuss the nature of the different physicochemical factors affecting the valence isomerism between 2H-pyrans (2HPs) and 1-oxatrienes, and we describe the most versatile synthetic methods reported in recent literature to access to 2HPs, with the only exception of 2HPs fused to aromatic rings (i.e., 2H-chromenes), which are not included in this review.
Keywords: 1-oxa-triene; 2H-pyran; Knoevenagel; cycloisomerization; dienone; heterocycles; oxa-electrocyclization; propargyl Claisen; synthesis; valence isomerism.