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In this review, we discuss the nature of the different physicochemical factors affecting the valence isomerism between 2<i>H</i>-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., 2<i>H</i>-chromenes), which are not included in this review.