Cycloaddition of 2<i>H</i>‐Pyran‐2‐thiones with Nitroso Derivatives. An Unexpected Cycloaddition‐Rearrangement Reaction

Abstract

Abstract Reaction of pyran‐2‐thiones 4 with nitroso derivatives led surprisingly to type‐ 8 ( 19 ) adducts which proved to be isomeric with the initially expected primary Diels ‐ Alder cycloadducts 5 . Methyl 2‐thioxo‐2 H ‐pyran‐5‐carboxylate ( 4f ), when reacted with nitrosobenzene at ‐10°, led quantitatively to the thieto‐oxazine intermediate 13 , which turned out to be the cornerstone of the complex cycloaddition‐rearrangement 5 → 8 reaction pathway ( Scheme 3 ). Differential scanning calorimetry, as performed for the 18a → 19a conversion, permitted to demonstrate that this multistep rearrangement is overall a higly exothermal process, the final product 19 representing an energy‐sink along this reaction pathway.