Diels-Alder cycloadditions using nucleophilic 3-(p-tolylthio)-2-pyrone. Regiocontrolled and stereocontrolled synthesis of unsaturated, bridged, bicyclic lactones

Captodative 3-(tolylthio)-2-pyrone (1) is shown to be reactive as a nucleophilic diene undergoing 2+4-cycloadditions with various electrophilic alkenes under sufficiently mild thermal conditions (≤90 o C) so that the initial bicyclic lactone adducts can be isolated on gram scale in moderate to very good yields (42-82%) without loss of CO 2 . These bicyclic adducts are formed regiospecifically and often with excellent stereoselectivity. These Diels-Alder cycloadditions are the first examples of a captodative unsaturated sulfide acting as an enophile