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 (less-than-or-equal-to 90-degrees-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 CO2. 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. NMR data (C-13) are presented correlating the electron density in the pyrone diene systems with their Diels-Alder reactivity, and some transformations of the bicyclic lactone adducts are shown to illustrate the value and versatility of these richly functionalized synthetic intermediates.