[2+2]-photocycloadditions of 2-aminopropenenitriles to two coumarins and to thia-analogous systems

Triplet excited coumarin (4) forms with high regioselectivity and stereosel ectivity "head to head" [2+2]-cycloadducts 6a,b,d by addition of 2-donor su bstituted propenenitriles 2a,b,d to its C3,C4 double bond. All adducts isol ated have donor-endo geometry. The structure of 6a has also been unambiguou sly confirmed by a single X-ray structural analysis. Direct irradiation of 3-(2-benzothiazolyl)coumarin (7) results in the formation of donor-exo and donor-endo cycloadducts 8a-c and 9a-c. The fluorescence of 7 is quenched by alkenes 2a-d with 2-(tert-butylthio)propenenitrile (2d) having both the lo west quenching efficiency and being totally unreactive in the cycloaddition . The photoreaction of two coumarin-thia-analogoues 10a,b with 2a neither g ave rise to the expected thietanes nor led to cycloaddition at C3,C4 but fo rmed substitution products 11a,b and 12a,b.