Photocycloaddition of 2H-1-benzopyran-3-carbonitriles and 2H-1-benzothiopyran-3-carbonitriles to alkenes and alkenynes

Both (intermolecular) photocycloadditions of 2H-1-benzopyran- and 2H-1-benz othiopyran-3-carbanitriles to 2,3-dimethylbut-2-ene and 2-methylbut-1-en-3- yne, and (intramolecular) photoisomerization of 4-(alkenyl)benzopyran-3-car booitriles were investigated. In contrast to 2H-1-benzopyran-3-carbonitrile (1), its thia analog 4 reacts with 2,3-dimethylbut-2-ene selectively, to a fford only cyclobuta derivative 2 In the presence of 2-methylbut-1-en-3-yne , both 1 and 4 behave alike to afford the all-cis-cyclobuta diastereoisomer s, 15 and 8, respectively, as main products, as well as minor amounts of cy clobutenes 17 and 10, respectively, which result from the addition of the t erminal C-atom of the acetylenic bond to C(3) of the heterocycle. 4-Methyl- 2H-1-benzopyran-3-carbonitrile (5) does not undergo photoaddition to the al kene or the alkenyne mentioned above, whereas the corresponding intramolecu lar [2 + 2] photocycloaddition of 4-(pent-4-enyl)benzopyran-3-carbonitrile (6b) to tetracycle 20 proceeds quantitatively.