Light-Induced Cycloaddition of Furan and addition of methanol to a 4-thiacyclohex-2-enone ( = 2,3-dihydro-4H-thiin-4-one) and a 4-thiacyclopent-2-enone ( = thiophen-3(2H)-one)

Irradiation of newly synthesized 2,2-dimethyl-2,3-dihydro-4H-thiin-4-one ( 1 ) in furan affords the two 4 + 2] cycloadducts 3 and 4 and the 2 + 2] cycloadduct 5 in a 5:4:1 ratio (Scheme 1). Irradiation of 1 in MeOH gives a 3:2 mixture of 5- and 6-methoxy-2,2-dimethylthian-4-ones 6 and 7 . Irradiation in CD₃OD affords the same (deuterated) adducts with the CD₃O and D groups trans to each other, results compatible with cis-addition of MeOH to a trans -configurated ground-state enone. Irradiation of the same enone in furan/MeOH 1:1 gives only the furan cycloadducts 3–5 and no MeOH adducts, suggesting that furan interacts with the (excited) triplet enone before the deactivation of this species to a ground-state (E)-cyclohexenone, which then reacts with MeOH. On irradiation in furan, the corresponding five-membered thiaenone, 2,2-dimethylthiophen-3(2H)-one ( 2 ) affords only one, cis-fused, 4 + 2] cycloadduct with ‘exo’-configuration, i.e. 8 , and 2 does not undergo solvent addition but rather cyclodimerization (→ 9 ) on irradiation in MeOH (Scheme 1).