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

Irradiation of newly synthesized 2,2‐dimethyl‐2,3‐dihydro‐4 H ‐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 CD 3 OD affords the same (deuterated) adducts with the CD 3 O 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(2 H )‐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 ).