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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)

✍ Scribed by Ronald Kowalewski; Paul Margaretha

Publisher
John Wiley and Sons
Year
1992
Tongue
German
Weight
333 KB
Volume
75
Category
Article
ISSN
0018-019X

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✦ Synopsis

Abstract

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).

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