Thiochromanones, photolysis

Photolysis of (96) results in the inversion of the hetero ring a formal 1,2-shift of sulfur generates the spirothietanone (100) which relaxes by migration of the carbonyl function. This interesting inversion is very selective carbons 1 and 3 remain in their original positions but the product is thiochromanone rather than an isothiochromanone (equation 42)... 

Oxidation of thiochromones does not lead to the sulfoxide or sulfone derivatives these are better prepared from the corresponding thiochromanones by oxidation followed by bromination and dehydrobromination. The 1-oxide is an extremely reactive species and gives a Diels-Alder adduct with butadiene. The 1,1-dioxide is also very reactive. Photolysis in benzene affords a 2 1 adduct in 30% yield (116) (79TL1097) via an initial [2 + 2] addition followed by a Diels-Alder reaction. Simple [2 + 2] addition is found with cyclohexene. 

Photolysis of 2-azido-4-thiochromanone 1,1-dioxide afforded the ring-expanded product (118), while (119) and (120) were formed by base-catalysed sulphimide rearrangements. 

Thiochromanones, on photolysis (350 nm) in methanol, undergo not photoisomerization, such as is observed with 4-isothiochromanones, but photodehydrogenation to 4-thiochromones in moderate (50—55%) yield. 2-(t-Butylthio)-benzaldehyde (177), prepared in near quantitative yield from 0-nitrobenzaldehyde as indicated in Scheme 40, is a useful new synthon for thiocoumarins (178 R = CO2H, CONH2, C02Et, CN, Bz, Ph, or 2-thienyl) and benzo[l,2-f> 5,4-f> ]bisthiopyrans (179). ... 

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