Thiochromones preparation

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. 

Ring synthesis by y- closure is a somewhat more versatile entry into this class of compound, though even then it is restricted to suitably activated precursors such as (142) (equation 101) (72T5197). Thiochromones and thioxanthones are probably most frequently prepared by this mode of closure, usually via an intramolecular Friedel-Crafts condensation of a carboxylic acid with an aromatic ring (equation 102) (71GEP2006196). 

The reaction of thiosalicylic acids with A,A-dialkylacetamides with an electron withdrawing group at the 2-position leads to 2-dialkylamino-3-substituted thiochromones. The use of A-alkylated piperidones produces the benzothiopyrano[2,3-Z)]pyridine ring system <05AJC864>. Cyclisation of alkyl 2-mercaptophenyl ketones, prepared from thiosalicylic... 

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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