Thiochromones and Thiocoumarins

Some thioflavones and isothioflavones have been reported. a-Epoxy-ketones of type (62 R = H or Me) yield 3-hydroxy-A -thiochromen-4- ies (63) on treatment with sulphuric acid.  

4-Methylthiocoumarin has been prepared by treating thiophenol with diketen followed by cyclization of the resulting thiol ester with polyphos-phoric acid. 4-Hydroxythiocoumarins undergo Michael additions with a/3-unsaturated ketones, and also form Mannich bases (64), from which the pyranothiocoumarin derivative (65) has been prepared.  

Thiochromones and Thiocoumarins. - Thiochromone is converted into a mixture of stereoisomers on irradiation in benzenoid solvents, and the difference between its behaviour and that of the sulphone has been ascribed mainly to the differing natures of the lowest excited state of each compound. Thiochromones are not reduced by sodium borohydride alone, but the AH-thiochromene is the main product in the presence of cerium(lll) chloride. 

Reduced 3-aryl-thiocoumarins have been synthesized from cyclohexane- 

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Oxidation of 2H-1 - benzothiopyrans with selenium dioxide leads to the thiaselenin system, together with thiocoumarins, thiochromones and other cleavage products. Careful choice of conditions can lead to 35% yield of the selenium insertion product as shown in equation (49) (75T2099). 

Ultraviolet and near ultraviolet spectra of a number of thiocoumarins have been recorded and discussed. Infrared spectra, in particular concerned with carbonyl absorptions, have been frequently collected,and comparisons made with those of thiochromones. All the usual spectra of thiocoumarin itself have been recorded and analyzed as well as the C-NMR spectrum and the fluorescent characteristics. ... 

The spectroscopic distinction between thiocoumarins and thiochromones is best made by observing the low-field 5-proton in the NMR spectrum of the latter and also the characteristic retro-Diels-Alder fragmentation always observed in the mass spectrum of thiochromones. Thiochromones also show distinctive, strong ultraviolet absorptions at 250-270 nm. ... 

The focus will be directed to five ring systems thiochromans (1), thiochromones (2), isothiochromans (3), benzothiopyrans (4—6), and, benzothiopyrylium salts (7, 8). It is difficult completely to segregate their chemistry since they are so interrelated cross-references will be employed as often as possible. Thiocoumarins and thioxanthones are excluded since these molecules differ considerably from those reviewed here. 

The ketones derived from thiopyrans are 2//-thiopyran-2-one 25 and 47/-thiopyran-4-one 26 and the benzologues are 2//-1 -benzothiopyran-2-one or thiocoumarin 27, 4//-l-benzothiopyran-4-one (thiochromone) 28, and 9//-thiox-anthen-9-one or simply thioxanth-9-one 29. The trivial names dihydrothiocoumarin and thiochroman-4-one are used for the partially reduced derivatives. l//-2-Benzothiopyran-l-one 30 is usually known as isothiocoumarin. The 2- and 3- aryl derivatives of 10, 28, and 31 are generally referred to as the thioflavonoids. 

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). ... 

This chapter follows essentially the same plan as in the previous Volume except that all benzothiopyrans (thiochromans, thiochromenes, thioisochromans, e/c.) have been covered in Section 7 and all benzothiopyrones (thiocoumarins, thiochromones, etc.) in Section 8. Iliiopyrylium salts are included in Section 6, regardless of whether or not they are monocyclic, and similarly all thiopyran 1,1-dioxides are treated in Section 5. 

Thioflavonoids, Thiocoumarins and Thioxanthones.- Heating methyl 2-mercaptobenzoate with a benzyl cyanide (e.g., 2-chloro-benzyl cyanide) in pyridine is a new method of synthesizing thiochromones such as (188 Ar=2-ClC6Hi, ) in good yield. Improved... 

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