Thiocoumarins

Single crystal-to-single crystal enantioselective photodimerizations of cou-marin (169a) and thiocoumarin (169b) were found to proceed efficiently in... 

The intramolecular version of this reaction provides a general method for the preparation of biologically active heterocycles such as coumarins, quinolinones, and thiocoumarins (yield, 50-91%) (Equation (48) and Table 2).47,47b The reaction tolerates various functional groups such as Br, GHO, etc. 

Such a coupling reaction when achieved in electrophilic media was shown to be of a general character. Thus, thiocoumarines could be also converted into corresponding TT-donor dimers in one step and in good yields. 

A review of Claisen rearrangements in aqueous solution has appeared. The synthesis of natural products utilizing tandem Diels-Alder additions with sigmatropic rearrangement processes has been reviewed, and a brief review of the regioselective synthesis of coumarins, quinolones and thiocoumarins with 3,4-fused pyran or furan ring systems by the Claisen rearrangement has been presented. ... 

Kaiser and Wulfers reported the first example of a thietane formation reaction involving photolysis of thiobenzophenone with olefins [27]. Since then a large number of systems have been reported. Thiobenzophenone and related compounds, xanthione and related compounds, the a,p-unsaturated thiones such as thiocoumarin, and adamantanethione have been most thoroughly examined. The wavelength dependence of these reactions has been systematically investigated in order to ascertain differences, if any, in the photochemistry of the thione S2 and T states. Scheme 1 shows one example of photocycloaddition of... 

Toda et al. reported that the topotactic and enantioselective photodimerization of coumarin and thiocoumarin takes place in single crystals without significant molecular rearrangements [49]. Molecular motion needs to be called upon to explain the photochemically activated cycloaddition reaction of 2-benzyl-5-benzylidenecyclopentanone. The dimer molecules, once formed, move smoothly in the reactant crystal to form the product crystal [50]. Harris et al. investigated the reactivity of 10-hydroxy-10,9-boroxophenanthrene in the solid state and the mechanism of the solid-state reaction was characterized by both X-ray diffraction and thermal analysis [51]. It was demonstrated that the solution chemistry of 10-hydroxy-10,9-boroxophenanthrene is different from that in the solid state, where it undergoes dimerization and dehydration to form a monohydride derivative. 

Tanaka K, Toda F, Mochizuki E, Yasui N, Kai Y, Miyahara I, Hirotsu K (1999) Enantiose-lective single-crystal-to-single-crystal photodimerization of coumarin and thiocoumarin in inclusion compounds with chiral host compounds. Angew Chem Int Ed Engl 38 3523-3525... 

The photocyclodimerization of coumarin (2//-l-benzo[b]pyran-2-one) in direct and sensitized experiments is known to proceed rather inefficiently in the absence of Lewis acids and in better yields in their presence.42,43 In contrast, 2-quinolone (22, X = NH)44 or 1-thiocoumarin (2//-l-benzo[b]thiopyran-2-one 22, X = S)45 afford anti head-to-head dimers 23 on direct irradiation in very good yields. Psoralens (7//-furo[3,2-g][l]benzopyran-7-ones) usually exhibit a behavior similar to that of coumarins.46... 

The chemistry of thiocoumarin (1) has for the most part been neglected, compared to that of coumarins and related systems. Apart from Elderfield s extremely brief coverage in 1950 and recent surveys in the Chemical Society s Specialist Periodical Reports,no reviews of the chemistry of... 

Formation of cinnamoyl esters of thiophenols is readily accomplished. These esters cyclize, with elimination of benzene, by the action of aluminum chloride (Scheme 3). An alternative approach, exemplified in Scheme 4, utilizes the ready thermal isomerization of thiochromones to thiocoumarins (see p. 121), the former being derived from phenylthioacrylic esters. ... 

Thiocoumarin itself has been made in this way from phenylthioacrylic acid, although in only 10% yield. Some thiocoumarin is also formed in the the aluminum chloride-catalyzed cyclization together with the thiochro-mone (see p. 121). 

Although 2-mercaptobenzaldehyde (6) is an unstable intermediate, prone to resinification on storage, a surprising amount of research has been conducted using it as a source of thiocoumarins, with many workers preferring to use it directly or store it in solution at 0°C or below. It was first synthesized by Friedlander and Lenk in 1912 (Scheme 5). °... 

Perkin reaction of this aldehyde with phenylacetic acid gives a mixture of the benzothiophene 13 (57%) and the thiocoumarin 14 (40%). ... 

In Section II,B several cinnamic acid derivatives were prepared from 2-mercaptobenzaldehydes and the corresponding ketones. These were cyclized either in situ or separately to give thiocoumarins. An alternative synthetic approach introduces the mercapto group as the last stage by way of a... 

Cyclization to thiocoumarin was accomplished with acetic anhydride, or by... 

And 5-acetylamino-2-mercaptocinnamic acid has been efficiently cyclized yielding 7- and 6-aminothiocoumarin, respectively, by this approach.These have been extensively utilized for the synthesis of other thiocoumarins (see p. 125). 

Thiochromenes (e.g., 15) are converted to thiocoumarins by oxidation with chromium trioxide in pyridine. In a similar way thiochromylium salts (e.g., 16) are converted to thiocoumarins by oxidation with either active... 

Finally, dithiocoumarins (e.g., 17, see p. 131) are converted into thiocoumarins by the action of hydrogen peroxide in acetic acid. ... 

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. ... 

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