Thioamides photoreactions

The formation of thietanes from thiones and olefins has been less exploited for synthetic purposes than the corresponding oxetane-forming reaction. It should be remarked that thiocarbonyl compounds very often undergo efficient photoreactions from the second excited singlet state S2 U4). One interesting synthetic application is found in the photochemical preparation of quinolines from N-thioamides (4.84)498). The primary photochemical step is assumed to be the intramolecular thietane formation. 

Sakamoto et al. reported the X-ray crystallographic data and the solid-state photoreaction of eight Ar,AT-disubstituted a,p-unsaturated thioamides 23a-h, which involves hydrogen abstraction by the alkenyl carbon atom conjugated with thiocarbonyls (Fig. 9) [44-46]. 

Table 10 Photoreaction of thioamides 24f-h in both solution and the solid-state...

In the solid-state photochemical reaction of N,N-disubstituted a,(3-unsatu-rated thioamides 24, a crystal-to-crystal nature was observed in 24c furthermore, absolute asymmetric transformation in the chiral crystalline environment was performed in the photoreaction of 24b, 24c, and 24e. 

The solid-state photoreaction of thioamide 1 was done under an atmosphere purged with dry argon. The solid sample was irradiated for 4 h as a powder prepared by grinding, and placed inside a Pyrex slide. When powdered thioamide 1 was irradiated in the solid state at 0 °C, at up to 19% conversion, optically active /i-thiolactam 2 of 31% ee was isolated. The optical purity was determined by HPLC using a chiral cell OD column (Daicel Chemical Ind.). 

An interesting crystal-to-crystal asymmetric photoreaction of a, 3-unsaturated thioamide to p-thiolactam (Scheme 5.53) with 94% ee in 96% yield of 58% conversion was reported by Sakamoto et al2 ... 

Direct excitation (A > 240 nm) of thiiran is followed by intersystem crossing to the lowest excited state. This species is capable of undergoing reversible addition to alkenes, thereby inducing Z, -isomerization inefficient irreversible addition has also been observed. The carbonyl sulphide, Ph2=6—S, has been proposed as a possible intermediate in the photodecomposition of diphenyloxathiiran, which is itself formed by irradiation of thiobenzophenone -oxide at 77 An unexpected photoinduced fragmentation and solvent incorporation was observed on irradiation of A-(6,7-dimethoxy-2-methyl-3-quinazolinio)ethoxythio-formamidate (289) in ethanol to give photoproducts (290)—(292). The photoreactions of some A-isoquinolinio(thioamidates) have been compared with those... 

Photoreaction of six-membered thioamides, such as pyridinethiones, pyrimidinethiones, pyridazineth-iones, thiouracils, thiouridines and their benzo-analogs (Figure 106.1), gives a variety of types of adducts in the presence of alkenes where thietanes are beheved to be intermediates. Most of the reactions are already documented in previous reviews. ""... 

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