Thietane formation

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. 

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... 

Scheme 3 shows the mechanism for the thietane formation, in which the six-membered 1,4-biradical BR is appropriate. There are two ways of cyclization to thietane 2, and each pathway gives an enantiomeric structure of thietanes, (1S,4R)- or (lR,4S)-2, respectively. The absolute structure of (-)-(M)-la and the major isomer (- -)-(lS,4R)-2a was determined by X-ray structural analysis using... 

When monothioimide le was irradiated in a benzene solution, [2+2] thietane formation proceeded, producing four p-lactams, syn-2e (A and B) and anti-2e (C and D) (Scheme 4). However, diastereoselectivity in the thietane formation... 

Sakamoto et al. reported an intramolecular [2+2] thietane formation in the solid state (Scheme 7). [26] Achiral A-(thiobenzoyl)methacrylamide 39 formed (E,Z)-conformation of the imide moiety, crystallized in a chiral space group Phhh, and the photolysis of single homochiral crystals at room temperature resulted in the formation of an optically active thietane-fused 3-lactam (40,75%) with 10% ee. The solid-state photoreaction proceeded even at -45°C to give higher ee value, 40% ee (conv. 30%, yield 70%). 

The photolysis of N(3- or 4-alkenyl)mono- or di-thioglutarimides seems to proceed in several steps involving initial intramolecular thietane formation between thiocarbonyl and N-substituted alkene. Photochemical cleavage of both C-S and C-C bonds of a thietane resulted in the formation of the corresponding indolizines or quinolizines (Equation 3) <2000H(53)2781>. 

Gotthardt and Lenz reported the diastereodifferentiating thietane formation in the photocycloaddition of thiobenzophenone 13 and xanthione 14 (Scheme 6) with (- )-menthyl methylacrylate 15 [88]. The photochemistry of thioketones is wavelength dependent, involving both excited triplet (TJ and singlet (S2) states that undergo independent processes. It was seen that the des obtained were greater (18%) for the product from the T] state than that from the S2 state. In-depth discussion of this topic is to be found in Chap. 5. 

Good yields of thietanes have been obtained by photolysis of a thiocarbonyl compound and an alkene. The thione is excited initially to one of two singlet states, Si (n -> 7T ) or Si (ir Thietane formation via the state proceeds through... 

Irradiation of crystalline 0-methyl A -(2,2-dimethylbut-3-enoyl)-A -phenyl-thiocarbamate (174 R R = -(CH2)4-) which exists in chiral space group P2 promotes intramolecular [2 + 2] thietane formation (175 same R R ) followed by rearrangement to give the optically active y-thiolactone (176 same... 

An interesting extension of the Patemo-Biichi reaction is the photochemical formation of thietanes. A recent example [125] of stereoselective thietane formation shows that sterics play a large role in the selectivity. Silyl thioketone 131 (see Scheme 48) was irradiated in the presence of several electron-poor alk-enes, which showed greater than a 12 1 propensity for the larger group on... 

Oda, K., Ishioka, X, Fukuzawa, Y, Nishizono, N., and Machida, M., Photochemistry of N-5-butenyl- and N-4-pentenylglutarimides intramolecular thietane formation and the fission of thietane ring, Heterocydes, 53, 2781, 2000. 

Oda, K., Machida, M., and Kanaoka, Y, Photochemistry of the nitrogen-thiocarbonyl systems. Photochemistry of N-but-3-enyl thiophthalimides. Intramolecular thietane formation, Chem. Pharm. Bull, 40, 585, 1992. 

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