1.4- Oxathiine ring

The most widely used fungicide of the carboxamide group, carboxin (9), contains an oxathiin ring. Its composition is 5,6-dihydro-2-methyl-l,4-oxathiin-3-carboxanilide (Von Schmeling and Kulka, 1966). Its synthesis starts with acetoacetanilide (7), which with sulfuryl chloride gives 2-chloro-3-oxobutyranilide... 

Most partially saturated ring systems, 2,3-dihydro-l,4-dioxin 10 (sometimes named as 1,4-dioxene), 2,3-dihydro-1,4-dithiin 11, 2,3-dihydro-l,4-oxathiin 12, 2,3-dihydro-l,4-benzodioxin or 1,4-benzodioxane 13, 2,3-dihydro-l,4-benzodithiin 14, and 2,3-dihydro-l,4-benzoxathiin 15 are well investigated. Ring numbering for compounds 10-12 is followed as shown, independently of the presence of substituents. 

Oxathiane 2-oxides aie fonned by the oxidative ring expansion of 2-alkylthio-2-benzylthiolane 1-oxides brought about by [bis(trifluoroacetoxy)iodo]benzene. That the reaction is only successful with the (lR )-diastereoisomeis is attributed to chelation between the nucleophilic S and O atoms and the hypervalent iodine <99EJ0943>. A diazo-mediated thiolane ring expansion is the key step in a synthesis of the acenaphtho-[U-b][l,4]oxathiine system <99JCS(P2)755>. 

Chemical shifts of protons of 1,2-dithiin (Table 4) show no evidence of a ring current, but borderline aromaticity of a 1,2-oxathiin 2,2-dioxide derivative has been inferred from the chemical shift of H-6 which falls midway between that for a proton in a nonaromatic cyclic ether and for H-2 of furan (64JOC1110). 13C shifts of 1,2-oxathiin 2,2-dioxides at the C-4 and C-6 positions have been considered anomalous but may be rationalized by invoking mesomeric withdrawal of electron density from these positions by the SO2 moiety (770MR(10)208>. 

The central ring of both dibenzo[l,4]- dithiins and -oxathiins is cleaved on treatment with Li and a catalytic amount of 4,4"-di-rert-butylbiphenyl (DTBB) to afford thiols after reaction of the dilithio intermediate with electrophiles. In certain instances, the initial product can be cyclised to the dibenzo- dithiepine and -oxathiepine <02CL726>. The dilithio salt from thianthrene reacts sequentially with two different carbonyl compounds to give a l,2-di(hydroxyalkyl)benzene. When C02 is used as the second electrophile, a phthalan results <02TL7205>. 

The /ra -6-fluoro-3,6-dihydro-1,2-oxathiin 2-oxide 126 prefers a conformation in which the ring oxygen lies almost in the plane of the four carbon centers and the S=0 bond resides in a pseudoequatorial orientation. The fluorine substituent adopts a stable pseudoaxial orientation. Quantum-chemical calculations suggest a stabilizing anomeric effect which was interpreted in terms of an n0 a C-F hyperconjugative interaction <2002CEJ1336, CHEC-III(8.10.3)688>. 

The cycloaddition reaction of o-thioquinones with acyclic 1,3-dienes is a finely tuned process that can involve either reagent behaving as the diene component. It has now been established that formation of a 2-spiro-linked thiopyran by a [2-1-4] reaction is kinetically controlled and that the [4-1-2] alternative leads to the thermodynamic product, a 1,4-benzoxathiin (Scheme 31). Cyclic dienes yield only the oxathiin <03T5523>. Incorporation of the thionocarbonyl diene unit into fiiran and pyran rings and reaction with carbohydrate glycols leads to tricyclic oxathianes, desulfurisation of which yields 2-deoxydisaccharides <03T4249>. 

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