1.4- Oxathiin

Various 1,4-oxathiins have been converted to the sulfoxides in a highly stereoselective reaction. The sulfoxides undergo a thermal retro-Diels-Alder reaction to ot,a -dioxosulrines which can behave both as electron deficient dienes and as dienophiles, affording different oxathiins and dihydrothiopyran 1-oxides, respectively <96T12233>. In like manner, o-thioquinone 5-oxides have been generated and they too exhibit diene and dienophile behaviour <96T12247>. 

Oxathiins derived from the cyclic diacylthione (60) and glycals by a cycloaddition reaction yield glycosides on desulfurization, offering an alternative approach to glycosyl transfer <96AG(E)777>. 

Oxathiine derivatives lb 301,304 Oxathizine fungicides la 44 Oxazepam la 364 Oxazolidinthione derivatives lb 301 Oxeladine citrate lb 327 Oxidation, aluminium isopropoxide la 59 -, atmospheric oxygen la 60 -, chromic acid la 59, 60 -, hydrogen peroxide la 59 -, iodine la 60... 

Thiazines heve been synthesized from the nitrile imine dimer 300 (Equation 105) <1980J(P1)2923>, 1,4-dithiin 1,1,4,4-tetraoxide 301 (Equation 106) <1982TL299>, 1,4-oxathiins (Equation 107) <1996JOC3894>, and the anthracene adduct 302 (Scheme 77) <1998S915>. 

Dioxins, 1,4-oxathiins, 1,4-dithiins, and annulated derivatives have been reviewed in 1997 <1997HOU(9a)l> More recently, synthetic methods for preparing 1,4-dioxins and their benzo- and dibenzofused derivatives <2004808(16)15, 20048L2449>, 1,4-dithiins <2004SOS(16)57> and 1,4-oxathiins, and their annulated analogs... 

Many crystal structures of host compounds or solvate compounds with 1,4-dioxane as well as a wide range of organic structures bearing a 1,4-dioxin, 1,4-oxathiin, or 1,4-dithiin core have been determined by single crystal X-ray diffraction. 

The UV photoelectron spectra of 1,4-dioxin, 1,4-dithiin, 1,4-oxathiin, and their dibenzo derivatives as well as the saturated compounds were detailed in CHEC(1984) <1984CHEC(3)958>. 

Dioxins, 1,4-oxathiins, and 1,4-dithiins have often been prepared by elimination reactions from saturated analogs as described in CHEC-II(1996) <1996CHEC-II(6)447>. Since then, a synthesis of tetramethyl l,4-dithiin-2,3,5,6-tetracarboxylate 241 has been reported in low yield (12%) by thermal decomposition of the 1,4,2,5-dithiadiazine system 240 in refluxing o-dichlorobenzene in the presence of DMAD <1997J(P1)1157>. Recently, 2,6-divinyl-l,4-dithiin 68 has been isolated from the reaction of l,4-bis(4-bromobut-2-ynyloxy)benzene with an excess of alumina-supported sodium sulfide. The formation of 68 has been presumed to take place via cyclic sulfide 242 <2003S849>. 

Dihydro-1,4-oxathiins (336 Z = 0) and 5,6-dihydro-1,4-dithiins (336 Z = S) are easily obtained from 1,3-oxathiolanes (335 Z = 0) and 1,3-dithiolanes (335 Z=S), respectively, by treatment with bromine (9IS223), A-bromosuccinimide (94T7265), chlorine (87JOC5374, 91S223), or sulfuryl chloride (88JCR(M)1401>. 

The monobenzo-fused derivatives of 1,4-dioxin, 1,4-oxathiin and 1,4-dithiin, (345), (346) and (347), can all be prepared by base-catalyzed reaction between the appropriate 1,2-disubstituted benzene and an a-haloketal via an intermediate 2-alkoxy-2,3-dihydro derivative (348). The pyrolysis of the acetoxy derivative (349) at 450°C gives (345 80%) (67ZC152). 2-Hydroxy-2-phenyl-l,4-benzodioxane, from catechol and phenacyl bromide, is dehydrated to (345) by thionyl chloride in pyridine. 

Dioxins and 1,4-dithiins have often been prepared by elimination reactions of appropriately substituted saturated analogues. However, in the more recent literature a number of examples of syntheses of 1,4-dithiins are described which utilize reactions involving unsaturated precursors. Examples of 1,4-oxathiins are relatively scarce and no general preparative routes have been developed. 

Various dihydro-1,4-oxathiins and dihydro-1,4-dithiins are reported to be useful in areas other than agriculture. Thus, 2,3-dihydro-l,4-dithiin-5,6-dicarboximides display a wide spectrum of biological activity (77SST(4)332) while derivatives of the basic structure (243) have been patented as tranquilizers, anticonvulsants and hypnotics (76GEP2550163). Sedative, myorelaxant and cataleptic properties are associated with certain 2,1-benzoxathiane 2,2-dioxides (75BRP1383459). 

In some cases it is the diene component from the retro reaction which is the desired product and extrusion of a volatile alkene such as ethylene is then ideal. Pyrolysis of the 1,4-oxathiin systems 21 proceeds in this way to give the a-oxothiones 22 which for R = Pr1, exists mainly as the enethiol tautomer 2323. Thermal extrusion of ethylene from 24 provides convenient access to the interesting fulvene 25 in quantitative yield24, and the corresponding reaction of 26 at 650 °C and 10-4 torr gives the cyclopentadienoben-zopyrene 28 in 95% yield, presumably by way of the intermediate 2725. 

The fully unsaturated compounds containing two oxygens (e.g., 1,4-dioxin 61), two sulfur atoms (e.g., 1,4-dithiin 62), and one each of oxygen and sulfur atoms (e.g., 1,4-oxathiin) are electron-rich and highly reactive toward electrophiles. Benzo derivatives are usually most reactive in the fused heteroring (84M113). 

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