2,6-Disubstituted 1,4-dithiins

Addition of sodium sulfide to alkynyl sulfides, S(C = CR)2, also yields 2,6-disubstituted 1,4-dithiins (75RTC163). 

The anion derived from protected 2-hydroxy-5,6-dihydro-1,4-dithiin is efficiently attacked by electrophiles to give the 2,3-disubstituted dithiin. Both deprotection and desulfurisation are readily accomplished in the latter case, the dithiin functions as a 3-carbon homologating agent <97JOC9369>. 

The addition of benzyl and t-butylmercaptans to diacetylene and its symmetric disubstituted homologs 57 affords l,4-di(benzylthio)- (59) or l,4-di(t-butylthio)-buta-1,3-dienes (61), respectively, from which 1,2-dithiins 55 are formed (65ZC352 67AG685 85KGS1443 96T12677 96USP5453500). 

Whilst the 3,6-disubstituted-l,2-dithiins undergo facile sulfur extrusion, the (-l-)-camphor analog 25 is extremely stable (Scheme 30) <1995T13247, 1994TL1973> even in 70 eV electron ionization (El) mass spectra, no peak... 

Application of the rod-like 3,6-disubstituted-l,2-dithiins in liquid crystal compositions and also as antifugal agents and anti-infectives was reported in the patent literature. 

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. 

An interesting photochemical synthesis of 2,5-disubstituted-thiophenes 38 stems from the first report of the photoinduced sulfur extrusion from variously 3,6-disub-stituted 1,2-dithiins 35a by exposure to daylight in ethyl acetate or in DCM, or by irradiation at 365 nm in methanol (Scheme 12.11) [29]. Other examples involve naturally occurring thiarubrines 35b [30], as well as variously substituted synthetic derivatives [ 31]. This photochemical reactivity has been generalized with an application in the synthesis of selenophene 40 obtained in almost quantitative yield from 39 [32]. 

Ring expansions using spiro 2,2-disubstituted-l,3-dithiolanes 314 turned out to be useful route for the constmction of larger rings bearing sulfur atoms like 1,4-dithiins 315 (Equation 30). 

Typical for disubstituted 1,2-dithietes is their valence isomerization, which results in the formation of 1,2-dithiones. The equilibrium favors the 1,2-dithiete with electron-withdrawing substituents such as CF3. The reaction with 2,3-dimethylbut-2-ene to give a hexasubstituted 2,3-dihydro-l,4-dithiine proceeds, however, as a (4 + 2) cycloaddition via the 1,2-dithione. 

Photochemically generated diatomic sulphur added to l,2-bis(methylene)-cyclohexane to afford a 3,6-dihydro-l,2-dithiin. The dihydrodithiin (20) was formed by thermal rearrangement of a 1,3-dithiin. Addition of disulphur monoxide to 2,3-disubstituted buta-1,3-dienes yielded the rather unstable dihydrodithiin 1-oxides (21). 

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