Dihydro-1,4-dithiins, formation

Although the formation of dithioacetals generally is a simple reaction, side reactions become prevalent when a reasonable leaving group is in the a-position to the carbonyl or to a conjugated double bond. In the reaction of 2-bromo-2-phenylacetophenone with ethanedithiol, 2,3-diphenyl-5,6 dihydro-l,4-dithiin (equation 8) was obtained . Similarly, the dihydro-dithiin (1) was obtained from 6-j3-acetoxy 4-cholesten-3-one (equation 9 f, Additional examples exist for the formation of dihydro-1,4-dithiins via halides , epoxides and even amides . 

The stereospecific formation of 5,6-dihydro-l,4-dithiins from the reaction of the 13-dithiete 60 with alkenes has been shown to proceed through its valence isomer, l,2-bis(methoxycarbonyl)ethane-13-dithione (Scheme 42) <99JOC8489>. 

Lithiated chiral dihydro-1,4-dithiins (152) have been recently utilized in the addition to prochiral aldehydes in the presence of Ti(OPi )4 (Scheme 23). Under such conditions (Z)-allylic alcohols have been obtained in good yields and with satisfactory enantioselectivity (ee >40%), after removal of the chiral sulfur-containing moiety of the addition product. The reaction is likely to proceed via the formation of a cyclic five-membered transition state (153) involving titanium, oxygen and one of the sulfur atoms. When Ti(OPi )4 is replaced by TiCLt, the addition becomes slower than the uncatalyzed reaction itself. 

The formation of naphthothiete (34) from dithiol 41 and naphthothiadiazine (42) is preceded by biradical intermediate 43 as proven by the photolysis of naphthothiadiazine (42) in carbon disulfide. After 15 min irradiation alongside naphthothiete (34) obtained in a 52% yield there also formed in a 22% yield naphtho[l,8-<7e]-2,4-dihydro-l,3-dithiin-2-thione (44) resulting from the reaction of biradical 43 with carbon disulfide (Scheme 7). 

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. 

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