1.2- Dithiolanes, ring contraction

As previously illustrated in Scheme 13, the anodic oxidation of dithianes involves a ring contraction affording 1,2-dithiolane [18]. 

An interesting synthetic approach to thietanes is the selective desulfurization of cyclic disulfides.The treatment of dithiolanes with a diethyl-aminophosphine results in a ring contraction to thietanes, (Eq. 19). This has been demonstrated with a-lipoic acid, a coenzyme with a dithiolane structure involved in the biological oxidation of pyruvic acid. The reaction is proposed to be initiated by the electrophilic attack of the phosphorus on the ring sulfur atom, resulting in the formation of an acyclic internal phosphonium salt, which by subsequent elimination of a phosphine sulfide, closes to the four-membered ring. °... 

Thietanes can also be prepared by ring contraction of higher-membered heterocyclic rings. Thermolysis of the dithiolane dioxide 129 in benzene leads to trans-2-phenyl-3-benzoylthietane (130) and benzalacetophenone (Eq. 20). 

Similar ring-contractions have been accomplished by treatment of a dithiolane with tris(A A -diethylamino) phosphine.l,2-Dithiolane-4-one, however, gave only polymeric material. The reaction proceeds by thiophilic attack by the nucleophilic cyanide or phosphine ... 

Treatment of 1,2-dithiolan-3-ones with triphenylphosphine or tris(dimethyl-amino)phosphine results in ring-contraction to /3-thiolactones. " Yields vary from 24 to 98% and a chlorine substituent can be tolerated, for example, the... 

The most practical methods of syntheses of 1,2-dithiolanes and other saturated derivatives appear to be those involving sulfur to sulfur bond formation by oxidative ring closure of dithiols, or thiolate displacement of a leaving group attached to another sulfur. This can be applied to many types <90SR257>, but in view of the availability of 1,3-dithianes, syntheses by their oxidative ring contraction offer attractive alternatives. 

Single and double alkylation of the 1,3-dithiane S-oxide 104 followed by acid hydrolysis results in ring-contraction to form 1,2-dithiolanes 105 and 106 (13RSCA21911). A new method for synthesis of trithiaazapentalene derivatives has allowed examination of the relative contribution of the... 

Chloromethyl Oxirans react with barium hydroxide that is saturated with HjS to give 3-hydroxy-thietans. Aldehyde-dimedone adducts give thietans (80) when treated with SCl2. Ring-contraction of a dithiolan with tris(diethylamino)-phosphine gives a 2-propyl-thietan. Treatment of (81) with triphenylphosphine and diethyl azodicarboxylate gives (82). 

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