The synthesis of simple substituted 3-sulfolenes via 2,5-dihydrothiophenes

Dihydrothiophenes are very easily oxidized to sulfolenes by a variety of oxidizing agents. Accordingly, a common approach to sulfolenes is to construct a substituted dihydrothiophene first, then oxidize it. The synthesis of dihydrothiophenes has been reviewed [11]. As the approaches to sulfolenes are very wide ranging, we illustrate general strategies which are frequently employed to provide common basic types of sulfolene unit. We also exemplify the preparation of commonly used sulfolene ring systems. 

Dienes with a mesomerically electron withdrawing group in the 2-position are often unstable and are prone to dimerization. 3-Sulfolenes bearing a mesomerically electron withdrawing group at the 3-position have therefore become established as useful stable precursors for their equivalent dienes. 

Perhaps the most commonly employed route to 3-carbomethoxy-3-sulfolenes is the elegant method reported by McIntosh and Sieler [83,84]. In the first step of the 

John Leonard, Andrew B. Hague and John A. Knight 

Analogues of a-mercaptoacetaldehyde were also used in this route to incorporate substituents at C-5. This approach to sulfolenes has been adapted by several other workers to provide useful diene precursors. In our laboratory we have recently developed a modification of the synthetic procedure to provide 3-carbomethoxy-3-sulfolene on a large scale, in one pot, and using cheaper reagents than the original method [74], 

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