Thiophene 2.5- diethyl-, ring synthesis

If diethyl oxalate is utilised as the 1,2-dicarbonyl component, with sodium methoxide as base, a 3,4-dihydroxy thiophene is generated as its disodium salt, usually trapped by double 0-alkylation, as indicated in Scheme 46. Note that in this situation, the product of the ring synthesis is the 2,5-diester [75, 76], not an ester-acid. 

The synthesis of the thiophene ring system by formation of a bond between the two /3-carbons of a diethyl sulfide derivative, or between the (3 -carbon and an ortho position of a phenylthio ethyl derivative, has been one of the most widely exploited methods for the preparation of thiophene derivatives. 

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