Thiophenes 3,4-dihydroxy-, tautomerism

Examples of the remaining potential 3,4-dihydroxy heterocycles are presently restricted to furan and thiophene. Although the parent 3,4-dihydroxyfuran apparently exists as the dioxo tautomer (86), derivatives bearing 2-alkyl or 2,5-dialkyl substituents prefer the keto-enol structure (87) (71T3839, 73HCA1882). The thiophene analogues also prefer the tautomeric structure (87), except in the case of the 2,5-diethoxycarbonyl derivative which has the fully aromatic structure (88) (71T3839). 

Mono- and disubstituted-3,4-dihydroxythiophenes can potentially exist in four tautomeric forms (116)-(119), For unsubstituted and for most disubstituted derivatives, the hydroxyketo form (117) or (118) is observed. In monosubstituted products (R H, R = H), (117) is the only one observed <71T3839>. These compounds also follow a trend similar to that found in the furan series with the exception of the parent compound 3,4-dihydroxyfuran which exists in the dioxo form <60JA3219>. The only thiophene derivative not to follow the trend is the 2,5-diethoxycarbonyl substituted compound which exists in the dihydroxy form (116 R, R = C02Et). 

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