Compounds with Two Potential Hydroxy Groups

The potentially 2,4-dihydroxy derivatives of furan and thiophene exist in the solid state and in polar solvents as the monoenols (98) (71T3839). However, in non-polar solvents the furan derivatives exist predominantly in the dioxo form (99). The 2,5-dioxo structure (100) is well established for X = 0, NR, 

S and Se (71BSF3547) and there is no evidence for intervention of any enolic species. The formal tautomer (101) of succinimide has been prepared and is reasonably stable (62CI(L)1576). 

Examples of the potential 3,4-dihydroxy heterocycles are presently restricted to furan and thiophene. Although the parent 3,4-dihydroxyfuran apparently exists as the dioxo tautomer (102), derivatives bearing 2-alkyl or 2,5-dialkyl substituents prefer the keto-enol structure (103) (71T3839, 73HCA1882). The thiophene analogues also prefer the tautomeric structure (103), except in the case of the 2,5-diethoxycarbonyl derivative which has the fully aromatic structure (104) (71T3839). 

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