3- Hydroxy-2-methylthiophene, tautomerism

In a related example, reaction of N-hydroxy-N-methylthiophene-2-carboximidamide 56 with DMAD gave a double Michael addition product 57, which when heated at reflux in xylenes, afforded hydroxypyrimidinone 60 in 57% overall yield (Scheme 6.21) [9f]. The mechanism invoked was opening of the oxa-diazole 57 to 58, followed by a [3,3]-Claisen-type rearrangement to 59, which, after tautomerization and cyclization, afforded 60. 

Dihydroxy-5-methylthiophene can also exist in four tautomeric forms. The NMR spectrum in CDCI3 shows a doublet atS = 1.55 (CH3), a double quartet at 4.10 (H-5), a broad signal at 6.1 (OH), and a doublet at 6.32 (H-4). No other peaks were observed, and together with the IR and UV data (Table 77) the true structure of 2,3-dihydroxy-5-methylthiophene is 3-hydroxy-5methyl-3-thiolene-2-one 229. 

Hydroxy-thiophenes are even more unstable than 2-hydroxy-thiophenes 3-hydroxy-2-methylthiophene exists as a mixture of hydroxyl and carbonyl tautomeric forms, with the former predominating. The parent can be prepared by gas-phase pyrolysis of a Meldrum s acid precursor it exists as a 2.9 1 mixture of keto and hydroxy tautomers and gradually dimerises. ... 

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