Chlorinated thiophenes. Part 2. Trihalogenated hydroxythiophenes; preparation, reactions and tautomeric properties

2-Hydroxy-3,4,5-trichlorothiophene (4), 3-hydroxy-4-bromo-2,5-dichlorothiophene (5) and 3-hydroxy-2,4,5-trichlorothiophene (6) have been synthesised. It was found that 4 exists in a carbonyl form, whereas in 5 and 6 the hydroxy forms were the major tautomers. Several derivatives of the trihalogenated hydroxythiophenes were prepared. O-Methylation was carried out with diazomethane and O-acetylation with acetyl chloride. Silylation of 4 was performed using N,O-bis(trimethylsilyl)acetamide. Compound 4 reacted with methylmagnesium iodide in an unexpected way by exchanging chlorine with magnesium iodide, giving 3,4-dichloro-2,5-dihydrothiophen-2-one (16) after hydrolysis. In an unsuccessful attempt to try to prepare thiophene isosteres of the infamous 2,3,7,8-tetrachlorodibenzo-p-dioxine, the new dimer 2,2′,3,3′,4,4′-hexachloro-2,2′,5,5′-tetrahydro-2,2′-bithiophen-5-one (15) was discovered, the crystal structure of which is reported.