Synthesis and properties of methyl 3,4-dihydro-4-oxo-1H-2-benzothiopyran-3-carboxylate and related thiopyrano-compounds

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Abstract
Methyl 3,4-dihydro-4-oxo-1H-2-benzothiopyran-3-carboxylate (2)[or its tautomer (3)] has been obtained by Dieckmann cyclisation of methyl(o-methoxycarbonylbenzylthio)acetate. It reacts with hydrazine and with phenylhydrazine to give 1,2-dihydro-5H-[2]benzothiopyrano[4,3-c]pyrazol-3-one (7) and the 2-phenyl derivative (6), respectively, but does not give an isoxazolone with hydroxylamine. Methylation of the pyrazolone (6) with various reagents has been investigated. Methyl 6,7-dihydro-7-oxo-4H-thieno[3,2-c]thiopyran-6-carboxylate (12) and methyl and ethyl 3,4-dihydro-8-nitro-4-oxo-1H-thiopyrano[4,3-b][1]benzothiophen-3-carboxylate [(15) and (16) or their respective tautomers] have been obtained by the appropriate Dieckmann cyclisations. Compound (16) may also be obtained in a single step by treatment of 2-bromo-2′-chloro-5′-nitroacetophenone with ethyl mercaptoacetate (2 mol. equiv.). Improved procedures for carrying out some well known chemical transformations are described.

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