Prepn. of thiophen carboxylic acids of formula (I) is effected by reaction of a 3-ketothiphan carboxylic acid (II) with a sulphonic acid: R4. SO2Y (III) to give a 3-sulphato deriv. then reacting the product with an alkali metal polysulphide. The resulting cpd. is treated with a sulphuric acid chloride or bromide or with Cl2 to give modified polysulphide which is finally treated with ahalogen and water. In the formula R is CO2R3 or H; R2 is H whe R' is -CO2R3; or R' is -CO2R3 when R' is H; R3 is H or an aliphatic group; X is halo; R4 is an aliphatic or aromatic group; Y is halo, the group OR3 or a group -O3S.R4. Provides simple, more economic process for prodn. of (I) which is intermediate for thiophen-saccharin sweetening agents. A typical cpd. methyl-3-chlorosulphonylthiophen-4-carboxylate, was prepd. by the 4-stage process from methyl 3-hydroxy-dihydrothiophen-4-carboxylate.
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