This invention provides a process for reducing an exocyclic double bond at the 5-position of a thiazolidinedione moiety of a thiazolidinedione precursor comprising the steps of: a) preparing a solution or suspension of the thiazolidinedione precursor in a non-ether solvent medium with a base and b) combining the solution or suspension with a dithionite source. Preferred solvent media include aqueous NN-dimethylformamide. Sodium dithionite is a preferred dithionite source. In particular the application discloses preparation processes for Pioglitazone Rosilitazone and Troglitazone.

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