The present invention relates to resolution of (cis,trans)5,6-dihydro-4H-4-ethylamino-6-methylthieno[2,3-b]thiopyran-2-sulfonamide-7,7-dioxide using dibenzoyl-L-tartaric acid monohydrate or di-p-toluoyl-L-tarrtaric acid monohydrate as a chiral resolving agent in presence of methanol to obtain hemitartarate salt, purifying it to obtain hemitartarate salt of 5,6-dihydro-4H-4(S)-ethylamino-6(S)methylthieno[2,3-b]thiopyran-2-sulfonamide-7,7-dioxide with de of >99%, Chemical purity >99.5% with cis isomer content of <0.1% and further converting into its pharmaceutically acceptable salts, preferably hydrochloride salt.
