The present invention relates to a process for preparing 5,6-dihydro- (S) -4- (ethylamino) - (S) -6-methyl-4H-thieno [2,3-b] thiopyran-2-sulfonamide-7,7-dioxide. The key step in the synthesis is the Ritter reaction, which unexpectedly occurs while maintaining the chirality of the final product.
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