Enantioselective synthesis of 5,6-dihydro-(S)-4-(ethylamino)-(S)-6-methyl-4H-thieno›2,3-B!thiopyran-2- sulfonamide 7,7-dioxide

A process for the preparation of a Compound of structural formula I:

• ##STR4## and its hydrochloride salt wherein chirality at C-6 is fixed, and the trans-stereochemical relationship between C-4 and C-6 substituents is retained, compound is crystalline and R and R.sup.1 are the same or different and are C.sub.1-3 alkyl, which comprises the steps of:
• A) treating a compound of formula II with ##STR5## a nitrile of formula RCN and a strong acid, such that the amount of water present, depending on the acid employed, is from about 0.5 to about 10 wt %, to form a compound of Structure III:
• ##STR6## B) treating III with chlorosulfonic acid to form a compound of Structure IV:
• ##STR7## C) treating IV with thionyl chloride to form a compound of Structure V:
• ##STR8## D) treating V with ammonia to form a compound Structure VI:
• ##STR9## E) treating VI with sodium borohydride and a strong acid, or borane-tetrahydrofuran or borane-dimethyl sulfide to form a compound of Structure VII ##STR10## F) isolation of VII as the maleate salt VIII;
• G) conversion of VIII to the hydro chloride salt of I;
• and
• H) purification of the hydrochloride salt of I.