Methylene chloride (60 mL), cis-4-hydroxy-5,6-dihydro-6-methyl-4-H-thieno-[2,3-b]-thiopyran-7,7-dioxide (5 g), N-ethyl-4-nitrobenzene-sulfonamide (6.86 g) and triphenyl-phosphine (8.42 g) are mixed under nitrogen atmosphere. The mixture is cooled to under −20° C. and diisopropyl azadicarboxylate (5.9 mL) is added, maintaining the temperature under −20° C. The temperature is allowed to rise to 20-25° C. The obtained mixture is treated with 5% NaOH (125 mL) and the resulting organic phase is vacuum distilled to dryness. The residue is chromatographed in a column (ethyl acetate/heptane, 2/1), obtaining 4.97 g of trans-4-[N-ethyl-N-(4-nitrobenzenesulfonyl)amino]-5,6-dihydro-6-methyl-4-H-thieno-[2,3-b]-thiopyran-7,7-dioxide.
