[C:1]([O:5][C:6]([NH:8][CH2:9][CH2:10][C:11]1[CH:15]=[CH:14][S:13][CH:12]=1)=[O:7])([CH3:4])([CH3:3])[CH3:2].C=O.O.[C:19]1(C)C=CC(S(O)(=O)=O)=CC=1>C1(C)C=CC=CC=1.C(OCC)(=O)C>[C:1]([O:5][C:6]([N:8]1[CH2:9][CH2:10][C:11]2[CH:15]=[CH:14][S:13][C:12]=2[CH2:19]1)=[O:7])([CH3:4])([CH3:2])[CH3:3]A solution of 1.753 g (7.711 mmol) of N-tert-butoxycarbonyl-2-(3-thienyl)ethylamine, 0.46 g (15.4 mmol) of powdered paraformaldehyde and 73 mg (0.39 mmol) of p-toluenesulfonic acid monohydrate in 150 ml of toluene was refluxed under dehydrating conditions for 0.5 hours in a Dean-Stark trap. The reaction mixture was cooled to room temperature and diluted with ethyl acetate, after which it was washed with aqueous sodium hydrogen carbonate; the organic layer was dried over anhydrous magnesium sulfate; the solvent was distilled off under reduced pressure. The resulting crude product was purified by silica gel column chromatography (hexane/ethyl acetate=15/1 to 9/1) to yield the desired product.
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