[N+:1]([CH2:3][C:4]([O:6][CH2:7][CH3:8])=[O:5])#[C-:2].[S:9]1[CH2:14][C:13](=O)[CH2:12][S:11][CH2:10]1.C(O)(=[O:18])C>O1CCCC1>[CH:2]([NH:1][C:3](=[C:13]1[CH2:12][S:11][CH2:10][S:9][CH2:14]1)[C:4]([O:6][CH2:7][CH3:8])=[O:5])=[O:18]A solution of potassium tert.-butylate (27.2 g) in tetrahydrofurane (200 cc) is cooled to 0° C. A solution of ethyl isocyanoacetate (25 g) in tetrahydrofurane (150 cc) is added dropwise. The mixture is stirred for a further one hour at 0° C. and a solution of 1,3-dithiacyclohexan-5-one (29.6 g) in tetrahydrofurane (375 cc) is then added. The reaction mixture is stirred for a further 90 minutes at 0° C. Acetic acid (50 cc) is added and the precipitate is filtered off. The solvents are evaporated off under reduced pressure (20 mm Hg) at 30° C. The residue is taken up with methylene chloride (2,500 cc). Extraction is carried out with a mixture of water (500 cc) and a saturated aqueous solution of sodium bicarbonate (300 cc). The organic phase is again washed with water (500 cc) and dried over sodium sulphate. The solution is treated with decolorising charcoal, filtered and then concentrated to dryness under reduced pressure (20 mm Hg) at 30° C. The residue is vigorously stirred with ethyl ether (400 cc). The precipitate obtained is filtered off. Ethyl α-formylamino-(4,5-dihydro-1,3-dithiin-5-ylidene)-acetate (18.5 g) is thus obtained. The filtrate is concentrated to dryness under reduced pressure (20 mm Hg) at 20° C. The residue is purified by chromatography on a column (diameter 3.7 cm; height 47 cm) containing silica (250 g). Elution is carried out with a mixture (two liters) of methylene chloride and ethyl acetate (85/15 by volume). A mixture (4.4 g) of ethyl α-formylamino-(4,5-dihydro-1,3-dithiin-5-ylidene)-acetate and ethyl α-formylamino-(1,3-dithiin-5-yl)-acetate is thus obtained.
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