added followed by 410 ml 4n aqueous hydrochloric acid the mixture was transferred to a separatory funnel the organic phase was separated and the aqueous portion was extracted 2 u00d7 100 ml with chloroform the combined organic extracts were washed 2 u00d7 50 ml with 1n aqueous hydrochloric acid and 1 u00d7 100 ml brine dried mgso 4 and concentrated to leave 38 g crude 8 after triturating with 100 ml absolute ethanol and filtrating the filter cake was washed with diethyl ether and dried in vacuo to leave 8 26 1 g 62 of sufficient purity to be used in the next step a mixture of threo n n 39 dicarbethoxy 2 3 diamino 1 4 butanediol 4 4 bismethanesulfonate 8 2 28 g 5 43 mmol and potassium thioacetate 1 55 g 13 6 mmol in 85 ml of dimethyl formamide was stirred at room temperature for 15 hours the majority of the dimethylformamide was removed under reduced pressure and the residue was taken up in 50 ml of wafter and extracted 3 u00d7 40 ml with chloroform the combined extracts were dried mgso 4 and concentrated to leave a residue which was chromatographed waters prep 500a one silica cartridge 30 ethyl acetate hexane to provide pure 9 1 45 g 70 m p 94 u00b0 97 u00b0 c a solution of threo 1 2 dimethylmercaptoethylene dicarbamic acid diethyl ester diacetate 9 1 67 g 4 39 mmol in 150 ml of concentrated hydrochloric acid was refluxed under nitrogen for 18 hours the reaction.