compound using one of the procedures described above for cyclization of a compound of formula iii yields the corresponding compound of formula i n is 1 the compounds formula i contain two asymmetrical carbon atoms and exist as mixtures of diastereoisomers the s or l configuration is preferred for the carbon alpha to the nitrogen atom 1 d 3 mercapto 2 methylpropanoyl l proline s s 5 425 g is taken into 200 ml of dichloromethane and added dropwise to a stirred solution of 1 cyclohexyl 3 2 morpholineoethyl carbodiimide metho p toluene sulfonate 50 g in 2 2 liters of dichloromethane under argon the reaction is stored for three days at room temperature the dichloromethane is removed in vacuo the residue is taken into ethyl acetate washed with 10 potassium sulfate water saturated sodium bicarbonate water dried over magnesium sulfate and concentrated to dryness in vacuo this material 1 7 g is crystallized from ethyl acetate hexane to yield 1 4 g of the title compound melting point 103 u00b0 104 u00b0 c a solution of 1 tert butyloxycarbonyl l proline 8 6 g and triethylamine 5 56 ml in dry tetrahydrofuran 300 ml is cooled to 15 u00b0 c under argon and treated with ethyl chloroformate 3 84 ml after stirring at 15 u00b0 c for 90 minutes sodium hydrosulfide 4 0 g is added and the stirring is continued for.