A method for the preparation of compound of formula, the method comprises a step STEP1 and a step STEP2, STEP2 is done after STEP1; STEP1 comprises a reaction REAC1, wherein compound of formula is reacted with PHOS in a solvent SOLV1; R is selected from the group consisting of -H, -CH 3, -CH 2 -SH, -CH 2 -COOH, -(CH 2) 2 -COOH, benzyl, H, -CH(CH 3)-CH 2 -CH 3, -(CH 2) 4 -NH 2, -CH 2 -CH 2 -(CH 3) 2, -(CH 2) 2 -S-CH 3, -CH 2 -C -NH 2, -(CH 2) 2 -C -NH 2, -(CH 2) 3 -NH-C -NH 2, -CH 2 -OH, -CH -CH 3, -CH(CH 3) 2, - 3 -NH-C -NH 2, and -(CH 2) 3 -NH 2; any functional residue of R, which is capable of reacting with phosgene, is protected by a suitable protecting group PG; R1 is selected from the group consisting of H, C 1-4 alkyl and benzyl; n is 0 or 1; PHOS is selected from the group consisting of phosgene, diphosgene, triphosgene and mixtures thereof; SOLV1 is selected from the group consisting of THF, 2-Me-THF, dioxan, ethyl acetate, dichlormethane, monoglyme, diglyme, triglyme, cyclopentyl methyl ether, acetonitril, and mixtures thereof; in STEP2 part of SOLV1 is exchanged against a solvent SOLV2 by removal of from 35 to 87.5 wt% of SOLV1, the wt% being based on the weight of SOLV1; SOLV2 can be added before, during or after removal of part of SOLV1; SOLV2 is selected from the group consisting of hexyl formate, pentyl acetate, butyl propionate, propyl butyrate, ethyl valerat, methyl caproate, pentyl formate, butyl acetate, propyl propionate, ethyl butyrate, methyl valerate, butyl formate, propyl acetate, ethyl propionate, methyl butyrate, and mixtures thereof. STEP2 is done after STEP1; STEP1 comprises a reaction REAC1, wherein compound of formula is reacted with PHOS in a solvent SOLV1; R is selected from the group consisting of -H, -CH 3, -CH 2 -SH, -CH 2 -COOH, -(CH 2) 2 -COOH, benzyl, H, -CH(CH 3)-CH 2 -CH 3, -(CH 2) 4 -NH 2, -CH 2 -CH 2 -(CH 3) 2, -(CH 2) 2 -S-CH 3, -CH 2 -C -NH 2, -(CH 2) 2 -C -NH 2, -(CH 2) 3 -NH-C -NH 2, -CH 2 -OH, -CH -CH 3, -CH(CH 3) 2, - 3 -NH-C -NH 2, and -(CH 2) 3 -NH 2; any functional residue of R, which is capable of reacting with phosgene, is protected by a suitable protecting group PG; R1 is selected from the group consisting of H, C 1-4 alkyl and benzyl; n is 0 or 1; PHOS is selected from the group consisting of phosgene, diphosgene, triphosgene and mixtures thereof; SOLV1 is selected from the group consisting of THF, 2-Me-THF, dioxan, ethyl acetate, dichlormethane, monoglyme, diglyme, triglyme, cyclopentyl methyl ether, acetonitril, and mixtures thereof; in STEP2 part of SOLV1 is exchanged against a solvent SOLV2 by removal of from 35 to 87.5 wt% of SOLV1, the wt% being based on the weight of SOLV1; SOLV2 can be added before, during or after removal of part of SOLV1; SOLV2 is selected from the group consisting of hexyl formate, pentyl acetate, butyl propionate, propyl butyrate, ethyl valerat, methyl caproate, pentyl formate, butyl acetate, propyl propionate, ethyl butyrate, methyl valerate, butyl formate, propyl acetate, ethyl propionate, methyl butyrate, and mixtures thereof. hexyl formate, pentyl acetate, butyl propionate, propyl butyrate, ethyl valerat, methyl caproate, pentyl formate, butyl acetate, propyl propionate, ethyl butyrate, methyl valerate, butyl formate, propyl acetate, ethyl propionate, methyl butyrate, and mixtures thereof.