H-Gln-Pro-Gly-OH (1 eq.) was added to H2O containing DIPEA (2.00 eq.). The slurry was stirred at 25° C. until a clear solution was observed and was then cooled to 0° C. Z-Val-OSu (1.1 eq.) was dissolved in MeCN at 25° C. until a clear solution was obtained and was then cooled to 0° C. The Z-Val-OSu solution was added to the solution of H-Gln-Pro-Gly-OH in such way that the temperature did not rise above 5° C. Then the mixture was stirred for at least 2 h. The peptide solution was concentrated and then diluted with a KHSO4 solution and stirred for a few minutes. This aqueous solution was washed twice with a mixture of IPE and AcOEt. The organic phases were discarded and the aqueous phase was extracted 2 times with a 20% i-BuOH in DCM solution. The organic phases were collected and concentrated under reduced pressure until water content in the evaporates was ≦1% weight. The solution of the protected tetrapetide was then precipitated in IPE at 25° C. Z-Val-Gln-Pro-Gly-OH (SEQ ID NO 16) was collected by filtration washed with IPE and dried under vacuum until IPE content in the peptide was <5% weight. The protected fragment was dissolved in methanol at 30° C. and Pd/C (0.02 eq.) was added to the peptide solution. The solution was stirred at 20° C. followed by the introduction of hydrogen under pressure (0.3 bar). After stirring for 3 hours the solution was filtered to remove the catalyst which was washed with methanol. After evaporation, the free tetrapeptide was then precipitated by transferring the solution into MeCN at 10° C. The solid was filtered and washed with MeCN. After drying under vacuum, 60% of H-Val-Gln-Pro-Gly-OH (SEQ ID NO 4) was recovered.
