Boc chemistry

[a_m_shaheen]

Hello everyone:
i am new to Boc chemistry, i used MBHA resin for my peptide and when i tried to cleave it using TMSOTf/TFA/m-cresol i have no PPt at all from MTBE or MTBE-hexane even if after cooling to extended period of time . i did not concentrate it down directly after cleavage because i am worry about the stability of my peptide. i wonder if anybody come across this before and know how to recover my peptide and if it is ok if i concentrate cleavage solution after doing the cleavage or i am right in my precaution .
thank you in advance

[harryntn]

Originally Posted by a_m_shaheen

Hello everyone:
i am new to Boc chemistry, i used MBHA resin for my peptide and when i tried to cleave it using TMSOTf/TFA/m-cresol i have no PPt at all from MTBE or MTBE-hexane even if after cooling to extended period of time . i did not concentrate it down directly after cleavage because i am worry about the stability of my peptide. i wonder if anybody come across this before and know how to recover my peptide and if it is ok if i concentrate cleavage solution after doing the cleavage or i am right in my precaution .
thank you in advance

I have successfully applied the method you are describing above on 2 syntheses of vasopressin analogues at scales of 150 gm amounts. What you are missing in the process is the work-up. Do NOT attempt to concentrate the solution and do NOT use MTBE or MTBE-hexane to "PPt" the product from the mixture. The beauty of this method is that 1) You can sample the cleavage mix over time to witness completeness of protecting group cleavage by removing a few drops from the mixture and adding water and neutralize to pH 5 with NaOH (5M) for HPLC monitoring, and 2) Workup by cooling the reaction mixture and cautiously neutral with 5M NaOH slowly to pH 5. Add more water, filter off the residual resin from the mix and you are ready to load the mixture directly onto a prep HPLC column. Start the loading with no acetonitrile present on the column. Pump for a period of time with water to remove all the salt---2-3 column volumes of pure water and then start your gradient to collect your product. It works. Start to finish, Boc-AA based, stepwise nonapeptide synthesis, cleavage and product lyophilization yielding 150 gm pure peptide from 300 gm starting resin (0.6 meq/gm) was 2 months time. Additionally, cleavage using the "cocktail" you described above, required 6 hours at ice temp to RT to complete the deprotection. By HPLC, the first step was the release from the resin, followed by the final removal of a Tosyl group on Arginine.

Good luck and I hope this helps out!

[a_m_shaheen]

thank you very much for this detailed reply. i will try it for sure.