We have to use Fmoc-Ser(Ac)-OH for the peptide synthesis (solid phase). We tried HOBt/HBTU/DIPEA combination. However, we didn't get any product. Tried to check the literature but it seems Fmoc-Ser(tBu) is used everywhere. We have to use Ac at the side chain, ie Fmoc-Ser(Ac)-OH. Before using the stronger coupling reagents, we are hoping to get some information/guidance. Is it really difficult to get the product with Ac protection?
Have you tried classical DIC, HOBt, DIPEA? I have little experience with solid phase chemistry so I've never worked with Ac protection, but I don't think it should give any special trouble...
Thanks for the suggestion Dr. Maitia Labora. No we didn't try..We tried only HOBt/HBTU/DIPEA. Since I didn't find any literature for Ac protection, I thought before screening all the coupling reagents better to get some Idea...I may have to go for the screening now..
Thanks!
I can't see that there should be any problem with Ac as protection group. What temperature are you doing it at as trans esterification could potentially be a problem at elevated temp. Another thing that is important to know before discussing possible problems is the nature of the residue you couple the Fmoc-Ser(Ac)-OH too as that can play a major role in the outcome.
Thanks Emil Cohrt..We are testing the reactions at room temperature..Will post the result if succeed...
Hello! The difficulty may be due to intramolecular interactions. Another downside may be the competition of activation in coupling with the Ac group. My suggestion: DIPEA + BOP reagent in DMSO / NMP (1:4). Adjust the pH around 8.5 or 9 (with DIPEA). Complete reaction after 2 h and do the ninhydrin test. Regards and keep me informed.
As Emil said, the problem might come from the previous residue in the resin. Try the coupling with the regular Fmoc–Ser(tBu)–OH and see if it goes OK to identify the source of your problem.
Also, if the Fmoc–Ser(Ac)–OH is used as a salt, then you might have a problem because some protons are needed in the mechanism (those usually come from the acid), I think that I remember seeing adding some catalytic PPTS in that case (there's a paper by Oliver Seitz on coupling a thiazole orange where they describe that issue, but I don't have the reference at hand).
Have you used fresh HBTU?, it can be stored for a few days as DMF solution, but I'd try it freshly prepared.
As for other coupling reagents, HATU works usually great in our hands, COMU is far less known, but also nice and effective.
this is the reference I was talking about
Kummer, S.; Knoll, A.; Socher, E.; Bethge, L.; Herrmann, A.; Seitz, O. Bioconjugate Chem. 2012, 23, 2051–2060.
Thanks a lot for the information Dr M. Eugenio Vazquez. Recently I tried two conditions with Fmoc-Ser(Ac)-OH; 1) HBTU/DIPEA and 2) DCI/HOBt. Though both reactions were ok. DCI/HOBt was clean without any impurity. Thanks for the valuable suggestions friends..
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