Does anyone know a good method for the introduction of carboxyfluorescein without using electrophilic auxiliaries (no HOBt, HOAt etc.) using only standard activating agents? (E. g. DIC, HBTU, COMU, you name it). It needs to be introduced on the N-alpha functionality (so far I have only found it on the epsilon-functionality for Lysine, which doesn't work for me).
Is the epsilon-N protected? It will always react faster than the alpha-N (more electron deficient and more hindered). Also, HBTU contains a 'masked' HOBt molecule.
Sorry for not being clear: I don't have Lysine in the sequence. Thanks for the tip with the "masked" HOBt.
Nucleophilicity of alpha N is less compared to epsilon N, but nevertheless it's enough to react with NHS esters or with acid/EDC. An alternative way is to modify alpha N with any alkyne NHS ester like propynyloxypropyonic acid oxysuccinimide ester [http://primetech.by/en/biocon_activated_esters.html, 140a], and then to click alkyne-modified peptide with FAM-azide.
..the hbtu makes in situ the hobt active ester of your amino acid and in this light, the addition of hobt is recommended to suppress all the hbtu driven side reactions (N guanidination, and cooh alkylation).
for successful acylation you need your cooh to be susceptible to the nucleophilic attack from the amine, which will not take place by simply heating the mixture, giving in this case the corresponding salt whose conversion in amide bond requires too severe conditions to be even remotely compatible with the concept of organic synthesis itself.
I do not understand why you are afraid of using the HOBT when making the acylation still via active esters :).
The alternative would be going for halogens activation, but you dont wanna that, do you? :) best regards
- Badges
- Science topic
- Similar topics
- Chemistry
- Organic Chemistry