[16859-20-4] · C9H10BF4NO2 · 2-Ethyl-7-hydroxybenzisoxazolium Tetrafluoroborate · (MW 251.00)
(reagent for the formation of activated esters in peptide synthesis1-3)
Physical Data: mp 137-138 °C.
Solubility: sol dipolar aprotic solvents and water.
Handling, Storage, and Precautions: the salt is not hygroscopic or light sensitive but releases HF when exposed to moist air.
The 2-ethylbenzisoxazolium cation has been studied as a peptide coupling reagent.1 A reagent which reacted rapidly and efficiently with carboxylic acids to produce a stable but activated ester was required. This ester must then react efficiently with peptide amines without competitive hydrolysis or racemization. 2-Ethyl-7-hydroxybenzisoxazolium tetrafluoroborate was found to be the most satisfactory derivative to fulfill these criteria. Treatment of a peptide carboxylate salt with the title reagent in pyridine-water at pH 4.5-5.0 gives excellent yields of the ester product (eq 1) with a half life of the reaction of ca. 1-2 min. This ester is formed by attack at the iminium cation before migration of the acyl group to give the more stable meta-acyloxy substituted product containing a strong intramolecular hydrogen bond.
Coupling of the active ester with amines occurs rapidly in the presence of 1,1,3,3-Tetramethylguanidine (TMG) (eq 2).2 Racemization is minimized at 0-10 °C and above 0.05 M concentration. Reaction times for 90% reaction in DMSO at 22 °C range from 15 to 90 min. Hindered amino acids react more slowly, e.g. Val + Val having a reaction time of 4 h. Proline reacts very slowly (9 h for 90% completion with ZGly). A wide range of peptide couplings can be successfully achieved with the title reagent, which compares favorably with 1-Hydroxybenzotriazole/1,3-Dicyclohexylcarbodiimide for peptide synthesis, particularly for the synthesis of small peptides.3
Miles S. Congreve
University of Cambridge, UK