2-Ethyl-7-hydroxybenzisoxazolium Tetrafluoroborate

[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.

Preparative Methods: from 2,3-dihydroxybenzaldehyde and Hydroxylamine-O-sulfonic Acid, followed by treatment with Triethyloxonium Tetrafluoroborate (78% yield for the two steps).1,3

Handling, Storage, and Precautions: the salt is not hygroscopic or light sensitive but releases HF when exposed to moist air.

Peptide Synthesis.

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


1. Kemp, D. S.; Wang, S.-W.; Mollan, R. C.; Hsia S.-L.; Confalone, P. N. T 1974, 30, 3677.
2. Kemp, D. S.; Wang, S.-W.; Rebek, J., Jr.; Mollan, R. C.; Banquer, C.; Subramanyam, G. T 1974, 30, 3955.
3. Kemp, D. S.; Wrobel, S. J., Jr.; Wang, S.-W.; Bernstein, Z.; Rebek, J., Jr. T 1974, 30, 3969.

Miles S. Congreve

University of Cambridge, UK



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