Boc/Bzl Solid-Phase Synthesis of Deltorphin II and Its Analogs without the Utilization of Anhydrous Hydrogen Fluoride

Objective: The disadvantages of the published methods for the preparation of peptide deltorphin II and its analogues hamper thorough biological investigations of this class of molecules. Aiming to develop a more productive synthetic method we investigated an approach where Boc/Bzl solid phase peptide synthesis technique was employed without the utilization of anhydrous hydrogen fluoride. Deltorphin II and its analogues were prepared in high yields and purity using the developed method and trifluoromethane sulfonic acid as a deprotection regent. Methods: Boc/Bzl solid phase peptide synthesis using accelerated and classical coulpling protocols was employed. A few strong Lewis acids were used in the final deprotection synthesis step. Results and Discussion: The toxicity and aggressive nature of hydrogen fluoride have resulted in the development of alternative strong Lewis acid-based reagents for the final deprotection and cleavage steps in Boc/Bzl peptide synthesis. Unlike hydrogen fluoride, these acids have high boiling points; however, the favorable physicochemical properties of most peptides allow them to be quite easily isolated from the cleavage cocktails by precipitation with ether. We found that this simple procedure is not suitable for the isolation of deltorphin II peptide and its analogs and developed and successfully implemented alternative methods of synthesis, isolation, and purification of these peptides. Conclusions: The use of strong Lewis acids as an alternative to anhydrous hydrogen fluoride may complicate the isolation of hydrophobic peptides by the standard techniques. An alternative method was proposed and successfully employed in the preparation of peptide deltorphin II and its three analogs. The developed procedures can be used to purify other hydrophobic peptides.