Synthesis and Application of Boronic Acid Derivatives

Abstract

Boronic acids are attractive synthetic intermediates and have been shown to be effective as inhibitors of various enzymes. In this project, the overarching goal is the selective inhibition of a protease present in the mitochondria known as human ClpXP. To study the potential selective inhibition of Human ClpXP using N-terminal peptidic boronic acid, we have designed a synthetic scheme that includes?-borylation of °,?-unsaturated carbonyl compounds using Cu(I) as catalyst, °-alkylation, saponification/hydrogenation, amidation, and oxidative removal of pinacolyl group with sodium periodate. A simple amidoboronic acid was also synthesized for stability studies. This compound, synthesized in 44% overall yield, could be used as a surrogate for N-terminal peptidic boronic acid to provide basic understanding of the stability of more elaborate N-terminal peptidic boronic acids. During the synthesis of this compound, published deprotection methods were not suitable to deprotect the pinacol group. A two-step protocol for pinacolyl boronic ester deprotection via a diethanolamine protected intermediate was successfully developed with the advantages of mild reaction conditions, tolerance to various functional groups, short reaction time and ease of product isolation. The current results will be useful for the deprotection of other boronic esters, such as pinanediol protected compounds, which are being used extensively in stereoselective synthesis.