Ozonolysis of Azoles and its Application to the Organic Synthesis

Abstract

Our recent studies concerning to the ozonolysis of various azoles are reviewed in this paper. While azoles are remarkably stable under the acidic, basic, oxidative and/or reductive conditions due to their aromatic characters, azoles are specifically labile toward ozone. These characters of azoles satisfy the requirement for the protection-deprotection of the functional group. Pyrroles are easily prepared from primary amines, and the side chains of these pyrroles are functionalized by base catalyzed alkylation, hydride reduction, and aminolysis. By the ozonolysis, pyrroles are converted into the corresponding amides. Through this series of the reactions, pyrroles are demonstrated to be useful as the protecting group of amino functions for the synthesis of α-aminoalcohols, α-aminoketones, α-aminoaldehydes, and some peptides. Oxazole ring is also useful as the protecting group of carboxyl group in peptide synthesis, and the ring opening is carried out by ozonolysis. In this deprotection, oxazoles are converted into acid anhydrides, which are the reactive forms of carboxyl group and are directly available for the further peptide formation. This is the most remarkable example that the protecting group takes a role of the precursor of the active form of the functional group.