イソオキサゾールの新規官能基化法の開発

抄録

Isoxazole is a five-membered heteroaromatic ring that is a highly important framework with widespread applications in pharmaceuticals and agrochemicals. Indeed, the isoxazole ring has desirable pharmacological activity because its two contiguous electronegative heteroatoms (nitrogen and oxygen atoms) contribute to hydrogen bonding with various target enzymes and receptors inaccessible by other ring systems. This unique and significant utility has led to great interest in the synthesis of functionalized isoxazoles, and various synthetic approaches have been developed for this class of compounds. Current synthetic approaches are classified into following four groups: (a) 1,3-dipolar cycloaddition, (b) condensation, (c) cycloisomerization, and (d) direct functionalization. Despite the recent much effort on synthetic researches, however, direct functionalization of isoxazoles (approach d) has still been immature to produce a variety of multifunctionalized isoxazoles. We herein describe our recent studies on direct functionalization of isoxazoles based on the generation of 4-isoxazolyl anion species and the subsequent gold (I)-catalyzed electrophilic aromatic substitution type reactions and rhodium (III)-catalyzed carboxylate-directed C-H activation. These developed functionalization methodologies enabled access to novel isoxazole derivatives that are not readily synthesized by other means. In addition, our interest in photolysis of isoxazoles led us to find photochemical conversion of isoxazoles into 5-hydroxyimidazolines.