Metal-carbonyl-induced reaction of 2-isoxazolines. Ring cleavage and reduction by pentacarbonyliron or nonacarbonyldi-iron

Abstract

3,5-Disubstituted 2-isoxazolines (1a–f) undergo N–O and C-4–C-5 bond-cleavage reaction with pentacarbonyliron and photoirradiation in methanol to give two fragments (aldehydes and ketones). The ketones could be derived from reduction of the complexed 1-substituted vinylnitrene intermediate and subsequent hydrolysis. Substituent effects have a significant influence on the course of the reaction. 2-lsoxazolines (1h and i) having no substituent at C-5 undergo N–O bond cleavage to give β-hydroxy- and β-methoxy-ketones, in addition to the above mentioned cleavage of the N–O and C-4–C-5 bonds. Furthermore, 2-isoxazolines (1j and k) bearing a substituent such as an acetoxy or ethoxy group at C-5 undergo N–O bond cleavage, and concomitant elimination of the substituent acetoxy or ethoxy group, leading to β-aminoenones. However, in the case of 2-isoxazolines (1l and m) bearing a piperidin-1-yl group at C-5, the N–O and C-4–C-5 bond-cleavage reactions predominate over the elimination of the C-5-substituent. Mechanisms are also proposed for the formation of the β-substituted ketones. Similar reactions were also effected thermally for several 2-isoxazolines with nonacarbonyldi-iron.