1,3-Dipolar Cycloadditions of 4‑Acetoxy Allenoates: Access to 2,3-Dihydropyrazoles, 2,3-Dihydroisoxazoles, and Indolizines

The
thermal 1,3-dipolar cycloadditions of 4-acetoxyallenoates 1 with various dipoles have been reported. When azomethine
imines and nitrones are used as the 1,3-dipole partner, the corresponding
reactions afford 2,3-dihydropyrazole and 2,3-dihydroisoxazole derivatives,
respectively. These reactions might proceed via a thermal 1,3-dipolar
cycloaddition and the subsequent elimination of HOAc. In addition,
allenoates 1 react well with in situ generated azomethine
ylides in which a similar cycloaddition pathway is followed by oxidative
aromatization to give indolizine derivatives.