A novel and practical synthesis of 3-unsubstituted indolizines

A novel and practical procedure for the preparation of 3-unsubstituted indolizines by 1,3-dipolar cycloaddition was developed. The requisite pyridinium N-methylides were generated simply from the corresponding N-(carboxymethyl)pyridinium halides. In the presence of MnO2, electron-deficient alkenes, instead of alkynes or vinyl bromides, were used successfully as dipolarophiles. This general method features cheap reagents, simple workup procedure and gives the products in moderate to high yields (57-92%).