A novel and practical synthesis of 3-unsubstituted indolizines

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