Nonstabilized N-unsubstituted azomethine ylides: a synthesis of indolizidine 239CD.

[formula: see text] Treatment of a (2-azaallyl)stannane with HF.pyridine generated a nonstabilized N-unsubstituted azomethine ylide, which was found to undergo an efficient and stereoselective dipolar cycloaddition with phenyl vinyl sulfone to produce a trans-2,5-dialkylpyrrolidine that was further transformed into the dendrobatid alkaloid indolizidine 239CD.