Reactions of Electrophilic Allenoates [and Isocyanates/Isothiocyanates] with a 2-Alkynylpyridine via a Free Carbene Intermediate

A pyridine containing a 2-alkynyl substituent armed with a carbene reporter group [R(1) = C(Me)(2)OAc] is shown to engage electrophilic allenes to generate intermediate free carbenes. Depending on the electron density at the carbene carbon atom, a feature that is modulated by the substituents on the allene substrate, the carbene will either rearrange or eject an acetate leaving group, leading to various types of indolizine-containing products. Iso(thio)cyanates react in an analogous fashion.