Electronic Effects in the Pt-Catalyzed Cycloisomerization of Propargylic Esters:  Synthesis of 2,3-Disubstituted Indolizines as a Mechanistic Probe

The initial 5-exo versus 6-endo cyclization of the acyl group onto the activated alkyne of propargylic esters has been found to be dependent on electronic effects of the acyl, alkyne, and propargylic carbon substituents. These electronic effects control the ratio of 2,3-disubstituted versus 1,3-disubstituted indolizine products formed when substrates bearing pyridines at the alkyne terminus are used.