One-Pot Two-Step Synthesis of 1-(Ethoxycarbonyl)indolizines via Pyridinium Ylides

Pyridinium salts Py+-CH2-EWG (EWG = CO2Et, CONEt2, CN, COMe, COPh) reacted with Michael acceptors Ar-CH=C(CO2Et)(Acc) (Acc = CO2Et, COMe, SO2Me, CONH2) at ambient temperature in the presence of base to give [3+2]-cycloadducts by stepwise [3+2]-cycloaddition of the intermediate pyridinium ylides. Treatment of the crude reaction mixtures with 1 equiv. of chloranil and atmospheric oxygen in the presence of sodium hydroxide gave 1-(ethoxycarbonyl)indolizines by dehydrogenation and elimination of the acceptor group (Acc). A good yield of indolizine was also obtained from Py+-CH2CN and iPr-CH=C(CO2Et)(2), which indicates that this method is not restricted to aromatic Michael acceptors. Structurally related isoquinolinium salts react with Michael acceptors analogously to give pyrrolo[2,1-a]isoquinolines.