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

Eur. J. Org. Chem., 2013, 28, 6379-6388 published on 12.08.2013
Eur. J. Org. Chem.
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.
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