Triflic Anhydride Mediated Cyclization of 5-Hydroxy-Substituted Pyrrolidinones for the Preparation of α-Trifluoromethylsulfonamido Furans

The reaction of α-angelica lactone with alkylamines under aqueous conditions afforded 5-hydroxy-5-methylpyrrolidinones in high yield. When the reaction was carried out under anhydrous conditions,
the only products obtained were the corresponding 4-oxopentanoic acid amides. Treatment of either
class of compound with triflic anhydride (Tf2O) in pyridine resulted in the formation of various
substituted sulfonamidofurans. The suggested mechanism involves initial formation of an iminium
ion which is subsequently transformed into a transient imino triflate. Cyclization of the highly
electrophilic imine onto the oxygen atom of the adjacent carbonyl group generates an imino
dihydrofuran intermediate. This species reacts further with another equivalent of Tf2O to give the
observed product. The nature of the Lewis acid used was found to affect the outcome of the cyclization
reaction. In certain cases, the sulfonamide furan was utilized as a cycloaddition substrate for the
synthesis of indolines and related heterocyclic systems.