Triflic anhydride mediated cyclization of 5-hydroxy-substituted pyrrolidinones for the preparation of alpha-trifluoromethylsulfonamido furans

The reaction of alpha-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 (Tf(2)O) 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 Tf(2)O 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.