| The spirofurooxindole framework is an important motif found in many bioactive natural products and pharmaceutical agents.As both naturally occurring and pharmaceutically useful spirofurooxindoles bear a chiral quaternary center,the development of asymmetric syntheses for the assembly of this class of skeleton is highly desirable.Although existing asymmetric annulation methods can eventually afford spirofurooxindoles with high stereoselectivity,most rely mainly on manipulation of oxindole derivatives,which are in turn prepared by multistep processes.We herein report a novel preparation of spirofurooxindoles catalyzed by a chiral iodoarene.The methodology features mild reaction conditions,high enantioselectivity and readily available starting materials.Literature reports show that the applications of chiral hypervalent iodine reagents in enantioselective oxidation reactions with high stereoselectivities have received much attention in the past decade.However,the utilization of catalytic organoiodine reagents in oxidative cross-coupling reactions using m CPBA or AcO2H as the terminal oxidant has rarely been studied in organic synthesis.In this regard,the development of chiral ayliodine-mediated enantioselective spirocyclization is highly in demand.We used ethyl 5-?N-methyl?phenyl?amino?-3,5-dioxopent derivatives as substrates,C-20d as the catalyst,m CPBA as the terminal oxidant in a mixed solvent of CH3CN and TFE at rt to obtain 16 spirofurooxindole derivatives.Finally,control experiments were carried out and a plausible mechanism was proposed.First,when 1.0 equiv of PIFA was used as the sole oxidant,the substrate was converted into the racemic intermediate through an intramolecular C-O bond formation,which was transformed into the optically active product through an intramolecular C-C bond formation with an equal ee vale under the optimized organocatalytic conditions.These results indicate that enantioselection occurred during the late stage C-C bond formation step. |
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