Inverse electron demand asymmetric cycloadditions of cyclic carbonyl ylides catalyzed by chiral Lewis acids-Scope and limitations of diazo and olefinic substrates

High enantioselectivities (94-96% ee) were obtained for the inverse electron-demand 1,3-dipolar cyclo-additions between cyclohexyl vinyl ether and 2-benzopyrylium-4-olate generated via Rh(2)(OAc)(4)-catalyzed decomposition of o-methoxycarbonyl-alpha-diazoacetophenone. The reactions were effectively catalyzed by Eu(OTf)(3), Ho(OTf)(3), or Gd(OTf)(3) complexes (10 mol %) of chiral 2,6-bis[(4S,5S)-4,5-diphenyl-2-oxazolinyl]pyridine. The reactions with the other electron-rich dipolarophiles such as ally! alcohol, 2,3-dihydrofuran, and butyl-tert-butyldimethylsilylketene acetal showed moderate enanantioselectivities (60-73% ee). Good to high enantioselectivities (73-97% ee) were also obtained for the cycloadditions between 3-acyl-2-benzopyrylium-4-olates, generated from methyl 2-(2-diazo-1,3-dioxoalkyl)benzoates and butyl or cyclohexyl vinyl ethers, in the presence of binaphthyldiimine (BINIM)-Ni(II) complexes (10 mol %). Under similar conditions, the reaction between methyl 2-(2-diazo-1,3-dioxohexyl)benzoate and 2,3-dihydrofuran was highly endo-selective, and moderately enantioselective (70% ee). For the BINIM-Ni(II)-catalyzed reactions of cyclohexyl vinyl ether, the use of an epoxyindanone as the 3-acyl-2-benzopyrylium-4-olate precursor revealed that the chiral Lewis acid can function as a catalyst for asymmetric induction. The scope of the cyclic carbonyl ylides was extended to those generated from 1-diazo-2,5-pentanedione derivatives, which were reacted with butyl or TBS vinyl ether and catalyzed using the (4S,5S)-Pybox-4,5-Ph(2)-Lu(OTf)(3) complex to give good levels of asymmetric inductions (75-84% ee). (C) 2010 Elsevier Ltd. All rights reserved.