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 cycloadditions between cyclohexyl vinyl ether and 2-benzopyrylium-4-olate generated via Rh₂(OAc)₄-catalyzed decomposition of o-methoxycarbonyl-α-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 allyl 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-Ph₂–Lu(OTf)₃ complex to give good levels of asymmetric inductions (75–84% ee).ArticleTetrahedron. 66(16):3070-3089 (2010)journal articl

Chiral Lewis Acid-Catalyzed Enantioselective Cycloadditions between Indoles and Cyclic Carbonyl Ylides Derived from Diazodiketone or Diazoketoester Derivatives

Asymmetric 1,3-dipolar cycloaddition reactions between <i>N</i>-methylindoles and several cyclic carbonyl ylides that were derived from diazodiketone or diazoketoester precursors in the presence of both achiral Rh and chiral lanthanoid metal catalysts are described. For the six-membered cyclic carbonyl ylides derived from 1-diazo-5-aryl-2,5-pentanedione precursors, the cycloaddition reactions were carried out using Rh₂(OAc)₄ (2 mol %) and the chiral Pybox-Ph₂–Lu­(OTf)₃ complex (10 mol %) as catalysts, resulting in high enantioselectivities (83% to >98% ee (<i>exo</i>)) along with relatively good <i>exo</i>-selectivities (<i>exo</i>:<i>endo</i> = 65:35 to 94:6) and yields (63–85%). For the five-membered cyclic carbonyl ylide derived from 1-diazo-2,4-pentandione precursor, the cycloaddition reaction with 5-bromo-1-methylindole was carried out in the presence of Rh₂(OAc)₄ (2 mol %) and the chiral Pybox-Ph₂–Er­(OTf)₃ complex (30 mol %) as catalysts, resulting in relatively good enantioselectivity (78% ee) and <i>endo</i>-selectivity (<i>endo</i>:<i>exo</i> = 81:19)