Highly stereoselective [4+2] cycloaddition of azlactones to β,γ-unsaturated α-ketoesters catalyzed by an axially chiral guanidine base

Chemistry - A European Journal

Volumn 17, Issue 6, 2011, Pages 1760-1763

  Terada, Masahiro ^a   Nii, Hiroyuki ^a  

^a TOHOKU UNIVERSITY   (Japan)

Author keywords

amino sugar; asymmetric catalysis; cycloaddition; guanidine; organocatalysis

Indexed keywords

[4 + 2] CYCLOADDITION; ASYMMETRIC CATALYSIS; AZLACTONES; BASE CATALYST; CYCLOADDITION REACTION; DIASTEREOSELECTIVE; ENANTIO; FACILE ACCESS; GUANIDINE; KETO ESTER; ORGANOCATALYSIS; REACTION PATHWAYS; REACTIVE SITE; STEREO-SELECTIVE; STEREOSELECTIVE MANNER;

AMINES; CATALYSIS; CYCLOADDITION; ESTERS; RATE CONSTANTS; REACTION KINETICS; STEREOSELECTIVITY;

SUGARS;

EID: 79551486283     PISSN: 09476539     EISSN: 15213765     Source Type: Journal    
DOI: 10.1002/chem.201003015     Document Type: Article

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42. 1Ha NMR spectroscopy of the crude material.
43. The reaction of β,γ-unsaturated α-ketoesters with a methyl substituent (instead of the aromatic counterpart) at the γ-position gave the product in moderate yield (56%) despite the fact that the starting reactant, methyl-substituted α-ketoester in THF, was consumed completely in 3a h at room temperature. A significant amount of byproducts was formed in this reaction, although the desired product could be obtained with moderate stereoselectivity (64%a cis, 54%a ee).
44. CCDC-787766 (3ab) contains the supplementary crystallographic data for this paper. This data can be obtained free of charge from The Cambridge Crystallographic Data Centre via.
45. It was confirmed that the cyclization/ring-opening reactions of 5 took place under the catalytic conditions with chiral guanidine base 4 (see Tablea 2, entriesa 7 and 8). In addition, at the beginning of the reaction, acyclic intermediate 5 could be detected by TLC even at room temperature, but this intermediate had completely disappeared by the end of the reaction.
46. It can be considered that kinetic optical resolution of enantioenriched acyclic product 5 by chiral guanidine catalyst 4 would occur during the following cyclization/ring-opening steps. However, it seems unlikely that these processes occur because of the nearly equal enantio-and diastereoselectivity observed in both cycloaddition product 3aa and acyclic intermediate 5.
47. We cannot completely rule out the possibility that the reaction proceeds by a concerted process, that is, by an inverse-electron-demand hetero-Diels-Alder reaction, in parallel to the stepwise process that is proposed as the most dominant mechanism in this transformation. Also see referencea [4].