Enantioselective [4+2]‐Cycloaddition Reaction of a Photochemically Generated o‐Quinodimethane: Mechanistic Details, Association Studies, and Pressure Effects
- 21 April 2004
- journal article
- research article
- Published by Wiley in Chemistry – A European Journal
- Vol. 10 (9) , 2179-2189
- https://doi.org/10.1002/chem.200306049
Abstract
1,2,3,4‐Tetrahydro‐2‐oxoquinoline‐5‐aldehyde (2) was prepared from m‐aminobenzoic acid and 3‐ethoxyacryloyl chloride (4) in 19 % overall yield. Compound 2 underwent a photochemically induced [4+2]‐cycloaddition reaction with various dienophiles upon irradiation in toluene solution. The exo product 10 a was obtained with acrylonitrile (9 a) as the dienophile, whereas methyl acrylate (9 b) and dimethyl fumarate (9 c) furnished the endo products 11 b and 11 c (69–77 % yield). The reactions proceeded at −60 °C in the presence of the chiral complexing agent 1 (1.2 equiv) with excellent enantioselectivity (91–94 % ee). The enantiomeric excess increases in the course of the photocycloaddition as a result of the lower product association to 1. The intermediate (E)‐dienol 8 was spectroscopically detected at −196 °C in an EPA (diethyl ether/isopentane/ethanol) glass matrix. The association of the substrate 2 to the complexing agent 1 was studied by circular dichroism (CD) titration. The measured association constant (KA) was 589 M−1 at room temperature (25 °C) and normal pressure (0.1 MPa). An increase in pressure led to an increased association. At 400 MPa the measured value of KA was 703 M−1. Despite the stronger association the enantioselectivity of the reaction decreased with increasing pressure. At 25 °C the enantiomeric excess for the enantioselective reaction 2 + 9 a→10 a decreased from 68 % ee at 0.1 MPa to 58 % ee at 350 MPa. This surprising behavior is explained by different activation volumes for the diastereomeric transition states leading to 10 a and ent‐10 a.Keywords
This publication has 108 references indexed in Scilit:
- Supramolecular Control of the Template‐Induced Selective Photodimerization of 4‐Methyl‐7‐O‐hexylcoumarinChemistry – A European Journal, 2003
- Enantioselective [6π]-Photocyclization Reaction of an Acrylanilide Mediated by a Chiral Host. Interplay between Enantioselective Ring Closure and Enantioselective ProtonationThe Journal of Organic Chemistry, 2003
- Enantioselective Norrish–Yang Cyclization Reactions of N-(ω-Oxo- ω-phenylalkyl)-Substituted Imidazolidinones in Solution and in the Solid StateChemistry – A European Journal, 2002
- Intermolecular hydrogen binding of a chiral host and a prochiral imidazolidinone: enantioselective Norrish–Yang cyclisation in solutionChemical Communications, 2001
- Chiral Photochemistry within ZeolitesChemistry – A European Journal, 2000
- Pressure and Temperature Control of Product Chirality in Asymmetric Photochemistry. Enantiodifferentiating Photoisomerization of Cyclooctene Sensitized by Chiral BenzenepolycarboxylatesJournal of the American Chemical Society, 1998
- Absolute Asymmetric Synthesis from Achiral Molecules in the Chiral Crystalline EnvironmentChemistry – A European Journal, 1997
- Intramolecular Hydrogen Bond Effect in Meta-cycloaddition of Arene to OlefinSynthetic Communications, 1994
- Photochemistry of 4,4-dimethyl-1-mesityl-2-pentyn-1-oneJournal of the American Chemical Society, 1987
- Studies on positive inotropic agents. II. Synthesis of (4-substituted 1-piperazinylcarbonyl)-2-(1H)-quinolinone derivatives.CHEMICAL & PHARMACEUTICAL BULLETIN, 1986