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Volumn 129, Issue 2, 2007, Pages 395-405

The role of achiral pyrazolidinone templates in enantioselective Diels-Alder reactions: Scope, limitations, and conformational insights

Author keywords

[No Author keywords available]

Indexed keywords

ACHIRAL PYRAZOLIDINONE; DIENOPHILES; LIGANDS; ROOM TEMPERATURE;

EID: 33846253679     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja066425o     Document Type: Article
Times cited : (69)

References (113)
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    • For the use of pyrazolidinone templates in Diels-Alder reactions of dihydropyridines, see: b
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    • For selected recent examples of enantioselective Diels-Alder reactions, see: c
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    • For detailed reaction conditions and substrate preparation procedures, see Supporting Information
    • For detailed reaction conditions and substrate preparation procedures, see Supporting Information.
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    • Analogous experiments corresponding to entries 2, 3, and 5 (substrates 7a, 7b, and 7d, respectively) in Table 1 employing Cu(OTf)2/(S,S)-2,2′-isopropylidene-bis(4-phenyl-2-oxazoline) as the chiral Lewis acid were also conducted. Although a similar trend of increasing enantioselectivity with increasing size of the fluxional group was observed, the enantioselectivities were significantly lower in each case. The following enantioselectivities and endo:exo ratios were obtained with this ligand: (1) for substrate 7a, 28% ee, 88:12 endo:exo; (2) for substrate 7b, 38% ee, 91:09 endo:exo; and (3) for substrate 7d, 46% ee, 86:14 endo:exo. The sense of absolute stereochemistry of the cycloadducts was identical to that obtained with ligands 9-13
    • Analogous experiments corresponding to entries 2, 3, and 5 (substrates 7a, 7b, and 7d, respectively) in Table 1 employing Cu(OTf)2/(S,S)-2,2′-isopropylidene-bis(4-phenyl-2-oxazoline) as the chiral Lewis acid were also conducted. Although a similar trend of increasing enantioselectivity with increasing size of the fluxional group was observed, the enantioselectivities were significantly lower in each case. The following enantioselectivities and endo:exo ratios were obtained with this ligand: (1) for substrate 7a, 28% ee, 88:12 endo:exo; (2) for substrate 7b, 38% ee, 91:09 endo:exo; and (3) for substrate 7d, 46% ee, 86:14 endo:exo. The sense of absolute stereochemistry of the cycloadducts was identical to that obtained with ligands 9-13.
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    • For a representative Pd-N bond length in a Pd(II)/bisoxazoline complex, see reference 27(1). For a representative PdlO bond length, see: (b) Bugarcic, Z. D.; Nandibewoor, S. T.; Hamza, M. S. A.; Heinemann, F.; van Eldik, R. Dalton Trans. 2006, 2984.
    • For a representative Pd-N bond length in a Pd(II)/bisoxazoline complex, see reference 27(1). For a representative PdlO bond length, see: (b) Bugarcic, Z. D.; Nandibewoor, S. T.; Hamza, M. S. A.; Heinemann, F.; van Eldik, R. Dalton Trans. 2006, 2984.
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    • Experimental details, analysis, and spectra for nOe experiments are provided in the Supporting Information
    • Experimental details, analysis, and spectra for nOe experiments are provided in the Supporting Information.
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    • 2, bisoxazoline 10. and pyrazolidinone substrate 7d showed no line broadening with lower temperature (from room temperature to -35 °C), suggesting nitrogen inversion is sufficiently rapid that it is not observed on the NMR time scale.
    • 2, bisoxazoline 10. and pyrazolidinone substrate 7d showed no line broadening with lower temperature (from room temperature to -35 °C), suggesting nitrogen inversion is sufficiently rapid that it is not observed on the NMR time scale.


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