The diastereoselectivity of the Diels-Alder reaction of dienes bearing stereogenic allylic substituents.
Item
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Title
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The diastereoselectivity of the Diels-Alder reaction of dienes bearing stereogenic allylic substituents.
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Identifier
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AAI9000736
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identifier
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9000736
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Creator
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Tripathy, Rabindranath.
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Contributor
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Adviser: Richard W. Franck
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Date
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1989
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Language
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English
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Publisher
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City University of New York.
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Subject
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Chemistry, Organic
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Abstract
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The face selectivity of the Diels-Alder reaction of dienes having a stereogenic allylic carbon has been examined. For the acyclic dienes, where the allylic substituents have free rotation, the diastereoselectivity depends on the nature of the dienophile. Whereas N-phenylmaleimide and maleic anhydride afforded adducts from like topicity, tetracyano ethylene and 4-phenyl-1,2,4-triazolin-3,5-dione yielded products from unlike approach. The stereochemistries of the products have been established by NOE studies and in some cases by x-ray structure determination. Our results and those of other groups demonstrate that the dienophiles have a significant effect on the face selectivities observed. None of the recent theories are adequate to rationalize the experimental results. A new rationalization is put forward which successfully explains the observed stereodifferentiation and also serves in a predictive manner for synthetic purposes. The rationale favors two reactive diene rotamers in the transition state and the major stereodifferentiation is controlled by the relative rotamer populations and is also a function of the dienophiles.;The allylic dienes where the stereogenic center is conformationally locked showed a different trend in the face selectivity. In these cases the predominant products resulted from anti (unlike) attack irrespective of the nature of the dienophiles. Our results are also complementary to the results obtained by other workers. The stereochemistries of our products have been established by coupling constant correlations and in one case by an x-ray structure determination. The rationalization of the observed diastereoselectivity has been based solely on the steric effect. It has also been postulated that the A value, a parameter used to define the steric size of different groups should not be used in intermolecular reactions. A more appropriate parameter (n) has been used to define the steric size of different groups in Diels-Alder reactions between semicyclic dienes with different dienophiles.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
