5-Dethia-5-oxacephams: toward correlation of absolute configuration and chiroptical properties |
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Authors: | Łysek Robert Borsuk Katarzyna Chmielewski Marek Kałuza Zbigniew Urbańczyk-Lipkowska Zofia Klimek Agata Frelek Jadwiga |
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Affiliation: | Institute of Organic Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland. |
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Abstract: | The relationship between chiroptical properties of differently substituted 5-dethia-5-oxacephams and their respective molecular structures was investigated. The amide chromophore of the beta-lactam unit in these compounds was found to be nonplanar with a shallow pyramidal configuration at the nitrogen atom. Due to the nonplanarity, the beta-lactam system becomes inherently dissymmetric, which is supported by a high magnitude of the n --> pi* CD band. It was also found that the helicity of the lactam moiety in investigated oxacephams is controlled by the absolute configuration at the C(6) carbon atom. On this basis, a helicity rule correlating a positive (negative) sign of the n right arrow pi Cotton effect with a negative (positive) O [double bond] C [bond] N [bond] C(6) torsional angle for policyclic beta-lactam derivatives possessing a nonplanar amide chromophore was formulated. |
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