Ring-Expanded Bicyclic 尾-Lactams: A Structure鈭扖hiroptical Properties Relationship Investigation by Experiment and Calculations

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• 作者：Magdalena Woz虂nica ; Aleksandra Butkiewicz ; Aneta Grzywacz ; Patrycja Kowalska ; Marek Masnyk ; Karol Michalak ; Roman Luboradzki ; Filipp Furche ; Holger Kruse ; Stefan Grimme ; Jadwiga Frelek
• 刊名：Journal of Organic Chemistry
• 出版年：2011
• 出版时间：May 6, 2011
• 年：2011
• 卷：76
• 期：9
• 页码：3306-3319
• 全文大小：1376K
• 年卷期：v.76,no.9(May 6, 2011)
• ISSN：1520-6904

文摘

In the present work, the validity of the helicity rule relating the absolute configuration of the bridgehead carbon atom in bicyclic 尾-lactams to the sign of the 220 nm band observed in their electronic circular dichroism (ECD) spectra is examined for ring-expanded cephalosporin analogues. To this end, a series of model compounds with a seven-membered ring condensed with the 尾-lactam unit was synthesized. A key step of their synthesis was either the ring-closing metathesis (RCM) or the free radical cyclization leading to the seven-membered ring with an S, O, or C atom at the 6 position in the bicyclic skeleton. To investigate the scope and limitations of the simple, empirically established helicity rule, a combination of ECD spectroscopy, variable-temperature ECD measurements, X-ray analysis, and time-dependent density functional theory (TD-DFT) calculations was used. A comparison of the experimental ECD spectra with the spectra simulated by TD-DFT calculations gives a reasonable interpretation of the Cotton effects observed in the 240鈭?15 nm spectral range. The results suggest that the helicity rule does not apply to the investigated compounds because of the planarity of their amide chromophore. Thus, these compounds do not constitute an exception to the rule that was established for bi- and polycyclic 尾-lactams with the nonplanar amide chromophore only.