title: Ring-expanded bicyclic β-lactams: a structure-chiroptical properties relationship investigation by experiment and calculations.
abstract: In the present work, the validity of the helicity rule relating the absolute configuration of the bridgehead carbon atom in bicyclic β-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 β-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 240-215 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 β-lactams with the nonplanar amide chromophore only.
authors: Institute of Organic Chemistry of the Polish Academy of Sciences, Kasprzaka 44, 01-224 Warsaw, Poland.
authors: Magdalena
authors: M
authors: Woźnica
authors: Aleksandra
authors: A
authors: Butkiewicz
authors: Aneta
authors: A
authors: Grzywacz
authors: Patrycja
authors: P
authors: Kowalska
authors: Marek
authors: M
authors: Masnyk
authors: Karol
authors: K
authors: Michalak
authors: Roman
authors: R
authors: Luboradzki
authors: Filipp
authors: F
authors: Furche
authors: Holger
authors: H
authors: Kruse
authors: Stefan
authors: S
authors: Grimme
authors: Jadwiga
authors: J
authors: Frelek
mesh_terms: D002942:Circular Dichroism; D004583:Electrons; D008968:Molecular Conformation; D056004:Molecular Dynamics Simulation; D011789:Quantum Theory; D013237:Stereoisomerism; D013696:Temperature; D047090:beta-Lactams / Q000737:chemistry*
chemical_list: D047090:beta-Lactams