Stereochemical Assingment of β -lactam Antibiotics and their Analogues by Electronic Circular Dichroism Spectroscopy

The present work examines the relationship between the structure and chiroptical properties of penicillins and cephalosporins and their oxa- and carbaanalogues utilizing the electronic circular dichroism (CD). For some of the model compounds the study is additionally supported by the X-ray diffraction analysis. The applicability of the helicity rule that correlates the molecular structure of compounds and their CD spectra in respect to all classes of β-lactam derivatives is tested. It is demonstrated that a variety of oxacephams, clavams, and their carbaanalogues conform to the rule. To validate the experimental data the CD spectra computed using time-dependent density functional theory (TDDFT) are compared to the experimental curves recorded for the representative β-lactam derivatives. The study reveals a high sensitivity of the CD spectra to the configuration at the bridgehead atom and a surprisingly high sensitivity to the molecular conformation and substitution of the five- or six-membered ring. The results demonstrate that such a combined treatment permits the determination of the absolute configuration with high confidence.