Vinyl ethers 10, 23, and 33 were attached to Wang resin through the p-oxyphenylsulfonyl linker. The [2+2] cycloadditions between chlorosulfonyl isocyanate and the polymerbound vinyl ethers 12, 24, and 34, followed by intramolecular alkylation of the β-lactam nitrogen atom, gave mixtures of the corresponding diastereomeric clavams 8/9 and 21/22 or the oxacephams 31/32, accompanied by the oxirane 7 or the
It is well known that the biological activity of clavams depends strongly on the absolute configuration at the ring junction carbon atom. Therefore, development of the efficient stereo-controlled synthetic methods for the new oxygen analogs of penams, and the structure-activity relationship studies call for a reliable determination of the absolute stereochemistry of newly synthesized compounds. Recently, we proposed an empirical helicity rule relating the configuration of the bridgehead carbon atom to the sign of the 240 nm band observed in the electronic circular dichroism (ECD) spectrum of clavams. In the present work, we investigate the validity of this structure-property relationship for several enantiomeric pairs of model compounds possessing an additional, interfering chromophore in the molecule. For this purpose a combination of the ECD spectroscopy and the time-dependent density functional theory (TD-DFT) is used. A comparison of the ECD spectra with the theoretical ones obtained by the TD-DFT calculations gives a reasonable interpretation of the Cotton effects observed in the 250-220 nm spectral range. Moreover, the calculations confirm validity of the helicity rule for systems studied here and demonstrate that ECD spectroscopy may be used as a highly sensitive probe of the three-dimensional molecular structure of clavams.
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