Abstract

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A conformational analysis of the (3S,5R,6R)-6-acetylamidopenicillanic acid was performed using AM1, omega, and PM3 conformers. Their geometries were optimized at the ab initio HF/STO-3G level. The AM1 semiempirical MO method yields only eight conformers, the PM3 and Omega methods give 47 conformers, while the ab initio HF/STO-3G method leads to 28 conformers. For comparing the geometry of these conformers three geometrical properties were considered: the pseudo-chirality at N atom of the exocyclic amidic group, the anti/syn conformation of the O and H atoms from the exocyclic amidic group, and the three puckering classes of the thiazolidinic ring. The HF/STO-3G geometries do not differ significantly from the experimental ones or from those obtained by other authors who used basis sets of higher performance. However, unlike the high performant basis sets which tend to equalize the S1..C9 and S1..C10 distances, the HF/STO-3G basis set is able to distinguish between these bond lengths, namely the S1..C9 distance is larger than S1..C10. From this conformational analysis it resulted that the HF/STO-3G conformers belonging to the puckering classes (a) and (b) have the most favourable geometrical properties needed for the conversion of penicillins to cephalosporins.

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