Crystal structures of anhydrous and hydrated ceftibuten

Abstract

Ceftibuten, C15H14N4O6S2, with the systematic name (6R,7R)-7-{[(Z)-2-(2-amino-1,3-thiazol-4-yl)-4-carboxybut-2-enoyl]amino}-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid, is a third generation, orally administered cephalosporin antibiotic with broad antimicrobial activity and stability against extended spectrum β-lactamases. Ceftibuten can exist in various hydration states and to better understand the location of the water molecules of crystallization and their effect on the structure, the crystal structures of anhydrous (I) and hydrated (II) ceftibuten were determined and both occur as zwitterions with proton transfer from the carboxylate group adjacent to the β-lactam ring to the N atom of the thiazole ring. The β-lactam ring in (I) is almost planar but the equivalent grouping in (II) is slightly buckled. In the extended structure of (I), O—H...O and N—H...O hydrogen bonds link the molecules into a three-dimensional network. In (II), O—H...Oc, N—H...Oc, O—H...Ow, N—H...Ow and Ow—H...Ow (c = ceftibuten, w = water) hydrogen bonds link the components into a three-dimensional network. A large void space is present within the anhydrous crystal structure that can accommodate between two and three molecules of water.