5-(4-Ammonio-2-Oxopyrimidine-1(2H)-yl)-1,3-Oxathiolane-2-Carboxylate (lamivudine acid) Semihydrate: The Six-Fold Symmetry Created by Hydrogen Bond Network

Abstract

The title compound, C8H9N3O4S × 1/2(H2O), which is an impurity found in a drug lamivudine, crystallizes in the hexagonal space group P62 with a = 10.208(1) A and c = 18.073(2) A. Such a rare packing is constructed by the hierarchical network of hydrogen bonds, which connect the molecules into chains, then into pairs of chains, and neighboring chains, oriented at the angle of 60°, make the final packing mode. The molecule exists in the crystal as the zwitterion, with negative charged carboxylate and positive ammonio groups. The oxathiolane ring is close to an envelope conformation, and both pyrimidine and carboxylate substituent are in the equatorial positions. The hexagonal packing is constructed by the hierarchical network of hydrogen bonds, which connect the molecules into chains, then into pairs of chains, and neighboring chains, oriented at the angle of 60°, make the final packing mode.