Optimization of interactions in crystal packing revealed by crystal structures [ethyl 2-(formylamino)-3-thien-2-yl-2- (thien-2-ylmethyl)propanoate and ethyl 3-(5-bromothien-2-yl)- 2-[(5-bromothien-2-yl)methyl]-2-(formylamino)propanoate]

The title compounds, C15H14NO3S2 (I) and C15H15Br2NO3S2 (II), are derivatives of Aib (a-aminoisobutyric acid) with thiophene rings substituted at the Ca position. The Ca substitution causes the backbone to assume an extended conformation in the crystal structure. N–H and C–H donors share the thiophene ring p system for X–H· · ·p interactions. The packings of the molecules are stabilized by intermolecular N–H· · ·O, C–H· · ·O, C–H· · ·p and C–H· · ·Br hydrogen bonds. Br· · ·O interactions and a weak dihydrogen bond have also been observed in the crystal structure of II. The packing adopted by II has maximized the number of interactions that are possible. q 2004 Elsevier B.V. All rights reserved