Crystal Structures of Diketopiperazines Containing \alpha-Aminoisobutyric Acid: Cyclo(Aib-Aib) and Cyclo(Aib-L-Ile)

The crystal and molecular structures of two \alpha-aminoisohutyric acid (Aib)-containing diketopiperazines, cyclo(Aib-Aib) 1 and cyclo(Aib- L-Ile) 2, are reported. Cyclo(Aib-Aib) crystallizes in the space group P1 with a = 5.649(3), b = 5.865(2), c = 8.363(1), \alpha = 69.89(6), \beta = 113.04(8), \gamma = 116.0(3), and Z = 1, while 2 occurs in the space group ${P2}_12_12_1$ with a = 6.177(1), b = 10.791(1), c = 16.676(1), and Z = 4. The structures of 1 and 2 have been refined to final R factors of 0.085 and 0.086, respectively. In both structures the diketopiperazine ring shows small but significant deviation from planarity. A very flat chair conformation is adopted by 1, in which the $C^{\alpha}$ atoms are displaced by 0.07 A on each side of the mean plane, passing through the other four atoms of the ring. Cyclo(Aib-Ile) favors a slight boat conformation, with Aib $C^{\alpha}$ and Ile $C^{\alpha}$ atoms displaced by 0.11 and 0.05 A, on the same side of the mean plane formed by the other ring atoms. Structural features in these two molecules are compared with other related diketopiperazines.