Adamantane-constrained novel cyclodepsipeptides:: Crystal structure and self-assembling properties of cyclo(Adm-Ser)2 and cyclo(Adm-Ser-Xaa)2, Xaa=Val/Ser

Crystallographic studies on three members of a novel class of cyclodepsipeptides, containing adamantane units in the cyclic framework, are described. While the 18-membered cyclo(Adm-Ser)(2) (1) shows a simple structure with no internal hydrogen bonds, the higher members, cyclo(Adm-Ser-Val)(2) (2) and cyclo(Adm-Ser-Ser)(2), (3), exhibit interesting secondary structural features in the solid state. The two boat-shaped conformers of 2 assemble in a head-to-head fashion, giving rise to dimeric structures with water-filled channels, and show ion-transport properties. The molecule 3 revealed a unique intramolecular antiparallel beta-sheet formed by complementary hydrogen bonding between the backbone NH groups and the side chain ester carbonyls. The self-assembly patterns in the solid state are a hydrogen-bonded chain in 1 and infinite ribbons in 2 and 3.