Cocrystal Engineering Sorts Out Ladderanes

A cocrystallization strategy commonly used in protein chemistry has been redirected to help elucidate the structures of a pair of complex ladderane isomers, possibly the first application of the technique to the structural characterization of small organic molecules (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1104352108). Ladderanes are rod-shaped molecules that contain a core set of fused cyclobutane rings. They are used as building blocks in optoelectronics and recently were discovered to be a structural motif in lipid natural products. Leonard R. MacGillivray and coworkers of the University of Iowa had prepared a pyridyl-terminated ladderane, but they were unable to pin down the structures of the molecule’s cis-trans isomers by the standard approach of multidimensional NMR. Although the researchers managed to obtain the X-ray crystal structure of one isomer, which is achiral, the other isomer, which is chiral, refused to cooperate, forming only an amorphous material. But adding 3,5-dinitrobenzoic acid, which forms hydrogen bonds with the ladderane’s pyridyl groups (shown), led to single crystals for the X-ray analysis. “We expect our work will spur others to follow similar cocrystallization strategies in synthetic organic chemistry,” MacGillivray says.