The Design of “Awkward” Molecules Expected to Yield Multiple Crystal Forms

The very simple alicyclic compounds 3,7-dimethylbicyclo[3.3.0]octane-endo-3,endo-7-diol 3 and 2,6-dimethylbicyclo[3.3.0]octane-endo-2,endo-6-diol 5 were selected for study in the knowledge that these are awkwardly shaped molecules that are likely to pack in the solid state with some difficulty. The design philosophy behind the choice of these particular test molecules is explained. We expected that these isomeric diols would probably yield more than one crystal form if crystallized from a range of different solvents. Dialcohol 3 was found to give three (benzene clathrate, hemihydrate, and hydrate), and dialcohol 5 to give two (racemic and kryptoracemate/false conglomerate), crystal forms. The X-ray structures of these five very different crystalline solids (formed by two very similar model compounds) are described and contrasted. Our findings demonstrate that awkward molecules can be identified and indicate that the formation of alternative crystal forms is likely to be a comparatively frequent occurrence under ambient conditions.