The complex packing pattern of cis-1,2-dimethylcyclobutane-1,2-diol

with ®ve molecules in the asymmetric unit. Of these, two molecules are the building blocks of columns with a complex hydrogen-bonding pattern in their hydrophilic core. The walls of the columns are formed by the lipophilic parts of the molecules. The remaining three molecules of the asymmetric unit build columns with a less complex hydrogen-bonding system. In terms of co-operativity, the most signi®cant feature is the formation of homodromic rings of six hydroxy functions. Comment On crystallization, the polar (hydrophilic) and apolar (lipo-philic) parts of amphiphilic molecules usually segregate into polar and apolar building blocks of the crystal structure, respectively. If the polar and apolar parts occupy volume fractions of different sizes, only the larger part normally establishes intermolecular contacts in all three dimensions, whereas the smaller part may be restricted to intermolecular contacts in zero (pairs of molecules, clusters) or one dimension (chains, columns). The diols are a particularly widespread class of compounds which illustrate this principle since, owing to the variety of residues that may be attached to the diol function, the size ratios between the polar and apolar parts may span an extended range. Moreover, the diols have attracted our interest as they are the parent acids of potentially chelating ligands. Examples of structurally characterized diols which are representative examples in terms of the size ratio between the polar and apolar molecular parts include 1,2-dicyclopentyl-ethane-1,2-diol (Betz et al., 2007) and bi-1,10-cycloheptyl-1,10-diol (Betz & KluÈ fers, 2007), in both of which the diol functions are assembled in hydrogen-bonded chains, and cis- and rac-trans-cyclohexane-1,2-diol (SillanpaÈaÈ et al., 1984) which exhibit hydrogen-bonded layers. Finite hydrogen-bonding patterns have been found with pinacol, for example, namely eight-membered homodromic rings in tetramers of the dio