Threefold helical assembly via hy-droxy hydrogen bonds: the 2:1 co-crystal of bi-cyclo-[3.3.0]octane-endo-3,endo-7-diol and bi-cyclo-[3.3.0]octane-endo-3,exo-7-diol.

Reduction of bi-cyclo-[3.3.0]octane-3,7-dione yields a mixture of the endo-3,endo-7-diol and endo-3, exo-7-diol (C8H14O2) isomers (5 and 6). These form (5)2·(6) co-crystals in the monoclinic P21/n space group (with Z = 6, Z' = 1.5) rather than undergoing separation by means of fractional recrystallization or column chromatography. The mol-ecule of 5 occupies a general position, whereas the mol-ecule of 6 is disordered over two orientations across a centre of symmetry with occupancies of 0.463 (2) and 0.037 (2). Individual diol hy-droxy groups associate around a pseudo-threefold screw axis by means of hydrogen bonding. The second hy-droxy group of each diol behaves in a similar manner, generating a three-dimensional hydrogen-bonded network structure. This hydrogen-bond connectivity is identical to that present in three known helical tubuland diol-hydro-quinone co-crystals, and the new crystal structure is even more similar to two homologous aliphatic diol co-crystals. © Chan et al. 2021.