Threefold helical assembly via hy­droxy hydrogen bonds: the 2:1 co-crystal of bi­cyclo­[3.3.0]octane-endo-3,endo-7-diol and bi­cyclo­[3.3.0]octane-endo-3,exo-7-diol

Reduction of bi­cyclo­[3.3.0]octane-3,7-dione yields a mixture of the endo-3,endo-7-diol and endo-3, exo-7-diol (C(8)H(14)O(2)) isomers (5 and 6). These form (5)(2)·(6) co-crystals in the monoclinic P2(1)/n space group (with Z = 6, Z′ = 1.5) rather than undergoing separation by means of fractional recrystallization or column chromatography. The mol­ecule of 5 occupies a general position, whereas the mol­ecule of 6 is disordered over two orientations across a centre of symmetry with occupancies of 0.463 (2) and 0.037 (2). Individual diol hy­droxy groups associate around a pseudo-threefold screw axis by means of hydrogen bonding. The second hy­droxy 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–hydro­quinone co-crystals, and the new crystal structure is even more similar to two homologous aliphatic diol co-crystals.