Crystallisation of C2-symmetric endo,endo-bicyclo[3.3.1]nonane-2-6-diols: supramolecular synthons and concomitant degrees of enantiomer separation

Abstract

Each of the three racemic C₂-symmetric endo,endo-bicyclo[3.3.1]nonane-2,6-diols 6–8 shows a dominant, but different, mode of crystallisation across a wide range of solvents. These outcomes depend on the type of supramolecular synthon employed and the resulting degree of enantiomer separation that takes place. Diol 6 utilises the centrosymmetric hydrogen bonded (O–H)₆ cycle and forms racemic crystals containing an intimate mix of the two enantiomers. In contrast, diol 7 uses (O–H)₄ cycles with concomitant formation of homochiral layers, but racemic crystals still result since adjacent layers have opposite handedness. Diol 8 forms a hydrogen bonded network by using cross-linked (O–H)₈ repeat helices and self-resolution yields a mixture of pure (+)- and pure (−)-crystals (a conglomerate). Since conglomerates may be separated into their pure enantiomers, the latter discovery offers a new preparative approach for obtaining chirally pure bicyclo[3.3.1]nonane compounds of value in organic synthesis.