Non-covalent interactions in the crystallization of the enantiomers of 1,7-dioxaspiro[5.5]undecane (olive fly sex pheromone) by enantiospecific cyclodextrin hosts, hexakis(2,3,6-tri-O-methyl)-alpha-cyclodextrin and heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin

The enantiomers of racemic olive fly sex pheromone 1,7-dioxaspiro[5.5]undec ane (1) have been isolated by crystallization with enantiospecific cyclodex trin hosts: (S)-(1) crystallizes with heptakis(2,3,6-tri-O-methyl)-beta -cy clodextrin (TM beta CD) and (R)-(1) with hexakis(2,3,6-tri-O-methyl)-alpha -cyclodextrin (TM alpha CD). The crystal structure of TM beta CD/(S)-(1) fr om synchrotron radiation data at 100 K, determined for the first time, prov es that TM beta CD crystallizes with only the (S)-enantiomer from the racem ic mixture. Comparison with the 100 K structure of TM alpha CD/(R)-(1) rede termined with synchrotron data has provided insight into the interactions b etween each of the hosts with the corresponding enantiomeric guests. Owing to the high resolution of the data and the unusually high quality of the cr ystals, localization of the H atoms has been achieved, a rare accomplishmen t for cyclodextrin X-ray structures. This made possible, apart from the geo metry of the complexes, the detailed description of a five-membered-ring wa ter cluster with very well ordered hydrogen bonding. The enantiospecificity exhibited by the described systems reveals the subtle differences of the w eak intermolecular forces involved in the selective binding of the two opti cal antipodes by the two hosts. The binding geometry in the two complexes i s different, but it is effected in both by numerous host-guest C-H . . .O i nteractions, resulting from induced rt of the hosts toward each of the enan tiomeric guests. In TM alpha CD/(R)-(1) two of these H . . .O host-guest di stances, directed toward the acetal O atoms defining the chirality of the g uest, are much shorter than the rest and also shorter than all the H . . .O distances in TM beta CD/(S)-(1). Moreover, (R)-(1) interacts not only with the enclosing host, but with other hosts in the crystal lattice, in contra st to (S)-(1) in the TM beta CD/(S)-(1) complex which is isolated inside ch annels formed by the host molecules. The above differences are reflected in the much higher binding constant of TM alpha CD/ (R)-(1) compared with tha t of TM beta CD/(S)-(1) (similar to 6800 and similar to 935 M-1, respective ly), determined by NMR in aqueous solution, and the ability of TM alpha CD to selectively precipitate (R)-(1) from racemic (1) in much higher yield th an TM beta CD precipitates (S)-(1).