Fine-control of helical tubuland inclusion properties through the pendant group approach

The helical tubulands are a family of alicyclic diols which crystallise in chiral space group P3(1)21 and whose lattices can contain guest molecules e nclosed within parallel tubes. New examples can be designed following a ser ies of rules based on molecular structure and symmetry. In order to probe t he latter requirement, and also to increase host-guest interactions, the be haviour of 2,5,8-trimethyltri cyclo[5.3.1.1(3.9)]dodecane-syn-2,syn-8-diol 4 was investigated using X-ray methods. Although molecules of 4 have no C-2 symmetry they still form the helical tubuland lattice on crystallisation. In the pure apohost 4 their symmetry-breaking C5 methyl groups protrude int o the tube volumes where they randomly adopt one of two equivalent orientat ions. Although the pendant methyl groups reduce the volume available for gu est inclusion, they create irregular tube wall surfaces and interact more e ffectively with guest molecules. The helical tubulate inclusion compounds o f 4 with diisopropyl ketone, benzene, toluene, and o-xylene show significan t local methyl group ordering around a given guest molecule which performs a crucial templating role in formation of the product. In all cases, howeve r, crystallographic C-2 symmetry results in the solid through overall disor der of these diol orientations. Hence this, rather than strict molecular sy mmetry, is the true symmetry requirement of the P3(1)21 helical tubuland la ttice. (C) 2000 Elsevier Science Ltd. All rights reserved.