Helical tubuland hosts

MOLECULAR DETER14INANTS OF A NEW FAMILY OF HELICAL TUBULAND HOST DIOLS... 

ABSTRACT. The alicyclic diols (1-5) constitute the first members of a family of novel helical tubuland hosts crystallising in space group P3 21 but possessing quite different canal shapes and dimensions. Consideration of their structural data has revealed two distinct sub-classes of these materials. The molecular features necessary for a diol to crystallise with the helical tubuland structure are defined and discussed. 

Following this initial discovery we embarked on a program of systematic synthesis in order to demonstrate that additional materials of similar structure could be obtained. Recently we have described the syntheses and crystal structures 4 of further alicyclic diols (2-4) of this- type which also adopt the crystal space group P3] 21. Their structural characteristics have been analysed and reported in detail. The previously unreported diol (5) also belongs to this new family of helical tubuland host diols. 

Scudder - Molecular deteminants of a new family of helical tubuland host dlols Blake, A. J. - See Bell, M. H. et al. 

This host network, termed the helical tubuland structure type 7,8), is unique. The walls of the canals are lined only by aliphatic hydrocarbon, and the hydrogen bonded spines are insulated from the guest canals. Powder diffraction and IR measurements indicate that when 1 is crystallised from acetonitrile the same host crystal structure occurs, but devoid of guest. This has the unusually low calculated density, 1.02 g cm 3. 

Therefore both symmetrical configurations of the hydroxy functions in the host molecule allow the helical tubuland structure. The unsymmetrical epimer anti-2,, syn-7-dihydroxy-2,7-dimethyltricyclo[4.3.1.13-8]undecane (10), the hybrid of 2 and 8, does not possess a molecular twofold axis (or pseudo twofold axis) or the conformation of C—O bonds of Fig. 5, and would not be expected to fit into the helical tubuland structure framework. Its crystal structure is indeed different, with infinite zig-zag sequences of hydrogen bonds constructing a non-including lattice 14). 

In the following analysis of the helical tubuland structure type we distinguish (i) the host variables, and (ii) the lattice variables, and examine the correlations between them 8). For these purposes we represent the essence of the host molecule as in Scheme 3. 

In fact there is substantial structural variability among the five instances of the helical tubuland structure described above, in the form of variable placements of the diol hosts along the twofold axes, and variable a dimensions of the lattice, both of which have marked influence on the size and shape of the canals. Full analysis of these effects is provided elsewhere 8) and the major results only are reported here. As a consequence of steric repulsions between the bridge on the syn face and methyl substituents R on adjacent molecules presenting an anti face to the canal, as shown in Fig. 8, the molecules presenting the anti face are moved along the twofold axes, farther... 

Comment is required on the proliferative terminology for the geometry. The terms canal , channel , tube and tunnel have been used by various authors to describe host cavities extended in one dimension without restriction on cross-actional shape. We prefer the terms proposed by Weber and Josel namely helical tubuland ... 

Unlike the above cases, the relationship between structural and supramolecular properties can be far more cryptic and not immediately apparent. One example is the key role played by the (at first glance unimportant) methyl groups of the helical tubuland diol hosts 59-62. Another is the closely related host behavior of hydroquinone 49 and the diol 102 in the presence of small guests. Although these are aromatic and aliphatic compounds, respectively, both of these hosts form clathrates in a similar manner namely by trapping small guests between two interpenetrated sublattices in their respective crystal structures. ... 

Our program of synthesis has been planned to define the molecular features required in a host diol molecule for it to crystallise with the helical tubuland structure. The following molecular determinants have been found to be necessary. 

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