Self-resolution

A particular point of interest included in these hehcal complexes concerns the chirality. The heUcates obtained from the achiral strands are a racemic mixture of left- and right-handed double heUces (Fig. 34) (202). This special mode of recognition where homochiral supramolecular entities, as a consequence of homochiral self-recognition, result from racemic components is known as optical self-resolution (203). It appears in certain cases from racemic solutions or melts (spontaneous resolution) and is often quoted as one of the possible sources of optical resolution in the biological world. On the other hand, the more commonly found process of heterochiral self-recognition gives rise to a racemic supramolecular assembly of enantio pairs (204). 

Self-assembly may occur with self-recognition, mixtures of components yielding defined superstructures without interference or crossover (see Section 9.5). A special case is self-resolution which results in the formation of homochiral supramolecular entities from racemic components (see Sections 9.4.2, 9.4.4 and 9.7). 

A related phenomenon can also occur when the crystal lattice packing is chiral. This intrinsic handedness can result in formation of a 1 1 mixture of enantiomeric crystals. In this case, although there has been self-resolution into (+)- and (—)-crystals, both molecular enantiomers remain unseparated in each crystal. The fundamental distinction is that a conglomerate single crystal contains only one molecular enantiomer and therefore would be optically active in solution, while, for the latter, a single crystal contains both molecular enantiomers and its solution would be optically inactive. 

Self-resolution of Enantiomeric Molecules 3.2.1 Enantiomeric self-resolution on crystallisation... 

The helical tubuland (HT) diols are a group of alicyclic dialcohols that show characteristic crystallisation behaviour [32,35], When a racemic sample is crystallised, then enantiomeric self-resolution occurs, yielding a conglomerate of (+)- and... 

An even more startling case of self-resolution was observed when a mixture of diols 12 and 13 was crystallised from toluene solution, and the helical tubulate... 

Enantiomeric self-resolution through guest complexation... 

Enantiomeric self-resolution through complexation with a hydrogen-bonding guest solvent has interesting potential as a means of separation. Unfortunately, little prediction of new examples seems possible with our current level of understanding. 

Self-resolution of Enantiomeric Crystals Containing Both Enantiomeric Molecules... 

As explained earlier (Section 3), a 1 1 mixture of (+)- and (—)-crystals may also be obtained where self-resolution of the molecular enantiomers has not taken place. This occurs because the crystallisation process generates a lattice that is itself handed. Two examples of this phenomenon are presented here. 

Crystallisation of racemic 15 from either chloroform or 1,1,2,2-tetrachloroe-thane yields achiral inclusion compounds that contain both host enantiomers. In contrast, when 15 is crystallised from tetrahydrofuran (THF), a mixture of (+)- and (—)- crystals is produced in space group /J2 2 2 [40], Once again, chirality arises from the lattice structure rather than from self-resolution of the host enantiomers. 

Angelin M, Vongvilai P, Fischer A, Ramstrom O (2010) Crystallization driven asymmetric synthesis of pyridine P-nitroalcohols via discovery-oriented self-resolution of a dynamic system. Eur J Org Chem 6315-6318... 

Surprisingly little work has been carried out on the resolution of homochiral helicates into the two enantiomers. Self-resolution upon crystallization has been observed for two homonuclear triple helicates [37,38], but there seem to be only two well-authenticated cases of enantiomeric resolution, both using antimonyl tartrate the complex [ 02(9)3] " , a dinuclear triple helix [39], and a trinuclear double helical complex of iron(ll) [Fe3(19)2] " with a tm-terpyridyl ligand [40]. The circular dichroism spectrum of [ 02(9)3] " is shown in Figure 13. 

Sonography is a reliable imaging technique to differentiate between simple and complex ovarian cysts. Unrelated to their sonographic appearance, ovarian cysts show a high potential for self-resolution. Conservative treatment with sonographic monitoring seems appropriate for most cases. 

---