Helical compound resolution

Finally, it is noted that suitable enantiomers of helicenes have been applied as selector for non-helical compounds. The disodium salt of P-( +)-7,10-dicarboxy hexahelicene coated on silicagel was successfully used in resolving the N-(2,4-dinitro-phenyl)-a-amino acid esters. Good resolutions were found for alanine, isoleucine, valine, phenylalanine and phenylglycine 94). 

In 1979 the bieyclic diol exo-2,ejco-6-dihydroxy-2,6-dimethylbicyclo[3.3.1]nonane (i) was prepared and observed to co-crystallise with various solvents, including ethyl acetate, chloroform, toluene, dioxane, and acetone. A crystal structure determination of the ethyl acetate compound revealed the occurrence of a helical canal host structure, containing ethyl acetate as guest (with 3 1 diol ethyl acetate stoichiometry), and that spontaneous resolution had occurred on crystallisation of the multimolecular inclusion compound 6>. 

This compound ((3R,3 i )-(P,P)-trans-1,1, 2,2, 3,3, 4,4 -octahydro-3,3 -dimcthyl-4,4 -biphenanthrylidene, (P,P)-trans-35) was prepared by McMurry coupling of (R)-3-methyl-4-keto-l,2,3,4-tetrahydrophenanthrene, which in turn was obtained through resolution or asymmetric alkylation methods.1671 X-ray analysis showed that (P,P)-trans-35 adopts a double helical structure, with the two methyl substituents in a pseudo-axial orientation. Calculations confirmed this preferred conformation for (P.P) -trans-35, and showed that (M,M)-trans-35, with both methyl groups in pseudo-equatorial orientations, was 8.6 kcal mol-1 less stable (Scheme 22). For the cis isomer, the same features were observed, with (M,M)-cis-35 (diequatorial Me-substitu-ents) less stable than (P,P)-cis-35 by 11.9 kcal mol-1. 

Synthetic polymers can be prepared to contain chirality as is the case for cellulose and other natural polymers. Chirality can be introduced into the monomer before polymerization to yield the chiral polymer. Alternatively polymerization of an achiral monomer in the presence of some chiral catalyst yields the chiral polymer. Polymethacrylates exhibiting chirality due to single-handed helicity have been prepared via polymerization in the presence of a chiral catalyst. These materials are used in liquid chromatography primarily under low-pressure conditions and have shown good resolution for compounds capable of hydrogen-bond formation. 

For the class of compounds discussed here, the self-assembly process always takes place in solution, where the components have sufficient mobility. For the characterization of the product the method of choice is X-ray crystallography, which requires the isolation of a crystalline solid. However, a total reliance on crystal structure determination poses several problems. The structural information available from single crystal diffraction is very complete, but it is quite often impossible to grow suitable crystals, and even when it is possible, disordered solvent and counter ions can give considerable problems in the resolution and refinement of the structure. A more fundamental question is whether the crystallization process, itself a form of self-assembly, has not resulted in a structural change. As an example we may quote the complex [Cu2(mimpy)2] which exists as isolated double helical units in solution, but which crystallises in columns with strong stacking interactions between individual units [5]. 

To make a knot is a task easily accomplished by a child playing with a rope, or even by a careless adult with shoelaces. For a chemist playing with molecules this is an intrinsically difficult problem and so far only a few of these topologically interesting compounds have been synthesized. A strategy that finally led to success is based on the self-assembly of double-helical copper phenanthroline complexes. The helix represents the core structure from which the trefoil knot 30 in Fig. 15 is obtained in a final cyclization step. It is important to note that a trefoil knot is chiral. The resolution of the two enantiomers was recently accomplished by fractional crystallization of the diastereoisomers obtained with a chiral counterion. 

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