Host stereoselective

The theory and development of a solvent-extraction scheme for polynuclear aromatic hydrocarbons (PAHs) is described. The use of y-cyclodextrin (CDx) as an aqueous phase modifier makes this scheme unique since it allows for the extraction of PAHs from ether to the aqueous phase. Generally, the extraction of PAHS into water is not feasible due to the low solubility of these compounds in aqueous media. Water-soluble cyclodextrins, which act as hosts in the formation of inclusion complexes, promote this type of extraction by partitioning PAHs into the aqueous phase through the formation of complexes. The stereoselective nature of CDx inclusion-complex formation enhances the separation of different sized PAH molecules present in a mixture. For example, perylene is extracted into the aqueous phase from an organic phase anthracene-perylene mixture in the presence of CDx modifier. Extraction results for a variety of PAHs are presented, and the potential of this method for separation of more complex mixtures is discussed. 

A regio- and stereoselective Beckmann rearrangement utilized diastereose-lective host guest interactions of the inclusion complexes 225 and 228 in a solid state reaction. Initially, a 1 1 mixture of the chiral host 223 and the racemic oximes 224 and 227, respectively, was treated with ultra sound in the solid state to induce the optical resolution. Then H2SO4 was added to start the Beckmann rearrangement, the corresponding c-caprolactams 226 and 229 were isolated in 68 % and 64 % yields and ee of about 80 % and 69 % (determined by HPLC analysis on chiracel OC) (Scheme 43) [46]. 

It is not easy to control the steric course of photoreactions in solution. Since molelcules are ordered regularly in a crystal, it is rather easy to control the reaction by carrying out the photoreaction in a crystal. However, molecules are not always arranged at an appropriate position for efficient and stereoselective reaction in their crystals. In these cases inclusion chemistry is a useful technique, as it can be employed to position molecules appropriately in the host-guest structure. Chiral host compounds are especially useful in placing prochiral and achiral molecules in suitable positions to yield the desired product upon photoirradiation. Some controls of the steric course of intramolecular and intermolelcular photoreactions in inclusion complexes with a host compound are described. 

The synthesis of the macrocycles 43 (Scheme 9) is an example of repetitive, highly stereoselective Diels-Alder reaction between bis-dienes 41 and bis-dienophiles 42, containing all oxo or methano bridges syn to one another. The consecutive inter- and intramolecular Diels-Alder reactions only succeed at high pressure. Obviously, both reactions are accelerated by pressure. The macrocycles are of interest in supramolecular chemistry (host-guest chemistry) because of their well-defined cavities with different sizes depending on the arene spacer-units. 

The most extensive study in the field of host-guest reactions in clathrates has been that of Lahav, Leiserowitz, and co-workers (56,241,243) on the choleic acids. The results of these combined chemical and crystallographic investigations are of possible importance for stereoselective steroid functionalization. In these studies potentially reactive guests were activated thermally or photochemically to produce species that attacked the walls of the channel at specific sites determined by the proximity, orientation, and reactivity of the host molecules at the wall relative to the activated guest species. 

Besides, information on intermolecular interactions has been derived in these studies from complexation-induced shifts (CIS). The chemical shift is an indicator for the shielding of a nucleus and thus for the electronic state of a specific proton. Since the electronic environment may change on complexation, CIS can be used to monitor where host-guest contacts may take place. If these interactions occur stereoselectively, the CIS will be different for the two guest enantiomers (AS distinct from 0) giving possibly some insight into the chiral recognition mechanism. 

The two-steps synthesis of thiophosphorylated cavitands is by far the best method to control the stereoselectivity of the resultant products. As for the P=0 partners, it is important to obtain the all-inward oriented P=S donating groups in high yields to benefit from cooperative effects of the P=S donor groups and the aromatic cavity in the formation of host-guest complexes. 

Mo, W) decreases with the increase in basic strength of the framework oxygen of the host zeolite [32]. The M(CO)3(Oz)3 (Oz = zeolite framework oxygen) species generated by the thermal treatment are active in the stereoselective hydrogenation of butadiene to cis-2-butene [33, 34]. 

The gas-solid addition of HCl or HBr to simple alkylated oxiranes requires their inclusion. (-)-(M)-Tri-o-thymotide (150) (P3i21) selectively enclathrates the oxiranes 151 or 153 with 13-14% ee for (-i-)-(3P)-151 or 51-5Wo ee for (-1-)-(2P,3P)-153. Isolated molecules in the chiral cages of 150 react stereoselectively with HCl or HBr gas to give the products 152 and 154, respectively, in almost stereopure form according to the optical rotations after quantitative reaction under the influence of the chiral environment [78] (Scheme 17). The host lattice is preserved during the reactions, but destroyed during liberation of the product molecules. 

Chemical reactions in the sohd state have intrinsic features different from those for reactions performed in solution or in the gaseous state. For example, sohd-state organic reactions often provide a high regio- or stereoselectivity because the reactions and the structiue of a product are determined by the crystal structure of the reactant, i.e., the reaction proceeds under crystaUine lattice control [1-8]. When the reactant molecules are themselves crystalhne (molecular crystals) or are included in host crystals (inclusion compounds), the rate and selectivity of the reaction are different from those obtained in an isotropic reaction medium. 

Stereoselective catalysis using biocatalysts (e.g. enzymes) and also of rationally designed small chiral molecules, deals essentially with the same principle the spatial and selective docking of guest molecules to a chiral host molecule to form complementary interactions to form reversible transient molecule associates (see the specific sections in this volume). The enantiomeric excess of a certain reaction and hence the result will be determined by the degree of chiral discrimination. Along the same theoretical lines the concepts of protein (enzyme, antibody, etc.) mimicks via imprinted" synthetic polymers should be mentioned and will be discussed further. 

Reversing stereoselectivity of a Diels-AIder reaction by preorgamzation of reactants within a porphyrinic host... 

Brnmaghim, J.L., Michels, M. and Raymond, K.N. (2004) Hydrophobic chemistry in aqueous solution Stabilization and stereoselective encapsulation of phosphonium guests in a supramolecular host. Eur. J. Org. Chem., 4552-4559. 

Fig. 4.70 Dendrocleft (according to Diederich et al.) for stereoselective host/guest interaction depending upon the size of the dendrimer scaffold surrounding the chiral core (marked with a red asterisk)...

---