Structural control, Inclusion

Axial symmetry in a chain is a major factor in determining its ability to form crystallites, and one method of altering the crystalline content is to incorporate some structural irregularity in the chain. The controlled inclusion of linear symmetrical... 

The diol derivatives of bicyclo[3.3.1]nonanes (Figure 9) are such systems, and the unique molecular shape of this building block has been used to form self-assembled supramolecular structures and inclusion complexes with various guest molecules. Some derivatives of this ring skeleton, the so-called tubuland diols, give controllable crystal structures with a variety of inclusion guests. 

On the other hand, the crystallization process of diolefin compounds often plays a significant role in determining their topochemical behaviour, by changing their crystal structure or by forming solvent inclusion complexes. Furthermore, topochemical photoreactions of crystals with )8-type packing are accompanied by thermal processes under moderate control by the reacting crystal lattice (see p. 140). These factors seriously complicate the whole reaction scheme. 

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. 

A theoretical study at a HF/3-21G level of stationary structures in view of modeling the kinetic and thermodynamic controls by solvent effects was carried out by Andres and coworkers [294], The reaction mechanism for the addition of azide anion to methyl 2,3-dideaoxy-2,3-epimino-oeL-eiythrofuranoside, methyl 2,3-anhydro-a-L-ciythrofuranoside and methyl 2,3-anhydro-P-L-eiythrofuranoside were investigated. The reaction mechanism presents alternative pathways (with two saddle points of index 1) which act in a kinetically competitive way. The results indicate that the inclusion of solvent effects changes the order of stability of products and saddle points. From the structural point of view, the solvent affects the energy of the saddles but not their geometric parameters. Other stationary points geometries are also stable. 

The change in the conformation of the control enzyme brought about by covalent modification alters the activity of the control enzyme and so regulates substrate flux through that step. This fact underlines the importance of the three-dimensional structure of an enzyme. The inclusion of phosphates may bring about quite a small architectural change to the protein structure but it is sufficient to affect substrate binding and therefore enzyme activity. 

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