Cyclodextrin, crystalline inclusion compounds

Several prominent types of host molecule, such as the steroidal bile acids and the cyclodextrins, are chiral natural products that are available as pure enantiomers. Chemical modification of these parent compounds provides an easy route to the preparation of large numbers of further homochiral substances. Since all these materials are present as one pure enantiomer, it automatically follows that their crystalline inclusion compounds must have chiral lattice structures. It is not currently possible to investigate racemic versions of these compounds, but the examples discussed previously in this chapter indicate that very different behaviour could result. 

The cyclodextrins (cycloamyloses) are torus-shaped molecules that can form crystalline inclusion compounds, recently attracting much attention as enzyme-site models. Their history has been seen in three phases. From 1891 to 1935 they were known as natural products, but with no recognition of their exact chemical structure. This recognition emerged in the second period, to about 1970, when most of their characteristics were also elucidated. The period from 1970 to the present has seen considerable research into their industrial use and production.239 Their inclusion compounds or complexes have found employment in such diverse fields as explosives, insecticides, pharmaceutical products, rust-prevention agents, and even baking powder. 

Fig. 7. Schemes of crystalline cyclodextrin inclusion compounds (a) channel type (b) cage herringbone type (c) cage brick type (58).

Channel Inclusion Compounds, p. 223 Chiral Guest Recognition, p. 236 Clathrate Hydrates, p. 274 Crown Ethers, p. 326 Crystalline Microporous Silicas, p. 380 Cyclodextrins, p. 398 Hofmann-Type Clathrates, p. 645 Hydrogen Bonding, p. 658 Hydroquinone, p. 679... 

The formation and properties of inclusion compounds of cyclodextrin with double crystalline PLLA-b-PCL diblock copolymers has been extensively discussed in a series of contributions by Tonelli et al. [64-67], where they highlight the changes that can be introduced in the structure and properties of the copolymers when they form these peculiar inclusion complexes. 

ABSTRACT 3-Cyclodextrin was found to form a crystalline inclusion complex with coumarin. The solid-state photoreaction of the inclusion compound was studied at 25-27 C, and compared with those of coumarin and of a mixture of coumarin with the cyclodextrin. Under irradiation at a wavelength longer than 300 nm, coumarin with or without 3-cyclodextrin and in the inclusion complex converted to a photodimer, cis-head-to-head dimer in the solid state. The conversion rate of coumarin was higher than that in the mixture, and the latter was higher than that in the complex. These indicate that 3-cyclodextrin retards the photodimerization of coumarin, but does not affect the course of the reaction in the solid state. 

R 500 C.C. Rusa, M. Wei, X. Shuai, T.A. Bullions, X. Wang, M. Rusa, T. Uyar and A.E. Tonelli, Molecular Mixing of Incompatible Polymers through Formation of and Coalescence from Their Common Crystalline Cyclodextrin Inclusion Compounds , J.Polym.Sci.,B, Pol.Phys., 2004,42,4207... 

Cyclodextrins form crystalline inclusion complexes with many organic compounds, including some gases, which are bound within the molecule. Formation of these complexes is called encapsulation. Encapsulation results in a change of the physico-chemical properties of encapsulated compounds (e.g. volatility of flavour-active compounds and their increased stability against oxidation and photodegradation). Cyclodextrins are, therefore, of greatest use as carriers (encapsulators) of odoriferous substances, emulsion stabilisers and are also used to remove bitter substances from citrus juices (see Section 8.3.5.1.1). 

Crystalline packing of host cyclodextrins in solid cyclodextrin-inclusion compounds... 

The selectivity of the Complex Formation is a very interesting subject, y - Cyclodextrin, (y - CD) has been found to form inclusion complexes with poly (methyl vinyl ether) (PMVE), poly(ethyl vinyl ether) (PEVE), and poly(n- propyl vinyl ether) (PnPVE) of various molecular weights to give stoichiometric compounds in crystalline states. However, a- cyclodextrin (a - CD) and (3 - Cyclodextrin ((3- CD) did not form complexes with poly (alkyl vinyl ether)s of any molecular weight, y -CD did not form complexes with the low molecular weight analogs, such as diethyl ether and trimethylene glycol dimethyl ether. 

Although y-cyclodextrin does not complex with PEG virtually in aqueous solution as mentioned above, it formed inclusion complexes with some PEG derivatives such as bis(3,5-dinitrobenzoyl)-poly(ethylene glycol) and bis(2,4-dinitrophenylamino)-poly(ethylene glycol) to give crystalline compounds in high yields. Fluorescently labeled derivatives, bis(l-naphthy-lacetyl)-PEG (PEG-1N2) and bis(2-naphthylacetyl)-PEG (PEG-2N2) were prepared to investigate further the nature of the complexes formed between cyclodextrins and PEG derivatives (Scheme 20). PEG-1N2 and PEG-2N2... 

Cyclodextrins, for example, form inclusion complexes with metallocenes such as ferrocene, cobaltocene and nickelocene to form crystalline compounds. The structures of the complexes are dependent on the size of the cyclodextrin. Although ferrocene and its derivatives are strongly bound in uncharged states, when they are oxidized, the complexes dissociate. Dendrimers containing ferrocene units at the ends of the molecule have been prepared. The dendrimers form large supramolecular structures that can be broken apart or assembled upon... 

In these solid inclusion componnds, the association of host and guest components is strictly a solid state phenomenon, and the examples covered in this chapter focns almost entirely on solid inclnsion componnds of this type. However, it is relevant to note that there is also another class of inclnsion compound in which the host is a molecnle containing an appropriate cavity or binding site that can inclnde gnest molecules. Such molecular host-guest complexes generally exist as associated entities both in the solntion state and in the crystalline state, and thns the incln-sion phenomenon is not exclusively a property of the solid state. Examples of host molecules of this type are crown ethers, cyclodextrins, cryptands, and calixarenes. 

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