DCA inclusion compounds

Fig. 14. Choleic acid inclusion chemistry (a) crystal stmcture of DCA inclusion compound with phenanthrene (b) view along a DCA inclusion helix accommodating DMSO and water guest molecules (oxygen and sulfur atoms and methyl groups are represented by open circles and large and small black...

Although much work has been devoted to examination of a wide range of DCA inclusion compounds, an efficient enantioresolution has not yet been achieved. This is because DCA channels have only small pockets on the walls of the... 

Recently, Cataldo and co-workers reported polymerization reactions of isoprene and 3-melhyl-1,4-pentadiene in DCA inclusion compounds, and compared the polymerization reactions of 2,3-dimethylbutadiene within the DCA and thiourea host structures. The polydimethylbu-tadiene prepared by inclusion polymerization in the DCA host structure was found to have a lower degree of regularity and crystallinity than that prepared by polymerization in the thiourea host structure. 

One of the earliest reports of reactions between the host and guest components in a solid organic inclusion compound, by Lahav, Leiserowitz, and coworkers, was motivated by the prospect of mimicking enzymatic reactions in nature (in which highly selective reactions occur between enzyme and substrate due to the strict orientational control imposed on the reaction centers by the enzyme/substrate complex). It was demonstrated that the constrained geometric environment provided by DCA inclusion compounds affords... 

In contrast to the reaction in acetone/DCA, irradiation of the diethyl ketone/DCA inclusion compound under argon gives only a single photo-addition product (product 6, Figure 25). When the same procedure is carried out in the presence of air, a further hydroxylation product (product 2, Figure 25) is also obtained. The change in orientation... 

In order to explore mechanistic aspects of these reactions, in particular the stereochemistry of the reaction products, photo-addition reactions of DCA inclusion compounds containing acetophenone and m-chloroacetophenone guest molecules were studied. In both cases, photo-addition takes place at position 5 of DCA, resulting in the formation of a new chiral center with S configuration. This process involves a 180° rotation of the acetyl group of the guest molecule, the driving force for which was elucidated by... 

Deoxycholic acid (DCA) (17) and apoeholic acid (ACA) (18) are typical examples of the bile acid family of materials, but with the unique property of forming inclusion compounds with a wide variety of guest molecules 92). Partly due to the cis ring junction between rings A and B, and partly due to the conformation of the steroidal side chain these compounds present a convex hydrophobic P-face and a concave hydrophilic a-face, as shown for DCA (19), a classical aid to the formation of inclusion compounds 93). 

DCA forms canal inclusion compounds, known as choleic acids, which most frequently have the orthorhombic space group P212121, or less frequently Pl l. In such crystals the DCA molecules hydrogen bond to each other to form an extended bilayer structure, thereby creating a hydrophobic canal between adjacent bilayers. The guest molecules present in these canals therefore tend to be non-polar or moderately polar molecules such as aromatic compounds, alkenes, ketones and certain carboxylic acids 92). Since the bilayers are held together only by van der Waals forces the canals are able to adopt different dimensions to accommodate the variety of... 

Deoxycholic acid (DCA), apocholic acid (ACA), and cholic acid (CA) form channel-type inclusion compounds with a wide variety of organic molecules. Of these DCA has been extensively investigated. 

DCA is the first bile acid whose inclusion ability was confirmed in the crystalline state. During the last century many research groups dealt with the inclusion compounds of DCA with various guest molecules, such as aliphatic, aromatic and alicyclic hydrocarbons, alcohols, ketones, fatty acids, esters, ethers, nitriles, peroxides and amines, and so on [2], In 1972, Craven and DeTitta first reported the exact crystal structure of DCA with acetic acid [3], Subsequent crystallographic studies made clear that most of DCA inclusion crystals have bilayer... 

In contrast to DCA, there were no detailed reports on the inclusion abilities of its related compounds. There are only a few descriptions of apocholic acid [5] (ACA, see later, in Figure 5) with a very similar bilayer structure to DCA. In 1986, Miyata and Miki et al. discovered lots of inclusion compounds of CA with the similar bilayer structures [6], On the other hand, it took a long time to determine the hexagonal crystal structures of CDCA inclusion compounds, and LCA exhibits no inclusion abilities as yet. In this way, it was confirmed that an increase or decrease of only one atom brings about completely different inclusion behaviors and crystal structures. This fascinating fact has given us adequate and continuous motivation to investigate the inclusion compounds of bile acid derivatives. 

An interesting phenomenon whereby achiral compounds occupy chiral cavities has been reported. Steroidal host compounds give rise to the attachment of definite chiral conformations of achiral compounds within cavities, making it possible to observe solid-state circular dichroism spectra. Gdaniec and Polonski reported this type of property for the inclusion compounds of DCA and CA with various aromatic ketones [40a] and benzil [40c], Furthermore, it is possible for the selected conformers to maintain their chiral state temporarily in solution. That is, soon after the inclusion compounds are dissolved, the chirality may be retained for some time. /V-Nitrosopiperidines were found to display this type of dynamic chiral recognition in DCA and CA inclusion compounds [40b], In this case, one can observe the decay of the circular dichroism signal after dissolution of these inclusion compounds in methanol. 

Porous host substructures with parallel channels are typical of many inclusion compounds formed by bile acids and their derivatives (see Deoxycholic, Cholic, and Apocholic Acids). In this class of compounds, host molecules are always optically active, and the resulting host networks are chiral. The best known among them are inclusion compounds of deoxycholic acid (DCA) (also known as choleic acidsj. With most guests. DCA molecules assemble via hydrogen bonds into a coiTugated... 

APA J has long been known to form the inclusion compounds similar to DCA. The crystal structures and inclusion abilities are nearly the same as shown in Fig. 4. They are served as hosts for inclusion polymerization. The former has larger channels than the latter. 

In common with the situation for DCA, the helical canal inclusion compounds of TOT have so far received little attention compared to the research activity on urea. Much interesting work has been carried out on TOT inclusion compounds with other crystal space groups including the study of monomeric carboxylic acid guests the inclusion of thermodynamically disfavoured conformers of guests and the photochemical reactivity of molecules jigse are discussed in a recent review... 

Figure 1 Molecular structures of some organic molecules that form the host structures in solid inclusion compounds (these specific host structures are encountered freqnently in this chapter) urea, thiourea, tri-orfto-thymotide (TOT), perhydrotriphenylene (PHTP), deoxycholic acid (DCA) and varions molecules related to deoxycholic acid, host A (l,l,6,6-tetraphenylhexa-2,4-diyne-l,6-diol), host B (frans-2,3-bis(hydroxydiphenylmethyl)-l,4-dioxaspiro[5.4]decane), and host C (irans-2,3-bis(hydroxydiphenylmethyl)-l,4-dioxaspiro[5.4]nonane).

The importance of the size of the host tunnel in controlling the stereoregularity of the product obtained in inclusion polymerization reactions has been demonstrated by Miyata and coworkers, who investigated the polymerization of 2,3-dimethylbutadiene in the host structures formed by several DCA derivatives (see Figure 1) with a variety of tunnel dimensions. The results are summarized in Table 1. The relative sizes of the tunnels in the inclusion compounds... 

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