Cyclodextrin functional groups

The living character of the ring opening metathesis polymerization described earlier in this review enables a simple preparation of functionalized norbornene-based monoliths. Adding one more in situ derivatization step that involves functional norborn-2-ene and 7-oxanorborn-2-ene monomers that react with the surface-bound initiator, the pores were provided with a number of typical functional groups such as carboxylic acid, tertiary amine, and cyclodextrin [58,59]. 

Recently, it was found that the commercially available heptakis(2,3,6-tri-0-methyl)-/i-cyclodextrin (permethylated /1-cyclodextrin, TRIMEB), induced nonequivalence in the NMR spectra, in CD3OD, of enantiomeric mixtures of trisubstituted allenes devoid of polar functional groups, thus affording a simple and general way to determinations of their enantiomeric purity63. 

Numerous examples of modiflcations to the fundamental cyclodextrin structure have appeared in the literature.The aim of much of this work has been to improve the catalytic properties of the cyclodextrins, and thus to develop so-called artificial enzymes. Cyclodextrins themselves have long been known to be capable of catalyzing such reactions as ester hydrolysis by interaction of the guest with the secondary hydroxyl groups around the rim of the cyclodextrin cavity. The replacement, by synthetic methods, of the hydroxyl groups with other functional groups has been shown, however, to improve remarkably the number of reactions capable of catalysis by the cyclodextrins. For example, Breslow and CO workersreported the attachment of the pyridoxamine-pyridoxal coenzyme group to beta cyclodextrin, and thus found a two hundred-fold acceleration of the conversion of indolepyruvic acid into tryptophan. 

Table 2 Different Types of Cyclodextrins with Their Functional Groups... 

A new strategy was found recently by the authors for the specific introduction of desired catalytic functional group(s) on cyclodextrins. 

In order to prepare certain excellent and sophisticated enzyme models, preparation of cyclodextrins having one or more functional groups is essential. At the same time, firm experimental evidence for their structures should be provided. 

The most difficult problem in the design of the enzyme model clearly lies in the difficulty of specific introduction of the functional groups into the host skeleton. For example, preparation of an enzyme model by use of fi-cyclodextrin often requires the introduction of two or more functional groups at certain positions among 7 primary (C6) and 14 secondary (C2 and C,) reactive positions. Unless one expects an accidental success by the use of any nonspecific functionalization, it is inevitably necessary for the host design to solve these problems. 

Thus, in the present stage, we can freely design an enzyme model by the use of cyclodextrin, although introduction of intermediate numbers of functional groups on cyclodextrin is not very easy. 

Introduction of a single catalytic group in cyclodextrin generally affords enzyme models as shown in many examples listed in Table XVI. Thus, reasonable acceleration and substrate specificity were observed in these models. However, monosubstituted cyclodextrins seem to have limitations and introduction of two or more functional groups is usually necessary for multiple recognition and for a sophisticated enzyme model. 

Another successful example of such guest design is the Diels-Alder reaction markedly accelerated by the cyclodextrin inclusion. As shown in Table XXIV, /J-cyclodextrin accelerates the addition of a small dienophile to cyclopentadiene, but inhibits that of N-ethylmaleimide to anthracene-9-carbinol (117). Thus, the guest design is a really helpful concept for the remarkable catalysis. However, there seems to be some limitation in the choice of reactions, if cyclodextrins have no special functional group for the... 

Sufficient chiral recognition also requires interaction of other functional groups around the chiral center with the lip of the cyclodextrin cavity, and possibly even with secondary cyclodextrin molecules. 

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