Cyclodextrins

Abstract

Many of the compounds mentioned previously within this work are entirely synthetic in origin and as such do not occur naturally. The cyclodextrins are somewhat different however since they have a natural origin, being synthesised originally by bacterial fermentation and now via the use of enzymes from a natural feedstock. We have already discussed how small rings can be incorporated into macrocycles, such as in crown ethers and calixarenes. The cyclodextrins, also known as cycloamyloses, represent further examples of macrocyclic compounds formed by the joining of small rings to give rise to a much larger ring system. We intend to provide a general overview of cyclodextrin chemistry within this chapter; for further detail the reader is recommended to study some of the works listed in the Bibliography.Amylose is a water-soluble polysaccharide which is made up of 1,4-linked glucose units. Together with amylopectin it is one of the major constituents of starch. Cyclodextrins are made up of a series of α-D-glucopyranoside sugars linked together to form a macrocycle and can be thought of as cyclic versions of amylose. The three most common members of this group occur naturally; these are the hexamer (α-cyclodextrin), heptamer (β-cyclodextrin) and octamer (γ-cyclodextrin), as shown in Figure 6.1. The cyclic pentamer has been successfully synthesised in the laboratory -as have larger members of the series.Cyclodextrins were initially isolated by Villiers 1 as long ago as 1891; Villiers coined the name 'cellulosine' for these materials. He digested starch with Bacillus amylobacter to obtain amongst other things about 0.3% of a crystalline material. Elemental analysis showed it to be a polysugar and to have two distinct crystal forms. However, the techniques for in-depth separation and characterisation of these materials (thought to have been αand β-cyclodextrin) were not then available. Later work by Schardinger, 2 again using bacterial digestion of starch, managed to produce materials similar to cellulosine in yields of up to 30% and to separate and characterise the natural αand β-cyclodextrins, which were then named 'Schardinger sugars' or 'Schardinger dextrins'. The early history of these and the γ-cyclodextrins, the elucidation of their composition from glucose (they can also be thought of as consisting of maltose; for example, α-cyclodextrin can be thought of as consisting of six glucose or three maltose units) and their cyclic structure have all been reviewed. 3 In 1998, the journal Chemical Reviews published a special issue on cyclodextrins, to which the reader is referred in the Bibliography.Macrocycles: C onstruction, Chemistr y and Nanotechnology Applications, First Edition.Frank D avis and S éamus Higs on.