Synthesis of cyclic oligomers of a modified sugar amino acid utilising dynamic combinatorial chemistry

Dynamic combinatorial libraries (DCLs) contain molecules that are generated from a set of building blocks, the connections between which are reversible. The connections may be covalent or noncovalent and suitably functionalised building blocks allow access to either DCLs of small molecules or cyclic oligomers. DCLs can change composition in the presence of a guest by shifting the equilibrium to increase the concentration of the DCL member(s) that best recognises (through molecular recognition) the guest.¹ Dynamic combinatorial chemistry (DCC) proceeds not to generate high yielding pure compounds, as is most often the desired outcome of conventional synthesis, but rather to access diversity and exploit the effective amplification of the 'best binder' by the addition of a suitable guest or host in a screening protocol. There are now several examples of receptor amplification in systems derived from DCLs generated in the presence of a guest.² Drawing inspiration from the extensively explored cyclodextrins as 'artificial' receptors in host–guest chemistry, many studies pursuing the synthesis of unnatural cyclic carbohydrate oligomers have been reported.³ These macrocycles retain the cluster of hydroxyl groups important for molecular recognition but incorporate unnatural covalent connections between building blocks. The unnatural connections impart different physical and chemical properties to the carbohydrate oligomers and in addition these modified connections can serve as a synthetic aid for cyclic oligomer synthesis.