Facile synthesis of per(6-O-tert-butyldimethylsilyl)-α-, β-, and γ-cyclodextrin as protected intermediates for the functionalization of the secondary face of the macrocycles.

Abstract

Per(6-O-tert-butyldimethylsilyl)-α-, β- and γ-cyclodextrin derivatives are well-known as synthetic intermediates that enable the selective mono-, partial, or perfunctionalization of the secondary face of the macrocycles. Although silylation of the primary rim is readily achieved by treatment with tert-butyldimethylsilyl chloride in the presence of pyridine (either alone or mixed with a co-solvent), the reaction typically results in a mixture containing both under- and oversilylated byproducts that are difficult to remove. To address this challenge in preparing a pure product in high yield, we describe an approach that centers on the addition of a controlled excess of silylating agent to avoid the presence of undersilylated species, followed by the removal of oversilylated species by column chromatography elution with carefully designed solvent mixtures. This methodology works well for 6-, 7-, and 8-member rings (α-, β-, and γ-cyclodextrins, respectively) and has enabled us to repeatedly prepare up to ⁓35 g of ≥98% pure product (as determined by HPLC) in 3 d. We also provide procedures for lower-scale reactions, as well as an example of how the β-cyclodextrin derivative can be used for functionalization of the secondary face of the molecule.