Interglycosidic acetals. Part 3. Synthesis and structure determination of cyclic monobenzylidene acetals of cyclodextrin derivatives bridging between two contiguous d-glucopyranosyl residues

Transacetalation of fully 6-O-pivaloylated α-, β-, and γ-cyclodextrins with benzaldehyde dimethylacetal in the presence of (+)-10-camphorsulfonic acid gave monobenzylidene acetals (4, 5, 6) in moderately good yields. Benzylation of the β-cyclodextrin derivative 5 followed by acid-catalyzed hydrolysis of the benzylidene group and acetylation afforded a di-O-acetyl-non-adeca-O-benzyl derivative 9. NMR spectroscopic analysis of 9, including two-dimensional HOHAHA and 1H13C correlation experiments revealed that the benzylidene group bridged the O-2 and O-3 positions of contiguous d-glucopyranosyl residues. Reductive ring-opening of the benzylidene acetal with lithium aluminum hydride/aluminum chloride afforded predominantly a 21-O-unprotected derivative 10 in good yield.