Mirror-image cyclodextrins

For over a century, naturally occurring cyclodextrins (CDs) have been investigated intensively and extensively. CDs possess inherently stable chiralities, which render them versatile players in diverse arenas of technology. Although naturally occurring CDs can be mass-produced by the enzymatic modification of amylose, their mirror-images have, however, remained inaccessible. Here we report the syntheses of three mirror-image CDs—namely, α-, β- and γ-l-CDs, which are composed of six, seven and eight α-1,4-linked l-glucopyranosyl residues, respectively. Hallmarks of their syntheses include the highly diastereoselective installations of multiple contiguous 1,2-cisl-glucopyranosidic linkages, the rapid assembly of linear oligosaccharides employing one-pot glycosylation strategies and three efficient diastereoselective cyclizations. The structures and inherent chiralities of all three synthetic l-CDs have been established unambiguously by single-crystal X-ray diffraction and induced electronic circular dichroism spectroscopy. The availability of l-CDs has enabled the elucidation of an unprecedented chiral self-sorting of a racemic modification of β-CDs in the solid state and an investigation of the chiral recognition of enantiomeric fenchone by α-l-CD. This research identifies a missing piece of the cyclodextrin jigsaw and sets the stage for scientists to explore the mirror-image world of naturally occurring CDs. (Figure presented.)