Stereoisomeric β-cyclodextrin (CD) dimers linked with a sulfur atom or an arene spacer were designed to create a tethered dual CD capsule for precisely manipulating the regioand enantioselectivities of the photocyclodimerization of 2anthracenecarboxylate (AC) to four stereoisomeric classical 9,10:9′,10′-cyclodimers and two nonclassical 5,8:9′,10′-cyclodimers. Among the dimeric CD hosts prepared, exo-3-thia-β-CD dimer formed 1:1 and 1:2 host−guest complexes with AC in aqueous solutions, the former of which hindered but the latter facilitated the AC photocyclodimerization with regio-and enantioselectivities much higher than those obtained with native β-CD or the rest of the β-CD dimers. The stereochemical outcomes turned out to be highly sensitive to and hence critically manipulable by the linking position and configuration of the connected saccharide units and the linker length, as well as the external variants, such as temperature, pH, and added salt. Eventually, the photocyclodimerization of AC mediated by the dimeric β-CD host gave enantiopure syn-head-to-tail-9,10:9′,10′-cyclodimer in 97−98% yield in a pH 5.1 buffer solution at 0.5 °C and also in an aqueous CsCl solution at −20 °C.
Three types of reactions of 2,3-anhydro-beta-cyclodextrins, namely nucleophilic ring-opening, reduction to 2-enopyranose, and reduction to 3-deoxypyranose, have been investigated to regio- and stereoselectively functionalize the secondary face of beta-cyclodextrin. Upon treatment with various nucleophiles, both 2,3-mannoepoxy and 2,3-alloepoxy-beta-cyclodextrins are found to undergo nucleophilic ring-opening reaction generating 3- and 2-modified cyclodextrin derivatives. In each case, the 3-position is more easily accessible than the 2-position. By using these ring-opening reactions, imidazolyl, iodo, azido, and benzylmercapto groups are selectively introduced to the secondary face of beta-cyclodextrin in place of the 2- or 3-hydroxyl groups. The functionalized cyclodextrins have either modified glucosidic subunits or modified altrosidic subunits that make the hydrophobic cavity slightly distorted from that of native beta-cyclodextrin. Thiourea also reacts with the cyclodextrin epoxides. In this case, thiirane and olefin species are generated instead of any ring-opening products. By ameliorating the reaction condition, cyclodextrin olefin, diene, and triene derivatives are prepared in moderate to good yields. Reduction of per[6-(tert-butyldimethyl)silyl]-beta-cyclodextrin permannoepoxide with lithium aluminum hydride produces the per(3-deoxy)-beta-cyclomannin. All these chemically modified cyclodextrins are structurally well characterized and most of them are expected to serve as versatile scaffolds for diverse purposes such as the construction of catalysts and development of synthetic receptors and molecular containers.
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