Azobenzene-capped cyclodextrin

Another type of photoactive capped cyclodextrin, azobenzene-capped cyclodextrin, is also reported, but regioisomer distribution is not mentioned and their treatment does not seem quite satisfactory. See Yoshimura, H., Saka, H., and Osa, T., J. Am. Chem. Soc. 101, 2779 (1979) Chem. Lett. 841, 1007 (1979) Chem. Lett. p. 29 (1980). 

Hamada, F. Fukushima, M. Osa, T. Ikeda, H. Toda, F. and Ueno, A. (1993) Azobenzene-capped y-cyclodextrin as photo-switchable host, Makromol. Chem., Rapid Commun. 14, 287-291. 

Capping of one side of the torus with an apolar moiety enhances the binding ability of a cyclodextrin. This effect is attributable to the increase in the apolar nature of the cavity due to the capping by the apolar groups. For example, capped )8-cyclodextrin (la) binds sodium l-anilino 8-naphthalenesulfonate 24 times better than parent j8-cyclodextrin [5]. The binding ability of another capped -cyclodextrin (lb) changes on photoirradiation, since the conformation of )8-cyclodextrin is varied due to the cis-trans photoisomerization of the azobenzene moiety [6]. 

Threading of two cyclodextrins onto a symmetrical dumbbell can occur head-to-head, head-to-tail, or tail-to-tail, defining a new class of diastereoisomeric [3]-rotaxanes, as shown schematically in Figure 2.12d. Anderson and co-workers have shown that end-capping of 4,4,-bis(diazonio)azobenzene chloride 43 with 2,6-dimethylphenol 44 in the presence of a-CDX produced the [3]-rotaxane 45 in 12% yield, in addition to the [2]-rotaxane 46 in 9% yield, and free dumbbell 47 in trace amounts (Figure 2.18).42 The stereochemistry of the [3]-rotaxane species is remarkable because the two cyclodextrin beads have their smallest rims facing to each other. Therefore the threading reaction was stereoselective. The reasons for the exclusive formation of the tail-to-tail stereoisomer are not clearly established. 

In another attempt, azobenzene-appended y-cyclodextrin 11 was prepared. Upon the irradiation with 320- to 390-nm light, 70% of the E-isomer in water isomerized to the Z-isomer, which exhibited an enhanced binding ability for (-)-borneol and (-l-)-fenchone. Benzo-18-crown-6 12 linked to a phenol was synthesized. Upon irradiation by UV light, the azo linkage isomerized to the Z-isomer. It slowly reverted to the E-form in the dark and rapidly upon irradiation with visible light. An enhanced extraction of Ca + by the Z-isomer has been observed. The Z structure constitutes a photoresponsive anion cap. ... 

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