Cyclodextrins ternary complex formation with

In analogy to the carboxylate binding by zinc-containing cyclodextrin 10 (see Sect. 2), Lewis acidic centers such as a copper(II) histamine unit may also serve for the chelation of the (deprotonated) 2-aminoacetate substructure of a-amino acids [51], Rizzarelli, Marchelli et al. used a respective j8-cyclodextrin derivative for the formation of the ternary complexes 36 and 37 with racemic... 

Regioselective reaction of phenols. Reaction of phenols with chloroform in aqueous alkaline solution catalyzed by P-cyclodextrin results in virtually complete attack at the para-position by dichlorocarbene to give, after hydrolysis, 4-hydroxybenzaldehydes. If the para-position is substituted, 4-(dichloromethyl)-2,5-cyclohexadienones are obtained as the major product. The selectivity results from formation of a ternary complex from P-cyclodextrin, chloroform, and the phenol. ... 

We have previously found that solvophobic interactions between side chain moieties of two different ligands co-ordinated to die same metal ion favor mixed complex formation by means of a favorable enthalpy contribution [47-49]. In the CDhm systems the cavity seems to play the same role as a side chain moiety and stereoselectivity is seen to be enthalpy driven. The entropy change seems to be less favorable for the D-enantiomers, the side chains of which are most probably included in the cavity. This could also be interpreted as resulting from the loss of internal rotational freedom of the side chain which predominates over the effect of cavity desolvation due to inclusion. Thermodynamic stereoselectivity was also found in the copper(U) ternary complexes with 6-deoxy-6-[4-(2-aminoethyl)imidazolyl]-p-cyclodextrin 2 (CDmh) [29], an isomer of CDhm, where the histamine is linked by the imidazole nitrogen (Table IE). In this case the copper(II) mixed complex with the D-isomer of tryptophan is less stable than the complex with the L-isomer. 

In order to obtain independent evidence for the involvement of the cyclodextrin cavity, fluorescence measurements were carried out for copper(II) ternary complexes with L- or D-tryptophan. In fact, the fluorescence spectrum of tryptophan has already been shown to be sensitive to the polarity of the microenvironment in which it is located and has been used in many studies as a probe for the conformation of proteins and peptides [53]. As for many fluorophores, the indole fluorescence of Trp is quenched by the copper(II) ion this effect has been used as a measure of the stability constants of copper(II) complexes [54, 55]. In a recent work, it has been shown that the fluorescence of dansyl derivatives of amino acids undergo enantioselective fluorescence quenching by chiral copper(n) complexes and that fluorescence measurements can be used for the study of enatioselectivity in the formation of ternary complexes in solution [56]. Bearing this in mind, we performed the same type of experiments by adding increasing amounts of the [Cu(CDhm)] + complex to a solution of D- or L-tryptophan [36]. The fluorescence titration curve shows that the artificial receptor inhibits the indole... 

Synergistic effects in terms of efficiency of CE enantioseparation have been observed when a second (not necessarily chiral) selector is added in the same buffer system. It has been demonstrated that a combination of 18-crown-6 and )-cyclodextrin can achieve or enhance enantioselective separations of nonpolar amines, which are rarely observed with cyclodextrins alone <1997JCH(781)129, 1997JCH(695)157>. The formation of a ternary sandwich complex (dual complex) is postulated to be responsible for such a beneficial effect. 

---