Cyclodextrin proximity

The importance of the proximity effect in cyclodextrin catalysis has been discussed on the basis of the structural data. Harata et al. 31,35> have determined the crystal structures of a-cyclodextrin complexes with m- and p-nitrophenols by the X-ray method. Upon the assumption that m- and p-nitrophenyl acetates form inclusion complexes in the same manner as the corresponding nitrophenols, they estimated the distances between the carbonyl carbon atoms of the acetates and the adjacent second-... 

The more useful types of chirally active bonded phases are those based on the cyclodextrins. There are a number of different types available, some of which have both dispersive or polar groups bonded close to the chirally active sites to permit mixed interactions to occur. This emphasizes the basic entropic differences between the two isomers being separated. A range of such products is available from ASTEC Inc. and a separation of the d and / isomers of scopolamine and phenylephrine are shown in figure 4. The separations were carried out on a cyclodextrin bonded phase (CYCLOBOND 1 Ac) that had been acetylated to provide semi-polar interacting groups in close proximity to the chiral centers of the cyclodextrin. The column was 25 cm long, 4.6 mm in diameter and packed with silica based spherical bonded phase particles 5pm in diameter. Most of the columns supplied by ASTEC Inc. have these dimensions and, consequently, provide a... 

Yamamoto and coworkers used two-dimensional, nuclear Over-hauser effect experiments (NOESY) to determine the proximity of particular protons situated on an included p-nitrophenolate ion to particular protons of a host alpha cyclodextrin molecule. The experiments showed cross-peaks connecting the H-3 resonance of alpha cyclodextrin to both meta and ortho proton resonances of the p-nitrophenolate ion, whereas H-5 of the alpha cyclodextrin gave a cross-peak only with the resonance of the meta proton thereof. As a consequence, it was unequivocally confirmed that the p-nitrophenolate ion is, in solution, preferentially included with its nitro group oriented to the narrow end of the alpha cyclodextrin... 

Another interesting enzyme model obtained by use of difunctionalized cyclodextrin having two imidazole groupings also afforded carbonic an-hydrase models as reported by the authors (69). Carbonic anhydrase has Zn2 + surrounded by three imidazoles in the active site and COz is bound to the active site is close proximity to Zn2+ with the assistance of hydrophobic... 

Supramolecular metallocatalysts consist in principle of the combination of a recognition subunit (such as a macrocycle, a cyclodextrin, a cyclophane, etc.) that selects the substrate(s) and of a metal ion, bound to another subunit, that is the reactive site. Complexed metal ions presenting free coordination positions may present a variety of substrate activation and functionalization properties. Heterotopic coreceptors such as 70 bind simultaneously a substrate and a metal ion bringing them into proximity, thus potentially allowing reaction between them. 

NMR ROESY experiments indicated molecular proximities (<5 A) between hydrogens 10, 11, and 12 of the aromatic ring of praziquantel and hydrogen H-3 and H-5, in the internal cavity of p-cyclodextrin (06JPB1428). Rotamers of 2-acyl-1,3,4,6,7,11 b-hexahydro-1 H-pyrazino [l,2-a]isoquinolin-4-ones were identified and analyzed by [H and 13C NMR spectroscopy (08EJO895). Commercial praziquantel tablets were analyzed by NMR spectroscopy (07JPB263). 

Cyclodextrins have been covalently modified for catalytic oxidation, such as compounds 57, 62-65 (Schemes 3.14 to 3.16) [44, 45]. Enantioselective epoxidation of styrene derivatives, and carene using 20-100 mol% of the CD-ketoester 57 has been achieved. The inclusion-complex formation was confirmed by aH NMR titration experiments, confirming the 1 1 substrate catalyst stoichiometry under the reaction conditions. In the oxidation of carene, NOE and ROESY experiments showed different behavior according to the size of the R group (Scheme 13.14). Evidence was found for the formation of inclusion complexes with compounds 58 and 59. On the other hand, compounds 60 and 61 proved to interact with the catalyst via a tail inclusion vide infra). The increased diastereoselectivity observed with compounds 58 and 59 might be explained by a closer proximity to the covalently linked dioxirane. 

The chemistry of interest when cyclodextrin or its derivatives are used as enzyme mimics involves two features. First of all, the substrate binds into the cavity of the cyclodextrin as the result of hydrophobic or lyophobic (4) forces. Then the bound substrate undergoes a reaction, which may involve the cyclodextrin as a reagent or as a catalyst. The speed of this reaction is promoted generally by the proximity induced by binding, and in addition the reactions are often selective because of geometric constraints in the transition state. This selectivity may involve the selective reaction of one potential substrate relative to another, selective production of one regiochemical isomer compared with another, or selective production of one stereoisomer relative to another. This last area, selective stereochemistry and asymmetric synthesis, is still one of the most neglected areas of cyclodextrin chemistry. 

Several new excipients are being evaluated in order to increase the solubility or improve the stability of parenteral drugs. Cyclodextrins have been tried for the above reasons. Currently, there are two FDA approved parenteral products that have utilized a and y-cyclodextrins. p-cyclodextrin is unsuitable for parenteral administration because it causes necrosis of the proximal kidney tubules upon IV and subcutaneous administration. Hydroxypropyl p-cyclodextrin (HPpCD) and sulfobutylether p-cyclodextrin (SBE-7-p-CD) have shown the most promise. Captisol is the trade name of SBE-7-p-CD and is anionic. Currently, two CaptisoF based small molecule IV and IM drug formulations are in Phase III clinical trials in the United States. One parenteral formulation that utilizes HPpCD (Cavitron ) is in Phase II/III clinical trials, and another (Sporanox) has been approved by... 

In a subsequent detailed study we saw that the chlorination of unbound anisole involved two molecules of HOCl, surely as their very reactive product CI2O, while the chlorination in the complex involved only a single molecule of HOCl. The obvious conclusion from this is that the HOCl reacts with a hydroxyl group of the cyclodextrin to make a hypochlorite that is able to donate a chlorine atom to the para position of bound anisole, which is the one accessible when the anisole is fully bound. Proximity makes up for the weak chlorinating ability of a hypochlorite. 

These. supramolecular catalysts showed high substrate selectivity in competition hydrogenation experiments and exceptional activity in the hydroformylation reactions. In contrast to the simple methylated P-cyclodextrin previously mentioned, even internal and cyclic olefins were converted into aldehydes. Such improvements were explained with the formation of an inclusion complex at the phase boundary, with the cylodextrin host fixing the substrate in the proximity of the catalytically active metal center (Fig. 

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