Cyclodextrins configuration

Although the chiral recognition mechanism of these cyclodexttin-based phases is not entirely understood, thermodynamic and column capacity studies indicate that the analytes may interact with the functionalized cyclodextrins by either associating with the outside or mouth of the cyclodextrin, or by forming a more traditional inclusion complex with the cyclodextrin (122). As in the case of the metal-complex chiral stationary phase, configuration assignment is generally not possible in the absence of pure chiral standards. 

Righetti and co-workers [11] were one of the first to demonstrate the utility of classical isoelectric focusing for the chiral separation of small molecules in a slab gel configuration. In their system, dansylated amino acids were resolved enan-tiomerically through complexation with (i-cyclodextrin. Preferential complexation between the cyclodextrin and the derivatized amino acid induced as much as a 0.1 pH unit difference in the pK s of the dansyl group. 

The transition state of concerted Diels-Alder reactions has stringent regio- and stereochemical requirements and can assume settled configurations if the reaction is carried out in a molecular cavity. Cyclodextrins, porphyrin derivatives and cyclophanes are the supramolecular systems that have been most investigated. 

Ekgorg-Ott et al. (1997). An interesting trend was discovered when considering the relative amount of D-theanine present in the samples. The teas of the highest grades consistently contained the lowest amounts of D-theanine. The theanine achiral-chiral system configuration included a C18 column operated in the reverse-phase mode and a y-cyclodextrin CSP in the polar organic mode. 

Phenylthiocarbamoyl derivatives of 18 chiral amino acids were separated on a C8 column connected in series to a phenylcarbamoylated (3-cyclodextrin column (Iida et al., 1997). The Cg column separated the derivatized amino acids from one another entering the chiral column. Under this configuration several enantiomers of adjacent amino acids coeluted resulting in poor resolution. However, this configuration was successful in determining the amino acid sequence and chirality of the amino acids in a D-amino acid containing peptide. 

In conclusion, the most important result is that the use of permethylated cyclodextrin as chiral solvating agent for NMR spectroscopy not only affords a simple and practical way for the determination of the stereochemical purities of trisubstituted allenes, but also allows one to simultaneously determine their absolute configuration. Indeed, TRIMEB induced only positive complexation shifts of all the allene protons, which are greater for the (S )-enantiomer than for the (R)-enantiomer, independent of the structure of the allene. This empirical correlation seems to be reliable since it has been satisfied by a large number of trisubstituted allenes. 

Investigations on the stereochemistry of chiral semiochemicals may be carried out by (gas) chromatographic separation of stereoisomers using chiral stationary phases, e.g. modified cyclodextrins [32]. Alter natively, formation of diastereomers (e.g. Mosher s ester or derivatives involving lactic acid etc.) may be followed by separation on conventional achiral stationary phases. Assignment of the absolute configuration of the natural product will again need comparison with an authentic (synthetic) reference sample. 

The stereospecific conversion of sulfinates into sulfoxides of known chirality has been applied as a general method for determining the absolute configuration of a wide range of optically active sulfinic esters. For example, the absolute configurations of a series of alkyl alkanesulfinates obtained by asymmetric synthesis (107) or resolution via 3-cyclodextrin inclusion complexes (106) were determined by this method. 

By capillary GC on a cyclodextrin-B column. b Absolute configuration tentatively assigned. At OX. d Absolute configuration by chemical correlation. e At 25 X. 

Fig. 3. Computer projections of p-cyclodextrin inclusion complexes of (a) (R)-propranolol and (b) (5)-propranolol from x-ray crystallographic data. Dotted lines represent potential hydrogen bonds (see text). The configurations shown represent the optimal orientation of each isomer on the basis of the highest...

Using a stationary phase based on a /3-cyclodextrin derivative (2,3-di-0-ethyl-6-O-te/T-bu ty klimethy Isily 1-/3-CD), complete separation of (R)- and (S)-18 (but not of (R)/(S)-19) is possible within 3.9 min. Because the configuration comprises two simultaneously operating GC units, about 700 exact ee determinations of (R)f(S)-lS are possible per day. Moreover, the corresponding values for the conversion and the selectivity factor E (or s) are likewise automatically provided in microtiter-format, which means that the ee of (R)/(S)-19 is also accessible. Of course, every... 

The influence of /i-cyclodcxtrin on the reduction of acetophenone by aqueous NaBHU is to induce a small ee of (, )-1 -phenylethanol. The addition of stoichiometric amounts of triethylamine has been found to invert the absolute configuration of the mam product, and substantially higher ee values (up to 56%) are then observed.241 hi a separate study, the stereoselectivity of the same reaction has been reversed through use of 6-dcoxy-6-(ethylenediamine)-/)-cyclodextrin.242... 

In the presence of /9-cyclodextrin, that binds to the R group of substituted epoxides, benzeneselenol reacts regio- and stereo-specifically to give an 80% yield of the j3-hydroxy selenide.42 Because the selenol only attacks the least substituted carbon with inversion of configuration, the reaction must occur by an SN2 mechanism. 

Compressibility factor, 61-63 Configuration factor, 33 Countercurrent chromatography, 74 Critical point data, 250 Cryogenic GC, 149 Cyclodextrin, 266... 

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