Absolute configuration cyclodextrins

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. 

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... 

D.W. Armstrong, TJ. Ward, A. Czech, B.P. Czech and R.A Bartsch, Synthesis, Rapid Resolution, and Determination of Absolute Configuration of Racemic 2,2 -Binaphthyldiyl Crown Ethers and Analogues via P-Cyclodextrin Complexation, J. Org. Chem., 50(26)(1985)5556. 

In complexation gas chromatography, semipreparative separations of spiroketals (among them pheromones) have been reported. The preparative invertomer separation of l-chloro-2,2-dimethylaziri-dine permitted the determination of chiroptical data, absolute configuration and inversion barrier. Very large separation factors a were observed for saturated hydrocarbons (cis- and tra s-pinane, camphene) on a mixture of a-cyclodextrin in form-amide impreg/nated on celite. The preparative enantiomer separation of a-ionone on (18) and of all-trans-perhydrotriphenylene on (25) has been described. The large separation factors observed for the inhalational anesthetics enflurane, isoflurane, and desflurane on (18) allowed their enantiomer separation for subsequent biomedical trials and acquisition of chiroptical data. The continuous preparative enantiomer separation of enflurane and... 

Chiral columns are also employed in pheromone research for the determination of the absolute configuration of stereoisomers. The configuration of 4-methylheptan-3-ol and 4-methylheptan-3-one, sex pheromones of the oak bark beetle, were determined using a fused silica column coated with octakis [6-0-methyl-2,3-di-0-pentyl]-7-cyclodextrin. The elution order of the ketone stereoisomers was determined according to the literature and the natural compound was found to be (S)-methylheptanone. The absolute configuration of the alcohol present in the insects was determined by comparison with authentic standards and was found to be (3R, 4S)-methylheptanol. 

Next, the asymmetric bromination of the cyclodextrin complex in the solid state was compared with the homogeneous reaction in (CH3)2S0 solution (see Table III). The optical yield from the homogeneous reaction was much smaller than that from the gas-solid reaction, although the absolute configurations were the same. This result shows that the gas-solid reaction is topochemically controlled by the crystalline lattice of the cyclodextrin complex. 

ABSTRACT. Studies of chiroptical properties need separation or at least enrichment of enantiomers. The separation is difficult with chiral con-formers of molecules with very low barriers of internal rotation. However, in association with cyclodextrin, these labile molecules can exhibit a strong Cotton effect in solution one chiral conformer is favoured by complexation. The cyclodextrins have the advantage to yield inclusion complex in solution as well as crystalline clathrates. Therefore, the absolute configuration of the guest can be obtained by determining the structure of the cyclodextrin. In this work, the first CD spectrum of 4-helicene is recorded And crystal structures of several clathrates of labile molecules are studied. 

UcceUo-Barretta G, Balzano F, Caporusso AM, lodice A, Salvadori P. Permethylated p-cyclodextrin as chiral solvating agent for the NMR assignment of the absolute configuration of chiral trisubstituted allenes. J. Org. Chem. 1995 60 2227-2231. 

In the presence of y-cyclodextrin, the dimers 10 and 11 were obtained with about 10% ee. The absolute configuration of the predominating enantiomer was not identified, but CD-spectra indicated that the two anthracenes prefer i -helicity in the complex. From the two l -hehcal complexes that lead to the enantiomers of 10, one seems to be stericaUy favored over the other because of fewer interactions between the substituents and the cyclodextrin. 

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