Absolute configuration, assignment

Similarly, 2-anthrylmethoxyacetic acid (2ATMA 64) can also be used as a chiral anisotropic reagent (Fig. 1 23).80 The advantage of using this compound is that the absolute configuration assignment can be accomplished from only one isomer of the ester without calculating the AS value (Ss — SR) ... 

This anisotropic distribution of the occluded additive provides a second independent method of confirming the absolute configuration assigned by means of the morphological changes, once the mechanism of adsorption is known. This principle will be met again in the growth of centrosymmetric crystals. 

Imines 123, generated from a-arylethylamines 122 (Ar = Ph, 4-B1C6H4 or 4-O2NC6H4) and aqueous formaldehyde, react with cyclopentadiene in situ at room temperature to afford mixtures of the bridged dihydropyridines 124 and 125 (equation 67), whose relative configurations were deduced by XH NMR experiments and their absolute configurations assigned by reference to the X-ray structure of the aziridinium derivative 126 (equation 68)60. 

Having explored the scope of this reaction, it was then possible to use an optically active phosphoramidate (232.d), derived from (5)-a-methyl-benzylamine, to perform a kinetic resolution procedure.134,138 Upon reaction of 221 with 0.5 equiv of the phosphoramidate anion 232.d, selective reaction with the 7 -isomer of 221 resulted in an enhancement in the proportion of 5-221 along with the expected ketenimine products (233). The absolute configuration of i -233.d was determined by X-ray crystallography, and thus all other absolute configurations assigned by comparison of CD and HPLC data.39,138... 

Determination of the absolute configurational assignment of chiral 1,3,5,7-cyclooctate-... 

In this section an approach is discussed whereby absolute configurational assignment is based on auxiliary- or reagent-controlled stereoselective key reactions. The stereochemical outcome of the reaction used for the assignment must be predictable, from either the absolute configuration of the auxiliary or the reagent, by a rule or (better) a well-defined model of the transition state220. 

Berson, J. A., and M. A. Greenbaum Asymmetric induction studies with optically active biphenyls. V. On the unreliability of absolute configurational assignments based on hydride reductions ofphenylglyoxylatcs. J. Amer. chem. Soc. 81, 6456 (1959). 

Compounds containing the (hydroxy)ethyleneamino link 8 were originally prepared in solution by reduction with NaBH4 of the carbonyl in the oxomethyleneamino link 7. 141,155,156 This reaction was later adapted to solid-phase conditions. 150,159 Two diastereomers, differing in the chirality of the hydroxylated carbon, have been characterized and their absolute configuration assigned by X-ray diffraction in some cases, 149156 but their separation is usually not possible. 142 ... 

The absolute configuration assignment of both chiral centers in 2-glycosyl-3-benzoylaziridines was achieved by comparing the CD properties of series cis- (116) and tram - (117) isomers289. 

Boyd DR, Sharma ND, Coen GP, Gray PJ, Malone JF, Gawronski J (2007) Enzyme-Catalysed Synthesis and Absolute Configuration Assignments of cw-Dihydrodiol Metabolites from 1,4-Disubstituted Benzenes. Chem Eur J 13 5804... 

Zacchino, S.A. and Badano, H. (1991) Enantioselective synthesis and absolute configuration assignment of erythro-(3,4-methylenedioxy-7-hydroxy-1 -allyl-3, 5 -dimethoxy)-8-0-4 -neolignan and its acetate, isolated from nutmeg (Myristica fragrans). Journal of Natural Products 54(1), 155-1 60. 

The application of chiroptical methods towards absolute configuration assignment is not limited to natural products, of course. Molecules targeted for their potential application as nano materials have also been studied, such as ladderanes (1) synthesized by Zuber et al. [213] which are discussed elsewhere in this chapter. 

Camps, R Contreras, J. Font-Bardia, M. Morral, J. Munoz-Torrero, D. Solans, X. Enantioselective synthesis of tacrine-huperzine A hybrids. Rreparative chiral MRLC separation of their racemic mixtures and absolute configuration assignments by x-ray diffraction analysis. Tetrahedron-Asymmetry, 1998, 9(5) 835-849. 

Aqueous solutions of the enantiomers of asyw-cfs-[Co(edda)(en)]Cl are stable to racemization at room temperature. The circular dichroism spectrum of the (-) isomer in water shows (Ae) 485 nm(-2.24), 357 nm (+0.89). The absolute configuration of the closely related (7 )-l,2-diaminopropane [(-)-pn] isomer, asym-cA-[Co(edda)[(-)-pn] ]C1, has been determined by x-ray crystallography.The results of this study confirm the absolute configuration assigned to the two. sym-cfs-[Co(edda)(en)]Cl enantiomers on the basis of their circular-dichroism spectra. ... 

On the basis of literature data, dealing with the stereochemistry of cyclobutyl-cyclopropyl-methyl cation rearrangements of related secondary cyclobutyl systems, the correlation leads also to absolute configurational assignments of the cyclopropane derivatives by a process retaining the configuration at Cl and inverting it at C3. 

We have recently developed chiral carboxylic acids as novel molecular tools useful for enantioresolution and simultaneous determination of the absolute configuration of various alcohols. These chiral molecular tools are very powerful for the facile preparation of enantiomers with 100% ee and also for the absolute configurational assignment. Methods using these chiral tools have been successfully applied to various compounds, and their methodologies and applications are explained throughout this chapter. 

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