Chiral probes

Scheme 3. Chirality probe approach to search for primary vinyl cation.

Although formation of primary vinyl cation was disproved by the chirality probe approach, a vinyl cationic intermediate can be generated from a primary substrate via participation if a more stable cation could result. Unsymmetrically substituted 2,2-dialkylvinyl iodonium salt 24 gave mainly rearranged products on solvolysis.15 The products involve those of the 1,2-shift of either of the alkyl groups on the p position (Scheme 4). Those formed from migration of the alkyl... 

Comparisons of product distributions in thermal and photochemical solvolyses show that the primary vinyl cation is not involved in thermolysis but is formed photochemically. The chirality probe approach using optically active 4-methylcyclohexylidenmethyl(aryl)iodonium tetrafluoroborate 19 was applied to the photosolvolysis.24 The rearranged product 4-methylcycloheptanone retained some optical activity, but the enantiomeric product in slight excess has a different structure depending on the iodoarene leaving group Arl of the substrate. The results indicate that the primary vinyl cation involved is not in a free, dissociated achiral form. 

Stereochemical probes of the specificity of substrates, products, and effectors in enzyme-catalyzed reactions, receptor-ligand interactions, nucleic acid-ligand interactions, etc. Most chirality probe studies attempt to address the stereospecificity of the substrates or ligands or even allosteric effectors. However, upon use of specific kinetic probes, isotopic labeling of achiral centers, chronfium-or cobalt-nucleotide complexes, etc., other stereospecific characteristics can be identified, aU of which will assist in the delineation of the kinetic mechanism as well as the active-site topology. A few examples of chirality probes include ... 

Chrominm(III) and Cobalt(III) Nucleotide Complexes. Coordination isomers of Cr(III)- and Co(III)-nucleotides can be separated and used to test for specificity. Since Cr(III) is diamagnetic, EPR and NMR experiments can supplement the chirality probes . ... 

Exchange-inert complexes of Cr(III) with nucleotide ligands are very stable toward hydrolysis. Such complexes have proven to be extremely useful as chirality probes in that different coordination isomers can be prepared and separated These nucleotide complexes have also proved useful as dead-end inhibitors of enzyme-catalyzed reactions. Because Cr(lII) is paramagnetic, distances can be measured by measuring the effects of Cr(lll) on the NMR signals of nearby atoms when the Cr(lll)-nucleotide complex binds to the surface of a mac-romolecule. See Exchange-Inert Complexes... 

A stereochemical property of compounds arising from the ability of an enzyme s active site to distinguish between two chemically identical substituents covalently bound to a tetrahedral center (usually carbon and, in some cases, phosphorus). Prochirality is also termed prostereoisomerism. The classical example is citrate with its two carboxymethyl group substituents. Likewise, the Cl carbon atom of ethanol has two prochiral hydrogens. See Chirality Chirality Probes... 

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS CHIRALITY PROBES Chirality sense,... 

PROCESSIVITY OF KINESIN ATPase PROCHIRALITY CHIRALITY CHIRALITY PROBES PROCOLLAGEN N-PEPTIDASE PRODUCT... 

CHIRALITY CHIRALITY PROBES PROCHIRALITY PROSTHETIC GROUP... 

Mei H-Y, Barton JK.Tris(tetramethylphenanthroline)rathenium(II) a chiral probe that cleaves A-DNA conformations. Proc Natl Acad Sci USA 1988 85 1339-43. 

If we need one pure enantiomer of butan-2-ol, we must find a way of separating it from the other enantiomer. The separation of enantiomers is called resolution, and it is a different process from the usual physical separations. A chiral probe is necessary for the resolution of enantiomers such a chiral compound or apparatus is called a resolving agent. 

If the imaginary replacement of either of two protons forms enantiomers, then those protons are said to be enantiotopic. The NMR is not a chiral probe, and it cannot distinguish between enantiotopic protons. They are seen to be equivalent by NMR. ... 

Leroy, E Bensel, N. and Reymond, J.L (2003) Fluorogenic cyanohydrin esters as chiral probes for esterase and lipase activity. Advanced Synthesis Catalysis,... 

Optically active macrobicyclic complexes may be employed as chiral eluents for the separation of optically active anions (Section 5.1) and as chirality probes for biological objects [307, 310, 312],... 

Also the size (five- or six-membered) and flexibility of the chelate ring has an important influence on the rate of the inversion of configuration process at tin. The latter effect was nicely illustrated by a recent study of the triorganotin bromides 58-61 (see Fig. 15) containing the chiral (—)-menthyl group as a chiral probe (105,106,124). 

Fig. 15. Pentacoordinate triorganotin bromides containing the (—)-menthyl group as a chiral probe.

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