Chiral stability

Among the peculiar features of 2-bromoamides there are the following i) possibility of substitution at the tertiary C-Br (RCO2H, RR NH, or a saccharide, as the nucleophiles) ii) chiral stability and stereochemical control at the secondary C-Br atom (RR NH, ROH or a saccharide as the nucleophiles) iii) the presence of bromine allows cyclic voltammetry and electroreduction at controlled potential both of starting compounds and relevant intermediates iv) the Ca polarity can be reversed upon electroreduction, and the resulting Ca enolate forms a C-C bond (CO2 as the electrophile). 

Buser H-R, MD Muller, ME Balmer (2002) Environmental behavionr of the chiral acetamide pesticide metalaxyl enantioselective degradation and chiral stability in soil. Environ Sci Technol 36 221-226. 

Several of the active ingredients were chiral, and the nature of the material used in the marketed product— racemate or single isomer—was considered and chiral stability issues discussed. 

Liu W, Qin S, Gan J (2005) Chiral stability of synthetic pyrethroid insecticides. J Agric Food Chem 53 3814-3820... 

Li Z, Zhang Z, Zhang L, Leng L (2009) Isomer- and enantioselective degradation and chiral stability of fenpropathrin and fenvalerate in soils. Chemosphere 76 509-516... 

Qin S, Budd R, Bondarenko S, Liu W, Gan J (2006) Enantioselective degradation and chiral stability of pyrethroids in soil and sediment. J Agric Food Chem 54 5040-5045... 

In an analogous approach, the chiral stabilized azomethine ylide 165, generated in situ via Lewis acid-catalyzed condensation of (53 )-5-phenylmorpholin-2-one 164 with 2,2-dimethoxypropane, was trapped diastereoselectively with singly and doubly activated dipolarophiles such as the acrylate (Scheme 24). The cycloadduct 84 was then employed to furnish enantiomerically pure 5,5-dimethylproline derivatives (see Section 11.11.6.3), <2001SL1836>. 

The molecular structure of a compound is very important. For example, one can usually deduce from the structure whether or not the compound will absorb UV radiation and be detectable with a UV detector. The molecular structure also reveals if the compound has ionizable functional groups and will require a mobile phase modifier if HPLC analysis is used. Examination of the molecular structure may also tell something about the chemical reactivity of the molecule. The molecular structure indicates whether the molecule contains any chiral centers. If the molecule is chiral and non-racemic, then an assay to determine chiral stability may be required. 

Also, the addition of copper plates in order to simulate the application of cupric vessels during lavender oil production does not influence the chiral stability of linalool and linalyl acetate. 

Reist, M., Carrupt, P. A., Erancotte, E., Testa, B. Chiral inversion and hydrolysis of thalidomide mechanisms and catalysis by bases and serum albumin, and chiral stability of teratogenic metabolites. Chem. Res. Toxicol. 1998, 11, 1521-1528. 

Chirally stabilized Pt-colloids can be prepared by reduction of an aqueous solution of PtCl, in the presence of protonated 10,11-dihydrocinchonidine (DH-CIN) [7], The positively charged alkaloid has not only a stabilizing effect on the platinum particles, it also can induce enantioselectivity in catalytic hydrogenations [7],... 

For entrapment, an aqueous solution of the chirally stabilized Pt-colloid is mixed with an aqueous solution of a polyanion while stirring (Figure 1). The resulting mixture is dropped onto a suitable surface (e. g. a polyethylene (PE) film) by a syringe equipped with a 1 mm capillary. Afterwards, the droplets were dried by e qx)sing flte film to air for at least 24 hours. Very plain lens-shaped particles of... 

Figure 1 Scheme of the heterogenization procedure for chirally stabilized platinum colloids 1) mixing the solutions, 2) dropping the mixed solutions on a suitable surftice e. g. a PE film, and drying the droplets afterwaids. 

Figure 2 Structure of different carbohydrates used for the heterogenization of chirally stabilized Pt-colloids.

Colloid catalysts with chiral stabilizing agents can be used as asymmetric catalysts in the reactions of prochiral compounds. One such catalyst is the well-known "Skita-catalyst"(colloidal Pt or Pd with gum-arabicum as an optical active polysaccharide as a protective colloid). These catalysts have been used very often in a number of hydrogenations of unsaturated compounds, including prochiral molecules, but never were their asymmetrizing action noted. Nevertheless, including chiral components as protective colloids in such catalysts allowed for the discovery of asymmetric effects in their action. Indeed, Balandin, Klabimovskii et al. found small... 

Vorlop et al. suggested a novel strategy for heterogenizing homogeneous and colloidal chiral catalysts based on the three-dimensional entrapment of catalyst by electrostatic attraction between a polyelectrolyte and an active metal in the polymeric matrix. Based on this method, they entrapped chirally stabilized Pt-colloid in alginate (polymannuronate) and conducted twenty-five hydrogenation cycles of EtPy. 

With respect to chiral stability amino acids are fairly insensitive to acids and bases. Racemization via enolization of carboxylic acids in acidic solutions involves protonation of the carbonyl oxygen... 

Chiral stabilized ylides were prepared by the reaction of the Bestmann ylide ((triphenylphosphoranylidene)ketene) with camphor-derived lactams that were used directly or after modification by alkylation in Wittig olefinations. Representative examples are shown in Scheme 92. The products can be regarded as chiral building blocks. ... 

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