Asymmetric yield

The synthesis of threonine can be made stereospecific using optically active complexes of the type L-[Co(en)2Gly]2+ but with low asymmetric yield.442 In the case of dipeptide complexes only... 

Both competing reductions consume the cofactor nicotinamide adenine dinucleotide (NADH) and thereby interfere with the redox balance of the cell and feedback on glycolysis where NADH is regenerated on the one hand, while on the other hand NAD+ is required to keep the glycolytic pathway running. The nonlinear dynamical model combines the network of glycolysis and the additional pathways of the xenobiotics to predict the asymmetric yield (enantiomeric excess, ee) of L-versus D-carbinol for different environmental conditions (Fig. 3.4). Here, the enantiomeric excess of fluxes vy and i>d is defined as... 

On the other hand, rhodium complexes with fully alkylated phosphine ligands were used for the hydrogenation of carbonyl compounds, a-ketoamides " and ketopantolactone (66% ee). When a rhodium complex with CyDlOP (23) was used, a slightly higher asymmetric yield (71% ee) was observed in the hydrogenation of a-ketoamides. Hydrogenation of A(-(a-ketoacyl)amino acid esters was... 

Asymmetric reduction of activated olefins has been performed using prochiral cumarin derivatives [414—416]. A maximum asymmetric yield reported was 17%, as in Eq. (62) [415], and an asymmetry induction mechanism has also been discussed [416]. Later, Schafer and coworkers carried out similar enantioselective electroreduction of various 4-substituted cumarins by systematic variation of the electrolysis conditions, and they obtained optical yields as high as 67% [Eq. (62)] [417-419]. Computational chemistry using semiempirical AMI demonstrates that ri-protonation of the intermediate anion by protonated yohimbine to form the (i )-isomer is energetically favored [418,419]. 

The asymmetric reduction of prochiral ketones to chiral alcohols has been extensively investigated [420 28], and a maximum asymmetric yield of 48.4% has been reported using 2-acetylpyridine [421]. 

The asymmetric reduction of C=N double bonds has been investigated using prochiral oximes [427-431]. The maximum asymmetric yield is 18% [428]. 

Not only asymmetric yields of products but also their absolute configurations vary drastically with cathode potential, supporting electrolyte, solvent, pH, cathode material. 

This method was applied to the asymmetric reduction of ketones [440 44] and imines [439,445]. Maximum asymmetric yields reported for the former and the latter are 20% [441] and 8.95% [445], respectively. A higher asymmetric yield (20.6%) was obtained in the hydrodimerization of a ketone [444]. It is a problem that lower asymmetric yields are obtained using much larger amounts of asymmetry inducers, which must play the role of supporting electrolyte. 

Seebach and Oei [446,447] reported the asymmetric hydrodimerization of acetophenone (a maximum asymmetric yield of 6.4%) in a chiral cosolvent. The use of small amounts of chiral crown ethers was attempted however, no significant asymmetric induction was observed [443]. It is interesting that in the presence of /5-cyclodextrin, head-to-tail coupling of acetophenone leads to optically active dimeric monoalcohol (ca. 24% ee), whereas the head to head coupling gives optically inactive pinacols [448,449]. 

Intramolecular asymmetric induction has also been used in electrochemistry as in the reduction of optically active alcohol esters or amides of a-keto [469,470] and unsaturated [471] acids and oximes [472] and in the oxidation of olefins [473]. A maximum asymmetric yield of 81% was obtained in the reduction of (5 )-4-isopropyl-2-oxazolidinone phenyl-glyoxylate [470]. Nonaka and coworkers [474,475] found that amino acid A-carboxy anhydrides were polymerized with various electrogenerated bases as catalyst to give the poly(amino acids) with high chirality in high yields. Conductive chiral poly(thiophenes) prepared by electropolymerization can be used for chiral anion recognition [476]. 

In simulation of the intramolecular asymmetric reduction using chiral but optically inactive (racemic) substrates, such as ketones [103,104] and gem-dihalopropanes [477], asymmeric yields higher than 80% could be obtained. It was also found that the intramolecular asymmetric reduction could be improved in asymmetric yield by using an optically active supporting electrolyte [445,478,479]. 

A number of 1,1-dihalocyclopropanes have been reduced electrochemically to monohalides using a mercury cathode in an appropriate solvent containing a supporting electrolyte, e.g. reduction of 38 (Table 4)112,130 ggg j.gj-g 128-130, 132, 133, 745, 758), but few reactions have been carried out on a preparative scale. One exception is 9,9-dibromobicyclo[6.1.0]nonane which is converted stereospecifically to exo-9-bromobicyclo[6.1.0]nonane (39) in better than 80% yield at a mercury cathode at 0°C using methanol as solvent and lithium chloride as supporting electrolyte. A reaction resulting in 17% asymmetric yield is described in ref 886. 

Figure 8.6 Asymmetric yields of induction for monomer 4 in Table 8.1, when grown in the presence of (+) dimer and (—) dimer as additives.

Seelig, F. F. (1971a). System-theoretic model for the spontaneous formation of optical antipodes in strongly asymmetric yield. J. Theor. Biol., 31, 335-61. 

The IR and Raman spectra of organosilicon compounds have been thoroughly studied and largely systematised. Shifts in the valence vibrations (symmetrical) and Vas (asymmetrical) yield essential information. If one or two methyl groups in tetramethylsilane (Vj = 598 cm", = 696 cm" ) are replaced by carbofunctional... 

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