Stereoselective products

Optically active iV-unprotected-2-pyrrolidinones 194 were obtained from selenocarboxylate or allylamine via radical cyclization and subsequent one-step cleavage of the C-O and C-N bond of the inseparable mixture of the two bicyclic oxyoxazolidinones 192 and 193 with -Bu4NF. The initial radical reaction is highly stereoselective. Products were obtained with ee up to 90%. The mandelic acid 195, which served as the chiral auxiliary in this method, was recovered with no loss of optical activity (Equation 33) <2003T6291>. 

Erdelyi, B., Szab6, A., Seres, G., Birincsik, L., Ivanics, J., Szatzker, G. and Poppe, L., Stereoselective production of (S)-l-aralkyl- and 1-arylethanols by freshly harvested and lyophi-lized yeast cells. Tetrahedron Asymm., 2006, 17, 268-274. 

However, orbital factors may override thermodynamic control. For example, the regiochemistry of nucleophilic attack on the bridged norcaradiene radical cation 122 shows a significant deviation from thermodynamic control. Although attack on the cyclopropane ring should be favored by both release of ring strain and formation of delocalized free radicals (cf. Scheme 6.8), methanol attacks 122 " selectively at C2 (and C5), generating 123 and 124. There is little stereoselectivity Products derived from 123 and 124 were formed in comparable yields. ... 

Nishizawa M, Shimizu M et al (1995) Stereoselective production of (+)-tram-chrysanthemic acid by a microbial esterase cloning, nucleotide sequence, and overexpression of the esterase gene of Arthrobacter globiformis in Escherichia coli. Appl Environ Microbiol 61 3208-3215... 

Although the hydrogen atoms are transferred one at a time, this reaction is fast enough that both of these atoms usually end up on the same side of the C=C double bond. This can t be seen in most alkanes produced by this reaction because of the free rotation around C—C bonds. Reduction of a cycloalkene, however, gives a stereoselective product. 

Reduction of 1,2-dimethyl-1-cyclopentene (6.5) with hydrogen and Ni or Pt catalyst gives stereoselective product, ds-l,2-dimethylcyclopentane (6.6). 

It is well known that silylation of allyl derivatives with vinylsilane catalyzed by a ruthenium hydride complex is accompanied by isomerization ofpropen-l-yl to propen-2-yl derivatives as well as homo-coupling of vinylsilane when equimolar amounts of the initial substances are used. If catalyst I was used in the SC of allyl amide and allyl amine with vinylsilanes, a S-fold excess of olefin to vinylsilane was used to stop homocoupling of vinylsilane, but simultaneously no more than 5% of isomerization of allyl compound was observed [19, 26]. When allyl boronate is used instead of allylamine under mild conditions (20 - 40 °C), the two reactions catalyzed by I and IV yield stereoselectively -product (see Scheme 4) [26]. 

Another example of a protease-catalyzed commercial process, which in this case uses the enzyme in a synthetic mode, is the completely regio- and stereoselective production of the low-caloric sweetener aspartame developed by DSM-TOSOH [15] (Fig. 7.9). Aspartame is a dipeptide consisting of the amino acids phenylalanine and aspartic acid, which are coupled by the enzyme thermolysin from Bacillus thermoproteolyticus. For an efficient coupling, relatively high temperatures are required and the amount of water in the system must be kept low to drive the reaction in the desired direction. Thermolysin, which is a metallo-endoprotease, meets these two requirements. It is thermostable, and it works in an organic solvent, which is required to keep the water activity low. In practice, however, organic solvents were not necessary, since the product aspartame forms an insoluble complex with unreacted D-Phe-OMe, which crystallizes out of the aqueous medium. 

However, the chiral bis-bisoxazolinate complexes did not lead to stereoselective products and only atactic polymers were produced. 

M. E. Jung, D. Sim, Stereoselective production of ( -amino alcohols and p-thioacyl alcohols via an application of the non-aldol aldol process. Tetrahedron Lett. 40 (1999) 8343. 

Tsao, C.C., S.J. Coulter, A. Chien, G. Luo, N.P. Clayton, R. Maronpot et al. (2001). Identification and localization of five CYP2Cs in murine extrahepatic tissues and their metabolism of arachidonic acid to regio- and stereoselective products. J. Pharmacol. Exp. Ther. 299, 39 7. 

The Julia reaction is under thermodynamic control as equilibration occurs under the reaction conditions. The stereoselective product is the E-alkene. 

The stereochemistry of the reaction is controlled by RNA-diene interactions. The RNA interacts strongly and stereoselectively with the cycloaddition products, requiring several structnral features to be present. Strong and stereoselective product inhibition is... 

Torus-shaped a-cyclodextrin, which is a molecule-discriminating molecule, has been used as an enzyme model, since it exhibits an enzyme-like behavior such as stereoselective acceleration leading to a stereoselective product,... 

Hydrogenation with a chiral rhodium complex as catalyst affords stereoselective products as shown in eq. (18.33). When the racemate of the raw material is hydrogenated in 54% conversion, the R isomer, which is less reactive, is obtained in 84% purity, and when the racemate is hydrogenated in 70% conversion, pure R isomer is obtained. HCOOH/NEts is used as the hydrogen source. The reaction shown in eq. (18.33) is the reverse reaction of that shown in Scheme 18.2 which is the hydrogenation of carbon dioxide [67]. 

The mechanism of this cycloaddition can be explained by FMO theory, in which one component acts as a HOMO and other as LUMO in a favorable low-energy TS to afford a stereoselective product. The Woodward-Hofifmann mles for electro-cyclic reactions are also applied to this cycloaddition reaction. The reaction of an alkene with a carbene is considered as a 4n electron process and of a conjugated diene with an electrophilic molecule as a 4n+2 electron process. Therefore, for thermal reaction of 4n electron process, conrotatory motion of the substituents from the termini of the n system will favor a low-energy TS to afford the product and in photochemical process, the reverse disrotatory mode of motion will be the favored path. Similarly, for a 4n + 2 electron process, disrotatory mode is a symmetry allowed process in thermal reaction and conrotatory mode for its photochemical reaction. 

Sanchez-Barba LF, Garces A, Femandez-Baeza J, et al. Stereoselective production of poly(rac-kctide) by ROP with highly efficient bulky heteroscorpionate alkyknagnesium initiators. Organometallics. 2011 30 2775-2789. 

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