Epimerization reactions additions

To corroborate that the epimerization reaction is responsible for the stereoerrors in the polypropylene chain, we treated 1-octene with complex 14 activated with MAO. The reaction resulted in the quantitative formation of frans-2-octene. In addition, the reaction of allylbenzene with the system 11/MAO at room temperature produced 100% conversion of the former to trans-methylstyrene. The isomerization results indicate that during the polymerization, the metal center in 14 is also able to migrate through the growing polymer chain, inducing branching and additional environments for the methyl ligands (Scheme 2) [124]. 

By adding only about ten additional statements to this program, a program for simulating reversible polymer modification and epimerization reactions was derived. Use of CSMP to generate plotted output and to evaluate kinetic parameters are also illustrated. 

Aminoacylation assays have revealed that overexpressed peptide synthetases are only incompletely pusttranslationally modified by pantetheine. Tyrocidine synthetase 1 expressed in E. coJi contains about 1.5% holo-enzyme (80). The reduced cofactitr content leads to decreased activities in aminoacylation and epimerization reactions, which require the cofactor (64,80,84)- Some evidence has been obtained that apo-eniymes may slightly differ from holo--en2ymes with respect to catalytic properties (80). Addition of 4 -phos-phopantetheine is thought to be catalyzed by a holo-enzyme synthase utilizing CoA ... 

Carboxylative TMM q cloaddition has also been realized with 3-methoxytropone and precursor (56) to produce an epimeric mixture of acids (122), which was employed in a synthetic study of the bicyclic diterpene sanadaol (123). The use of bi-dentate ligand tpdp (12) and high pressure did not improve the reaction. However, the addition of MesSnOAc as a co-catalyst did produce a better yield of (122) (Scheme 2.33) [16]. 

A mixture of 50 g of betamethasone, 50 cc of dimethylformamide, 50 cc of methyl orthobenzoate and 1.5 g of p-toluenesulfonicacid Is heated for 24 hours on oil bath at 105°C while a slow stream of nitrogen is passed through the mixture and the methanol produced as a byproduct of the reaction is distilled off. After addition of 2 cc of pyridine to neutralize the acid catalyst the solvent and the excess of methyl orthobenzoate are almost completely eliminated under vacuum at moderate temperature. The residue Is chromatographed on a column of 1,500 g of neutral aluminum oxide. By elution with ether-petroleum ether 30 g of a crystalline mixture are obtained consisting of the epimeric mixture of 170 ,21 -methyl orthobenzoates. This mixture is dissolved without further purification, in 600 cc of methanol and 240 cc of methanol and 240 cc of aqueous 2 N oxalic acid are added to the solution. The reaction mixture is heated at 40°-50°C on water bath, then concentrated under vacuum. The residue, crystallized from acetone-ether, gives betamethasone 17-benzoate, MP 225°-231°C. 

Commercial A -acetylneuraminic acid aldolase from Clostridium perfringens (NeuAcA EC 4.1.3.3) catalyzes the addition of pyruvate to A-acetyl-D-mannosamine. A number of sialic acid related carbohydrates are obtained with the natural substrate22"24 or via replacement by aldose derivatives containing modifications at positions C-2, -4, or -6 (Table 4)22,23,25 26. Generally, a high level of asymmetric induction is retained, with the exception of D-arabinose (epimeric at C-3) where stereorandom product formation occurs 25 2t The unfavorable equilibrium constant requires that the reaction must be driven forward by using an excess of one of the components in order to achieve satisfactory conversion (preferably 7-10 equivalents of pyruvate, for economic reasons). 

Due to the reversibility of this nitroaldol reaction, the easy epimerization at the nitro-sub-stituted carbon, and the often low yields in reactions with nitro compounds other than ni-tromethane, few stereoselective additions have been reported. Highly stereoselective reactions are known for the synthesis of cyclic systems (see Section 1.3.5.6.6.). 

Enolates derived from various imino compounds have been sulfinylated in reactions analogous to those shown by equations (14) and (15). Some representative examples are shown in equations 16-18. Here again, these compounds have been utilized in asymmetric syntheses. Addition of sulfinate ester 19 to a THF suspension of a-lithio-N,N-dimethylhydrazones, derived from readily available hydrazones of aldehydes and ketones, leads to a-sulfinylhydrazones in good yield, e.g. 53 and 54 (equations 16 and 17)85,86. Compounds 53 and 54 were obtained in a 95/5 and 75/25 E/Z ratio, respectively. The epimer ratio of compound 53 was 55/45. Five other examples were reported with various E/Z and epimeric ratios. 

GC analysis revealed > 290 1 selectivity anti. syn) in the diethylzinc addition. The 4-acetoxy-l,3-dioxane 152 used in the above experiments was a 24 1 mixture of diastereomers, epimeric at the 2-position. This implies that the acetal stereocenter undergoes isomerization to the most stable oxocarbenium ion prior to reaction with Et2Zn. Conclusive evidence for this was obtained when submission of compound 156 to the identical conditions produced 155 as the major product (Eq. 26). 

Epimerization at C-5 of hexuronic acids is a reaction that proceeds both on the polymer and on the sugar nucleotide level. In addition to the three pairs of parent acids, namely, the u-glucolL-ido-, D manno/L-gulo-, and D-ga-lacto/L-altro-he uron c acids, the 2-amino-2-deoxy acids belonging to the last two and the 2,3-diamino-2,3-dideoxy acids belonging to the middle pair have been found. 

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