Methyl groups stereospecific

As a part of the total synthesis of (+ )-corydalic acid methyl ester (12), a reaction of a cyclic enolate with an imine has been applied. The 2-toluamide enolate 9, which in this case is substituted at the methyl group, adds stereospecifically to imine 10, affording mainly tram-iso-quinolone 11 with a d.r. (transjds) > 95 525. 

The oxidation by strains of Pseudomonas putida of the methyl group in arenes containing a hydroxyl group in the para position is, however, carried out by a different mechanism. The initial step is dehydrogenation to a quinone methide followed by hydration (hydroxylation) to the benzyl alcohol (Hopper 1976) (Figure 3.7). The reaction with 4-ethylphenol is partially stereospecific (Mclntire et al. 1984), and the enzymes that catalyze the first two steps are flavocytochromes (Mclntire et al. 1985). The role of formal hydroxylation in the degradation of azaarenes is discussed in the section on oxidoreductases (hydroxylases). 

Due to the presence of the a-methyl groups, these agents exist as optical isomers. Both isomers usually produce DOM-like effects, and the R (-)isomers constitute the eutomeric series. In this regard then, the effects of these agents are stereoselective, but not stereospecific. In general, the R (-)isomers are twice as potent as their racemates and about 5 to 8 times more potent than their S (- )enantiomers. Some representative data are provided in table 1. 

Scheme 4.9 gives some examples of the use of boranes in syntheses of alcohols, aldehydes, ketones, amines, and halides. Entry 1 demonstrates both the regioselec-tivity and stereospecificity of hydroboration, resulting in the formation of trans-2-methylcyclohexanol. Entry 2 illustrates the facial selectivity, with the borane adding anti to the endo methyl group. 

Similarly, Z E -hexadiene gives a product that is stereospecific with respect to the diene methyl groups. 

There s a whole area of chemistry dealing with the spatial configurations of organic molecules called stereochemistry. To get into this area, you have to have molecules that have an asymmetrical carbon atom. That s one that has four dissimilar atoms or groups attached to it. PP has that condition on a repeating basis—the methyl groups on every ocher backbone carbon. Such a polymer can be stereoregular or stereospecific. 

By considering the three-dimensional shape of estrone (see Box 3.19), we can appreciate that nucleophilic attack from the upper face is hindered by the methyl group adjacent to the ketone. Therefore, the nucleophile can only approach from the lower face, and the product is formed stereospecifically. 

The stereoselectivity of the reaction can be rationalized in terms of the relative stabilities of the products 133 and 134. The stereoselectivity was more pronounced in the cyclohexane series. The stereoselectivity increases with increasing size of substituent R, and the reaction becomes a stereospecific process for the rert-butyl derivatives. The equatorial position of a bulkier substituent is preferred, and thus in the phenyl series the steric requirement of the phenyl group is smaller than that of the methyl group. 

The synthesis in Scheme 13.40 uses stereospecific ring opening of the epoxide to establish the stereochemistry of the C-4 methyl group. The starting material can be made by enantiospecific epoxidation of the corresponding allylic alcohol.137... 

The first naturally occurring tricyclo[6.3.0.0 ]undecane to be synthesized was isocomene (757), a colorless oily sesquiterpene hydrocarbon isolated from several plant sources. In 1979, Paquette and Han reported an efficient, stereospecific approach starting with a preformed bicyclic enone, to which the third five-membered ring was appended with proper attention to stereochemistry and position of unsaturation (Scheme LXXX) The pivotal steps are seen to be the stannic chloride-induced cyclization of aldehyde 732 and the conjugate addition of lithium dimethylcuprate to 733 which sets the stereochemistry of the last methyl group. 

Polypropylene (PP) is a stereospecific polymer prepared by polymerization using an organometallic catalyst system. Commercial polypropylenes have up to 95% isotactic content, which means that pendant methyl groups are almost all on the same side of the chain. 

Polypropylene (PP) is a stereospecific polymer prepared by polymerization using organo-metallic catalyst system. Commercial PPs have up to 95% isotactic content, which means that pendant methyl groups are almost all on the same side of the chain. When PP is exposed to ionizing radiation, free radicals are formed that cause chemical changes. Because PP is highly crystalline, these radicals are relatively immobile and consequently may not be available for reaction for long periods of time.96... 

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