Product stereoselectivity

The palladium-catalyzed silastannation of bis(allene)s gives trans-cyclized product stereoselectively (Equation (121)).162 This stereochemical outcome stands in contrast to the distannation of the bis(allene)s, which affords m-cyclized product. 

Photocycloaddition proceeds between allylsilane and A-methylphthalimide to yield a mixture of [2 + 2] and [4 + 2] adducts along with the allylated product291. Intramolecular cycloadditions of the vinylsilanes with the cyclopentenone moieties in 200 furnish good yields of cyclic products stereoselectively (equation 164)292. In the presence of 1,4-dicyanonaphthalene, diallylsilane 201 undergoes an intramolecular photocycloaddition reaction in an aromatic solvent to give a four-membered ring product (equation 165)293. 

Olah et al.724 have shown that Nafion-H induces the ring opening of oxiranes under mild conditions to afford various products. Substituted oxiranes undergo hydrolysis or alcoholysis to yield 1,2-diols or 1,2-diol monoethers, when treated with Nafion-H under mild conditions in the presence of water or alcohols, respectively. Cycloalkene oxides give the corresponding trans products stereoselectively [Eq. (5.273)]. 

The symmetry of the ligand can influence the product stereoselectivity and enantio-selectivity. Enantiomerically pure chemicals are extremely important for the agrochemical, pharmaceutical, and food industries. Many of the bidentate ligands used in these processes feature C2-type symmetry, dividing the space around the metal center into two empty quadrants and two full quadrants (see Section 3.1.3 on asymmetric homogeneous catalysis) [61,62], Ligand symmetry is also important in polymerization catalysis [63], where it can influence the polymer s tacticity (Figure 3.22). 

The selectivity of a productive reaction refers to the relative amounts of P, P at the time of observation. The ratio of the amounts of P and P which are formed is the ratio of the corresponding rate constants, if the stereoselective is a pair of corresponding reactions53. If, however, the productive stereoselective reaction is a more complex kinetic scheme, then the ratio of the amounts of any two stereoisomeric products, P and P , which depends on time and pairs of the appropriate kinetic constants, has a positive lower bound and a finite upper bound. Both of these bounds are the ratios of two rate constants54. However, since the free enthalpy difference of stereoisomeric transition states is due to different non-bonded interaction and does not, as a rule, exceed 3 kcal/mole, and since the rate constant ratio depends on the free enthalpy difference, this ratio has a rather low upper bound. Accordingly, the stereoselectivity of productive reactions is generally low (50—90% relative yield of the preferred product in most cases). 

Recently, Inoue et al. have established the concept of multidimensional control of asymmetric photochemistry [58]. Applying this strategy, the product stereoselectivity can be inverted by environmental factors such as the temperature, pressure or solvent, and this control has been interpreted in terms of the contributions of both enthalpy (AH+) and entropy (AS+). Although originally designed for enantiodifferentiating photo sensitization of cycloalkenes [58], Miranda et al. were able to apply it to PET cyclizations of o-allylaniline derivatives [59]. In particular, irradiations at different temperatures revealed a significant entropy-controlled diastereoselectivity for compound 72 and the equipodal temperature was found at 292 K (Sch. 34). 

A number of families consisting of one or more distinct proteins comprise this enzyme group. Many xenobiotics are preferentially metabolized by a particular CYP or CYP family however, they may also be substrates for other CYPs. In these instances, the differences in enzyme activity are in the rate at which the oxidation occurs, the site of the modification on the parent compound (regioselectivity), and the steric configuration of the product (stereoselectivity). 

Radical Addition and S 2 Displacement (Athene a-Functionalized Product) Stereoselective radical additions to Af-enoylsultams occur at the a-position, while additions to the p-position are essentially nonselective. The Sn2 displacement of y-bromo-N-enoylsultams with higher order cyanocuprates occurs with good n -face selectivity (90-96% de). ... 

Mechanistical studies, for example, on the stereochemistry of the I -hydroxylation of (R)- and (S)-r-deuterated-phenylelhane by purified cytochrome P450lm2 have demonstrated that the product stereoselectivity most probably results from constraints of the substrate binding site provided by the protein environment, rather than the intrinsic hydroxylation mechanism of cytochrome P450 (White et al., 1986). 

Testa, B. (1988). Substrate and Product Stereoselectivity in Monooxygenase-Mediated Drug Activation and Inactivation, Biochem. Pharmacol., 37 85-92. 

Indirect electroreductive cyclization of y-bromo-Q, yS-unsaturated esters using Co(III) or NI(II) redox mediator provides six-membered ring products stereoselectively, as in Eq. (33) [227]. 

Bioreduction of ketones often leads to (he creation of an asymmetric center and. thereby, two possible stereoisomeric alcohols. " For example, reduction of acetophenone by a soluble rabbit kidney reductase leads to the enantiomeric alcohols (5)(-)- and (R)( + )-mcthylphen lcarbinol. with the (.V)(-) isomer predominating (3 1 ratio). The preferential formation of one stereoisomer over the other is termed product stereoselectivity in drug metabolism. " Mechanistically, ketone reduction involves a "hydride" transfer from the reduced nicotinamide moiety of the cofactor NADPH or NADH to (he carbonyl carbon atom of the ketone. It is generally agreed that this step proceeds with considerable stereoselectivity." Consequently, it is not surprising to find many reports of xenobiotic ketones that are i uced prefer-emi ly to a predominant stereoisomer. Often, ketone reduction yields dcohol metabolites that arc pharmacologically active. 

Chirality may feature in one of the following ways (a) substrate stereoselectivity (b) product stereoselectivity (c) inversion of configuration (d) loss of chirality. 

Products formed at different rates in product-selective reactions may also share various types of relationships. Thus, they may be analogs, regioisomers, or stereoisomers, resulting in product selectivity (narrow sense), product regioselectivity or product stereoselectivity (e,g, product enantioselectivity). Note that the produet selectivity displayed by... 

Product stereoselectivity occurs when ste-reoisomeric metabolites are generated (l)dif-ferently (in quantitative and/or qualitative terms) and (2) from a single substrate with a suitable prochiral center or face. Examples of metabolic pathways producing new centers of chirality in substrate molecules include ketone reduction, reduction of carbon-carbon double bonds, hydroxylation of prochiral methylenes, oxygenationof tertiary amines to N-oxides, and oxygenation of sulfides to sulfoxides. Product stereoselectivity may be a re-... 

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