Stereochemical adsorption

Amprenavir (APV, Agenerase) is the most recently approved HIV protease inhibitor. It is smaller and stereochemically less complex than the other drugs in this class. Adsorption of this compound was found to be impaired by high fat meals. Common side effects of Amprenavir are nausea, vomiting, diarrhea, rash and a tingling sensation around the mouth. 

The stereochemical outcome of the reaction of steroidal enone 39 changed when adsorbed on alumina [43], Photocycloaddition in methanol occurred from the a side (83 17). In contrast, the fi product was favored when the reaction was carried out on alumina. The observed fi selectivity can be accounted for by the preferential adsorption of the sterically less hindered a side of the enone to the gel surface. 

The concept that the stereochemistry of reduction of an unsaturated hydrocarbon is determined at the adsorption stage of the reaction is evident in each of the above accounts, and others could be cited. However, the development of techniques which permit the identification of different product-controlling reactions directs one to consider the stereochemical consequences of various postulated reaction sequences and this will be discussed in Section IV. 

Both stereoisomers were formed, implying a loss of stereochemical integrity during the formation of the second carbon-carbon bond. When the reaction was conducted on ZnO, surface-related processes affected both the rate and stereochemistry. The effect of various quenchers could be explained as competitive adsorption at active sites, with or without interference with electron transfer. A reaction scheme involving formation of dimer, both in the adsorbed state and in solution, was proposed, the former route being the more important On CdS, the reaction could sometimes be induced in the dark as well because of the presence of acceptor-iike surface states. Neither particle size, surface area, nor crystal structure appeared to significantly influence the dimerization observations parallel to those found in the CdS photoinduced dimerization of N-vinylcarbazole... 

Infrared spectroscopic studies regarding the adsorption of pyridine on both anatase and rutile have been reported (136, 176, 194, 216,217). Hydrogen-bonded pyridine is readily desorbed on pumping at room temperature, whereas pyridine held by coordinatively unsaturated Ti4+ ions is thermally stable up to approximately 400°C. As ammonia, pyridine forms two distinct coordinately held species (176, 217) indicating the existence of two types of Lewis acid sites, which should correspond to Ti4+ ions in different stereochemical environments. According to Primet et al. (176), the more stable species is chemisorbed on type... 

The present volume continues our effort to provide diverse exposure. We include two articles devoted to stereochemical aspects of catalytic reactions (J. K. A. Clarke and J. J. Rooney R. L. Augustine), and one (J. D. Morrison, W. F. Masler, and M. K. Neuberg) devoted to the control of a yet more subtle level of chemical structure asymmetry (or optical activity) a comprehensive review of liquid phase organic oxidation catalysis (R. A. Sheldon and J. K. Kochi) a review of specific adsorption and poisoning action as a means to learn more about active sites (H. Knozinger) and some of the latest considerations to catalysis of molecular orbital theory (R. C. Baetzold). 

Multilayers of sterols,3 chlorophyll,4 and proteins5 have also been built. Those of sterols show specific adsorptive powers for digitonin, according to the stereochemical configuration of the hydroxyl group those of chlorophyll showed no fluorescence, although the quantity deposited was sufficient to give marked fluorescence in solution. 

It has been generally accepted that the orientation of adsorption of an unsaturated molecule onto the catalyst is controlled by a steric interaction or hindrance between the substrate and the catalyst in other words, the adsorption at a less hindered side of the substrate is more favored.149 The stereochemical outcomes of many hydrogenations have thus been explained by syn addition of hydrogen (from the catalyst) to the substrate at a less hindered side. Unless isomerization or some other opposing factors are concerned, such a theory may be successfully applied to those cases where the adsorption of substrate or the formation of half-hydrogenated state is the key step that... 

The relation between the rate of each reaction step and the steric stability of the intermediate species must also be considered to rationalize the stereochemical results. Consider the electrochemical reduction [Eq. (2)] of an alkyl halide (R-X) to the corresponding alkane (R-H) via a radical anion [(R-X) ], radical (R ), and anion (R ) mechanism (ECEC). The order of configurational stability may be R-H > (R-X) > R > R- in general, but the stability of R- is not always lower than that of R when the adsorption of R is weakened by electrostatic repulsion by the cathode. The situation is more compli-... 

Baizer and coworkers established the most brilliant industrial electroorganic synthesis of the hydrodimerization of acrylonitrile to adiponitrile. They extended this hydrodimerization to a variety of activated olefins and in some cases [41 3] paid attention to the stereochemistry of products. However, their stereochemical data were not enough to discuss the stereochemical course of the reaction. Afterward, an attempt was made to provide a working hypothesis in the hydrodimerization of cinnamates by considering an orientated adsorption of radical anion intermediates on a cathode surface, but this was not persuasive because of a lack of experimental data on the stereochemistry of both the starting olefins and products. Recently Utley and coworkers [44-46] have reported stereochemical data of hydrodimers derived from a variety of cinnamic acid esters with chiral alcohol components. 

Some fundamental studies were made earlier. It was found that the hydrodimerization of an O, j6-unsaturated ketone (enone) was at least partially stereospecific, with the d, I dimer formed from the /ranj-enone and mainly the meso dimer formed from the cw-enone. Kanetsuna and Nonaka [50] also examined both the stereoselectivity and stereospecificity of the hydrodimerization in detail. It was indicated that either the reaction mechanism or the stereochemical course is changed by the nature (protic or aprotic) of the reaction media. Particularly, the latter is greatly influenced by the manner of adsorption of an anion radical intermediate on the cathode. It was also postulated to explain the experimental results that the radical anion is adsorbed at the )6-position, where the radical p electron is delocalized and the adsorption is affected by the nature of the media. 

The stereospecificity, stereoselectivity, and regioselectivity of the fluoroacetamidation of a variety of olefins have been studied in detail from synthetic and mechanistic aspects by Laurent and coworkers, as in Eq. (51) [331-334]. The significant stereochemical results observed may be rationalized by considering coplanar adsorption on the anode of carbo-nium cation intermediates derived from the olefins. 

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