Reactions stereochemistry

Interpretation of reaction stereochemistry has focused attention on the role played by bridged halonium ions. 

Benzene-sensitized photolysis of methyl 3-cyclohexene-1-carboxylate in acetic acid leads to addition of acetic acid to the double bond. Only the trans adducts are formed. What factor(s) is (are) responsible for the reaction stereochemistry Which of the two possible addition products, A or B, do you expect to be the major product ... 

Palladium, (diammine)bis(thiocyanato)-isomerism, 1, 185 Palladium, dichlorobis(amine)-substitution reactions stereochemistry, 1, 318 Palladium, dichlorobis(pyridine)-substitution reactions, 1, 314 Palladium, dinitritobis(triisopropylphosphine)-substitution reactions, I, 314 Palladium, ethylene-synthesis... 

Enantioselective enolate alkylation can be done using chiral auxiliaries. (See Section 2.6 of Part A to review the role of chiral auxiliaries in control of reaction stereochemistry.) The most frequently used are the A-acyloxazolidinones.89 The 4-isopropyl and 4-benzyl derivatives, which can be obtained from valine and phenylalanine, respectively, and the c -4-methyl-5-phenyl derivatives are readily available. Another useful auxiliary is the 4-phenyl derivative.90... 

It is sometimes difficult to distinguish clearly between these mechanisms, but determination of reaction stereochemistry provides one approach. The tme one-step insertion must occur with complete retention of configuration. The results for the two-step process will depend on the rate of recombination in competition with stereorandomization of the radical pair intermediate. 

Enzymatic Reactions, Stereochemistry of, by Use of Hydrogen Isotopes (Arigoni and Eliel) 4 127... 

The Holton, Nicolaou, Danishefsky, and Wender syntheses of baccatin III structures employ various cyclic intermediates and take advantage of stereochemical features built into these rings to control subsequent reaction stereochemistry. These syntheses also provide numerous examples of the selective use of protective groups to differentiate between the several hydroxy groups that are present in the intermediates. 

The control of reactivity to achieve specific syntheses is one of the overarching goals of organic chemistry. In the decade since the publication of the third edition, major advances have been made in the development of efficient new methods, particularly catalytic processes, and in means for control of reaction stereochemistry. For example, the scope and efficiency of palladium- catalyzed cross coupling have been greatly improved by optimization of catalysts by ligand modification. Among the developments in stereocontrol are catalysts for enantioselective reduction of ketones, improved methods for control of the... 

Ai initio calculations of the Diels-Alder reaction leading to the formation of ethyl esters of hexahydropyrano[3,4-( ]-pyrroles have been carried out. The calculations predicted the same reaction stereochemistry that was experimentally obtained - thermodynamically favored products resulted from an jco-cycloaddition reaction whereas OTrfb-cycloaddi-tion favored the kinetic products (Equation 1) <2003T8955>. 

Another aspect of the reaction is the stereochemistry. There are three points of interest— C-1, C-3 and C-5 in a 1,4-pentadiene type di-7t-methane reactant. If C-5 is taken as the carbon of that vinyl group which remains in the photoproduct, that stereochemistry—cis or trans—is retained32. The reaction stereochemistry at C-3 (the methane carbon ) has been shown to be inversion of configuration33-34 (equations 9a and 9b). The situation at C-l is... 

The evidence for such a mechanism results from both the reaction stereochemistry and also from the observation of minor diphenyltoluene by-products (Scheme 8). The major pathway is outlined using heavy arrows. This can be seen to afford the stereospecificity of equations 14a and 14b. Additionally, each of the diradical intermediates and intermediate excited states—B, C, E, F and G—undergo a minor extent of internal bond fission [i.e. Grob or 2,3- (1,4) fragmentation] to afford a diphenyltoluene with the corresponding ring skeleton. The basis for the choice of a main pathway versus the minor ones comes from the observed stereochemistry. 

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