Stereoselective reactions characterized

A stereospecific chemical reaction is one in which starting substrates or reactants, differing only in their configuration, are converted into stereoisomeric products. Note, with this definition a stereospecific reaction has to be stereoselective whereas the inverse statement (that is, with respect to a stereoselective reaction or process) is not necessarily true. 2. Referring to reactions that act on only one stereoisomer (or, have a preference for one stereoisomer). Thus, many enzyme-catalyzed reactions are stereospecific, and characterization of that stereospecificity is always an issue to be addressed for a particular enzyme. 

The description of a stereoselective reaction primarily requires characterization of enantiomeric and/or diastereomeric products by their configuration (not their stereochemistry , see Introduction). Problems have not arisen with enantiomers but difficulties (see enumerations in refs 1 and 2) are. or perhaps were, apparent for diastereomers, a focal point having been acyclic compounds, in particular aldol addition products. These are pertinent examples to illustrate the problems and their various solutions very well ... 

The concept of stereospecificity has been introduced to characterize the stereochemical course of pairs of highly stereoselective reactions. Another term, stereoconvergence, is also very useful in this connection. 

It has been estimated that biochemists have isolated and characterized over two-thirds of the projected 2000 naturally occurring enzymes (26). Enzymes are known to catalyze a myriad of different chemical reaction types. Table 1 summarizes the dassiOcation of enzymes based on function and provides examples of the kind of stereoselective reactions exhibited by... 

Combination with a second C3BN ring leads to diastereoisomers which can be separated (46a,b). In the diastereoisomer (46a) the metal M uses the same ring sites, whereas in (46b) different sites are coordinated. Many examples have been described by Schmid where both isomers are formed and could be separated and characterized by single x-ray structure analysis <85MI 317-01 >. This chiral property of 1,2-azaborolyl rings in combination with metal atoms has some interesting consequences with respect to stereoselective reactions. 

Claisen rearrangement proceeds via a chair-like transition state (584), which was reported to be favored over the boat by 2.5-3.0 kcal moT (10.5-12.6 kJ moT ). 7 jn different work, Houk and co-workers estimated that the chair transition state for this reaction is more stable than the boat transition state by 6.6 kcal mol (27.6 kJ mol ). The synthetic applications of the Claisen rearrangement were reviewed by Ziegler and the stereoselectivity that characterizes this reaction is apparent in those applications. generic Claisen rearrangement is illustrated by heating an allyl vinyl ether such as 585, which rearranges to 586 via chair... 

Pentacoordinate Allylsilicates Characterization and Highly Stereoselective Reaction with Aldehydes... 

The fact that carbenes are characterized by their stereospecific transformations has effectively paved the way for numerous innovative developments. The discovery that rhodium acetate can function as a catalyst for carbene generation from diazoacetates [15, 16] with much higher efficiency than copper salts, which had been traditionally used, provided an opening into the vast possibilities offered by transition metal catalysis. The increasing number of novel, stereoselective reactions of carbenoid species have become an indispensable toolbox for the modern practitioner of chemical synthesis [17-22],... 

The fruitful relationship between experiment and theory has pushed carbene chemistry further toward the direction of reaction control that is, regio- and stereoselectivity in intra- and intermolecular addition and insertion reactions. The interplay between experiment and modem spectroscopy has led to the characterization of many carbenes that are crucial to both an understanding and further development of this held. 

For the overwhelming majority of nitronates, the reactions with monosubstituted olefins are characterized by the head-to-head approach of the olefin, that is, the substituent R is present at the C-5 atom. A general conclusion about stereoselectivity of this reaction (endo or exo approach of olefin to the dipole (Scheme 3.127)) cannot be drawn. However, the exo approach prevails for nitronates. (Possible factors responsible for discrimination of the facial approach will be discussed below in Section 3.4.3.5). 

In particular, a series of isoxazolines (211) fused to the five-membered ring was synthesized (193) (Scheme 3.149, Eq. 1). As a rule, the reactions are characterized by high stereoselectivity (dr > 20 1). 

When choosing a concrete scheme for the synthesis of a complicated substrate in terms of the strategy proposed by Prof. Denmark, not only the characteristic features of the target product but also other factors should be taken into account. As a mle, intramolecular cycloaddition reactions proceed more easily and are characterized by higher stereoselectivity, but the starting substrates are much more difficult to synthesize. 

The C,C-coupling reactions of ambident nucleophile CH2=CHCH=(OMe)O.S occur regioselectively at the terminal C=C bond, but the reactions are characterized by low stereoselectivity. 

The very fact that chemical catalysis involves the formation of an adduct opens up possibilities of selectivity, particularly stereoselectivity, that are absent in redox catalysis. Several examples of homogeneous chemical catalysis are described in the following section, illustrating the improvements that can be achieved when passing from redox to chemical catalysis. It remains true that redox catalysis has several useful applications that have already been discussed, such as kinetic characterization of fast follow-up reactions (Section 2.3) and determination of the redox properties of transient radicals (Section 2.6.4). 

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