Diastereoselective synthesis, definition

On the other hand, selective, usually applied to a synthesis, means that of all the possible isomers only one isomer is obtained. However, if the reaction product was/is a mixture of isomers one could speak then of the "degree of selectivity". Since usually one of the isomers will be the predominant isomer, we may say that the reaction (or the synthesis) is selective with respect to this particular isomer. As in the case of "specificity", we may refer to "regioselectivity" or to "stereoselectivity" (either diastereoselectivity or enantioselectivity) and may say, for instance, that a synthesis is 80% diastereoselective. According to the most updated terminology "diastereomers" are all the "stereoisomers" that are not "enantiomers", so geometrical isomers are also included in such a definition. 

One can define diastereoselectivity as the formation of diastereoisomers in a non-statistical ratio in any chemical transformation (formation of transition states included). Such a definition concerns equilibrium as well as nonreversible reactions. An asymmetric synthesis in a restricted sense can be considered as a reaction leading to a product containing at least one new stable dissymmetric center with a definite chirality. Such a reaction may take place in the coordination sphere of a metal ion. First of all the following question has to be answered which are the structural properties in the architecture of the coordination sphere that lead to the following phenomenona ... 

Alcohols can be obtained from many other classes of compounds such as alkyl halides, amines, al-kenes, epoxides and carbonyl compounds. The addition of nucleophiles to carbonyl compounds is a versatile and convenient methc for the the preparation of alcohols. Regioselective oxirane ring opening of epoxides by nucleophiles is another important route for the synthesis of alcohols. However, stereospe-cific oxirane ring formation is prerequisite to the use of epoxides in organic synthesis. The chemistry of epoxides has been extensively studied in this decade and the development of the diastereoselective oxidations of alkenic alcohols makes epoxy alcohols with definite configurations readily available. Recently developed asymmetric epoxidation of prochiral allylic alcohols allows the enantioselective synthesis of 2,3-epoxy alcohols. 

A second area concerns the use of the word chiral. This has a specific definition and can only be properly applied to three-dimensional objects. Thus it is incorrect to refer to a chiral centre/axis or a chiral synthesis . The former can be a chirality centre/axis or a centre/axis of chirality although for most purposes we prefer to replace these terms by stereogenic centre/axis (which is, however, not equivalent). [51 As a collective term for enantio- and diastereoselective... 

However, diastereoselective transformations like this are not to be discussed within this monograph, as they do not fulfill the criteria of asymmetric synthesis, according to Marckwald s definition (in today s language) this would mean [...] those reactions, or sequences of reactions, which produce chiral nonracemic substances from achiral compounds with the intermediate use of chiral nonracemic materials, but excluding a separation operation [35]. Thus, diastereoselective conversions not included for that reason in this book are, for example, aldol additions, Mannich reactions, and Michael additions of enolates to ketones, imines, and cx,P-unsaturated carbonyl compounds, respectively, with any chiral skeleton. For such stereoselective enolate reactions that are not asymmetric syntheses, the reader is referred to the literature, which treated this topic in a comprehensive manner [36]. 

Professor Barry M. Trost has provided useful definitions of various kinds of selectivity that apply to nearly all chemical transformations (136). The ideal reaction is perfectly chemoselective ( performs a wanted stractural change and none other ), regioselective ( orients the reacting partners in correct fashion ), diastereoselective ( creates the correct orientations of the various parts of the molecule with respect to each other ), and enantioselective ( enables the formation of a molecule of one-handedness or a mirror image isomer ). None yet meets all of these goals. Considerable challenges and opportunities remain for practitioners of synthesis. 

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