Stereoselective reactions, definition examples

Following our definition of stereospecificity given above, it follows that both stereoselective reactions (transformations (155-163a/b) and nonstereoselective ones (transformations 157a/b-163a/b) can be stereospecific. Therefore, the first part of the statement "all stereospecific processes are stereoselective, but all stereoselective processes are stereospecific " is not true. By our definition, all stereospecific processes are not stereoselective for example, stereospecific transformations 157a/b-163a/b can be nonstereoselective. 

However, if both maleic and fumaric acid gave the dl pair or a mixture in which the dl pair predominated, the reaction would be stereoselective but not stereospecific. If more or less equal amounts of dl and meso forms were produced in each case, the reaction would be nonstereoselective. A consequence of these definitions is that if a reaction is carried out on a compound that has no stereoisomers, it cannot be stereospecific, but at most stereoselective. For example, addition of bromine to methylacetylene could (and does) result in preferential formation of trans-1,2-dibromopropene, but this can be only a stereoselective, not a stereospecific reaction. 

An interesting pericyclic-anionic-pericyclic domino reaction showing a high stereoselectivity is the cycloaddition-aldol-retro-ene process depicted in scheme 20.1581 The procedure presumably starts with a [4+2]-cycloaddition of diene 98 and S02 in presence of a Lewis acid. After opening of the formed adduct reaction with (Z)-silyl vinyl ether 99 leads to a mixture of alk-2-enesulfinic acids 101. It follows a retro-ene reaction which affords a 7 3 mixture of the products 102 and 103. The reaction described by Vogel et al is a nice example for the efficient generation of polypropionate chains with the stereoselective formation of three stereogenic centers and one (0-double bond in a three-component domino reaction in its strict definition. 

The term stereoselective is often confused with the term stereospecific, and the literature abounds with views as to the most satisfactory definition. To offer some clarification, it is perhaps timely to recall a frequently used term, introduced a decade or so ago, namely the stereoelectronic requirements of a reaction. All concerted reactions (i.e. those taking place in a synchronised process of bond breaking and bond forming) are considered to have precise spatial requirements with regard to the orientation of the reactant and reagent. Common examples are SN2 displacement reactions (e.g. Section 5.10.4, p. 659), E2 anti) elimination reactions of alkyl halides (e.g. Section 5.2.1, p.488), syn (pyrolytic) elimination reactions (Section 5.2.1, p.489), trans and cis additions to alkenes (e.g. Section 5.4.5, p. 547), and many rearrangement reactions. In the case of chiral or geometric reactants, the stereoisomeric nature of the product is entirely dependent on the unique stereoelectronic requirement of the reaction such reactions are stereospecific. 

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]. 

In this book we have attempted to amalgamate the various definitions described above. Thus, in many circumstances we have made efforts to include the first example of a certain stereoselective transformation, even if the selectivity was modest. We have incorporated early examples of auxiliaries or catalysts that helped to guide and define the development of the field, although they may have subsequently been surpassed. In trying to include exemplars of many of the reactions employed by the practioners of the field, we have inevitably faced the difficulty that any number of prototypes could have been selected, all of equal merit. Under these circumstances, there is no way of getting around the fact that the selection becomes subject to the authors own background and personal biases. 

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