Reaction stereoselective

Chiral compounds are very important substances. Many natural products, medicinal compounds, and biomolecules exist as a single, optically active stereoisomer. Furthermore the opposite enantiomer or diastereomer may not have any physiological activity or may, in fact, have a detrimental physiological effect. There is therefore great interest in reactions in which only one stereoisomeric form of a compound is produced by a particular synthetic sequence. 

Since the stereochemical changes in each reaction of the sequence are known, a particular amino acid starting material (R or S) will give a particular configuration in the product. In this strategy of asymmetric synthesis, all or part of die final molecular skeleton is derived from the chiral precursor. While simple, diis strategy is limited by the size of the chiral pool and by the types of reactions which occur stereospecifically at tetrahedral centers. 

It is much more common for reactions to produce new chiral centers from achiral starting materials. Consequently, if we are to use the whole arsenal of synthetic methods available to us and at the same time produce single stereoisomers, then we must be able to control (or at least understand) the stereochemistry of reactions occurring at achiral centers. 

Since chiral centers are most commonly tetrahedral, die conversion of trigonal centers to tetrahedral centers by some type of addition process is die most common way in which new chiral centers are created. The reaction of carbonyl 

The Re-Si nomenclature enables the faces of a carbonyl group to be differentiated stereochemically however, the carbonyl group itself is achiral. Moreover, the Re-Si designation is not indicative of the stereochemistry of the chiral center produced by addition. In the above example hydride addition to the Si face gives 

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The condensation of aldehydes or ketones with secondary amines leads to "encunines via N-hemiacetals and immonium hydroxides, when the water is removed. In these conjugated systems electron density and nudeophilicity are largely transferred from the nitrogen to the a-carbon atom, and thus enamines are useful electroneutral d -reagents (G.A. Cook, 1969 S.F. Dyke, 1973). A bulky heterocyclic substituent supports regio- and stereoselective reactions. 

Open-chain 1,5-polyenes (e.g. squalene) and some oxygenated derivatives are the biochemical precursors of cyclic terpenoids (e.g. steroids, carotenoids). The enzymic cyclization of squalene 2,3-oxide, which has one chiral carbon atom, to produce lanosterol introduces seven chiral centres in one totally stereoselective reaction. As a result, organic chemists have tried to ascertain, whether squalene or related olefinic systems could be induced to undergo similar stereoselective cyclizations in the absence of enzymes (W.S. Johnson, 1968, 1976). 

About ten years later in J. Am. Chem. Soc., Vol. 113, No. 23 (1991) and Vol. 114, No, 21 (1992) we still find simple target molecules. Stereoselective reactions, mostly alkylations, prevail now. How can the problems of regio- and stereoselectivities be solved Read the papers and learn ... 

Many stereoselective reactions have been most thoroughly studied with steroid examples because the rigidity of the steroid nucleus prevents conformational changes and because enormous experience with analytical procedures has been gathered with this particular class of natural products (J. Fried, 1972). The name steroids (stereos (gr.) = solid, rigid) has indeed been selected very well, if one considers stereochemical problems. We shall now briefly point to some other interesting, more steroid-specific reactions. 

Silyl ethers serve as preeursors of nucleophiles and liberate a nucleophilic alkoxide by desilylation with a chloride anion generated from CCI4 under the reaction conditions described before[124]. Rapid intramolecular stereoselective reaction of an alcohol with a vinyloxirane has been observed in dichloro-methane when an alkoxide is generated by desilylation of the silyl ether 340 with TBAF. The cis- and tru/u-pyranopyran systems 341 and 342 can be prepared selectively from the trans- and c/.y-epoxides 340, respectively. The reaction is applicable to the preparation of 1,2-diol systems[209]. The method is useful for the enantioselective synthesis of the AB ring fragment of gambier-toxin[210]. Similarly, tributyltin alkoxides as nucleophiles are used for the preparation of allyl alkyl ethers[211]. 

In describing the stereochemical features of chemical reactions, we can distinguish between two types stereospecific reactions and stereoselective reactions. A stereospecific reaction is one in which stereoisomeric starting materials aflFord stereoisomerically different products under the same reaction conditions. A stereoselective reaction is one in which a single reactant has the capacity of forming two or more stereoisomeric products in a particular reaction but one is formed preferentially. 

Some stereospecific reactions are listed in Scheme 2.9. Examples of stereoselective reactions are presented in Scheme 2.10. As can be seen in Scheme 2.9, the starting materials in these stereospecific processes are stereoisomeric pairs, and the products are stereoisomeric with respect to each other. Each reaction proceeds to give a single stereoisomer without contamination by the alternative stereoisomer. The stereochemical relationships between reactants and products are determined by the reaction mechanism. Detailed discussion of the mechanisms of these reactions will be deferred until later chapters, but some comments can be made here to illustrate the concept of stereospecificity. 

The stereoselective reactions in Scheme 2.10 include one example that is completely stereoselective (entry 3), one that is highly stereoselective (entry 6), and others in which the stereoselectivity is modest to low (entries 1,2,4, 5, and 7). The addition of formic acid to norbomene (entry 3) produces only the exo ester. Reduction of 4-r-butylcyclohexanone (entry 6) is typical of the reduction of unhindered cyclohexanones in that the major diastereomer produced has an equatorial hydroxyl group. Certain other reducing agents, particularly sterically bulky ones, exhibit the opposite stereoselectivity and favor the formation of the diastereomer having an axial hydroxyl groi. The alkylation of 4-t-butylpiperidine with benzyl chloride (entry 7) provides only a slight excess of one diastereomer over the other. 

If R and R are different, the two faces of the double bond become nonequivalent, permitting stereoselective reactions at the double bond. These effects have been explored, for example, using 4-silyl-2-pentenes. Reactions such as epoxidation and hydroboration proceed by preferential addition fiom the face opposite the bulky silyl substituents. 

In addition to being regioselective, alcohol dehydrations aie stereoselective. A stereoselective reaction is one in which a single starting material can yield two or more stereoisomeric products, but gives one of them in greater amounts than any other. Alcohol dehydrations tend to produce the more stable stereoisomer of an alkene. Dehydration of 3-pentanol, for exanple, yields a mixture of trans-2-pcntcnc and d5-2-pentene in which the more stable trans stereoisomer predominates. 

The hydrogenation of 2-methyl(methylene)cyclohexane is an example of a stereoselective reaction, meaning one in which stereoisomeric products are formed in unequal fflnounts from a single starting material (Section 5.11). 

A common misconception is that a stereospecific reaction is simply one that is 100% stereoselective. The two terms are not synonymous, however. A stereospecific reaction is one which, when cariied out with stereoisomeric starting materials, gives a product from one reactant that is a stereoisomer of the product from the other. A stereoselective reaction is one in which a single starting material gives a predominance of a... 

Stereoselective reaction (Sections 5.11 and 6.3) Reaction in which a single starting material has the capacity to form two or more stereoisomeric products but forms one of them in greater amounts than any of its stereoisomers. Terms such as addition to the less hindered side describe stereoselectivity. 

Synthesis and chemistry of substituted l-azabicyclo[1.1.0]butanes 97SL1029. Synthesis of aziridines via stereoselective reactions with imines 99PAC1033. 

The primary objective of this review is to provide an integrated analysis of the contributions from structural, mechanistic and preparative studies toward the successful development of a modern, stereoselective reaction. The synthetic aspects of this useful transformation have been extensively reviewed elsewhere [6]. 

Most asymmetric induction processes with Chital auxiliaries involve a stereo-differentiating reaction that affords one diastereomet as the primary product To obtain the desired enantiomer, the Chiral auxiliary must be removed Highly dia-stereoselective reactions between otganocoppet reagents and allylic substrates with... 

Keller, H. J., and Soos,-Z. G. Solid Charge-Transfer Complexes of Phenazines. 127, 169-216 (1985). Kellogg, R. M. Bioorganic Modelling — Stereoselective Reactions with Chiral Neutral Ligand Complexes as Model Systems for Enzyme Catalysis. 101, 111-145 (1982). 

In 1999, Bob Atkinson wrote [1] that aziridination reactions were epoxida-tion s poor relation , and this was undoubtedly true at that time the scope of the synthetic methods available for preparation of aziridines was rather narrow when compared to the diversity of the procedures used for the preparation of the analogous oxygenated heterocycles. The preparation of aziridines has formed the basis of several reviews [2] and the reader is directed towards those works for a comprehensive analysis of the area this chapter presents a concise overview of classical methods and focuses on modern advances in the area of aziridine synthesis, with particular attention to stereoselective reactions between nitrenes and al-kenes on the one hand, and carbenes and imines on the other. 

Since alkenes are relatively impotent precursors to aziridines, especially with regard to stereoselective reactions, substantially greater advances have been made in this field by means of the addition reactions between imines and a range of car-bene equivalents. 

In contrast to the 2-butenylboranes, 2-butcnylboronates have found widespread application in acyclic diastereoselective synthesis owing to their ease of preparation (Section 1.3.3.3.3.1.1.), configurational stability and highly stereoselective reactions with aldehydes3 4. The results of reactions of substituted allylboronates and representative achiral aldehydes are summarized in Table 1. 

Finally, 2-allyl-4,5-tra ,s-diphenyl-l,3-bis(4-methylphenylsulfonyl)-l,3,2-diazaborolidincs have been used74. The 2-propenyl derivative undergoes highly stereoselective reactions with achiral aldehydes (95 - 97% ee) the ( )-2-butenyl derivatives (91-95% ee) and the analogous 2-chloro- and 2-bromo-2-propenyl derivatives (84-99% ee) also give excellent results in reactions with achiral aldehydes. 

Butenyl(trimethyl)silane reacts with aldehydes in the presence of titanium(IV) chloride to give sj/77-products with excellent stereoselectivity. Reactions of the (Z)-2-butenylsilane are less stereoselective although jyn-products are still preferred23. 

Due to the reversibility of this nitroaldol reaction, the easy epimerization at the nitro-sub-stituted carbon, and the often low yields in reactions with nitro compounds other than ni-tromethane, few stereoselective additions have been reported. Highly stereoselective reactions are known for the synthesis of cyclic systems (see Section 1.3.5.6.6.). 

Stereoselective reactions of this type known at present only deal with four- or five-membered cyclic iV-acyliminium ions. The reactions with carbon nucleophiles usually lead to rra/u-substi-tuted compounds with very high stereoselectivity due to steric control by the substituent already present in the ring. 

Some examples of stereoselective reactions, involving bond formation at the least hindered side, opposite to an alkyl substituent, are shown below. 

Bioorganic modelling. Stereoselective reactions with chiral neutral ligand complexes as model systems for enzyme catalysis. R. M. Kellogg, Top. Curr. Chem., 1982,101,111-145 (93). 

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