Chiral compounds reactions

The remarkable stereospecificity of TBHP-transition metal epoxidations of allylic alcohols has been exploited by Sharpless group for the synthesis of chiral oxiranes from prochiral allylic alcohols (Scheme 76) (81JA464) and for diastereoselective oxirane synthesis from chiral allylic alcohols (Scheme 77) (81JA6237). It has been suggested that this latter reaction may enable the preparation of chiral compounds of complete enantiomeric purity cf. Scheme 78) ... 

The main strategy for catalytic enantioselective cycloaddition reactions of carbonyl compounds is the use of a chiral Lewis acid catalyst. This approach is probably the most efficient and economic way to effect an enantioselective reaction, because it allows the direct formation of chiral compounds from achiral substrates under mild conditions and requires a sub-stoichiometric amount of chiral material. 

The power ofbiocatalysts for the production of chiral compounds can be elevated to a second stage when proper use of the process leads to a larger number of different enantioenriched products from the same reaction. Some efforts have been made in this direction because the enzyme can be selective to either nucleophile or acyl donor. The elegancy of the reaction is increased when the process is carried out for the resolution of both. Scheme 7.20 depicts this possibility [38]. 

They react at different rates with other chiral compounds. These rates may be so close together that the distinction is practically useless, or they may be so far apart that one enantiomer undergoes the reaction at a conveni t rate while the other does not react at all. This is the reason that many compounds are biologically active while their enantiomers are not. Enantiomers react at the same rate with achiral compounds. ... 

The advantages of such biotransformation processes are (1) the relatively high yields which can be achieved with specific enzymes, (2) the formation of chiral compounds suitable for biopharmaceuticals, and (3) the relatively mild reaction conditions. Key issues in industrial-scale process development are achieving high product concentrations, yields and productivities by maintaining enzyme activity and stability under reaction conditions while reducing enzyme production costs. 

The hydrolytic kinetic resolution (HKR) of terminal epoxides using Co-salen catalysts provides a convenient route to the synthesis of enantioemiched chiral compounds by selectively converting one enantiomer of the racemic mixture (with a maximum 50% yield and 100% ee) (1-3). The use of water as the nucleophile makes this reaction straightforward to perform at a relatively low cost. The homogeneous Co(III) salen catalyst developed by Jacobsen s group has been shown to provide high... 

Over the years of evolution, Nature has developed enzymes which are able to catalyze a multitude of different transformations with amazing enhancements in rate [1]. Moreover, these enzyme proteins show a high specificity in most cases, allowing the enantioselective formation of chiral compounds. Therefore, it is not surprising that they have been used for decades as biocatalysts in the chemical synthesis in a flask. Besides their synthetic advantages, enzymes are also beneficial from an economical - and especially ecological - point of view, as they stand for renewable resources and biocompatible reaction conditions in most cases, which corresponds with the conception of Green Chemistry [2]. 

In reactions of chiral aldehydes, TiIV compounds work well as both activators and chelation control agents, a- or A-oxygcnated chiral aldehydes react with allylsilanes to afford chiral homoallylic alcohols with high selectivity (Scheme 22).85 These chiral alcohols are useful synthetic units for the synthesis of highly functionalized chiral compounds. Cyclic chiral 0,0- and A/O-acetals react with allylsilanes in the same way.86,87 Allenylsilanes have also been reported as allylation agents. 

Asymmetric allylic substitution reactions have been studied for many years because they provide valuable chiral compounds. Regardless of the alkylating agent used, there are two major goals in these reactions (i) to minimize the amount of Sn2 products, and (ii) to maximize the enantiomeric purity of the Sn2 products. Various approaches have been investigated to achieve these goals. Recently, the efforts of several research groups have been focused on the... 

The transition metal-catalyzed hydrovinylation has been reviewed by RajanBabu who focused mainly on asymmetric reactions, affording chiral compounds.146 The vinylarenes are the most investigated substrates for the hydrovinylation reaction due to the high appeal of the final products in medicinal or polymer chemistry fields.1... 

As mentioned in Section 1.2, the presence of an asymmetric carbon is neither a necessary nor a sufficient condition for optical activity. Each enantiomer of a chiral molecule rotates the plane of polarized light to an equal degree but in opposite directions. A chiral compound is optically active only if the amount of one enantiomer is in excess of the other. Measuring the enantiomer composition is very important in asymmetric synthesis, as chemists working in this area need the information to evaluate the asymmetric induction efficiency of asymmetric reactions. 

Under normal conditions, the two enantiomers of a chiral compound have exactly the same boiling and melting points and the same solubility in normal achiral solvents. Their chemical reactions are also identical under achiral conditions. However, under chiral conditions, the enantiomers may behave very differently. For example, physical property or chemical reactivity may change significantly under chiral conditions. 

Recently, two new axially chiral compounds 44 and 45 have been prepared [44, A-acryl-A-allyl-o-t-butylanilide 45, A-(o-t-butylphenyl)-2-methyl-maleimide]. This represents the first instance of using nonbiaryl axially chiral ligands in asymmetric Dield-Alder reactions. In the presence of iodine, high endo-facial and diastereofacial selectivities have been obtained in 44/45-medi-ated reactions.10... 

The enantioselective addition of a nucleophile to a carbonyl group is one of the most versatile methods for C C bond formation, and this reaction is discussed in Chapter 2. Trifluoromethylation of aldehyde or achiral ketone via addition of fluorinated reagents is another means of access to fluorinated compounds. Trifluoromethyl trimethylsilane [(CF SiCFs] has been used by Pra-kash et al.87 as an efficient reagent for the trifluoromethylation of carbonyl compounds. Reaction of aldehydes or ketones with trifluoromethyltrime-thylsilane can be facilitated by tetrabutyl ammonium fluoride (TBAF). In 1994, Iseki et al.88 found that chiral quaternary ammonium fluoride 117a or 117b facilitated the above reaction in an asymmetric manner (Scheme 8-42). 

Enantioselective heterogeneous catalytic reactions are of growing interest, as optically pure chiral compounds are of great importance in different areas like fine chemical, pharmaceutical, agrochemical, flavor, and fragrance industries. 

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