Catalytic asymmetric synthesis synthetic applications

Sulfoximines are versatile reagents for diastereoselective and asymmetric synthesis. They continue to find many synthetic applications as both nucleophilic and electrophilic reagents. While the nucleophilic character of sulfoximine reagents has been well exploited,1 the use of the sulfoximine group as a nucleofuge is more recent and adds to the synthetic use of these compounds. The palladium(0)-catalyzed chemistry of allylic sulfoximines and the use of chiral sulfoximines as ligands in catalytic asymmetric synthesis are areas of recent development that have potentially useful applications. Further work is required to understand the factors that determine the diastereoselection and the stereochemical outcomes of these reactions. These studies will result in enhanced product diastereo- and enantioselectivities and make these reagents even more attractive to the wider synthetic chemistry community. 

Witayakran S, Ragauskas AJ (2009) Synthetic applications of laccase in green chemistry. Adv Synth Catal 351(9) 1187-1209. doi 10.1002/adsc.200800775 Wolfer J, Bekele T, Abraham CJ, Dogo-Isonagie C, Lectka T (2006) Catalytic, asymmetric synthesis of 1,4-benzoxazinones a remarkably enantioselective route to a-amino acid derivatives fiom o-benzoquinone imides. Angew Chem 45(44) 7398-7400. doi 10.1002/anie. 200602801... 

In a catalytic asymmetric reaction, a small amount of an enantio-merically pure catalyst, either an enzyme or a synthetic, soluble transition metal complex, is used to produce large quantities of an optically active compound from a precursor that may be chiral or achiral. In recent years, synthetic chemists have developed numerous catalytic asymmetric reaction processes that transform prochiral substrates into chiral products with impressive margins of enantio-selectivity, feats that were once the exclusive domain of enzymes.56 These developments have had an enormous impact on academic and industrial organic synthesis. In the pharmaceutical industry, where there is a great emphasis on the production of enantiomeri-cally pure compounds, effective catalytic asymmetric reactions are particularly valuable because one molecule of an enantiomerically pure catalyst can, in principle, direct the stereoselective formation of millions of chiral product molecules. Such reactions are thus highly productive and economical, and, when applicable, they make the wasteful practice of racemate resolution obsolete. 

The development of catalytic asymmetric reactions is one of the major areas of research in the field of organic chemistry. So far, a number of chiral catalysts have been reported, and some of them have exhibited a much higher catalytic efficiency than enzymes, which are natural catalysts.111 Most of the synthetic asymmetric catalysts, however, show limited activity in terms of either enantioselectivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activate only one side of the substrate in an intermolecular reaction, whereas the latter can not only activate both sides of the substrate but can also control the orientation of the substrate. If this kind of synergistic cooperation can be realized in synthetic asymmetric catalysis, the concept will open up a new field in asymmetric synthesis, and a wide range of applications may well ensure. In this review we would like to discuss two types of asymmetric two-center catalysis promoted by complexes showing Lewis acidity and Bronsted basicity and/or Lewis acidity and Lewis basicity.121... 

In conjunction with the chiral anion TRIP (156) (10 mol%), diamine 157 (10 mol%) can be used in the catalytic asymmetric epoxidation of a,p-unsaturated ketones (>90% ee) [196], while the secondary amine 158 (10 mol%) can be used for the epoxidation of both di- and trisubstituted a,P-unsaturated aldehydes (92-98% ee) (Fig. 15) [211], The facile nature of these reactions, using commercially available peroxides as the stoichiometric oxidant, together with the synthetic utility of the epoxide products suggests application in target oriented synthesis. 

As with any modern review of the chemical Hterature, the subject discussed in this chapter touches upon topics that are the focus of related books and articles. For example, there is a well recognized tome on the 1,3-dipolar cycloaddition reaction that is an excellent introduction to the many varieties of this transformation [1]. More specific reviews involving the use of rhodium(II) in carbonyl ylide cycloadditions [2] and intramolecular 1,3-dipolar cycloaddition reactions have also appeared [3, 4]. The use of rhodium for the creation and reaction of carbenes as electrophilic species [5, 6], their use in intramolecular carbenoid reactions [7], and the formation of ylides via the reaction with heteroatoms have also been described [8]. Reviews of rhodium(II) ligand-based chemoselectivity [9], rhodium(11)-mediated macrocyclizations [10], and asymmetric rho-dium(II)-carbene transformations [11, 12] detail the multiple aspects of control and applications that make this such a powerful chemical transformation. In addition to these reviews, several books have appeared since around 1998 describing the catalytic reactions of diazo compounds [13], cycloaddition reactions in organic synthesis [14], and synthetic applications of the 1,3-dipolar cycloaddition [15]. 

So what is left to be accomplished During the current decade one can expect further asymmetric applications and catalyst designs for metathesis reactions, a maturing of chiral catalyst development for cyclopropanation and insertion with increasing synthetic applications, and decreased reliance on traditional Fischer carbenes in synthesis. Major changes remain for ylide applications, especially those that can be enantioselective, in catalytic carbene chemistry, and advances in nitrene chemistry that are comparable to those achieved over the years in carbene chemistry are in their infancy. 

The catalytic asymmetric hydrosilation of a prochiral ketone to the corresponding chiral silyl ether followed by a mild hydrolysis is in principle an attractive route for the preparation of chiral alcohols that has die advantage that it would not require high hydrogen pressure to effect the reduction (1-6). Despite the synthetic potential only limited application of this technique to the synthesis of complex organic molecules has been made (7-9). This is in part due to the relatively low optical... 

Among the reactions catalyzed by titanium complexes, the asymmetric epoxidation of allylic alcohols developed by Sharpless and coworkers [752, 807-810] has found numerous synthetic applications. Epoxidation of allylic alcohols 3.16 by ferf-BuOOH under anhydrous conditions takes place with an excellent enantioselectivity (ee > 95%) when promoted by titanium complexes generated in situ from Ti(0/ -Pr)4 and a slight excess of diethyl or diisopropyl (R,R)- or (iS, 5)-tartrates 2.69. The chiral complex formed in this way can be used in stoichiometric or in catalytic amounts. For catalytic use, molecular sieves must be added. Because both (RJ )- and (5,5)-tartrates are available, it is posable to obtain either enantiomeric epoxide from a single allylic alcohol. Cumene hydroperoxide (PhCMe20OH) can also be used in place of ferf-BuOOH. This method has been applied to industrial synthesis of enantiomeric glycidols [811, 812]. 

This year has again emphasized the growing importance of organo-transition metal complexes in organic synthesis. In catalysed reactions the major advances have been in asymmetric catalysis with the first reports of chiral induction in catalytic epoxidation and further reports on improved catalysts for asymmetric hydrogenation and allylic alkylation. The formation of carbon-carbon bonds continues to attract attention, and several novel and potentially useful synthetic applications of organometallic complexes have been reported. 

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