Synthetic applications asymmetric transformation

The Sharpless-Katsuki asymmetric epoxidation (AE) procedure for the enantiose-lective formation of epoxides from allylic alcohols is a milestone in asymmetric catalysis [9]. This classical asymmetric transformation uses TBHP as the terminal oxidant, and the reaction has been widely used in various synthetic applications. There are several excellent reviews covering the scope and utility of the AE reaction... 

By using the Sharpless dihydroxylation, a variety of compounds have been transformed to diols with high enantiomeric excesses. The asymmetric dihydroxylation has a wide range of synthetic applications. As an illustration, the dihydroxylation was used as the key step in the synthesis of squalestatin 1 (3.8) (Scheme 3.2).74... 

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]. 

Unless specified otherwise, all reductions included in this chapter gave good yields of >90% enantiomeric excess (ee) products. Not all products of enzyme-catalyzed reactions meet the minimum % ee levels normally required for asymmetric synthetic applications. However, protocols exist for improving ee s of imperfectly specific enzyme-mediated transformations. 

The second approach involves photochemical transformation of chiral molecules with production of new asymmetric centers. When all the possible diastereoisomers are not formed in equal amounts but can be separated easily, synthetic applications can be defined. Sometimes, as a special improvement, a chiral and removable appendage can be fixed onto the prochiral substrate A as a preliminary step, as shown in Eq. 1 ... 

While silyl enol ethers 21 and 23 were subjected to similar reaction conditions (Tables 1.5 and 1.6), the allylic C—H bond could also be functionalized by metal carbenoids to afford silyl-protected 1,5-dicarbonyls 22 and 24 respectively, which can be viewed as an equivalent of an asymmetric Michael reaction. Although the double bond is highly electron-rich and readily undergoes cyclopropanation in the presence of most other metal carbenoids, by using aryldiazoacetates 1 as carbene precursors, cyclic silyl enol ethers 21 were readily transformed into their corresponding allylic C—H bond insertion products 22 (22 ) in excellent yields, excellent ee and moderate de (Table 1.5). Noticeably, while acyclic silyl enol ethers 23 were subjected to the reaction, excellent diastereoselectivity (>90% de) was obtained, which shows great potential in synthetic applications (Table 1.6). 

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