Epoxides catalytic stereoselective

It should be noted that the related imine-oxaziridine couple E-F finds application in asymmetric sulfoxidation, which is discussed in Section 10.3. Similarly, chiral oxoammonium ions G enable catalytic stereoselective oxidation of alcohols and thus, e.g., kinetic resolution of racemates. Processes of this type are discussed in Section 10.4. Whereas perhydrates, e.g. of fluorinated ketones, have several applications in oxidation catalysis [5], e.g. for the preparation of epoxides from olefins, it seems that no application of chiral perhydrates in asymmetric synthesis has yet been found. Metal-free oxidation catalysis - achiral or chiral - has, nevertheless, become a very potent method in organic synthesis, and the field is developing rapidly [6]. 

Until now, three main F-C transformations have been used for catalytic stereoselective formation of benzylic carbon stereocenters - 1,2-addition of arenes to carbonyl (C=X, X O, NR) moieties, 1,4-addition of arenes to electron-deficient C-C double bonds, and ring-opening reaction of epoxides. 

In 2001, K. B. Sharpless won the Nobel Prize in Chemistry for his work on asymmetric aminohydroxylation and asymmetric epoxidation °. These stereoselective oxidation reactions are powerful catalytic asymmetric methods that have revolutionized synthetic organic chemistry. 

W. Adam, P. L. Alsters, R. Neumann, C. R. Saha-Moller, D. Seebach, A. K. Beck, R. Zhang, Chiral hydroperoxides as oxygen source in the catalytic stereoselective epoxidation of allylic alcohols by sandwich-type polyoxometalates Control of enantioselectivity through a metal-coordinated template, /. Org. Chem. 68 (2003) 8222. 

The emergence of the powerful Sharpless asymmetric epoxida-tion (SAE) reaction in the 1980s has stimulated major advances in both academic and industrial organic synthesis.14 Through the action of an enantiomerically pure titanium/tartrate complex, a myriad of achiral and chiral allylic alcohols can be epoxidized with exceptional stereoselectivities (see Chapter 19 for a more detailed discussion). Interest in the SAE as a tool for industrial organic synthesis grew substantially after Sharpless et al. discovered that the asymmetric epoxidation process can be conducted with catalytic amounts of the enantiomerically pure titanium/tartrate complex simply by adding molecular sieves to the epoxidation reaction mix-... 

Since the seminal contributions by Nugent and RajanBabu the field of reductive C - C bond formation after epoxide opening via electron transfer has developed at a rapid pace. Novel catalytic methodology, enantio- and stereoselective synthesis and numerous applications in the preparation of biologically active substances and natural products have evolved. In brief, a large repertoire of useful and original reactions is available. These reactions are waiting to be applied in a complex context ... 

The chapter on alicyclic stereoselection has been splitted in two chapters (9 and 10). Chapter 10, which is exclusively devoted to Sharpless asymmetric epoxidation and dihydroxylation, has been rewritten de novo. The most recent advances in catalytic and stereoselective aldol reactions are incorporated in Chapter 9. 

A new stereoselective epoxidation catalyst based on a novel chiral sulfonato-salen manganese(III) complex intercalated in Zn/Al LDH was used successfully by Bhattacharjee et al. [125]. The catalyst gave high conversion, selectivity, and enantiomeric excess in the oxidation of (i )-limonene using elevated pressures of molecular oxygen. Details of the catalytic activities with other alkenes using both molecular oxygen and other oxidants have also been reported [126]. 

The origin of stereofacial selectivity in electrophilic additions to methylene-cyclohexanes (2) and 5-methylene-l,3-dioxane (3) has been elucidated experimentally (Table 2) and theoretically. Ab initio calculations suggest that two electronic factors contribute to the experimentally observed axial stereoselectivity for polarizable electrophiles (in epoxidation and diimide reduction) the spatial anisotropy of the HOMO (common to both molecules) and the anisotropy in the electrostatic potential field (in the case of methylenedioxane). The anisotropy of the HOMO arises from the important topological difference between the contributions made to the HOMO by the periplanar p C-H a-bonds and opposing p C—O or C—C cr-bonds. In contrast, catalytic reduction proceeds with equatorial face selectivity for both the cyclohexane and the dioxane systems and appears to be governed largely by steric effects. ... 

A -tritylaziridine-2-(5)-carboxaldehyde. The application of a novel, sequential, trans-acetalation oxonium ene cyclization has delivered a stereoselective synthesis of the C-aromatic taxane skeleton, and a combinatorial sequence of the regioselective propiolate-ene, catalytic enantioselective epoxidation and carbonyl-ene cyclization reactions has been used to complete the synthesis of the A-ring of a vitamin D hybrid analogue. 

Coates used [Cp2Ti(THF)2] and [(salphen)Al(THF)2] as Lewis acid to convert a variety of epoxides to racemic p-lactones, and substituted aziridines to p-lactams in high yields under mild conditions. PO is selectively converted to p-BL in 95% yield in 4 h at 60°C [117]. However, only racemic p-BL can be obtained from racemic PO. In order to get enantiopure molecules from racemic precursors, the catalytic system has to be stereoselective. This can generally be achieved by the use of a chiral stereo-inducing Lewis acid, which effects a kinetic resolving activation [119, 120]. However, examples of the chiral resolution of PO are rare. 

The authors also investigated the mode of activation of these BINOL-derived catalysts. They proposed an oligomeric structure, in which one Ln-BINOL moiety acts as a Brpnsted base, that deprotonates the hydroperoxide and the other moiety acts as Lewis acid, which activates the enone and controls its orientation towards the oxidant . This model explains the observed chiral amplification effect, that is the ee of the epoxide product exceeds the ee of the catalyst. The stereoselective synthesis of cw-epoxyketones from acyclic cw-enones is difficult due to the tendency of the cw-enones to isomerize to the more stable fraw5-derivatives during the oxidation. In 1998, Shibasaki and coworkers reported that the ytterbium-(f )-3-hydroxymethyl-BINOL system also showed catalytic activity for the oxidation of aliphatic (Z)-enones 129 to cw-epoxides 130 with good yields... 

Table 17) with two substituents in position C3 the oxygen transfer by the chiral hydroperoxides occurred from the same enantioface of the double bond, while epoxidation of the (ii)-phenyl-substituted substrates 142c,g,i resulted in the formation of the opposite epoxide enantiomer in excess. In 2000 Hamann and coworkers reported a new saturated protected carbohydrate hydroperoxide 69b , which showed high asymmetric induction in the vanadium-catalyzed epoxidation reaction of 3-methyl-2-buten-l-ol. The ee of 90% obtained was a milestone in the field of stereoselective oxygen transfer with optically active hydroperoxides. Unfortunately, the tertiary allylic alcohol 2-methyl-3-buten-2-ol was epoxidized with low enantioselectivity (ee 18%) with the same catalytic system . 

Berkessel and Sklorz screened a variety of potential co-ligands for the Mn-tmtacn/H202 catalyzed epoxidation reaction and found that ascorbic acid was the most efficient one. With this activator the authors could oxidize the terminal olefins 1-octene and methyl acrylate with full conversion and yields of 83% and 97%, respectively, employing less than 0.1% of the metal complex (Scheme 86). Furthermore, with E- and Z-l-deuterio-1-octene as substrates, it was shown that the oxygen transfer proceeded stereoselectively with almost complete retention of configuration (94 2%). Besides the epoxidation, also the oxidation of alcohols to carbonyl compounds could be catalyzed by this catalytic system (see also Section in.C). 

Also, the reaction pathways of the Corey-Chaykovsky epoxidation reaction have been compared quantum-chemically <1999JOC4596>. As models for one transition state, 1,3-oxathiane compounds such as 52, suitably substituted to allow comparison with experiment (Equation 3), were calculated and these predicted both the absolute stereochemistry of the main product 53 and the distribution of the other stereoisomers, as supported by experimental results. Thus, this theoretical study was able to identify the transition state which proved to be responsible for the stereoselectivity of the catalytic Corey-Chaykovsky epoxidation reaction. 

The second example demonstrated immobilization via ship in a bottle , ionic, metal center, and covalent bonding approaches of the metal-salen complexes. Zeolites X and Y were highly dealuminated by a succession of different dealumi-nation methods, generating mesopores completely surrounded by micropores. This method made it possible to form cavities suitable to accommodate bulky metal complexes. The catalytic activity of transition metal complexes entrapped in these new materials (e.g, Mn-S, V-S, Co-S, Co-Sl) was investigated in stereoselective epoxidation of (-)-a-pinene using 02/pivalic aldehyde as the oxidant. The results obtained with the entrapped organometallic complex were comparable with those of the homogeneous complex. 

Preparation of nonracemic epoxides has been extensively studied in recent years since these compounds represent useful building blocks in stereoselective synthesis, and the epoxide functionality constitutes the essential framework of various namrally occurring and biologically active compounds. The enantiomericaUy enriched a-fluorotropinone was anchored onto amorphous KG-60 silica (Figure 6.6) this supported chiral catalyst (KG-60-FT ) promoted the stereoselective epoxidation of several trans- and trisubstituted alkenes with ees up to 80% and was perfectly reusable with the same performance for at least three catalytic cycles. 

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