Sharpless asymmetric epoxidation process

To test the power of the Sharpless asymmetric epoxidation process, epoxidation of 32 with (-)-(iS, S)-diisopropyl tartrate (DIPT) was also performed and did indeed give the epoxide in the C-20 section. 

Asymmetric Epoxidation Reactions. While Ti(0-i-Pr)4 clearly has the capacity to bring about the nucleophilic ring-cleavage of 2,3-epoxy alcohols (see above), it remains the preferred species for the preparation of the titanium tartrate complex central to the Sharpless asymmetric epoxidation process (see, for example, eq 7). Since f-butoxide-mediated ring-opening of 2-substituted 2,3-epoxy alcohols (a subclass of epoxy alcohols particularly sensitive to nucleophilic ring-cleavage) is much slower than by isopropoxide, the use of Ti(0-f-Bu)4 is sometimes recommended in place of Ti(0-i-Pr)4. However, with the reduced amount of catalyst that is now needed for all asymmetric epoxidations, this precaution appears unnecessary in most instances. 

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-... 

The second synthesis of crystalline 43 was reported by Mori as summarized in Scheme 62 [93]. The building block (4.R,5S)-A was prepared by an enzymatic process, while another building block C was synthesized via Sharpless asymmetric epoxidation. Coupling of A with C gave D, which was cyclized under Op-polzer s conditions to give crystalline E. When E was oxidized with Dess-Martin periodinane or tetra(n-propyl)ammonium perruthenate or Jones chromic acid, crystalline 43 was obtained. Swern oxidation or oxidation with 2,2,6,6-tetramethylpiperidin-1 -oxyl of E afforded only oily materials. Accordingly, oxidation of E to 43 must be executed extremely carefully. A synthesis of oily 43 was reported by Gil [94]. 

Jacobsen epoxidation turned out to be the best large-scale method for preparing the cis-amino-indanol for the synthesis of Crixivan, This process is very much the cornerstone of the whole synthesis. During the development of the first laboratory route into a route usable on a very large scale, many methods were tried and the final choice fell on this relatively new type of asymmetric epoxidation. The Sharpless asymmetric epoxidation works only for allylic alcohols (Chapter 45) and so is no good here. The Sharpless asymmetric dihydroxylation works less well on ris-alkenes than on trans-alkenes, The Jacobsen epoxidation works best on cis-alkenes. The catalyst is the Mn(III) complex easily made from a chiral diamine and an aromatic salicylaldehyde (a 2-hydroxybenzaldehyde). 

The Sharpless asymmetric epoxidation of allyl alcohol gives the glycidol derivative 61 in 90% ee after in situ tosylation of 60 [63]. This process is working on a multiton-a-year scale (Arco Co., USA), facilitating the synthesis of a variety of /0-blockers. Asymmetric dihydroxylation of the allyl ether 63 catalyzed by a combined system of OSO4 and the cinchona alkaloid-based ligand 65 allows the commercial synthesis of the propranolol intermediate 64 in 91 % (Sepracor Co., USA) [64]. 

Asymmetric oxidations have followed the usual development pathway in which face selectivity was observed through the use of chiral auxiliaries and templates. The breakthrough came with the Sharpless asymmetric epoxidation method, which, although stoichiometric, allowed for a wide range of substrates and the stereochemistry of the product to be controlled in a predictable manner [1]. The need for a catalytic reaction was very apparent, but this was developed and now the Sharpless epoxidation is a viable process al scale, although subject to the usual economic problems of a cost-effective route to the substrate (see later) [2]. The Sharpless epoxidation has now been joined by other methods and a wide range of products are now available. The pow er of these oxidations is augmented by the synthetic utility of the resultant epoxides or diols that can be used for further transformations, especially those that use a substitution reaction (see Chapter 7) [1]. 

Two examples of such processes are shown in Scheme 1.6. One is the titanium TADDOLate-catalyzed addition of diethylzinc to myrtenal (see Section 4.3, [52] the other is the Sharpless asymmetric epoxidation (see Section 8.2.2, [58,63]). In both cases, the diastereoselectivity for the reaction of the substrate with an achiral reagent is low (65-70% ds), while the catalysts have enantioselectivities of >95% with achiral substrates. In these cases of double asymmetric induction, the catalyst completely overwhelms the facial bias of the chiral substrate. 

Other metal-based epoxidation catalysts have been explored to overcome some of the hmitations of the Sharpless procedure. One drawback with the Sharpless asymmetric epoxidation is the slightly lower ees often obtained when using cis-olefin substrates. The group of Yamamoto have achieved highly enantioselective epoxidations of ds-alkenes using vanadium(V) oxytriisopropoxide in the presence of C2-symmetric bishydroxamic acid ligands such as (4.23). In contrast to the Sharpless procedure this process is not hampered by the presence of air or... 

Other than the L-DOPA, there have been two other significant processes developed for industrial applications. The first of these uses a Sharpless asymmetric epoxidation, one of the most widely applied asymmetric transition metal catalysed transformations, to convert allyl alcohol (41) into (5)-glycidol (43), a valuable chiral building block, developed by ARCO Chemical Company (Scheme 4.12) [29]. Most of the successful applications of transition metal mediated asymmetric... 

Henegar KE, Cerula M. Process development for (S,S)-reboxetine succinate via a Sharpless asymmetric epoxidation. Org. Process Res. Dev. 2007 11 354-358. 

The most widely used system in industry for catalytic asymmetric epoxidation is known as Sharpless Asymmetric Epoxidation (SAE) (Figure 14.13). While key mechanistic insights will not be covered here as they have been described in detail in previous reviews, our focus will be on the application of this directed epoxidation in an industrial setting. Notably, SAE has been utilised for process-scale preparation of raw materials and as part of... 

Asymmetric epoxidation is another important area of activity, initially pioneered by Sharpless, using catalysts based on titanium tetraisoprop-oxide and either (+) or (—) dialkyl tartrate. The enantiomer formed depends on the tartrate used. Whilst this process has been widely used for the synthesis of complex carbohydrates it is limited to allylic alcohols, the hydroxyl group bonding the substrate to the catalyst. Jacobson catalysts (Formula 4.3) based on manganese complexes with chiral Shiff bases have been shown to be efficient in epoxidation of a wide range of alkenes. 

Related catalytic enantioselective processes It is worthy of note that the powerful Ti-catalyzed asymmetric epoxidation procedure of Sharpless [27] is often used in the preparation of optically pure acyclic allylic alcohols through the catalytic kinetic resolution of easily accessible racemic mixtures [28]. When the catalytic epoxidation is applied to cyclic allylic substrates, reaction rates are retarded and lower levels of enantioselectivity are observed. Ru-catalyzed asymmetric hydrogenation has been employed by Noyori to effect the resolution of five- and six-membered allylic carbinols [29] in this instance, as with the Ti-catalyzed procedure, the presence of an unprotected hydroxyl function is required. Perhaps the most efficient general procedure for the enantioselective synthesis of this class of cyclic allylic ethers is that recently developed by Trost and co-workers, involving Pd-catalyzed asymmetric additions of alkoxides to allylic esters [30]. 

The selective oxidation of hydrocarbons, particularly that of alkanes, remains a challenge. It is not surprising, therefore, that the problems of oxidation processes are addressed in several books,1043-1045 reviews,1046-1057 and a journal special issue,1058 as well as in international conferences1059-1064 devoted to the topic. For the advances in chirally catalyzed oxidation processes, including asymmetric epoxidation and osmylation, Sharpless was one of the recipients of the 2001 Nobel Prize in Chemistry. 

The potential of a catalytic process for use on a large scale can be a good indication of its efficiency. During recent decades there has been an increasing tendency to apply asymmetric catalytic processes in industry [1], The asymmetric Noyori hydrogenation [2] and the Sharpless and Jacobsen-Katsuki epoxidation [3] are representative examples of impressive developments in this field [1]. 

Catalytic, asymmetric epoxidations are one of the most important asymmetric processes. In 1980 Katsuki and Sharpless reported a stoichiometric asymmetric epoxidation of allylic alcohols, a method that was later improved to become a catalytic process.9 Moreover, catalytic asymmetric epoxidations of unfunctionalized olefins using salen-manganese complexes have been reported independently by several groups.10-12 In striking contrast to these successful achievements, an efficient catalytic asymmetric epoxidation of enones with broad generality has not been developed.13-22... 

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