Alkenes Sharpless asymmetric epoxidation

Further variations on the epoxyketone intermediate theme have been reported. In the first (Scheme 9A) [78], limonene oxide was prepared by Sharpless asymmetric epoxidation of commercial (S)-(-)- perillyl alcohol 65 followed by conversion of the alcohol 66 to the crystalline mesylate, recrystallization to remove stereoisomeric impurities, and reduction with LiAlH4 to give (-)-limonene oxide 59. This was converted to the key epoxyketone 60 by phase transfer catalyzed permanganate oxidation. Control of the trisubstituted alkene stereochemistry was achieved by reaction of the ketone with the anion from (4-methyl-3-pentenyl)diphenylphosphine oxide, yielding the isolable erythro adduct 67, and the trisubstituted E-alkene 52a from spontaneous elimination by the threo adduct. Treatment of the erythro adduct with NaH in DMF resulted... 

Hie first of Sharpless s reactions is an oxidation of alkenes by asymmetric epoxidation. You met vanadium as a transition-metal catalyst for epoxidation with r-butyl hydroperoxide in Chapter 33, and this new reaction makes use of titanium, as titanium tetraisopropoxide, Ti(OiPr)4, to do the same thing. Sharpless surmised that, by adding a chiral ligand to the titanium catalyst, he might be able to make the reaction asymmetric. The ligand that works best is diethyl tartrate, and the reaction shown below is just one of many that demonstrate that this is a remarkably good reaction. 

The reaction was also successful for substituted salicylaldehydes. When Jacobsen came to develop his asymmetric epoxidation, which, unlike the Sharpless asymmetric epoxidation, works for simple alkenes and not just for allylic alcohols, he chose salens as his catalysts, partly because they could be made so easily from salicylaldehydes. For example ... 

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). 

Figure 2.19 Oxygen transfer to the alkene using the Kalsuki-Sharpless asymmetric epoxidation system.

The aminocyclitol moiety was synthesized in a stereocontrolled manner from cis-2-butene-l,4-diol (Scheme 40)112 by conversion into epoxide 321 via Sharpless asymmetric epoxidation in 88% yield.111 Oxidation of 321 with IBX, followed by a Wittig reaction with methyl-triphenylphosphonium bromide and KHMDS, produced alkene 322. Dihydroxylation of the double bond of 322 with OSO4 gave the diol 323, which underwent protection of the primary hydroxyl group as the TBDMS ether to furnish 324. The secondary alcohol of 324 was oxidized with Dess-Martin periodinane to... 

Asymmetric epoxidation The catalytic asymmetric epoxidation of alkenes has been the focus of many research efforts over the past two decades. The non-racemic epoxides are prepared either by enantioselective oxidation of a prochiral carbon-carbon double bond or by enantioselective alkylidenation of a prochiral C=0 bond (e.g. via a ylide, carbene or the Darzen reaction). The Sharpless asymmetric epoxidation (SAE) requires allylic alcohols. The Jacobsen epoxidation (using manganese-salen complex and NaOCl) works well with ds-alkenes and dioxirane method is good for some trans-alkenes (see Chapter 1, section 1.5.3). 

The Sharpless asymmetric epoxidation is an enantioselective reaction that oxidizes alkenes to epoxides. Only the double bonds of allylic alcohols—that is, alcohols having a hydroxy group on the carbon adjacent to a C=C —are oxidized in this reaction. 

An asymmetric route affording a low yield of 3-carboxysultam 281 (R = R = R = H) has been described (see Section 4.05.7.3). As intermediate, compound 429 is prepared through intramolecular ring opening of the epoxide 428 obtained by Sharpless asymmetric epoxidation of the corresponding alkene <2000JOC7690>. 

The hrst of Sharpless s reactions is an oxidation of alkenes by asymmetric epoxidation. You met vanadium as a transition-metal catalyst for epoxidation with f-butyl hydroperoxide in Chapter 33,... 

Starting with geraniol (126), Sharpless asymmetric epoxidation [68] proceeded in 95% yield and with 93% ee (see Scheme 29) [69]. Hydrobromination of the alkene was achieved under standard conditions using NBS in THF and water (5 1) to afford a 64% yield of diastereomeric bromo alcohols (127). Cyclization to the tetrahydropyran diol was accomplished by treating 127 with camphorsulphonic acid in diethyl ether, followed by diol cleavage with sodium periodate to give a mixture of aldehydes 128 and 129. Upon... 

Epoxides are key chiral synthetic intermediates and their enantioselective preparation by oxidation of achiral alkenes is a key reaction in many synthetic strategies. Sharpless asymmetric epoxidation is suitable for most allylic alcohols [26, 27], but few general procedures exist for unfunctionalized olefins. Jacobsen s manganese salen-mediated epoxidation is suitable for and gives good selectivities with Z-olefins (85 to 90% ee) [28]. The enzyme chloroperoxidase... 

The Sharpless asymmetric epoxidation (sec. 3.4.D.i) exploits this chelation effect because its selectivity arises from coordination of the allylic alcohol to a titanium complex in the presence of a chiral agent. The most effective additive was a tartaric acid ester (tartrate), and its presence led to high enantioselectivity in the epoxidation.23 An example is the conversion of allylic alcohol 40 to epoxy-alcohol 41, in Miyashita s synthesis of the Cg-Ci5 segment of (-t-)-discodermolide.24 in this reaction, the tartrate, the alkenyl alcohol, and the peroxide bind to titanium and provide facial selectivity for the transfer of oxygen from the peroxide to the alkene. Binding of the allylic alcohol to the metal is important for delivery of the electrophilic oxygen and... 

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... 

The Sharpless asymmetric epoxidation is reliable, but it works only for allylic alcohols. There is an alternative, however, which works with simple alkenes. The method was developed by Eric Jacobsen and employs a manganese catalyst with a chiral ligand built from a simple diamine. The diamine is not a natural compound and has to be made in enantiomeric form by resolution, but at least that means that both enantiomers are readily available. The diamine is condensed with a derivative of salicylaldehyde to make a bis-imine known as a salen. ... 

The asymmetric dihydroxylation is much less fussy about the alkenes it will oxidize than Sharpless asymmetric epoxidation. Osmium tetroxide itself is a remarkable reagent, since it oxidizes more or less any sort of alkene, electron-rich or electron-poor, and the same is true of the asymmetric dihydroxylation reagent. The following example illustrates both this and a synthetic use for the diol product. 

The Sharpless asymmetric epoxidation of allylic alcohols (one of the reactions that helped K. Barry Sharpless earn his part of the 2001 Nobel Prize) offers a good example of an enantioselective technique that can be used to create either enantiomer of an epoxide product. This reaction uses a diester of tartaric acid, such as diethyl tartrate (DET) or diisopropyl tartrate (DIPT), as the source of chirality. The dialkyl tartrate coordinates with the titanium tetraisopropoxide [Ti(Oi-Pr)4] catalyst and t-butyl hydroperoxide (r-BuOOH) to make a chiral oxidizing agent. Since both enantiomers of tartaric acid are commercially available, and each enantiomer will direct the reaction to a different prochiral face of the alkene, both enantiomers of an epoxide can be synthesized. 

The target molecule must first be rotated to fit the Sharpless asymmetric epoxidation model (need to have allylic alcohol group in bottom right position). The retrosynthesis of the epoxide leads to an alkene with the stereochemistry shown. Since the oxidation has occurred at the top face, the (+) enantiomer of diethyl tartrate (DET) is required. Note that only the alkene with the aUyUc hydroxyl group is oxidized in the Sharpless epoxidation. 

Epoxides, derived from alkenes, are important synthetic precursors to statine. Sharpless asymmetric epoxidation of 5-methyl-l-hexen-3-ol 6.173) gave 6.174... 

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