Evans reagent

Imide Systems. Imide compounds 22 and 23, or Evans reagents, derived from the corresponding oxazolidines are chiral auxiliaries for effective asymmetric alkylation or aldol condensation and have been widely used in the synthesis of a variety of substances. 

As shown in Scheme 9.31, the (S)-enolate (100a), from Evans reagent 100. reacts on its Re face if the metal is not coordinated to the oxazolidone carbonyl group at the time of electrophilic attack, which is the normal situation in an uncatalysed boron enolate aldol reaction (see Scheme 9.14) and on the Si face if the metal is coordinated to the oxazolidone carbonyl group (lOOb), which is the normal situation in enolate alkylation (see 9.3.2). 

These easily-made carbonates have become useful intermediates for (i) the direct synthesis of cyclic carbonates via the Heck reaction [82] (ii) optically active carbonates by enantioselective hydrogenation [83] and (iii) to oxazolidinones [84, 85] as an alternative route to the Evans reagent [86-88]. 

D1BAH (2 cquiv.) to the (2S)-hydroxy ketone 2 with high diastcreoselcctivity (>95% de). The products can be converted to the a /t -l,3-diol 4 by reduction with tctramethyl-ammonium triancetoxyborohydridc (Evans reagent) followed by desulfurization. 

In the study of biosynthesis of polyketide antibiotics. Cane and Luo applied the Evans aldol reaction to prepare a key intermediate 41 from aldehyde 40 and Evans reagent 20. °... 

Chiral 2-oxazolidones are useful recyclable auxiliaries for carboxylic acids in highly enantioselective aldol type reactions via the boron enolates derived from N-propionyl-2-oxazolidones (D.A. Evans, 1981). Two reagents exhibiting opposite enantioselectivity ate prepared from (S)-valinol and from (lS,2R)-norephedrine by cyclization with COClj or diethyl carbonate and subsequent lithiation and acylation with propionyl chloride at — 78°C. En-olization with dibutylboryl triflate forms the (Z)-enolates (>99% Z) which react with aldehydes at low temperature. The pure (2S,3R) and (2R,3S) acids or methyl esters are isolated in a 70% yield after mild solvolysis. 

A key step in the synthesis of the spiroketal subunit is the convergent union of intermediates 8 and 9 through an Evans asymmetric aldol reaction (see Scheme 2). Coupling of aldehyde 9 with the boron enolate derived from imide 8 through an asymmetric aldol condensation is followed by transamination with an excess of aluminum amide reagent to afford intermediate 38 in an overall yield of 85 % (see Scheme 7). During the course of the asymmetric aldol condensation... 

Evans and Colombo35 are developing a different route to enantiomerically pure sulfoxides involving intermediate N-sulfinyl oxazolidinones 5. Reaction of these dia-stereomerically pure species with organometallic reagents gives enantiomerically pure sulfoxides, as illustrated by equation 5 carried out on a multigram scale. 

It has been pointed out that, nevertheless, H2O2 is not a very good reagent for this reaction, since it gives sulfonic acids (19-25) as well as disulfides Evans, B.J. Doi, J.T. Musker, W.K. J. Org. Chem., 1990, 55, 2337. 

A dual isotope labeling technique [85] has been used to measure membrane permeability in plant cells, based on the selective permeabiHty of the membranes of living cells to tritiated water and carbon-14 labeled mannitol. Kieran [29] showed that the results of the dual isotope labeling and Evan s Blue staining methods correlated well as indicators of cell viability however, the latter was preferable in terms of reagent cost and ease of analysis. 

Evans, D.A., Hurst, K.M., Truesdale, L.K., and Takacs, J.M., The carbonyl insertion reactions of mixed tervalent phosphorus-organosilicon reagents, Tetrahedron Lett., 2495, 1977. 

Robinson, P.L., Barry, C.N., Kelly, J.W., and Evans, S.A., Diethoxytriphe-nylphosphorane a mild, regioselective cyclodehydrating reagent for conversion of diols to cyclic ethers. Stereochemistry, synthetic utility, and scope, /. Am. Chem. Soc., 107, 5210, 1985. 

As with the above pyrrolidine, proline-type chiral auxiliaries also show different behaviors toward zirconium or lithium enolate mediated aldol reactions. Evans found that lithium enolates derived from prolinol amides exhibit excellent diastereofacial selectivities in alkylation reactions (see Section 2.2.32), while the lithium enolates of proline amides are unsuccessful in aldol condensations. Effective chiral reagents were zirconium enolates, which can be obtained from the corresponding lithium enolates via metal exchange with Cp2ZrCl2. For example, excellent levels of asymmetric induction in the aldol process with synj anti selectivity of 96-98% and diastereofacial selectivity of 50-200 116a can be achieved in the Zr-enolate-mediated aldol reaction (see Scheme 3-10). 

Chiral a,/ -unsaturated A-acyloxazolidinones have been regarded as a complement for type I dienophile reagents. Evans et al.4 reported a Diels-Alder reaction promoted by dialkyl aluminum chloride. In this reaction, chiral a,/ -unsaturated A-acyloxazolidinones were used as highly reactive and diaster-eoselective dienophiles. The stereogenic outcome of the Diels-Alder adducts... 

Evans et al. prepared organozinc reagent 16 from quantitative metalation of iodide 15 by using an activated Zn/Cu couple in the presence of an ester functionality. Treating 2-iodoimidazole 17 with three equivalents of 16 in the presence of PdCl2(Ph3P)2 afforded adduct 18, which was then transformed into diphthine (19) in five additional steps [17]. 

Evans et al. (220) have also shown that this reaction is amenable to a catalyst recycling protocol. This cycloaddition is tolerant of a variety of solvents including hexanes, conditions under which Complex 266c is apparently insoluble. Nevertheless, in the presence of adsorbent (florisil), this reaction proceeds at reasonable rates to provide the cycloadduct in undiminished yields and selectivities. Indeed, the catalyst could be efficiently recycled by removal of the supernatant liquid and recharging the flask with fresh solvent and reagents. Under this protocol, five cycles may be executed with only a slight diminution in rate and no effect on selectivities, Eq. 182. 

Evans and co-workers carried out a detailed study of Fenton s reagents (FeS04 H202) as a redox initiator, where OH are known to be the chain initiating species. The mechanism is as under ... 

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