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Evans oxazolidinone-mediated

The Dias approach involves the use of three very efficient Evans oxazolidinone-mediated syn-aldol condensations. The other key steps involve a coupling reaction between the lithium acetylide-ethylenediamine complex and a tosylate followed by a methylation and a selective reduction to establish the C12-C13 (E) double bond. [Pg.44]

Scheme 33 illustrates the use of two standard persistent auxiliaries. The Evans oxazolidinone 33-1 [83] is highly versatile, i.e., suitable for enolate reactions and double bond additions alike. In the enolate alkylation case [reaction (99)] the high diastereoselectivity depends on the formation of a chelate 33-2 which fixes the reaction site in a defined conformation in which one of the diastereofaces is efficiently shielded. The removal of the auxiliary requires the chemoselective cleavage of the exo cyclic amide bond which is sometimes difficult to achieve. In boron mediated aldoltype additions [Scheme 34, reaction (100)] no chelate can be formed so that the extremeley high diastereoselectivity with which the syn-adduct 34-1 is formed must be due to some other effect, presumably allyl 1,3-strain on the stage of the enol borinate 34-1. [Pg.79]

Guided by the success of the Evans and related auxiliaries, several attempts were made to use enantiomerically pure a-bromoacyl oxazolidinones for stereoselective Reformatsky reactions. Fukuzawa and coworkers developed the reaction of various bromoacetyl oxazolidinones 323 as an alternative to an asymmetric acetate aldol addition. The conversion was mediated by samarium iodide and yielded P-hydroxy carbonyl compounds 325 with high diastereoselectivity in optimal combinations of auxiliary group and aldehyde. Among the different auxiliaries, the geminal dimethyl- and diphenyl-substituted ones performed better than the original Evans oxazolidinones. The stereochemical outcome was rationalized by assuming that an O-bound samarium(III) enolate reacts via a chair-like... [Pg.200]

As one of the pioneering works on asymmetric synthesis, Evans and coworkers reported diastereoselective Diels-Alder reaction of acryloyl oxazolidinones mediated by Et2AlCl [140]. Intramolecular version of this cycloaddition was also reported, in which interesting stereochemical difference depending on substituents at oxazo-lidinone ring was found. As an another excellent example, Oppolzer and coworkers reported diastereoselective Diels-Alder reaction of chiral dienophiles equipped with camphorsultam auxiliary [141]. Recently, these diastereoselective reactions were applied to enantioselective synthesis of ceralure Bl, known as Mediterranean fruit fly attr actant (Scheme 6.118) [142]. [Pg.305]

Evans has utilized Cu(ll)-bis(oxazoline)-mediated Diels-Alder reactions as the key step in several total syntheses. In 1996, Evans and co-workers used bu-box ligand 3 in the intramolecular Diels-Alder reaction of oxazolidinone 87 to form cycloadduct 88 enantioselectively, as shown in Figure 9.31. Compound 88 was subsequently converted to (-)-isopulo upone 89. [Pg.552]

The asymmetric total synthesis of cytotoxic natural product (-)-FRI 82877 was accomplished by D.A. Evans and co-workers." " To establish the absolute stereochemistry, a boron mediated aldol reaction was utilized applying (R)-4-benzyl-A/-propionyl-2-oxazolidinone" as a chiral auxiliary to yield the syn aldol product. [Pg.163]

L. Wessjohn and co-workers successfully applied the CrCl2-mediated Reformatsky reaction for the synthesis of C1-C6 fragment of epothilones. In their approach, they utilized the Evans (R)-4-benzyl-oxazolidinone chiral auxiliary to control the absolute stereochemistry. The chromium-Re/brmafsAy reaction between the (R)-4-benzyl-3-(2-bromoacetyl)-oxazolidinone and 2,2-dimethyl-3-oxo-pentanal occurred with complete chemoselection providing the product with 63% yield and as a single diastereomer. [Pg.375]

Oxazolidinones have proven to be extremely useful auxiliary groups in a variety of synthetic reaction types. Thus, the Evans auxiliaries are useful in the control of configuration in enolate alkylations and concerted cycloadditions, to name a few of the more important applications. Sibi and his collaborators at North Dakota State University have pioneered the use of these auxiliary groups in radical transformations mediated by Lewis acids [21-24]. Consider the general conformational questions that arise in a carboximide, such as 12, derived from an oxazolidinone (Eq. 18). The conformer 12 is disfavored by steric factors while 13 and 14 have similar steric demands. In the absence of any chelating Lewis acid, one expects that 13 would be of lower energy than 14 because of the opposed dipoles of the anti carbonyls in this conformation. [Pg.426]

The boron-mediated aldol reaction of an Evans s acyl oxazolidinone with an aldehyde affords the Evans-syn aldol adduct 9. The process proceeds via formation of the Z enolate that reacts with the aldehyde, presumably through a well ordered six-membered, chair-shaped Zimmerman-Traxler model... [Pg.533]

An alternative to the boron-mediated aldol addition of oxazolidinone 210 was developed also by Evans and coworkers by using the direct formation of chlorotitanium enolates that were generated by treatment with titanium tetrachloride in the presence of Hiinig s base or TMEDA. The stereochemical... [Pg.171]


See other pages where Evans oxazolidinone-mediated is mentioned: [Pg.134]    [Pg.321]    [Pg.535]    [Pg.298]    [Pg.73]    [Pg.281]    [Pg.421]    [Pg.8]    [Pg.162]    [Pg.611]    [Pg.285]    [Pg.552]   


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Evans

Evans oxazolidinones

Oxazolidinone

Oxazolidinones

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