Ketones chiral deriv

The use of chiral ketones for the protection of diols serves two purposes first, diol protection is accomplished, and second, symmetrical intermediates are converted to chiral derivatives that can be elaborated further, so that when the diol is deprotected, the molecule retains chirality. ... 

Asymmetric reduction of ketones. Pioneering work by Ohno et al. (6, 36 7, 15) has established that l-benzyl-l,4-dihydronicotinamide is a useful NADH model for reduction of carbonyl groups, but only low enantioselectivity obtains with chiral derivatives of this NADH model. In contrast, this chiral 1,4-dihydropyridine derivative (1) reduces a-keto esters in the presence of Mg(II) or Zn(II) salts in >90% ee (equation I).1 This high stereoselectivity of 1 results from the beneficial effect... 

Additions to Chiral Derivatives of a,/3-Unsaturated AIdehydes and Ketones 208... 

The (diastereoselective) conjugate addition of silylcuprate reagents to a variety of chiral derivatives of a,(3-unsaturated carboxylic acids can be used to prepare optically active p-silyl esters.258 259 Best results are obtained with substrates of type (25). The (related) p-silyl ketones, which also constitute valuable building blocks for (acyclic) stereoselective synthesis, are now accessible in high ee via palladium-catalyzed enantioselective 1,4-disiiylation of a,p-unsaturated ketones (Scheme 76).260... 

Using other hypervalent iodine compounds or different reagent combinations, various functional groups can be introduced in the a-position of ketones. a-Tosylations of ketones can be achieved directly using [hydroxy(tosyloxy)-iodo]benzene 6. The major drawback is the low regioselectivity observed in these reactions, although the a-tosylation of silyl enol ethers circumvents this problem. In the last few years some efforts have been done in the synthesis of chiral hypervalent iodine compounds [48, 53-55,113-117], but only a few of them have been used successfully in stereoselective synthesis. With chiral derivatives of type 59 it is possible to a-tosylate propiophenone with about 40% ee [56,118,119]. 

An example of the utilization of a bridged bicyclic ketone for preparation of an acyclic moiety is the stereoselective synthesis of the C-21 to C-27 segment of rifamycin-S, a member of the ansamycin family of antibiotics (Scheme 18). Rao et alP used ketone (61), derived from furan, to prepare lactone (62). Exhaustive reduction of (62) provided the segment (63), which contains five chiral centers of lifamycin-S. 

Chiral ketone 51 (derived from glucose, and therefore available as either enantiomer) was introduced in 2000 as an asymmetric epoxidation catalyst specifically for r-olefins, giving good results for a range of unsubstituted cycloalk-enes (Table 5, entries 6-12) <2000JA11551, 2002JOC2435>. 

Ge, H. Q. Chiral ketone catalysts derived from D-fructose. Synlett 200A, 2046-2047. 

Yamamoto et al. have been trying to synthesize limonene by a biomimetic cyclization of nerol (37) derivatives. The principle is to use the chiral derivative 523 and an alkylaluminum. After much experimentation,the best conditions were with the reagent 524 in CCI3F, when 77% ee of (-I-)-limonene (285) was obtained.It is widely believed still that the Wolff-Kishner reduction of optically active carvone (281) will yield optically active limonene (e.g., see Ref. 16, p. 152), a reaction reported by Friedman and Miller." This was already doubtful (Friedman and Miller gave no experimental evidence), since Jeger et al. had shown in 1950 that the Wolff-Kishner reaction with a,p-unsaturated ketones displaces the double bond, but Akhila and Banthorpe have now shown conclusively that (-)-carvone (281) yields ( )-limonene [( -285] by the Wolff-Kishner reaction. ... 

The first highly enantioselective construction of a-methyl-p-hydroxycarbonyl units, described by Masamune et al., used alkenyloxyboranes (29) prepared by enolization of ethyl ketone (30) derived in three steps from enantiomerically pure mandelic acid. Various dialkylboryl triflates are used with diiso-propylethylamine for enolization. The alkenyloxyboranes (29) exhibit striking stereoselectivity as chiral reagents in reactions with representative aldehydes. With judicious choice of the alkyl ligands on the... 

Silylation of azaallyllithium reagents derived from hydrazones unlike silylation of enolates seems to occur mainly on cartwn. While chiral (S)-l-amino-2-methoxymethylpyrrolidine (SAMP) aldehyde hydrazones (c/. equation 4) alkylate to a greater extent on nitrogen to form an azaallylsilyl reagent, ketones give predominant C-silylation. In the case of chiral ketone hydrazones derived from (5)-(4), a-silylated ketone hydrazones are produced in these reactions with consistently high ee ( 6%) ... 

Chiral -substituted carbonyl compounds have been prepared in high optical purity (64-98%) by the use of organoaluminium chemistry. Thus treatment of an acetal derived from an a, 6-unsaturated aldehyde and R,R- tartaric acid diamide with a trialkylalane gives largely the 1,4- addition product from which the desired ketone is derived by ozonolysis (Scheme 12). 1... 

High yields of 2-substituted chromans are readily attained from the asymmetric intramolecular oxa-Michael addition reaction of phenols bearing an (f -a,P-unsaturated ketone or thioester moiety mediated by a cinchona-alkaloid-urea-based bifunctional organocatalyst (140BC119). Molecular iodine-catalyzed reaction of phenols with a,P-unsaturated alcohols affords a wide range of 2,2-disubstituted chromans (14T5221). Chiral derivatives result from the intramolecular allylic alkylation of phenols bearing an... 

Further work on the preparation of chiral a-amino-acids reported in the past year (see also the section on asymmetric hydrogenation) includes an extension of the utility of anions derived from lactim ethers (228) in the synthesis of such compounds by condensations with aldehydes and ketones chiral inductions are somewhat lower than in the alkylations of (228) reported previously (4, 320). Enzyme-mediated hydrolysis of 5(4H)-oxazolones by chymotrypsin or subtilisin gives a-acylamino-acids with good enantiomeric enrichments, especially if the substrate carries bulky substituents. Schiff s bases of a-amino-esters can be enriched enantiomerically to an extent of up to 70% by sequential deprotonation with a chiral lithium amide and protonation with an optically pure tartaric acid. ... 

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