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Methyl ketones, asymmetric aldol reaction

By the use of chiral oxazolidines derived from a chiral norephedrine and methyl ketones, an asymmetric aldol reaction proceeds in a highly enantioselective manner. In the case of ethyl or a-methoxy ketones, the corresponding anti aldol products were obtained with high diastereo- and enantioselectivities. A chiral titanium reagent, generated from... [Pg.290]

The development of enantioselective aldol reactions has been widely studied in conjunction with the synthesis of natural products. Highly enantioselective aldol reactions have been achieved by employing chiral enolates of ethyl ketones and propionic acid derivatives.(1) On the other hand, achieving high asymmetric induction in the asymmetric aldol reaction of methyl ketones is still a problem.(2)... [Pg.290]

Based on this assumption, the asymmetric aldol reaction of chiral 1,3-oxazolidines 1 of methyl ketones was examined. It was found that the corresponding aldol products were obtained in good optical purity when divalent tin chloride was used as an additive metal salt. [Pg.291]

Table 1. Asymmetric Aldol Reaction of Methyl Ketones... Table 1. Asymmetric Aldol Reaction of Methyl Ketones...
Dialkylboron trifluoromethanesulfonates (triflates) are particularly useful reagents for the preparation of boron enolates from carbonyl compounds, including ketones, thioesters and acyloxazolidinones.4 Recently, the combination of dicylohexylboron trifluoromethanesulfonate and triethylamine was found to effect the enolization of carboxylic esters.5 The boron-mediated asymmetric aldol reaction of carboxylic esters is particularly useful for the construction of anti (3-hydroxy-a-methyl carbonyl units.6 The present procedure is a slight modification of that reported by Brown, et al.2... [Pg.107]

Asymmetric aldol reactions.4 The borane complex 3 can also serve as the Lewis acid catalyst for the aldol reaction of enol silyl ethers with aldehydes (Mukaiyama reactions).5 Asymmetric induction is modest (80-85% ee) in reactions of enol ethers of methyl ketones, but can be as high as 96% ee in reactions of enol ethers of ethyl ketones. Moreover, the reaction is syn-selective, regardless of the geometry of the enol. However, the asymmetric induction is solvent-dependent, being higher in nitroethane than in dichloromethane. [Pg.314]

Recent success was achieved in carrying out direct catalytic asymmetric aldol reactions of aldehydes with unmodified ketones [22]. No preconversion of the ketone moiety to a more reactive species such as an enol silyl ether or enol methyl ether is necessary. [Pg.150]

The same group further developed the asymmetric aldol reaction of A -methoxy-A -methyl-a-isocyanoacetamide (a-isocyano Weinreb amide) with aldehydes (Sch. 25). The reaction of the Weinreb amide 96 with acetaldehyde in the presence of 86c Au(I) catalyst gives the optically active tram-oxazoline 98 (E = CON(Me)OMe R = Me) with high diastereo- and enantioselectivities similar to those of 95 [49], The oxazoline can be transformed into A,0-protected /3-hydroxy-a-amino aldehydes or ketones. [Pg.590]

Having developed an efficient catalytic asymmetric nitroaldol reaction, we next applied our attention to a direct catalytic asymmetric aldol reaction. The aldol reaction is generally regarded as one of the most powerful carbon-carbon bond-forming reactions. The development of a range of catalytic asymmetric aldol-type reactions has proven to be a valuable contribution to asymmetric synthesis. In all these catalytic asymmetric aldol-type reactions, however, preconversion of the ketone moiety to a more reactive speeies such as an enol silyl ether, enol methyl ether or ketene silyl... [Pg.935]

In 2001 Barbas III et al. reported the amino acid-catalysed direct asymmetric aldol reaction between ketones and aldehydes. Using the benchmark condensation reaction between acetone and p-nitro-benzalde-hyde, the authors tested many different amino acids as organocatalysts, including (5 )-ot-2-methyl-proline 7a (Scheme 11.2). In this reaction however 7a proved to be much less reactive than (S)-proline (1), as well as slightly less enantioselective. Compound 7a was also found to be less efficient than 1 in the direct organocatalytic asymmetric a-oxidation of cyclohexanone with iodosobenzene, as reported by Cordova et ah in 2005 (Scheme 11.3). ... [Pg.264]

Scheme 13.46 Direct catalytic asymmetric aldol reactions of methyl ketones and hydroxyketones promoted by (S)-LLB-KOH. Scheme 13.46 Direct catalytic asymmetric aldol reactions of methyl ketones and hydroxyketones promoted by (S)-LLB-KOH.
This reaction is of great importance in its chiral variant as an asymmetric aldol reaction. There are many practical methods for stereochemical control in either the non-catalytic or catalytic variant of this reaction. Two stereogenic centers are formed in a single reaction step, with an exception when the terminal methyl group reacts as an enol component (Scheme 4.8) or symmetrical ketone and formaldehyde as a carbonyl component (Scheme 4.9). Aldehydes are much more convenient than ketones as an electrophilic carbonyl counterpart and are preferably used. In Scheme 4.9 pairs of syn- and anti -diastereomers formed in an asymmetric aldol reaction are presented. [Pg.73]

In 1999, Shibasaki et al. reported on the direct catalytic asymmetric aldol reaction (Scheme 8.36), which was not necessary to preconvert the ketone moiety into the more reactive species such as an enolate ion and enol ether." The addition of bulky aldehyde 248 into the mixture of ethyl methyl ketone 249 and LaLi3tris(/ -binaphthoxide) [(/ )-LLB)] afforded aldol adduct 250 in excellent stereoselectivity. However, this reaction required a large amount of ketones (50 equiv), and catalyst (20 mol%) were required. They improved the conditions to reduce the amount of ketone (5 equiv) and catalyst (8 equiv) by using the hetero-polymetallic asymmetric catalyst (Scheme 8.37). The addition of the catalytic amount of potassium bis(trimethylsilyl) amide (KHMDS) and H2O was found to be effective to the catalysis. Adduct 253 was converted into ester 254 by the... [Pg.234]

Extensive stmcture activity relationship (SAR) studies in this series revealed that unsymmetrical substitution on the heterocyclic ring and hence the introduction of chirality on the central carbon atom led to increased potency. Such asymmetrical dihydro-pyridines can be prepared by stepwise variation of the Hantzsch synthesis, based on the hypothetical alternate route to nifedipine. Thus, aldol condensation of methyl acetoacetate with 2,3-dichlorobenzaldehyde (13-1) gives the cinnamyl ketone (13-2). Reaction of that with the enamine (13-3) from ethyl acetoacetate gives the calcium channel blocker felodipine (13-4) [14]. [Pg.330]

Yamamoto has recently described a novel catalytic, asymmetric aldol addition reaction of enol stannanes 19 and 21 with aldehydes (Eqs. 8B2.6 and 8B2.7) [14]. The stannyl ketones are prepared solvent-free by treatment of the corresponding enol acetates with tributyltin methoxide. Although, in general, these enolates are known to exist as mixtures of C- and 0-bound tautomers, it is reported that the mixture may be utilized in the catalytic process. The complexes Yamamoto utilized in this unprecedented process are noteworthy in their novelty as catalysts for catalytic C-C bond-forming reactions. The active complex is generated upon treatment of Ag(OTf) with (R)-BINAP in THF. Under optimal conditions, 10 mol % catalyst 20 effects the addition of enol stannanes with benzaldehyde, hydrocinnamaldehyde, or cinnamaldehyde to give the adducts of acetone, rerf-butyl methyl ketone (pinacolone), and acetophenone in good yields and 41-95% ee (Table 8B2.3). [Pg.518]

Stereoselective reduction of a-alkyl-3-keto acid derivatives represents an attractive alternative to stereoselective aldol condensation. Complementary methods for pr uction of either diastereoisomer of a-alkyl-3-hydroxy amides from the corresponding a-alkyl-3-keto amides (53) have been developed. Zinc borohydride in ether at -78 C gave the syn isomer (54) with excellent selectivity ( 7 3) in high yield via a chelated transition state. A Felkin transition state with the amide in the perpendicular position accounted for reduction with potassium triethylborohydride in ether at 0 C to give the stereochemi-cally pure anti diastereoisomer (55). The combination of these methods with asymmetric acylation provided an effective solution to the asymmetric aldol problem (Scheme 6). In contrast, the reduction of a-methyl-3-keto esters with zinc borohydride was highly syn selective when the ketone was aromatic or a,3-unsaturated, but less reliable in aliphatic cases. Hydrosilylation also provided complete dia-stereocontrol (Scheme 7). The fluoride-mediated reaction was anti selective ( 8 2) while reduction in trifluoroacetic acid favored production of the syn isomer (>98 2). No loss of optical purity was observed under these mild conditions. [Pg.11]

In recent years the synthetic potential and mechanistic aspects of asymmetric catalysis with chiral Lewis base have been investigated. Aldol addition reactions between trichlorosilyl enolates with aldehydes have been also intensively studied. Now, full investigations of the trichlorosilyl enolates derived from achiral and chiral methyl ketones, in both uncatalysed and catalysed reactions with chiral and achiral aldehyde acceptors have been reported. The aldol addition is dramatically accelerated by the addition of chiral phosphoramides, particularly (137) and proceed with good to high enantioselectivity with achiral enolates and aldehydes (Scheme 34). ... [Pg.130]


See other pages where Methyl ketones, asymmetric aldol reaction is mentioned: [Pg.109]    [Pg.290]    [Pg.363]    [Pg.241]    [Pg.421]    [Pg.1305]    [Pg.124]    [Pg.252]    [Pg.255]    [Pg.199]    [Pg.121]    [Pg.2209]    [Pg.83]    [Pg.4]    [Pg.490]    [Pg.388]    [Pg.842]    [Pg.214]    [Pg.34]    [Pg.291]    [Pg.39]    [Pg.243]    [Pg.31]    [Pg.233]    [Pg.349]    [Pg.703]    [Pg.905]    [Pg.192]   
See also in sourсe #XX -- [ Pg.291 , Pg.292 ]




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Aldol ketones

Asymmetric aldol reactions

Asymmetrical ketones

Ketones aldol reactions

Ketones, methyl aldol reactions

Methyl ketones, asymmetric aldol

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