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Ketones nonracemic

JVH-Aldols are also obtained from ( Jo-2-inethylbicyelo[2.2.1]hept-5-en-2-yl)ethyl ketone. Subsequently, the thennolabile norbonenyl group can be removed by retro-Diels Alder reaction531. For applications in syntheses of nonracemic aldols, see Section 1.3.4.4. [Pg.469]

The asymmetric Michael addition of chiral nonracemic ketone enolates has most frequently been used as part of the Robinson annulation methodology in the synthesis of natural products171-172. The enolates are then derived from carbocyclic chiral ketones such as (+)-nopinone, (-)-dihydrocarvone, or (-)-3-methylsabinaketone. [Pg.971]

Several methods for asymmetric C —C bond formation have been developed based on the 1,4-addition of chiral nonracemic azaenolates derived from optically active imines or enamines. These methods are closely related to the Enders and Schollkopf procedures. A notable advantage of all these methods is the ready removal of the auxiliary group. Two types of auxiliaries were generally used to prepare the Michael donor chiral ketones, such as camphor or 2-hydroxy-3-pinanone chiral amines, in particular 1-phenylethanamine, and amino alcohol and amino acid derivatives. [Pg.980]

A series of ethynyl ketones and ethynylketoesters were reduced enantioselectively to the corresponding nonracemic propargyl alcohols using a secondary alcohol dehydrogenase from... [Pg.154]

An interesting example of the transfer of center chirality to helicity is the work by Ogawa et al., based on an asymmetric aromatic oxy-Cope rearrangement to provide nonracemic [5]helicenes (Fig. 15.8) [75]. The starting material with center chirality, bicyclo[2,2,2]ketone (-)-21 (>98% ee), was obtained by enzymatic resolution. In the annelation step, the phenanthrene derivative was subjected to aromatic oxy-Cope rearrangement, to afford a pentacyclic product in 47 % yield. The corresponding [5]helicene 22 was obtained in 7 % overall yield (> 98 % ee) after six steps. [Pg.554]

Jorgensen et al. reported that C2-symmetric bis(oxazoline)-copper(II) complex 25 also acts as chiral Lewis acid catalyst for a reaction of allylic stannane with ethyl glyoxylate [37]. Meanwhile, p-Tol-BINAP-CuCl complex 26 was shown to be a promising chiral catalyst for a catalytic enantioselective allylation of ketones with allyltrimethoxysilane under the influence of the TBAT catalyst [38]. Evans and coworkers have developed (S,S)-Ph-pybox-Sc(OTf)3 complex 27 as a new chiral Lewis acid catalyst and shown that this scandium catalyst promotes enantioselective addition reactions of allenyltrimethylsilanes to ethyl glyoxylate [39]. But, when the silicon substituents become bulkier, nonracemic dihydrofurans are predominantly obtained as products of [3+2] cycloaddition. [Pg.119]

Chiral Bronsted acids can also promote the asymmetric addition of allylic tin reagents to carbonyl compounds. Baba and coworkers have found that a stoichiometric amount of (fl)-BINOL (37) acts a chiral promoter for the allylation of unactivated ketones with tetraallyltin and in the presence of MeOH, the corresponding nonracemic tertiary homoallylic alcohols are obtained with up to 60% ee [50]. Later, Woodward et al. improved this process and achieved a catalytic enantioselective allylation of aryl ketones by employing (fl)-monothio-binaphthol 36 as a chiral Bronsted catalyst [49]. For instance, in the presence of 20 mol% of the chiral acid 36 and 40 mol% of H20 in toluene, acetophenone (42) was allylated by a 0.7 0.3 mixture of tetraallyltin (41) and butyltriallylltin (55) to give the (jR)-enriched allylated product 56 almost quantitatively with 89-86% ee (Scheme 8). [Pg.121]

In some cases, a third control to be secured is that of the enantioselectivity, such as for the deprotonation of prochiral ketones with nonracemic bases (Scheme 10). [Pg.534]

Although chiral nonracemic enolates of type 21 and 22 are expected to exist under particular conditions, their half-lives to racemization are usually too short to effect actual asymmetric reactions. To realize an asymmetric transformation via a chiral enolate of type 21, chiral ketone 23 was designed that would generate a chiral enolate with a significantly long half-life to racemization (Scheme 3.8). Steric interactions of C(2)-OMe with C(2,)-OEt and C(8 )-H in the enolate would prevent free rotation of the C(l)-C(l ) bond as well as coplanarity of the enolate double bond with the naphthalene ring. Thus the enolate was expected not to exist as an achiral planar enolate, but it could be a chiral enolate with a chiral C(1)-C(T) axis. The rotational barrier about the C(1)-C(T) bond may be estimated by analogy with 2,2/-disubstituted biphenys because of similarity of the local steric environment around the chiral axes. The half-life to racemization was assumed to be about a few years at —78°C from the reported rotational barrier of various 2,2/-disubstituted biphenys ( 80 kJ/mol).16... [Pg.181]

The enantioselective cleavage of strained ethers (epoxides and oxetanes) by organolithiums, mediated by chiral, nonracemic ligands (as well as additions to aldehydes and ketones), has been reviewed by Goldfuss <2005S2271>. [Pg.267]

Asymmetric a-amination of enolates has also been described. For example, treatment of a-silyl ketone 109 with LDA followed by addition of oxaziridine 65a gave the A -BOC-amino ketone 110 in 29% yield and 88% de <1998TA3709>. Asymmetric amination of the prochiral enolate of 111 with chiral nonracemic oxaziridine 112 afforded amino ester 113 in 51% yield and 21% de <2001TA535>. [Pg.574]

Enantioselective Alkylation. Both antipodes of this chiral amine have been used in the enantioselective alkylation of ketones and aldehydes via their respective chiral, nonracemic lithioe-namines (eq 1). The enantioselectivity in alkylation results from the induced rigidity of the lithioenamine upon chelation with the methoxy group, providing the bias necessary to influence the direction and rate of entry of the electrophile. [Pg.56]

Enolboration of Ketones and Opening of meso-Epoxides. Methyl alkyl ketones have been successfully enolized by IpciBX (X = OTf or Cl) in the presence of a tertiary amine. The corresponding enolborinates have been used in asymmetric aldol condensations (eq 5). The reagent has also been applied to the enantioselective opening of mcj o-epoxides to form the corresponding nonracemic chlorohydrins (eq 6). ... [Pg.194]

Another important asymmetric epoxidation of a conjugated systems is the reaction of alkenes with polyleucine, DBU and urea H2O2, giving an epoxy-carbonyl compound with good enantioselectivity. The hydroperoxide anion epoxidation of conjugated carbonyl compounds with a polyamino acid, such as poly-L-alanine or poly-L-leucine is known as the Julia—Colonna epoxidation Epoxidation of conjugated ketones to give nonracemic epoxy-ketones was done with aq. NaOCl and a Cinchona alkaloid derivative as catalyst. A triphasic phase-transfer catalysis protocol has also been developed. p-Peptides have been used as catalysts in this reaction. ... [Pg.1176]

Several approaches are available for the synthesis of enantiomerically enriched alcohols from ketones.The three main strategies to obtain enantiomerically enriched (nonracemic) material are listed below. [Pg.124]


See other pages where Ketones nonracemic is mentioned: [Pg.478]    [Pg.486]    [Pg.284]    [Pg.305]    [Pg.226]    [Pg.184]    [Pg.185]    [Pg.235]    [Pg.184]    [Pg.185]    [Pg.443]    [Pg.173]    [Pg.595]    [Pg.596]    [Pg.395]    [Pg.68]    [Pg.95]    [Pg.23]    [Pg.121]    [Pg.116]    [Pg.653]    [Pg.183]    [Pg.268]    [Pg.209]    [Pg.193]    [Pg.227]    [Pg.877]    [Pg.17]    [Pg.185]    [Pg.408]    [Pg.1173]    [Pg.1176]    [Pg.305]    [Pg.30]    [Pg.35]   


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Nonracemic

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