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Halo ketones Darzens’ reaction

The dissonant charge pattern for 2,5-hexanedione exhibits a positive (-1-) polarity at one of the a-carbons, as indicated in the acceptor synthon above. Thus, the a-carbon in this synthon requires an inversion of polarity Umpolung in German) from the negative (-) polarity normally associated with a ketone a-carbon. An appropriate substrate (SE) for the acceptor synthon is the electrophilic a-bromo ketone. It should be noted that an enolate ion might act as a base, resulting in deprotonation of an a-halo ketone, a reaction that could lead to the formation of an epoxy ketone Darzens condensation). To circumvent this problem, a weakly basic enamine is used instead of the enolate. [Pg.7]

More recently, the same group has used a simpler and more easily prepared chiral ammonium phase-transfer catalyst 99 derived from BINOL in asymmetric Darzens reactions with a-halo amides 97 to generate glycidic tertiary amides 98 (Table 1.13). Unfortunately the selectivities were only moderate to low [48]. As mentioned in Section 1.2.3.1, tertiary amides can be converted to ketones. [Pg.24]

The Darzens reaction, which is described on p T 253, is a good example of eyclisation to give a three-membered ring. The enolate of an a-halo ester, e.g. (27), adds to a ketone to give (26) v,/hieh cannot be isolated as it cyclises so readily. [Pg.344]

Darzens reaction, the reaction between a carbonyl compound and an a-halo ester in the presence of a base, consists of an initial aldol-type addition and a subsequent intramolecular Sn2 reaction, forming an epoxide as its final product. Its high stereoselectivity thus relies on the stereoselectivity of the nucleophilic addition of an a-halo ester onto the carbonyl substrate, which can be either an aldehyde or a ketone. [Pg.475]

Makosza and co-workers have reported the preparation of epoxides from a-halo carbanions and ketones, according to the Darzens reaction, under PT conditions, using TEBA72,73 or dibenzo-18-crown-6.74 The ratio of isomers depends on the reaction conditions.75,76 Asymmetric induction has been reported in the Darzens reaction using chiral catalysts.77,78 The use of several chloro carbanions as well as K2C03 and Na2C03 in the solid state has also been studied. [Pg.188]

Synthesis of Optically Active Epoxides. Alkaloids and alkaloid salts have been successfully used as catalysts for the asymmetric synthesis of epoxides. The use of chiral catalysts such as quinine or quinium benzylchloride (QUIBEC) have allowed access to optically active epoxides through a variety of reaction conditions, including oxidation using Hydrogen Peroxide (eq 5), Darzens condensations (eq 6), epoxidation of ketones by Sodium Hypochlorite (eq 7), halohydrin ring closure (eq 8), and cyanide addition to a-halo ketones (eq 9). Although the relative stereochemistry of most of the products has not been determined, enan-tiomerically enriched materials have been isolated. A more recent example has been published in which optically active 2,3-epoxycyclohexanone has been synthesized by oxidation with t-Butyl Hydroperoxide in the presence of QUIBEC and the absolute stereochemistry of the product established (eq 10). ... [Pg.498]

Treatment of a, ) -unsaturated ketones with a-halo esters often results in oxirane (205) formation (Darzens reaction). However, the reaction of the dienone 203 with ethyl bromoacetate (204) produces the bicyclo[4.1.0]heptenone derivative 206 (equation 55) ... [Pg.474]

This reaction was first reported by Nenitzescu in 1931. It is the formation of an a,p-unsaturated ketone directly by aluminum chloride-promoted acylation of alkenes with acyl halides. Therefore, it is known as the Darzens-Nenitzescu reaction (or Nenitzescu reductive acylation), or Nenitzescu acylation. Under such reaction conditions, Nenitzescu prepared 2-butenyl methyl ketone from acetyl chloride and 1-butene and dimethylacetylcyclohex-ene from acetyl chloride and cyclooctene. However, in the presence of benzene or hexane, the saturated ketones are often resolved, as supported by the preparation of 4-phenyl cyclohexyl methyl ketone from the reaction of cyclohexene and acetyl chloride in benzene, and the synthesis of 3- or 4-methylcyclohexyl methyl ketone by refluxing the mixture of cycloheptene and acetyl chloride in cyclohexane or isopentane. This is probably caused by the intermolecular hydrogen transfer from the solvent. In addition, owing to its intrinsic strain, cyclopropyl group reacts in a manner similar to an oleflnic functionality so that it can be readily acylated. It should be pointed out that under various reaction conditions, the Darzens-Nenitzescu reaction is often complicated by the formation of -halo ketones, 3,)/-enones, or /3-acyloxy ketones. This complication can be overcome by an aluminum chloride-promoted acylation with vinyl mercuric chloride, resulting in a high purity of stereochemistry. ... [Pg.851]

Scheme 16.34 Asymmetric phase-transfer catalytic Darzen reaction of a-halo ketones. Scheme 16.34 Asymmetric phase-transfer catalytic Darzen reaction of a-halo ketones.
Darzens glycidic ester condensation generally involves the condensation of an aldehyde or ketone 2 with the enolate of an a-halo ester 1 which leads to an a,P-epoxy ester (a glycidic ester) (3). Thus the reaction adds two carbons to the electrophile however, the reaction has been primarily developed as a one-carbon homologation method. That is, subsequent to the condensation, the ester is saponified and decarboxylation ensues to give the corresponding aldehyde or ketone 5.2... [Pg.15]

Aldehydes and ketones condense with a-halo esters in the presence of bases to give ot,p-epoxy esters, called glycidic esters. This is called the Darzens condensation. The reaction consists of an initial Knoevenagel-type reaction (16-41), followed by an internal Sn2 reaction (10-13) ° ... [Pg.1230]

S Darzen s 1 a-Halo ester XCH — COOR Aldehyde, ketone Epoxidation (Sn reaction) follows... [Pg.938]

Darzens condensation. Hinckley found that the condensation of a-halo esters with ketones when effected with sodium hydride dispersion proceeds smoothly and without usual side reactions. [Pg.1272]

The Darzens glycidic ester condensation involves the condensation of an aldehyde or ketone with an a-halo ester, in the presence of a base, to afford an a,3 epoxy ester (a glycidic ester ). The first synthesis of a glycidic ester was reported by Erlenmeyer in 1892 and is illustrative of the general reaction, as shown in equation (1). The reaction was subsequently developed and generalized by Darzens. Glycidic esters, in addition to undergoing transformations normally associated with epoxides, afford upon hydro-... [Pg.409]


See other pages where Halo ketones Darzens’ reaction is mentioned: [Pg.115]    [Pg.1230]    [Pg.115]    [Pg.955]    [Pg.115]    [Pg.115]    [Pg.128]    [Pg.128]    [Pg.422]    [Pg.422]    [Pg.841]    [Pg.2038]    [Pg.422]    [Pg.312]    [Pg.32]    [Pg.586]    [Pg.586]    [Pg.128]    [Pg.410]    [Pg.410]    [Pg.586]    [Pg.354]    [Pg.409]    [Pg.410]   
See also in sourсe #XX -- [ Pg.955 ]




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