P hydroxycarbonyl

Selective reduction of the oximino fragment in products (224) or (225) gives rise to poorly studied y-amino alcohols (226). In addition, desoximation of (225) can produce the corresponding p-hydroxycarbonyl derivatives (227). In addition, oximino alcohols (225) can be oxidized to y-nitro alcohols (228). 

A convenient procedure was developed for transformations of /V-siloxyisoxa-zolidines into functionalized p-hydroxycarbonyl compounds with aqueous titanium trichloride (185, 189) (Scheme 3.164). 

The aldol reaction is well established in organic chemistry as a remarkably useful synthetic tool, providing access to p-hydroxycarbonyl compounds and related building blocks. Intensive efforts have raised this classic process to a highly enantioselective transformation employing only catalytic amounts of chiral promoters, as reviewed in the previous section (Chap. 29.1). While some effective applications have been reported, most of the methodologies necessarily involve the preformation of latent enolates 2, such as ketene silyl acetals, using... 

The aldol reaction is one of the most useful and classical reactions in organic synthesis, since the resulting p-hydroxycarbonyl compounds are highly useful building blocks for pharmaceuticals and natural products.14 The asymmetric version of this reaction has been developed to a significant extent. We focus here on the asymmetric classical aldol reaction using a silver catalyst and related reactions.15... 

Another nucleophilic addition involving a charged nucleophile is the Aldol reaction. This involves the nucleophilic addition of enolate ions to aldehydes and ketones to form p-hydroxycarbonyl compounds ... 

Problem 17.20 Write structural formulas for the p-hydroxycarbonyl compounds and their dehydration products formed by aldol condensations of (a) butanal, b) phenylacetaldehyde, (c) diethyl ketone, (t/) cyclohexanone, (e) benzaldehyde. ... 

The Lewis acid mediated rearrangement of epoxy silyl ether (69) affords the corresponding P-hydroxycarbonyl compound (70 Scheme 26). ... 

Karrer, P. Hydroxycarbonyl compounds. I. A new synthesis of hydroxyaldehydes. Helv. Chim. Acta 1919, 2, 89-94. 

The direct asymmetric aldol reaction between unmodified aldehydes and ketones plays an important role in nature as a source of carbohydrates and it is used for the synthesis of chiral p-hydroxycarbonyl compounds. This reaction was performed by using (5)-proline/poly-(diallyldimethylammonium) hexafluorophosphate heterogeneous catalytic system 36. The catalyst was simply prepared by mixing a suspension of the commercially available polyelectrolyte 34 in methanol with a solution of (,S )-prolinc (35) in the same solvent (Scheme 3.11). 

Over the past decade the enantio- and diastereo-selective aldol methodology has been developed mainly with the purpose of synthesizing polyketide-type natural products such as macrolides and iono-phore antibiotics. These natural products have the basic structural units that result from propionate and acetate addition a-methyl-P-hydroxycarbonyl (27a-d) and (3-hydroxycarbonyl (28a and 28b). Discussions on the construction of these units will be followed by comments on the stereoselective assembly... 

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... 

Following route II, it can be seen that the primary H2O addition to the fiiran C-2/C-3 bond can also occur in an opposite direction to a, i.e. according to the retrostep d. This leads to the intermediate 7 for which the bond cleavage O/C-2 (e) is retroanalytically rationalized and leads to the y-hdXo-p-hydroxycarbonyl system 9. A retroaldol operation (g) provides the same starting materials 10 and 11 as the retrosynthesis I route II, however, suggests as the first step aldol addition of 10 and 11 to give 9 followed by intramolecular Sn cyclization of the enolate 9 to the dihydrofuran 7 and conversion into fiiran by dehydration. 

Dehydration o1 primary nitro compounds (Mukeuyama reaction) affords nitrile oxides, which may dimerize to yield furoxans, or otherwise be trapped by suitable dipoiatophiles such as double or triple bond systems, leading to the formation of various heterocyclic systems, S. The latter have been used for further derivatization in the heterocyclic series, or in return as precursors of acyclic products after ring cleavage, for example, 1,3-amino alcohols 6 or p-hydroxycarbonyl compounds, 9. 

The Aldol condensation is the coupling of an enolate ion with a carbonyl compound to form a p-hydroxycarbonyl, and sometimes, followed by dehydration to give a conjugated enone. A simple case is addition of an enolate to an aldehyde to afford an alcohol, thus the name aldol. 

Sol 2. (b) It is an example of a widely used masked-aldol reaction. 1,3-Dipolar cycloaddition of nitrile oxides with an alkene (or alkyne) gives a cyclic product, isoxazoline (or isoxazole). These cyclic compounds are readily cleaved by reduction of the N—O bond and subsequent hydrolysis of the resulting imine to give aldol-type p-hydroxycarbonyl (or Claisen-type P-dicarbonyl) products. 

An important general reaction of enolate ions involves nucleophilic addition to the electrophilic carbonyl carbon atom of the aldehyde or ketone from which the enolate is derived. A dimeric anion 18 results, which may then be neutralized by abstraction of a proton to produce a p-hydroxycarbonyl compound, 19 (Eq. 18.10). If the reaction is performed in hydroxylic solvents such as water or an alcohol, the source of the proton may be the solvent, whose deprotonation will regenerate the base required for forming the enolate ion. Thus, the overall process is catalytic in the base that is used. 

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