Hydroxy carbonyls

Cleavage reactions of carbohydrates also occur on treatment with aqueous base for prolonged periods as a consequence of base catalyzed retro aldol reactions As pointed out m Section 18 9 aldol addition is a reversible process and (3 hydroxy carbonyl com pounds can be cleaved to an enolate and either an aldehyde or a ketone... 

Reaction of aldehydes or ketones to give /3-hydroxy carbonyl compounds... 

If the initially formed /3-hydroxy carbonyl compound 3 still has an a-hydrogen, a subsequent elimination of water can take place, leading to an o ,/3-unsaturated aldehyde or ketone 4. In some cases the dehydration occurs already under the aldol reaction conditions in general it can be carried out by heating in the presence of acid ... 

An alkene activated by an electron-withdrawing group—often an acrylic ester 2 is used—can react with an aldehyde or ketone 1 in the presence of catalytic amounts of a tertiary amine, to yield an a-hydroxyalkylated product. This reaction, known as the Baylis-Hillman reaction, leads to the formation of useful multifunctional products, e.g. o -methylene-/3-hydroxy carbonyl compounds 3 with a chiral carbon center and various options for consecutive reactions. 

All three elimination reactions--E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particularly common. The substrate is usually an alcohol, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carbonyl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the OH group is assisted by simultaneous protonation. 

StepS 9-1° of F Sure 29-7 Dehydration and Dephosphorylation Like mos /3-hydroxy carbonyl compounds produced in aldol reactions, 2-phospho glvcerate undergoes a ready dehydration in step 9 by an ElcB mechanism (Section 23.3). The process is catalyzed by enolase, and the product i... 

The Mukaiyama aldol reaction can provide access to a variety of (3-hydroxy carbonyl compounds and use of acetals as reactants can provide (3-alkoxy derivatives. The issues of stereoselectivity are the same as those in the aldol addition reaction, but the tendency toward acyclic rather than cyclic TSs reduces the influence of the E- or Z-configuration of the enolate equivalent on the stereoselectivity. 

A great deal of the interest in isoxazolines stems from their use in the synthesis of acyclic compounds (19). The approach to (3-hydroxy carbonyl compounds via... 

Diastereomeric 1,3-amino alcohols 1 have been obtained by reduction of 4,5-dihydroisoxa-zoles350-353. 3C chemical shifts allow a stereochemical differentiation due to the formation of energetically preferred chelated conformations. Similar to /3-hydroxy carbonyl compounds and 1,3-diol derivatives, the chemical shifts of the backbone carbons are larger in the syn 1,3-amino alcohols than in the awn -isomers353. 

Problem 17.26 Some condensations have little synthetic value because they give mixtures of /3-hydroxy-carbonyl compounds. Illustrate with (a) a mixture of two aldehydes, each with an a H (/>) an unsymmetrical ketone having an H on each a C. ... 

The aldol condensation is the reaction of an aldehyde or ketone enolate with an aldehyde or ketone to give a /3-hydroxy aldehyde or ketone. A simple aldol reaction is one in which the enolate nucleophile is derived from the carbonyl electrophile. Very often the /3-hydroxy carbonyl product dehydrates to give an... 

The reaction of enolates with aldehydes or ketones to produce /3-hydroxy carbonyl derivatives is a very common and a very useful way to make carbon-carbon bonds. A fundamental stereochemical feature of the reaction is diat two new chiral centers are produced from achiral starting materials. Hence syn and anti diastereomers will be produced, each as a pair of enantiomers. This is shown schematically for the reaction of a propionate enolate with isobutyraldehyde. Because they have different energies, the syn and anti diastereomers will be... 

Because these asymmetric aldol reactions are ideal methods for constructing (3-hydroxy carbonyl compounds in optically active form, the development of an asymmetric aldol reaction without the use of an organostannane would be advantageous. Yamagishi and coworkers have reported the Mukaiyama aldol reaction using trimethylsilyl enol ethers in the presence of the BINAP-AgPF6 complex to afford the adducts with moderate enantioselectivities (Table 9.9).18 They have also assigned... 

DI0XY BNATED SKELETONS 3-HYDROXY CARBONYL COMPOUNDS... 

Synthesis The synthesis uses rather more vigorous conditions than those which gave the (3-hydroxy carbonyl compounds. In fact (Bull. Chem. Soc. Japan, 1 952, 25, 5k, Chem. Abs., 195 ) A 5143) you can either treat the [3-hydroxy compound with HCl in acetic acid or do the condensation in base ... 

By analyzing an input molecule, substructures are found which point to chapters in the reaction library. There, descriptions of changes in these substructures are contained which will occur in the course of a certain reaction. Implementing these changes leads to synthetic precursors. Thus, the recognition of the substructure of a /3-hydroxy-carbonyl group would point to the chapter aldol condensation in the reaction library. There the structural changes of a retro-aldol condensation are contained and result in the two carbonyl compounds as precursors (see Fig. 1). As the reac-... 

In the presence of acid or base catalysts the aldol reaction is reversible, and the (3-hydroxy carbonyl products may revert to the initial aldehyde or ketone reactants. In the absence of such catalysts these aldol products are perfectly stable and isolable compounds. 

In the aldol reaction, two molecules of an aldehyde or ketone react with each other in the presence of base to form a 3-hydroxy carbonyl compound. For example, treatment of acetaldehyde with aqueous OH forms 3-hydroxybutanal, a P-hydroxy aldehyde. 

Significant features of these active borinic acid catalysts are that they are strong Lewis acids and have a hydroxy group on the boron atom. Dehydration is strongly favored in THF. The reaction usually proceeds smoothly, and a,/3-enones are obtained in high yields as ( ) isomers. In reactions of a-substituted-/3-hydroxy carbonyl compounds, a,/3-enones are preferentially obtained from anti aldols, whereas most syn aldols are recovered. This dehydration thus represents a useful and convenient method for isolating pure syn aldols from syn anti isomeric mixtures (Eq. 106). 

Oshima and Nozaki generated the aluminum enolate regiospecifically by treatment of a-halo carbonyl compounds with Bu3SnAlEt2 subsequent reaction with aldehydes or ketones under mild conditions gave /3-hydroxy carbonyl compounds [117]. This subsequent aldol reaction is accelerated by the addition of catalytic Pd(PPh3)4 (Sch. 81). 

Cleavage and isomerization- Fructose 1,6-bisphosphate is cleaved in step 4 into two, three-carbon monophosphates, one an aldose and one a ketose. The bond between carbons 3 and 4 of fructose 1,6-bisphosphate breaks, and a C-O group is formed. Mechanistically, the cleavage is the reverse of an atdol reaction (Section 23.2) and is carried out by an aldolase enzyme. (A forward aldol reaction joins two aldehydes or ketones to give a iil-hydroj carbonyl compound a retro aldol reaction cleaves a /3-hydroxy carbonyl compound into two aldehydes or ketones.)... 

Aldol reaction (Section 23.1) The carbonyl condensation reaction of an aldehyde or ketone to give a /3-hydroxy carbonyl compound. 

Normally alcohols are stable to strong bases, but with /3-hydroxy carbonyl compounds (where the removal of an a-hydrogen atom occurs readily [III to IV]) the subsequent elimination of a hydroxyl group might follow. 

Asymmetric allyation of carbonyl compounds to prepare optically active secondary homoallyhc alcohols is a useful synthetic method since the products are easily transformed into optically active 3-hydroxy carbonyl compounds and various other chiral compounds (Scheme 1). Numerous successful means of the reaction using a stoichiometric amount of chiral Lewis acids or chiral allylmetal reagents have been developed and applied to organic synthesis however, there are few methods available for a catalytic process. Several reviews of asymmetric allylation have been pubHshed [ 1,2,3,4,5] and the most recent [5] describes the work up to 1995. This chapter is focussed on enantioselective allylation of carbonyl compounds with allylmetals under the influence of a catalytic amount of chiral Lewis acids or chiral Lewis bases. Compounds 1 to 19 [6,7,8,9,10,11,12,... 

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