Hydroxy ketones from aldehydes

This method can be used in conjunction with addition of cyanide to prepare a-hydroxy aldehydes from ketones.90... 

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... 

While the use of strong bases has changed the way in which many condensation reactions are carried out, it is important to remember the types of products that are produced from them. Recall that the aldol condensation yields ft -hydroxy aldehydes or ketones which are easily dehydrated to ,/3-unsaturatcd aldehydes or ketones. 

The hydroxynitrile lyase (HNL) class of enzymes, also referred to as oxynitrilases, consists of enzymes that catalyze the formation of chiral cyanohydrins by the stereospecific addition of hydrogen cyanide (HCN) to aldehydes and ketones (Scheme 19.36).275 279 These chiral cyanohydrins are versatile synthons, which can be further modified to prepare chiral a-hydroxy acids, a-hydroxy aldehydes and ketones, acyloins, vicinal diols, ethanolamines, and a- and P-amino acids, to name a few.280 Both (R)- and (.S )-selective HNLs have been isolated, usually from plant sources, where their natural substrates play a role in defense mechanisms of the plant through the release of HCN. In addition to there being HNLs with different stereo-preferences, two different classifications have been defined, based on whether the HNL contains a flavin adenine dinucleotide (FAD) co-factor. 

Hydroxy aldehydes or ketones normally exist in equilibrium with the corresponding 2-hydroxytetrahydrofurans, which afford 2,3-dihydrofurans on dehydration. Similarly, 7-aminoketones spontaneously yield pyrrolines. From a mechanistic viewpoint the-initial cyclizations may be regarded as being initiated by nucleophilic attack of the heteroatom upon the carbonyl group. While such reactions occur with facility, their synthetic applicability is largely determined by the accessibility of appropriate precursors. A selection of examples of this type of ring closure is provided in Scheme 12. 

Hemiacetals are formed from hydroxy aldehydes " or ketones which are usually first in a protected form and are deblocked prior to cyclization. Ozonolysis of cyclohexene derivative 5 and reductive workup, followed by hydrolysis of the acid chloride formed in situ and borane reduction of the carboxylic acid, leads to the six-membered hemiacetal 6. a precursor for fluo-rinated sugars that are potentially versatile as molecular probes in the elucidation of biochemical processes. ... 

The hydroxy acids obtained from aldehydes will contain the group,. —CH(OH) —COOH, and are thus secondary alcohols while those obtained from ketones will contain the group, =C(OH) —COOH, and are tertiary alcohols. These reactions are of especial importance in connection with the poly-hydroxy aldehydes and ketones, which, as we shall find, are the sugars. The cyan hydrines, or hydroxy acid nitriles, which are the intermediate products in these reactions, are not isolated as such, the reaction being completed without interruption. 

From To — Akanes CycJoaikanes Akenes Akynes Aryls Halogen compounds Alcohols Phenols Ethers, Quinones B, 5 and Si compounds P and BI compounds Nttro. Nitroso, Azo. Azoxy, Hydrazo Azides Amines Organometahic compounds Adehydes Ketones Acids. AnHyd rides. Esters Amides, Amidines. Nitriles Hydroxy-aldehydes or -ketones. Sugars. Hydroxy acids Ammo acids. Peptides Heterocycles Nucleosides Miscellaneous, including heterocycles... 

The Rubottom oxidation1 is the peracid-mediated oxidation of trimethylsilyl enol ethers to afford a-silyloxy- or a-hydroxy aldehydes or ketones.2,3 Use of an aqueous workup generally affords the hydroxy compounds, whereas nonaqueous workups provide the silyloxy derivatives. For example, the enolsilane 1 derived from cycloheptanone was converted to 2 in 77% yield by treatment with /w-CPBA followed by workup with 10% aqueous sodium hydroxide. Omission of the aqueous workup afforded 3 in 85% isolated yield,1 ... 

Lithiated 1,3-dithianes react readily with epoxides to give thio-acetals of 13-hydroxy aldehydes or ketones. Addition of carbonyl electrophiles to 2-lithio-1,3-dithiane is efficient and provides a method for preparing a-hydroxy carbonyl compounds. For example, the ketone 120 can be converted into the hydroxy aldehyde 122 via the alcohol 121 (1.108). The dithiane approaches the ketone from the less-hindered convex face of the fused ring system. 

An alternative method for the formation of aldehydes or ketones makes use of the complexes formed from a methyl sulfide with chlorine or A-chlorosuccinimide (NCS), in what is called the Corey-Kim oxidation. With dimethyl sulfide the salt 28 is generated and reacts with the alcohol to give the alkoxysulfonium salts and hence, on treatment with a base, the carbonyl compound. This reaction has fovmd particular application in the oxidation of 1,2-diols in which one alcohol is tertiary, to give a-hydroxy-aldehydes or ketones without rupture of the carbon-carbon bond. For example, the aldehyde 32 is formed in good yield from the diol 31 using dimethyl sulfide and NCS followed by addition of triethylamine (6.29). This transformation is thought to depend on the preferential five-membered transition... 

The resonant anion can act as a typical carbanion and add to C=0 or C=N bonds, just as a Gtignard reagent does. In a true aldol condensation, the anion after formation proceeds to interact with another molecule of the starting aldehyde or ketone, giving a product that is a beta-hydroxy aldehyde or ketone known as an aldol (Scheme 4.29). Frequently, water is eliminated from this compound to produce an a, P-unsaturated carbonyl compound this process is made easy by the resonance stabilization of such conjugated systems. 

Success in solving what are often called fun in base problems has a lot to do with recognition, of seeing the clues that are present in almost every problem. We will have more to say on this subject later, but for now remember that all P-hydroxy aldehydes and ketones and all a,P-unsaturated aldehydes and ketones come from aldol condensations. The quotes around all are there just to remind you that there will inevitably be exceptions. 

The nuclear magnetic resonance spectra of SchifF bases formed from primary amines and ortAo-hydroxy aldehydes and ketones show that the Schiff bases derived from l-hydroxy-2-acetonaphthone and from 2-hydroxy-1-naphthaldehyde exist as keto amines (7a) although their formation involves loss of most of the resonance energy of one of the aromatic rings When R is a phenyl group, the phenol-imine tautomer (7b) predominates Schiff bases derived from ortho-hydroxy aldehydes and ketones have the phenol-imine structure (8) . Evidently, in such compounds the keto-amine tautomer... 

For example, meso-tetraaryltetrabenzoporphyrin zinc complex results from heat treatment of the corresponding 3-benzylidenisoindolinone with zinc benzoate l Appropriately substituted methine and azomethine isoindolinones featuring substructure 24 were also converted into stable metal chelate pigments of general structure 26 via a metal template reaction with aromatic and heterocyclic a-hydroxy aldehyde or ketone hydrazone derivatives incorporating the structural element 25 . ... 

Like hydration, these addition reactions are governed by equilibria that usually favor the starting carbonyl compound. Hemiacetals, like hydrates, are therefore usually not isolable. Exceptions are those formed from reactive carbonyl compounds such as formaldehyde or 2,2,2-trichloroacetaldehyde. Hemiacetals are also isolable from hydroxy aldehydes and ketones when cyclization leads to the formation of relatively strain-free five- and six-membered rings. 

RibA also tolerates many other aldehyde acceptors, and can even utilize other donor nucleophiles, for example, propionaldehyde, acetone, and fluoroacetone, but at much lower rates, causing limitations at preparative scale (Table 10.18) [165-167]. For RibA from E. coU, nonphosphorylated acceptors show activities as low as 0.03-0.4% of that of the natural substrate. Therefore, the yields in some instances are low (<20%) and require large amounts of enzyme catalyst. On the other hand, 2-hydroxyaldehydes are relatively good acceptors, among which the o-isomers are better substrates than the r-isomers (Table 10.18). RibA-catalyzed reactions provide access to (3-hydroxy aldehydes and ketones with the stereoselective creation of one stereogenic center. 

A cyanohydrin is an organic compound that contains both a cyanide and a hydroxy group on an aUphatic section of the molecule. Cyanohydrias are usually a-hydroxy nitriles which are the products of base-cataly2ed addition of hydrogen cyanide to the carbonyl group of aldehydes and ketones. The lUPAC name for cyanohydrias is based on the a-hydroxy nitrile name. Common names of cyanohydrias are derived from the aldehyde or ketoae from which they are formed (Table 1). 

Mandelic Acid.—The reaction furnishes a simple and general method for obtaining hydroxy-acids from aldehydes or ketones by the aid of the cyanhydrin. The formation of the cyanhydrin may be effected in the manner described or by the action of hydrochloric acid on a mixture of the aldehyde or ketone with potassium cyanide, or, as in the case of the sugais, by the use of liquid hydrocyanic acid and a little amme-nia. 

Starting from a 3-hydroxy-1,5-diene 8, the rearrangement is not reversible, because the Cope product 9 tautomerizes to an aldehyde or ketone 10, and is thereby removed from equilibrium ... 

The aldol reaction yields either a /3-hydroxy aldehyde/ketone or an a, 3-unsatu-rated aldehyde/ketone, depending on the experimental conditions. By learning how to think backward, it s possible to predict when the aldol reaction might be useful in synthesis. Whenever the target molecule contains either a /3-hydroxy aldehyde/ketone or a conjugated enone functional group, it might come from an aldol reaction. 

The addition of H2S to an aldehyde or ketone can result in a variety of products. The most usual product is the trithiane (8). ° ot-Hydroxy thiols (5) can be prepared from polychloro and polyfluoro aldehydes and ketones.Apparently, compounds 5 are stable only when prepared from these compounds, and not even for all of them. Thioketones (6) can be prepared from certain ketones, such as diaryl ketones, by treatment with H2S and an acid catalyst, usually HCl. They are often unstable and tend to trimerize (to 8) or to react with air. Thioaldehydes are even less stable and simple ones apparently have never been isolated, though t-BuCHS has been... 

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