Carbonyl compounds, a-hydroxy

Here the carbanion (87), obtained from the action of base (usually eOH) on an oc-H atom of one molecule of a carbonyl compound (88), adds to the carbonyl carbon of another (88) to yield a -hydroxy-carbonyl compound. Thus with ethanal, CH3CHO, the product is 3-hydroxybutanal (89)—aldol itself ... 

Davis et al.111 developed another method for reagent-controlled asymmetric oxidation of enolates to a-hydroxy carbonyl compounds using (+)-camphor-sulfonyl oxaziridine (147) as the oxidant. This method afforded synthetically useful ee (60-95%) for most carbonyl compounds such as acyclic keto esters, amides, and a-oxo ester enolates (Table 4-20). 

Oxidation of silyl enol ethers. Oxidation of silyl enol ethers to a-hydroxy aldehydes or ketones is usually effected with w-chloroperbenzoic acid (6, 112). This oxidation can also be effected by epoxidation with 2-(phenylsulfonyl)-3-( p-nitrophenyl) oxaziridine in CHC1, at 25-60° followed by rearrangement to a-silyloxy carbonyl compounds, which are hydrolyzed to the a-hydroxy carbonyl compound (BujNF or H,0 + ). Yields are moderate to high. Oxidation with a chiral 2-arene-sulfonyloxaziridine shows only modest enantioselectivity. 

There are very few reactions of real synthetic significance which proceed via condensation of two 1,3-electrophile-nucleophile species. Probably the most important of this latter type of reaction is the synthesis of pyrazines by self-condensation of an a-acylamino compound to the dihydropyrazine followed by aromatization (equation 132). The a-acylamino compounds, which dimerize spontaneously, are normally generated in situ, for example by treatment of a- hydroxy carbonyl compounds with ammonium acetate or by reduction of a-azido, -nitro or -oximino carbonyl compounds. Cyclodimerization of a-amino acids gives 2,5-dioxopiperazines (equation 133), many derivatives of which occur as natural products. Two further reactions which illustrate the 1,3-electrophile-nucleophile approach are outlined in equations (134) and (135), but su i processes are of little general utility. 

Both the alio and manno epoxides (9) or (7), or their debenzyliden-ated analogs, undergo oxidation with methyl sulfoxide and boron trifluoride,104 but, unlike 5,6-anhydrohexofuranose derivatives (see p. 163), they do not yield a simple a-hydroxy carbonyl compound, as would be expected by analogy with simple oxiranes instead, the rearranged diulose (46) is obtained in low yield, possibly by elimination of methanol from the intermediate (47). 

The direct catalytic enantioselective a-aminoxylation of carbonyl compounds is a synthetically useful method for the preparation of versatile a-hydroxy carbonyl compounds. We have developed the direct catalytic enantioselective a-aminoxyla-... 

The preparation of a-hydroxy carbonyl compounds has been accomplished by the oxidation of enolates using both oxygen6 and MoC PyHMPA-(MoOPh).7 Acyl anion equivalents offer another route to this useful class of compounds. The procedure presented here for the synthesis of 6-hydroxy-3,5,5-tr1methyl-2-cyclohexen-l-one illustrates the use of MCPBA oxidation of an enol silyl ether as a method for obtaining an a-hydroxy enone. The procedure is a scaleup of a published synthesis. ... 

Hydroxy carbonyl compounds a-Hydroxy carbonyl compounds... 

In summary, the organocatalytic asymmetric a-aminooxylation of aldehydes and ketones with proline as catalyst is a highly enantioselective means of preparation of a-hydroxy carbonyl compounds, and their derivatives. Because this field has been developed only recently, more examples and work on extension of organocatalyst screening and process development can be expected in the near future. 

The role of the acid catalyst during the oxidation of epoxides with DMSO has been explored by DFT studies of three acids, namely H30+, Li+, and Mg2+. Stationary points have been obtained at the B3LYP/6-31+- -G(d,p) level of theory and the reaction barriers have been evaluated through tree-energy calculations. The mechanism proceeds in two steps, namely ring opening followed by an intramolecular proton transfer that leads to an a-hydroxy carbonyl compound.88 The epoxidation... 

Abstract Synthetically useful examples of carbonylations of aldehydes that enable the synthesis of structurally diverse carbonyl compounds such as -amino acids, a-hydroxy carbonyl compounds, furanones, and lactones will be highlighted in this chapter. 

A study on the chiral complexation preferences of three series of cyclic a-hydroxy carbonyl compounds (Scheme 3.26) has been carried out using DFT and MP2 calculations. The dimer formation study has been preceded by a conformational exploration of the isolated monomers of the parent compounds. In addition, three possible dimers have been considered initially in the dimers of the parent compound, discarding those with the highest energies for the rest of the derivatives. 

I. Alkorta et al., Chiral recognition in cyclic a-hydroxy carbonyl compounds A theoretical study. J. Phys. Chem. A 109, 3262-3266 (2005)... 

Fig. 3.6.10. G eneral access to p-amino-a-hydroxy carbonyl compounds (i) 5 equiv. BrCH2CN, toluene, 150 °C, MW, 15 min ...

Hydrocarbons 407 can be used as precursors in hydroxyalkylation reactions of a,p-unsaturated carbonyl compounds 408 in the presence of molecular oxygen (Fig. 95) [438]. The reactions are catalyzed by 0.3-1 mol% Co(acac)3 and 30 mol% 398. Alkylated a-hydroxy carbonyl compounds 409 and a-keto carbonyl compounds 410 were obtained in 42-98% yield in a 1-100 1 ratio. The reactions showed a considerable induction period. Co(acac)2, in contrast, initiated the reaction very quickly, but conversion soon ceased. The success using the unreactive Co(III) complex consists of gradual reduction to catalytically active Co(II). 

Each step is the same as the aldol sequence with acetaldehyde, and the product is again a hydroxy-carbonyl compound, but this time a hydroxy-ketone. 

N-Sulfonyloxaziridines are an important class of selective, neutral, and aprotic oxidizing reagents.11 Enantiopure N-sulfonyloxaziridines have been used in the asymmetric hydroxylation of enolates to enantiomerically enriched a-hydroxy carbonyl compounds,9 11-13 the asymmetric oxidation of sulfides to sulfoxides,14 1S selenides to selenoxides,16 sulfenimines to sulfinimines,17 and the epoxidation of alkenes.18... 

Enolate hydroxylation cf. 11, 108).- Enolates of ketones or esters are oxidized by this oxaziridine to a-hydroxy carbonyl compounds. Yields are highly dependent on the base they are highest with potassium hexamcihyldisilazide. Yields are generally higher than those obtained with the Vedejs reagent (MoOPH, 8, 207). 

Examples of fluorides prepared from a-hydroxy carbonyl compounds using DAST are given in Table 4. 

Table 4. Fluorination of a-Hydroxy Carbonyl Compounds with DAST...

---