O-Ketols

BREDERECK Imidazole synthesis Synthesis at imidazoles Irom formamide (acetamide) and a-diketones, o-ketols, o amnnketones, a-oximinoketones... 

E. Forne and J. Pascual, J.C.S. Perkin I, 1978, 288. It appears also that the X-ray structures of the a//o-ketol, bishydropulegone, and the related dehydrobispulegone have been determined by D. Rogers etal. but never published (D. Rogers, personal communication see also this reference and Vol. 7, p. 29, ref. 288). 

Hydroxylations. Titanium enolates i urnish o-ketols (60-97%) " on exposure -apid (-78°C, 1 min). 

Clxavage of vk iots to auhoxytic rtcHfs. In the presence of catalytic amounts of NaiWO,-211 0 and H,PO, and at a pH of 2,1 IjO (40%) owdizes a variety of water-S(riuble l,2carboxylic acids at 9(r via an o-ketol, Addition of a phase>transfer catalyst permits oxidation of watcrriitsoluble dtols. 

Polymer-supported thiazolium salts have been found to have high catalytic activity in the formose reaction, the main product being hydroxyacetone. Use of synthetic a-ketols as catalysts in the formose reaction led to selective formation of trioses, especially with o-ketols bearing electronegative substituents." Incubation of D-glucose with bakers yeast in the presence of benzyl mercaptan gave 5-benzyl thioglycerate in preparatively useful yields fermentation experiments with... 

This compartmentatlon would be lost during maceration of the tissue, as was demonstrated in corn seedlings by the five-fold increase in o-ketol concentration within ten minutes after mincing the tissue (33). It could be that Isomerases function to convert excess toxic fatty acid hydroperoxides to Innocuous ketol products as a protective mechanism. 

The addition of the a-carbon of an enolizable aldehyde or ketone 1 to the carbonyl group of a second aldehyde or ketone 2 is called the aldol reaction It is a versatile method for the formation of carbon-carbon bonds, and is frequently used in organic chemistry. The initial reaction product is a /3-hydroxy aldehyde (aldol) or /3-hydroxy ketone (ketol) 3. A subsequent dehydration step can follow, to yield an o ,/3-unsaturated carbonyl compound 4. In that case the entire process is also called aldol condensation. 

In aqueous solution the rate law for oxidation of simple a-ketols, such as acetoin and benzoin and various aldoses and ketoses o. sog-sioj markedly different from that of Wiberg and Nigh (vide supra), viz. 

This enzyme [EC 5.3.1.15], also referred to as lyxose ketol-isomerase, catalyzes the interconversion of D-lyx-ose and o-xylulose. 

In neutral medium, and in the presence of air and cupric acetate, the ketol group is first oxidized into a ketoaldehyde that then condenses with o-phenylenediamine to produce a quinoxaline derivative. This derivative absorbs strongly in the near-UV range [82], with all of the corticosteroids affording equivalent sensitivity levels [72]. 

Ring fused products can be elaborated from isoxazolines (80S757). Several nitrocyclo-alkenes (516) were prepared and reacted with phenyl isocyanate to generate the intermediate nitrile oxides which underwent internal cycloaddition to afford the tricyclic isoxazolines (517). Cleavage of the N—O bond by hydrogenation in the presence of a catalytic amount of Raney nickel and subsequent hydrolysis afforded the /3-ketol (518 Scheme 113). 

The decarboxymethylation of substituted o -hydroxy-o -carbomethoxy hexacyclic substituted ketones (43), one of these used as an advanced intermediate in the synthesis of the alkaloid aspidophytine, can be effected by heating with Mgl2 in CH3CN in good yields (75-84%) (Scheme 12).34 The reaction was shown to proceed through a novel a-hydroxy /3-dicarbonyl to a-ketol ester rearrangement mechanism, which is supported by the isolation of the carbonate (45) intermediate. 

Decarboxylation must lead to incorporation of a proton from the solvent. The oxidative decarboxylation of 6-O-phosphono-D-gluconate gives D-erythro-pentulose-1 (S)-t 5-phosphate (with inversion of configuration).45 The R-isomer can be obtained46 from D-ribose 5-phosphate by using D-ribose 5-phosphate ketol isomerase (E.C. 5.3.1.6). 

L I E B t G Benzylic Aad Rearrangement Benzyto acids by rearrangement o( diketones (also a-ketol rearrangement)... 

MS and GC-MS. A variety of derivatives have been used to protect labile functional groups of the free prostaglandins prior to gas chromatography and direct fragmentation in GC-MS. The 9,11-/3-ketol grouping of the E series is particularly sensitive in vapour phase analysis and commonly 0-methyloxime TMS ether derivatives or O-methyloxime acetates have been employed in GC-MS (e.g. [426]). An alternative approach is to dehydrate the ketol under controlled conditions to the more stable enone analogues of the B series [427,428]. The cyclic n-butylboronate of the 9,11-diol in the F series when used with methyl ester TMS ethers confers good GC properties and some simplification in... 

The reaction is assumed to occur via the formation and cyclization of a ketyl, 156, followed by the addition of a second electron and two protons, leading to an unstable a-hydroxyimine 157. Hydrolysis affords the ketol 154, while elimination of water, followed by a second reduction of the resulting o, )6-unsaturated imine and hydrolysis, leads to the ketone 155 (Scheme 16). 

O, y6-Unsaturated ketones of the structure RR C=CHCOCH3 yield on treatment with sodium amalgam a mixture of products, including tetrahydrofurans (V) and dihydrofuran derivatives (VI), glycols (VII), e-diketones (VIII), and ketols (XI) [85-87]. The ratio between the products depends on R and R bulky groups as found in steroids favor formation of glycols rather than of e-diketones [88]. The reactions leading to these products are similar to those discussed in Chapter 10. 

NaBH4 reduction with the help of CeCl3 -7H20 to obtain threo derivatives 232 (O Scheme 61). An enzymatic route for the synthesis of L-fucose analogs modified at the non-reducing end is reported by Fessner et al. [86], Using 2-Hydroxy-2-methylpropanal 233 and dihydroxyacetone phosphate 234 as substrates, branched fucose derivative 237 has been prepared via recombinant L-fuculose 1-phosphate aldolase (FucA) and L-fucose ketol isomerase (Fuel) in E. coli (O Scheme 62). 

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