Methyl diketones, cleavage

Both D-[l- C]xylose and D-[5- C]arabinose were exposed to a concentrated phosphate buffer solution (pH 6.7). 1-Hydroxy-2-propanone (ace-tol) was distilled from the heated solution. Radioassay indicated that similar labeling [3- C] occurred in the acetol from both pentoses, with loss of the configurational difference thus, a 3-ketopentose or its enediol was suggested as an intermediate. Further work with 3-0- and 6-0-methyl-D-glucose and with 1-0-methyl-D-fructose indicated that /3-elimination from a 3-ketose or, in the case of a hexose, from a 3-ketose or a 4-ketose, or both, tautomerization of the resulting a-diketone to a /3-diketone, and hydrolytic cleavage are essential steps in the formation of acetol. 

The preferred route for reducing the molecular weight of PVA involves chain scission at the 1,3-diketone site (see Fig. 6). As the diketone element is chemically not very stable, a spontaneous degradation of oxidised PVA was also discussed [80]. Nevertheless, the preferred degradation pathway is most likely the biochemical process because enzymes were identified that showed high activity with diketone substrates [81], especially with oxidised PVA. The p-diketone hydrolase (BDH EC 3.7.1.7) hydrolyses aliphatic p-diketones to form methyl ketones and carboxylic acids in equimolar amounts [82]. The enzymatic cleavage of C-C bonds in p-diketones is not well studied [83]. BDH enzymes could be isolated from different PVA-degrading strains, purified, characterised and cloned [84]. 

When heated with trimethyloxonium fluoroborate in 1,1-dichloroethane, dilactone 842 experiences cleavage of both lactone rings to give a mixture of diene diesters, catalytic reduction of which produced 853 Reductive methylation of 853 proceeded with installation of the methyl groups on the exterior face for obvious steric reasons. Acyloin condensation followed by ferric cloride oxidation furnished a-diketone 854 which proved to be highly responsive to photoexcitation. However, irradiation of 854 did not provide 855 as expected. Rather, a most unusual reaction pathway was followed to deliver diol 856. 

Base-catalysed conjugate addition of nitropropane to methyl vinyl ketone occurred smoothly to give the nitroketone. Formation of the salt with sodium methoxide was followed by oxidative cleavage of the C-N linkage with ozone. The product was a 1,4-diketone which was isolated without further aldol reaction by this route. 

Some modifications of the previous processes (23,36,72) were also reported (78,79) for the preparation of the ( )- and (—)-ethyl esters 29 and the corresponding ( )-methyl ester 30, another emetine precursor (34,37,61,72). The latter ( )-precursor 30 was alternatively synthesized by Takano et al. (80) who utilized cleavage of an a-diketone monothioketal intermediate. Birch reduction of the starting ( )-tetrahydroprotoberberine... 

A more direct method of cleavage has been developed which consists of heating 502 overnight in toluene with a dispersion of sodium and chlorotrimethylsi-lane.352 418 This procedure led efficiently to 506 whose hydrolysis with dry methanol (kinetic quenching) resulted in the production of a mixture of dihydro diesters rich in 507 (77 %). The structurally rigid 508 and related a-diketones have subsequently been elaborated from this diester as starting material.418) Related endo.endo diesters bearing external methyl and methylthio substituents (509) have also been synthesized.419 ... 

The rearrangement of 1-methoxy-substituted bicyclo[2.2.2]octenones (66a-d) follows a mechanistic variant of the ODPM reaction, which includes two consecutive single-step phototransformations, i.e. the ODPM path to (67a-d), again triplet sensitized, and conversion to the final 1,4-diketones (68a-d) cf. Scheme 13).5-1U05-108 jjjg second step involves cleavage of the transient cyclopropane in (67a-d) accompanied by loss of the methoxy methyl, most likely via methyl radicals, and transfer of a hydrogen atom from the solvent. 

Therefore several reactions were subjected to various antibody catalyses, e. g., ester and enol ether cleavage, transesteiification, ketone reduction. Cope rearrangement, ring closure via epoxide opening, or Diels-Alder cycloaddition [74, 75]. An exceptional reaction is the antibody-catalyzed Robinson annulation of triketone 28 to the Wieland-Miescher ketone 29 on a preparative scale. Surprisingly, even the alkylation of diketone 27 with methyl vinyl ketone was catalyzed by the same antibody, but at moderate rates (Scheme 15) [76]. 

Alkylation of lithiated l-(l-ethoxyprop-2-enyl)benzotriazole leads to enones after hydrolytic removal of the heterocycle addition of the lithiated species to cyclohexenone then hydrolytic cleavage of the heterocycle prodnces an nnsaturated l,4-diketone. ° Addition of the same anion to methyl but-2-enoate generates an anion in which the benzotriazole is displaced intramolecularly and a cyclopropane resnlts. ... 

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