Oxidations acetals, dimethyldioxirane

Direct oxidation of oximes is prospective promising procedure for the generation of nitrile oxides. Mercury(II) acetate (54), dimethyldioxirane (55), ceric... 

We have found that the secret to a successful synthesis of the alkenyl glycosides 64 lies in obtaining the pure -acetate 65 [47]. Condensation of the acetate 65 with the appropriate alkenol,in the presence of a small amount (5-20 mol% depending on the scale) of trimethylsilyl triflate, gave the alkenyl glycosides 64 in excellent yield. Oxidation of the alkene 64 with dimethyldioxirane, followed by deacetylation, then gave the putative enzyme inhibitors 60. 

Acetyl-protected 1,2,3,4-tetrahydropyrazines 105, which are prepared by treatment of 2,3-dihydropyrazine with acetic anhydride and zinc (Scheme 27), undergo photooxidation to produce new dioxetanes 106 <1995JA9690>. Upon thermolysis, the dioxetanes 106 decompose quantitatively to tetraacyl ethylenediamines 107. Dimethyldioxirane oxidation of tetrahydropyrazine 105 affords novel epoxide 108, which is also generated by deoxygenation of dioxetane 106 with dimethyl sulfide. In 2,3,4,5-tetrahydropyrazine 1-oxide 109, which is prepared... 

In an interesting paper by Bernini et al., compounds with a flavonoid structure have been selectively oxyfunctionalized at the C-2 carbon atom by dimethyldioxirane (DMD). Products obtained in this way appeared to be useful starting materials to access anthocyani-dins. An example of this route is presented in Scheme 10.1. Here, 2,4-cw-flavane-4-acetate (A) was oxidized by DMD at room temperature, affording the corresponding C-2 hydroxy derivative (B) as the only product (63% yield). Further treatment of B with silica gel eliminated acetic acid to give C quantitatively. Then C was easily transformed into the flavylium salt (D) by simple addition of a 37% solution of HCl in water. 

The most widely used and, presumably, the most chemoselective reagents for the epoxidation of nucleophilic C—C double bonds are the peroxycarboxylic acids (see Houben-Weyl, Vol. IV/ 1 a, p 184, Vol. Vl/3, p 385, Vol. E13/2, p 1258). Using chloroform as solvent, epoxidation rates are particularly high79. Reactive or acid/base sensitive epoxides can often be obtained with dimethyldioxirane (see Houben-Weyl, Vol. R13/2, p 1256 and references 15, 16, 87-90), peracid imides (see Houben-Weyl, Vol. IV/1 a, p 205, Vol. VI/3, p 401, Vol. E13/2, p 1276) (prepared in situ from nitriles and hydrogen peroxide), hydroperoxy acetals (see Houben-Weyl, Vol. El3/2, p 1253) or peroxycarbonic acid derivatives (see Houben-Weyl, Vol. IV/la, p 209 and references 17-19) as oxidants. For less reactive alkenes, potassium hydrogen persulfate is a readily available reagent for direct epoxidation20. 

In the case of the rhenium-catalyzed oxidation of methoxy- and hydroxy-substituted substrates, there is some complementary work concerning the general mechanism of the arene oxidation [10b, 11]. Since the major products in the oxidation of such arenes or phenols are the quinones, the formation of intermediary epoxides seems to be a predominant reaction step. When p-substituted phenols such as 2,6-di( -butyl)-4-methylphenol are treated with the MTO/H2O2 oxidant and acetic acid as solvent, the formation of hydroxydienones is observed. This is also reported for the oxidation using dimethyldioxirane as oxidant [20]. Since an arene oxide intermediate was postulated for the dioxirane oxidation, a similar mechanism is plausible here [11], e. g., for the oxidation of l,2,3-trimethoxy-5-methylbenzene (Scheme 3) or 2,6-di(f-butyl)-4-methyl-phenol. 

At least one methylenecyclopropane derivative with two electron-withdrawing substituents on the double bond has been isolated and unequivocally characterized by spectroscopic methods. Methyl 2-cyclopropylidene-3-(phenylsulfinyl)acetate (30) is prepared by oxidation of the phenylsulfanyl derivative 29 with dimethyldioxirane under strictly anhydrous conditions. Compound 30 is a marginally stable crystalline material at room temperature in the absence of nucleophiles, but it rapidly adds water on standing in air or upon attempted chromatography on silica gel. " It also undergoes rapid cycloadditions (see Section 5.2.3.2.1.2.). 

O3, AcOEt, —78°C, 94% yield. These conditions are used to convert an acetal to an ester." Oxone" and dimethyldioxirane" can also be used to generate esters from 1,3-dioxolanes, but oxone does not always result in oxidation." ... 

Dimethyldioxirane, acetone, CH2CI2, 0°C, 24h, >95% yield." Although ketone dioxolanes are cleaved to ketones, aldehyde dimethyl acetals will gives the ester, but the generahty of the later process has not been established beyond the acetal of benzaldehyde. Ethers are also oxidized under these conditions. 

Oxidative debenzylation using dimethyldioxirane proceeds well with benzyl ethers of primary and secondary alcohols and the method is compatible with silyl ethers. Isopropylidene acetals are stable but benzylidene acetals are cleaved. The deprotection of p-bromo. p-cyano and 2-naphthylmethyl ethers can also be accomplished. 

The addition of BF3-OEt2 to an a-phosphorylated imine results in the 1,3-transfer of a diphenylphosphinoyl group, with resultant migration of the C-N=C triad. This method is less destructive than the thermal rearrangement. The decomposition of dimethyldioxirane in acetone to methyl acetate is accelerated with BF3 OEt2, but acetol is also formed. Propene oxide undergoes polymerization with BF3-OEt2 in most solvents, but isomerizes to propionaldehyde and acetone in dioxane. ... 

Methyl 2-iodoxybenzoate can be further converted into the diacetate 490 or a similar bis(trifluoroacetate) derivative by treatment with acetic anhydride or trifluoroacetic anhydride, respectively [658]. Single-crystal X-ray diffraction of methyl 2-[(diacetoxy)iodosyl]benzoate 490 revealed a pseudo-benziodoxole stmcture with three relatively weak intramolecular l - O interactions. The esters of 2-iodoxyisophthalic acid (e.g., 491) have been prepared by oxidation of the respective iodoarenes with dimethyldioxirane. X-Ray stmctural analysis of diisopropyl 2-iodoxyisophthalate 491 showed intramolecular I - O interaction with the carbonyl oxygen of only one of the two ester groups, while NMR spectra in solution indicated equivalency of both ester groups [658]. 

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