Esters with ruthenium tetroxide

The conversion of primary alcohols and aldehydes into carboxylic acids is generally possible with all strong oxidants. Silver(II) oxide in THF/water is particularly useful as a neutral oxidant (E.J. Corey, 1968 A). The direct conversion of primary alcohols into carboxylic esters is achieved with MnOj in the presence of hydrogen cyanide and alcohols (E.J. Corey, 1968 A,D). The remarkably smooth oxidation of ethers to esters by ruthenium tetroxide has been employed quite often (D.G. Lee, 1973). Dibutyl ether affords butyl butanoate, and tetra-hydrofuran yields butyrolactone almost quantitatively. More complex educts also give acceptable yields (M.E. Wolff, 1963). 

Ethers in which at least one group is primary alkyl can be oxidized to the corresponding carboxylic esters in high yields with ruthenium tetroxide. Molecular oxygen with a binuclear copper (II) complex " or PdCVCuCVCO " also converts ethers to esters. Cyclic ethers give lactones. " The reaction, a special case of 19-14,... 

In the acetonitrile modification reported by Sharpless and coworkers, hydrolysis t parently does not take place to any tqipreciable extent. Consequently the yield of ester can be significantly increased (equation 11). This improved procedure, along with some minor variants, therefore appears to be the method of choice for effecting the oxidation of ethers with ruthenium tetroxide, and has been widely adopted. 

Ethers in which at least one group is primary alkyl can be oxidized to the corresponding carboxylic esters in high yields with ruthenium tetroxide. Molecular... 

Hydroxy carboxylic acids and their esters are oxidized to keto acids and their esters, respectively. o-Nitromandelic acid is oxidized to o-nitro-phenylglyoxylic acid with a dilute solution of potassium permanganate at room temperature in 54% yield [886], The oxidation-of ethyl 3-hydroxy-cyclobutanecarboxylate with ruthenium tetroxide and sodium periodate in... 

S223). G. y-3,6-piperidazinedicarboxylic acid 116 could be prepared from the adduct 114 which was oxidized with ruthenium tetroxide at 0°C to give, after esterification, the ester 115. Hydrolysis of the latter afforded 116 (95CPB535). 

Sequential alkylation of the lithium enolate of 897 with methyl iodide and then allyl bromide furnishes dialkylated malate 938 with 96 4 diastereoselectivity. The hydroxyl group is removed via a xanthate ester to give 939, which is then alkylated with 4-bromo-l-butene to give 940 as the major isomer. The mixture is hydrolyzed with base and heated with urea to yield imides 941 and 942 in a 3 1 ratio. After isolation of pure 941 by crystallization, the remaining undesired diastereomer 942 is epimerized to 941 with potassium tert-hutoxide. Oxidation of 941 with ruthenium tetroxide-sodium periodate and esterification of the resulting acids furnishes imide 943. This sequence has produced more than 10 g of 943 in a single run. 

Cyclic sulfites (68) also are opened by nucleophiles, although they are less reactive than cyclic sulfates and require higher reaction temperatures for the opening reaction. Cyclic sulfite 77, in which the hydroxamic ester is too labile to withstand ruthenium tetroxide oxidation of the sulfite, is opened to 78 in 76% yield by reaction with lithium azide in hot DMF [82], Cyclic sulfite 79 is opened with nucleophiles such as azide ion [83] or bromide ion [84], by using elevated temperatures in polar aprotic solvents. Structures such as 80 generally are not isolated but as in the case of 80 are carried on (when X = N3) to amino alcohols [83] or (when X = Br) to maleates [84] by reduction. Yields are good and for compounds unaffected by the harsher conditions needed to achieve the displacement reaction, use of the cyclic sulfite eliminates the added step of oxidation to the sulfate. 

The diols (97) from asymmetric dil droxylation are easily converted to cyclic sii e esters (98) and thence to cyclic sulfate esters (99).This two-step process, reaction of the diol (97) with thionyl chloride followed by ruthenium tetroxide catalyzed oxidation, can be done in one pot if desired and transforms the relatively unreactive diol into an epoxide mimic, ue. the 1,2-cyclic sulfate (99), which is an excellent electrophile. A survey of reactions shows that cyclic sulfates can be opened by hydride, azide, fluoride, thiocyanide, carboxylate and nitrate ions. Benzylmagnesium chloride and thie anion of dimethyl malonate can also be used to open the cyclic sulfates. Opening by a nucleophile leads to formation of an intermediate 3-sidfate aiuon (100) which is easily hydrolyzed to a -hydroxy compound (101). Conditions for cat ytic acid hydrolysis have been developed that allow for selective removal of the sulfate ester in the presence of other acid sensitive groups such as acetals, ketals and silyl ethers. 

Compound 14 has been oxidised with osmium tetroxide to the glycol 15 which has been protected as the acetonide 16. Oxidation in basic medium with ruthenium oxide afforded a ketoacid which has been transformed into the corresponding methyl ester 17. 

The rates of hydrolysis of monomeric ruthenium(n) chloride complexes which are generated electrochemically have been reported and are much more rapid than those observed for the corresponding ruthenium(m) species. It is suggested that Ru chloride complexes may exist as unstable intermediates when the ruthenium(iii) species are reduced. Ruthenium(n) dimethyl sulphoxide complexes have been shown to be the products of refluxing the hydrated ruthenium(iii) chloride with DMSO. The use of ruthenium tetroxide as an oxidant has also been described. In the oxidation of tetrahydrofuran in aqueous perchloric acid, an inverse hydrogen-ion dependence is observed, the proposed mechanism involving hydride abstraction with recombination to yield an ester. A scheme consistent with isotope-labelling data may be represented as... 

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