Esters by oxidation

Oxidation of phenyl hexyl sulphide with sodium metaperiodate gave also only a trace amount of the corresponding sulphoxide72. On the other hand, Hall and coworkers73 prepared benzylpenicillin and phenoxymethyl penicillin sulphoxides from the corresponding benzyl esters by oxidation with sodium metaperiodate in dioxane solution with a phosphate buffer. A general procedure for the synthesis of penicillin sulphoxides was reported later by Essery and coworkers74 which consists in the direct oxidation of penicillins or their salts with sodium metaperiodate in aqueous solution at pH 6.5-7.0. 1-Butadienyl phenyl sulphoxide 4475 and a-phosphoryl sulphoxides 4576 were also prepared by the same procedure. 

The direct conversion of alcohols and amines into carbamate esters by oxidative carbonylation is also an attractive process from an industrial point of view, since carbamates are useful intermediates for the production of polyurethanes. Many efforts have, therefore, been devoted to the development of efficient catalysts able to operate under relatively mild conditions. The reaction, when applied to amino alcohols, allows a convenient synthesis of cyclic urethanes. Several transition metal complexes, based on Pd [218— 239], Cu [240-242], Au [243,244], Os [245], Rh [237,238,246,247], Co [248], Mn [249], Ru [224,250-252], Pt [238] are able to promote the process. The formation of ureas, oxamates, or oxamides as byproducts can in some cases lower the selectivity towards carbamates. 

Methyl carhoxylates.2 The reagent (1) is alkylated by alkyl halides under phase-transfer conditions with NaOH as base. The product is converted to a methyl ester by oxidation followed by a Pummerer-type rearrangement (equation I). 

Oxidation of chiral esters of phenylacetic acids. These esters can be converted into acetates of mandelic esters by oxidation with DDQ in acetic acid. The reaction is diastereoselective when carried out on esters of chiral alcohols, of which 8-phenylmenthol is the most useful. The presence of substituents on the phenyl group has slight effect on the diastereoselectivity, which depends on formation of a donor-acceptor complex between the substrate and the quinone with removal of a hydrogen atom. The acetoxy group then enters from the opposite, more bulky face. 

The products are converted into P-hydroxy carboxylic esters by oxidation with NBS in CHjCh/CjHjOH at 0°. 

The products can be converted into chiral cyanohydrins or a-hydroxy esters by oxidation and -elimination of the chiral side chain (equatjon 1). Alternatively, the cyano group can be reduced (BH,-THF) prior to oxidation and (i-elimination for a synthesis of ehiral 3-amino seeondary alcohols. 

Primary and secondary allylic alcohols and saturated secondary alcohols are oxidized to the corresponding carbonyl compounds quickly and in high yield at room temperature in DMF. There is no appreciable overoxidation of allylic alcohols in DMF, but primary saturated alcohols are readily oxidized to their corresponding acids. Recently, it has been reported that aldehydes may be converted to methyl esters by oxidation with PDC in the presence of methanol. Preparation of other esters, or methyl esters direct from the alcohol, proved to be less efficient. 

Both of these rings may be synthesized by using an mizyme. Carbamoyloxy and nitrile groups are converted into this ring in the presence of baker s yeast or catalase. The second ring is formed from 2-carbamoyloxybenzoic acid esters by oxidation with rat liver microsomal extract in ethanol at pH 7.4 and a temperature of 20-22°C The process takes about 8 h under aerobic conditions and yields are better than those of some chemical methods. 

The preparation of sulphonate esters by oxidation of sulphur(II)- and sulphur(IV)-containing moieties has rarely been reported. This is probably because of the wide range of other synthetic routes that are available (as described in Sections III. A and III.C) and the fact that yields in the oxidative processes so far reported are usually low. There are, however, a few noteworthy attempts at this method of preparation and these are covered below. 

Quermann, R., Maletz R., Schafer, H. J. (1993). Conversion of fatty acid methyl esters into dialkylated succinic esters by oxidative coupling of their enolates, Liebigs Ann. Chem., p. 1219. 

One interesting application of this reaction is the preparation of mixed esters by oxidation of silyl ethers in the presence of aldehydes. 

Another important derivative of m-cresol used in the manufacture of plant protection agents is m-phenoxytoluene, which can be produced from m-cresol and chloro- or bromobenzene at temperatures of 200 °C, with copper catalysts. m-Phenoxytoluene is converted into m-phenoxybenzoic acid methyl ester by oxidation with a cobalt acetate/KBr catalyst and subsequent esterification m-phenoxybenzoic add methyl ester serves as an intermediate in the production of m-phenoxybenzaldehyde, which is used as the raw material in the production of the synthetic pyrethroid insecticide, fenvalerate (see Chapter 6.3.2). The cyanohydrin is formed in-situ, then made to react with 2-isopropyl-(4-chlorophenyl) acetic acid chloride to yield fenvalerate, which was developed by Sumitomo Chemical in 1972. Pyrethroid insecticides are distinguished by their low toxicity and high activity. 

By feeding to Corydalis incisa ( )-tetrahydrocoptisine labeled with tritium at C-8 and C-14, it has been demonstrated that this tetrahydroprotoberberine can be converted to corynoline, corydalic acid methyl ester, and corydamine. Additionally, tetrahydrocorysamine acts as a precursor for corynoline and corydalic acid methyl ester by oxidative fission of the C-6 to N-7 bond (Scheme 22.3). ... 

An improved method has been described for the preparation of glyoxylic acid esters by oxidative cleavage of the corresponding tartrates use of ethereal periodic acid necessitates no work-up procedure, obviating the problem of water solubility of the lower glyoxylates. j8-Keto-csters are dehy ated to the corresponding acetylenic esters by oxidation of the derived 5-pyrazolones with thallium(in) nitrate (Scheme 36). 

Sano K, Yamamoto T, Yamamoto A. Preparation of Ni- or Pt-containing cyclic esters by oxidative addition of cyclic carboxylic anhydrides and their properties. Bull Chem Soc Jpn. 1984 57 2741-2747. 

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