Ethers to esters

Ethyl 2-tert-butylthroacetate Ethyl 2-tert-butylsulfonylacetate J[107] 

Methyl-p-tolyl sulfide Methyl-p-tolyl sulfone S [99] 

5 Oxidations of Amines, Amides, Ethers, Sulfides, Phosphines, Arsines, Stibines 

---

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). 

The oxidation of ethers to esters according to the reaction offers many possibilities for the modification of functionality in open chain or cyclic systems. An example is the... 

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,... 

Conversion of MEM ethers to esters. 2-Methoxyethoxymethyl (MEM) ethers are converted into carboxylic esters by reaction with an anhydride in the presence of FeCl3 (0.4 equiv.) (equation I). Selective cleavage is possible in the presence of a benzyl ether but not in the presence of a f-butyl ether. Aromatic rings, if present, can undergo acylation. 

Halogens are frequently used as oxidation agents and, under two-phase conditions, they can either be employed as ammonium complex halide salts [3], or in the molecular state with or without an added quaternary ammonium catalyst [4]. Stoichiometric amounts of tetra-n-butylammonium tribromide under pH controlled conditions oxidize primary alcohols and low-molecular-weight alkyl ethers to esters, a,cyclic ethers produce lactones [3], and secondary alcohols yield ketones. Benzoins are oxidized to the corresponding benzils (80-90%) by the tribromide salts in acetonitrile in the presence of benzoyl peroxide [5]. 

Fig. 1.11 Reaction scheme for stoicheiometric oxidation of ethers to esters by RuO [388]...

Traws-RuCljCdppp) (dppp=(l,2-diphenylphosphino)propane, sometimes abbreviated as (dpp) in the literature) is made from K2[Ru(H30)Cl5] and (dppp) in refluxing EtOH. Oxidation of a wide range of ethers to esters or lactones was effected by franx-RuCl Cdppp) or [RuCl(dppp)2]Vaq. Li(C10)/CH3Cl2 (Table 5.2) [931]. 

Abstract This chapter principally concerns oxidations of organic substrates containing N, O, S, P, As and Sb. Oxidations of amines are covered first, including primary amines to nitriles or amides secondary amines to imines or other products tertiary amines to N-oxides or other prodncts (Section 5.1) and the oxidation of amides (5.2). Oxidation of ethers to esters or lactones follows (5.3), then of sulfides to sulfoxides or sulfones (5.4) and of phosphines, arsine and stibines to their oxides (5.5). A final section (5.6) concerns such miscellaneous oxidations not covered by other sections in the book. 

Table 5.2 Oxidation of ethers to esters or lactones and suMdes to sulfoxides or sulfones... 

Despite these early successes, few synthetic chemists appeared to recognize the potential of metallic oxidants for the oxidation of ethers to esters and lactones, and only a few further developments were published prior to 1980. Since that time, as the chemistry of these reagents has become better understood, their use has increased dramatically to the stage where they are now the reagents of choice for many applications, including complex natural product synthesis. 

DJERASSI RYLANDER Oxidation Ru04 in oxidative cleavage of phenols or alkenes oxidation ot aromatics to quinones oxidation ol or of ethers to esters... 

Zinc dichromate tiihydrate, ZnCr207<3H20, is obtained as an orange-red solid by adding zinc carbonate to a cold solution of chromium trioxide in dilute sulfuric acid [660]. The applications are oxidations of acetylenes lo a-diketones, of aromatic hydrocarbons to quinones, of alcohols to aldehydes, and of ethers to esters and the oxidative regeneration of carbonyl compounds from their oximes [660]. 

A -Bromosuednimide is used in the dehydrogenation of hydrazo compounds to azo compounds [744] and in the oxidative degradation of a-hydroxy acids to aldehydes or ketones [745]. This reagent also oxidizes alkyl trimethylsilyl ethers to esters or ketones [744],... 

The applications of ruthenium tetroxide range from the common types of oxidations, such as those of alkenes, alcohols, and aldehydes to carboxylic acids [701, 774, 939, 940] of secondary alcohols to ketones [701, 940, 941] of aldehydes to acids (in poor yields) [940] of aromatic hydrocarbons to quinones [942, 943] or acids [701, 774, 941] and of sulfides to sulfoxides and sulfones [942], to specific ones like the oxidation of acetylenes to vicinal dicarbonyl compounds [9JS], of ethers to esters [940], of cyclic imines to lactams [944], and of lactams to imides [940]. 

Acylations. Catalyzed by graphite the Friedel-Crafts acylation as well as the transformation of ethers to esters by acyl halides in refluxing 1,2-dichloroethane are realized. Although aliphatic chlorides are inferior to aromatic halides this method is quite general as shown by the synthesis of benzoates (e.g., allyl benzoate from allyl benzyl ether and methyl benzoates from methyl f-butyl ether). 

Conversion of ethers to esters.2 The reagent in the presence of an excess of water converts ethers of the type RCH2OR into carboxylic acid esters. Benzyl ethyl ether... 

Eq. 3.1) and intramolecular mode. Selective oxidation of ethers to esters by irradiation in the presence of benzil and oxygen was reported by Seto [29]. By this procedure, a tricyclic y-lactone is prepared from the corresponding fused tetrahy-drofuran as shown in Scheme 3, Eq. (3.2). The mechanism presumably involves a hydrogen abstraction by an acylperoxyl radical. 

---