Carboxylic acid esters hindered

Sodium salts of carboxylic acids, including hindered acids such as mesitoic, rapidly react with primary and secondary bromides and iodides at room temperature in dipolar aprotic solvents, especially HMPA, to give high yields of carboxylic esters.679 The mechanism is Sn2. Another method uses phase transfer catalysis.680 With this method good yields of esters have been obtained from primary, secondary, benzylic, allylic, and phenacyl halides.681 In another procedure, which is applicable to long-chain primary halides, the dry carboxylate salt and the halide, impregnated on alumina as a solid support, are subjected to irradiation by microwaves in a commercial microwave oven.682 In still another method, carboxylic acids... 

If the alcoholate or the alkylating reagent contains a carboxylic acid ester, acylation of the alcoholate can compete with alkylation. This potential side reaction does not cause trouble in the examples sketched in Scheme 6.14 (first and third reactions), because these esters are sterically hindered and devoid of a hydrogen (no ketene formation can occur) but, as illustrated in Scheme 6.15, less hindered esters can readily undergo transesterification with alcoholates. 

Acyl iodides.1 This reagent converts acyl chlorides into acyl iodides at 25°. In combination with iodine (1 1) it also converts carboxylic acids, esters, and anhydrides into acyl iodides in generally high yield. This reaction in combination with an alcohol is a useful method for transesterification of hindered alcohols. 

A new microwave-assisted protocol for the generation of diversely substituted 3,4-dihydropyrimidine-5-carboxylic acid esters 40 has been developed by Kappe and co-workers [88, 89] using trimethylsilyl chloride (TMSCl) as a mediator for the Biginelli MCR. This involved the reaction of S-ethyl acetothioacetate or ethyl acetoacetate, an aromatic aldehyde and (monosubstituted) urea or thiourea as building blocks. Also sterically hindered aromatic and heterocyclic aldehydes... 

Combination of silicon hydrides with catalytic amounts of a ruthenium(II) complex in tetrahydrofuran, chloroform or benzene has afforded a new reducing system capable of efficient reduction of a,p-unsatu-rated carboxylic acids, esters, amides, etc. Addition of a weak proton source, such as a sterically hindered phenol significantly increases reaction rates. The ruthenium mixture was found to exhibit the same regioselectivity observed with the above-described palladium systems. 

Transesterification. The alkyl group of carbamates can be exchanged using Ti(OPri) as catalyst. [Note that transesterification of common esters of functionalized carboxylic acids with hindered alcohols such as menthol, bomeol, and isopinocampheol in the presence of Ti(OEt) has been reported. ]... 

Esterification. One use for the reagent is for esterification of carboxylic acids, either for protection of the carboxyl function or preparation of characterizing derivatives. Carboxylic acids, even hindered tertiary acids, react with diphenyidiazomethane to give benzhydryl esters the protective group is removed when required by hydrogenolysis.° Acetylsalicylic acid reacts with 1.5 equiv. of reagent in refiuxing... 

The reagent has been used successfully for conversion of hydroxy carboxylic acid esters into the corresponding chloro esters.4 Bromo esters are obtained if CBr4 is used. The replacement occurs with inversion. The method is applicable to hindered a-hydroxy esters. Hooz and Gilani5 state that the reaction is more rapid if tri-n-octylphosphine is used in place of triphenylphosphine. 

Carboxylic acids from hindered esters COOR COOH... 

Potassium tert-butoxide/dimethyl sulfoxide Carboxylic acids from hindered carboxylie acid esters... 

Synthesis.— The perennial interest in the esterification of carboxylic acids, particularly hindered ones, continues. Isopropenyl esters, prepared by the zinc(ii)-catalysed addition of the hindered acids to propyne, undergo ready acid-catalysed exchange with alcohols, as shown in Scheme 24,... 

Cleavage of stericaUy hindered carboxylic acid esters with retention of phenolether groups... 

Its remarkable selectivity results from its high sensitivity to steric hindrance the less hindered double bonds in polyenes always reacting best. Moreover, this catalyst is compatible with a variety of functional groups such as the carboxylic acid, ester, ether, cyano- and nitro groups. The thiophene ring is not an inhibitor of the Wilkinson catalyst, in contrast to many other classical transition metals systems [60,61]. 

Total and preferential esterification—Sterically hindered carboxylic acid esters s. 14, 296... 

The hydrolysis of silicic acid esters, as well as that of carboxylic acid esters, can proceed either relatively quicker by acid-catalyzed pathway or slower by basic catalysis, involving an 8 2 attack on the silicon center, both much quicker for alkoxysilanes than for common organic esters. In the view of poor miscibility of alkoxysilanes with water, they are usually subjected to hydrolysis in mixed alcohol-water media. It has to be mentioned that the covalent nature of bonding between silicon and oxygen hinders the exchange of alkoxide residues in alcohol solutions, that is, Si(OMe)4 remains essentially pure methoxide with relatively higher reactivity even when used in ethanol solution and so on. It is important to keep in mind that even the hydrolysis of alkoxysilanes, silicon alkoxides, is not an irreversible reaction and both depolymerization and realkoxylation of silica do take place [18,19]. 

Esterification. The esterification of rosin provides important commercial products for the adhesive industry. Rosin esters are formed by the reaction of rosins with alcohols at elevated temperatures. Because the carboxyl group of the resin acids is hindered by attachment to a tertiary carbon, esterification with an alcohol can only be accomplished at elevated temperatures. This hindrance is in turn responsible for the high resistance of the resin acid ester linkage to cleavage by water, acid and alkali. 

The methyl ester (100, R = CH3), derived from this A-nor acid by treatment with diazomethane, is different from the ester (102) obtained either by Favorskii rearrangement of 2a-bromo-5a-cholestan-3-one (101) or by the action of cyanogen azide on 3-methoxy-5a-cholest-2-ene (103) followed by hydrolysis on alumina. The ketene intermediate involved in photolysis of (99) is expected to be hydrated from the less hindered a-side of the molecule to give the 2j -carboxylic acid. The reactions which afford (102) would be expected to afford the 2a-epimer. These configurational assignments are confirmed by deuteriochloroform-benzene solvent shifts in the NMR spectra of esters (100) and (102). ... 

PhCH20COCl, Na2C03, H20,0°, 30 min, 72% yield. Alpha-omega diamines can be protected somewhat selectively with this reagent at a pH between 3.5 and 4.5, but the selectivity decreases as the chain length increases [H2N(CH2) NH2, n = 2, 71% mono n = l, 29% mono]. Hindered amino acids are protected in DMSO (DMAP, TEA, heat, 47-82% yield). These conditions also convert a carboxylic acid to the benzyl ester. ... 

Sterically hindered derivatives of isoxazole carboxylic acids have yielded a goodly number of antibiotics. Chlorination of the oxime of the appropriately substituted benzaldehydes (15) leads to the intermediates, ( 16. Condensation of the chloro oximes with ethyl acetoacetate in base gives the esters (17) of the desired isoxazole carboxylic acids. Alternately, the esters... 

This procedure provides a convenient method for the esterification ol a wide variety of carboxylic acids. The reaction proceeds smoothly with sterically hindered acids6 and with acids which contain various functional groups. Esters are obtained in high purity using Kugelrohr distillation as the sole purification technique. In cases where traces of dichloromethane present no problems, the crude product is usually pure enough to be used directly in subsequent reactions. Methyl and ethyl ethers of phenols may also be prepared by this procedure (see Note 8). 

Carbonyl compounds, such as aldehydes [103, 179], (thio)ketones [31, 94, 180-183], carboxylic acids, and esters [183, 184] with 1 are reduced to alcohols after hydrolysis [5], except in stericaUy hindered cases (see Section 8.5) [185, 186]. Under the same experimental conditions the regioselective reduction of the oxirane ring with 1 gives also the corresponding alcohol [183, 187]. 

Alkyl halides in the presence of silver oxide will convert any non-hindered carboxylic acid (or its salt) to the corresponding alkyl ester in minutes, and phenolic or thiol groups will also be alkylated rapidly [436]. Hydroxyl groups are alkylated slowly an ot always to completion. The alkyl halides most frequently uJQp are the lower molecular weight aliphatic... 

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