Mesitoate esters

Trimethylbenzoate (Mesitoate) Ester 34. Methyl Carbonate 36. 2,2,2-Trichloroethyl Carbonate 39. Allyl Carbonate... 

The stereochemical assignment is based on the comparison of the product with the pure cis and trans isomers (128 and 129) of the enamine prepared by the base-catalyzed elimination of the mesitoate esters of the d/-erythro-2-(4-morpholino)-l,2-diphenylethanol (130) and the <7/-threo-2-(4-morpho-lino)-l,2-diphenylethanol (131). 

The preparation of a pair of stereoisomeric enamines to which configurations were assigned has been reported 125). When the mesitoate esters 171 and 172... 

Consequently, a microwave effect can be important when steric effects are involved in a reaction, as exemplified by the increased magnitude of the effect for saponification of hindered mesitoic esters relative to benzoic esters [55] (vide supra). 

Rapid and easy reactions occur, even with the most hindered mesitoic esters, which are otherwise practically nonsaponifiable under classical conditions [65],... 

The use of mesitoate esters in the elucidation of reaction mechanisms has been pioneered by Burrows and Topping (1969,1970). This system has been used to suppress the competitive intermolecular reaction by steric bulk effects and to detect participation by the identification of the products formed. Under identical conditions (pH 11.28 at 30°C in 9.5% ethanol-water), 2-acetylphenyl mesitoate [41]is hydrolysed 130 times more readily than 4-acetylphenyl mesitoate, clearly indicating intramolecular catalysis. However, the products of hydrolysis provided no clue to the mechanism of... 

To a flask which was previously base-washed and dried is added 32 ml of 1.5 M potassium f-butoxide solution in dimethyl sulfoxide. (NOTE The potassium f-butoxide (MSA Research Corp.) is sublimed under 0.1 mm pressure at 155°-160°C, transferred to a brown bottle in a dry nitrogen atmosphere, sealed with a serum cap, dry dimethyl sulfoxide added, and the resulting solution only removed with a syringe.) To the latter solution is added 1.99 gm (0.0046 mole) of the mesitoate ester of < -f/ ra>-2-(4-morpholino)-l, 2-diphenyl-ethanol, the reaction is stirred for 3 hr at room temperature under a nitrogen atmosphere, and the mixture poured in 150 ml of ice-cold water. The resulting solution is extracted with ether, dried, and concentrated to afford 1.16 gm (94%) of crude enamine, m.p. 78°-81°C. The enamine is dissolved in the minimum amount of methanol at temperatures below 50°C, quickly cooled, and then filtered to afford 0.79 gm (65%), m.p. 103°-104°C. 

More practical methods involve the dehydrohalogenation [100], dehydro-cyanation [101], or stereospecific bimolecular -elimination of mesitoate esters using potassium t-butoxide in dimethyl sulfoxide [102] as described in Eqs. (40H42). 

Mixtures of Z and E stereoisomers have been obtained in most syntheses. This undoubtedly holds in the presence of an acid catalyst. The tendency of the E-enamine to isomerize to the Z-enamine was readily observed on numerous occasions under unexpectedly mild conditions. Munk and Kim357 summarized the requirements for the stereospecific synthesis of enamines. First, if the introduction of the double bond is to be the final step of the synthesis, it must be stereospecific in character. Second, once the enamine is formed it must retain its stereochemical integrity under the conditions employed in the double-bond-forming step. The base-induced bimolecular -elimination reaction fulfills both these requirements. Indeed, treatment of the mesitoate esters of ( )-threo- (99) and ( )-erythro- (100) l-(4-morpholino)-l,2-diphenylethanol with... 

Trimethylbenzoate (Mesitoate) Ester 2,4,6-Me3C6H2C02R (Chart 2) Formation... 

Mesitoic ester saponification is extremely difficult under classical conditions. It can be conveniently performed within 5 h at 85 °C with improved yield using the solvent-free technique that, in addition, does not require expensive catalysts [Eq.(50)]. 

Three additional protecting groups in this category are the mesitoate ester (OCOC6H22,4,6-trimethyl, O— Mes), the pivaloyl ester (OCO—f-Bu), and the para-methoxybenzoyl ester (OCO—C6H4-4-Me). The mesitoate ester is formed by reaction of an alcohol with mesitoyl chloride in the presence of pyridine or triethylamine. The main advantage of this ester is its stability to hydrolysis (pH 1-12), nucleophilic attack,... 

A direct route to the many naturally occurring 1,5-diene systems is the coupling of two allyl units. To the available methods for this procedure can be added the reaction of an allylic halide with an allyl-lithium generated in situ by reduction of a mesitoate ester of the corresponding allylic alcohol (437). Only the mesitoate-derived allylic portion is subject to the allylic... 

It is possible to omit the solvent altogether. Mesitoic esters are slow to form and slow to hydrolyze under ordinary conditions, owing to steric hindrance. Using solid KOH and Aliquat 336, hydrolysis can be done with the organic reactants and products serving as the liquid phase (Eq. 9.11) [39]. When the hydrolysis is carried out with water and hydrocarbon liquid phases, yields are low. Successful esterifications are carried out by mixing the acid, KOH, alkyl bromide or iodide, and Aliquat 336 at 20-85°C to afford mesitoates in 88-98% yield [39]. 

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