Methyl and Isopropyl Esters

A notable difference in reactivity between the various esters is observed, e.g. methyl esters and isopropyl esters give a much faster reaction than ethyl esters. 

Cleavage of esters and lactones (8,447-448).1 SN2-Cleavage of esters and lactones with NaSeC6H5in HMPT-THF is considered to be the most effective known method. The order of reactivity of related reagents is NaSeC6H5-18-crown-6-THF > LiSe(, H5 HMPT THF > LiSeC6H5 THF. Methyl esters are cleaved more readily than ethyl esters benzyl, isoamyl, and isopropyl esters are cleaved in high yield, but more slowly. Methyl esters are cleaved easily, irrespective of steric hindrance. Amides are completely inert. 

Glucose-grown cells of G. candidum SC 5469 have also catalyzed the stereoselective reduction of ethyl-, isopropyl-, and tertiary-butyl esters of 4-chloro-3-oxobutanoic acid and methyl and ethyl esters of 4-bromo-3-oxobutanoic acid. A reaction yield of >85% and e.e. of >94% were obtained. NAD+-depen-dent oxido-reductase responsible for the stereoselective reduction of P-keto esters of 4-chloro- and 4-bromo-3-oxobutanoic acid was purified 100-fold. The molecular weight of purified enzyme is 950,000. The purified oxido-reductase was immobilized on Eupergit C and used to catalyze the reduction of (39) to S-( - )-(40). The cofactor NAD+ required for the reduction reaction was regenerated by glucose dehydrogenase. 

FAs are subjected to Ag-TLC usually in the form of methyl esters. The methods for methylation and transmethylation are simple, easy to perform, and with practically 100% yield. Methyl esters are particularly suitable when Ag-TLC is used as a complementary method with GC. Butyl and isopropyl esters were employed for the fractionation of butterfat FAs by Ag-TLC because these derivatives provided better resolution in GC. Conversion of positionally isomeric 18 1 and 20 1 FAs into phenacyl esters ensured complete resolution of the components by Ag-TLC however, this is not possible when using methyl esters. 

Kumar and coworkers described the Michael addition of thiophenol 13 to 2-phenylacrylates 19 using a catalytic amount of cinchona alkaloids [16]. Among the four natural alkaloids, quinine 20 and quinidine 3 afforded the best results with opposite enantioselectivity (Scheme 7.11). Methyl or isopropyl ester substrates 19a and 19b gave comparable selectivities whereas more sterically demanding esters (e.g., tBu or CH(iPr)2) gave lower optical inductions. Based on the previous considerations and a computational analysis, the authors suggested a transition state... 

Murakami and coworkers reported a further use for this rhodium migration [80, 81]. Instead of protonolysis, they noticed that the aryl rhodium species after the vinylic to aryl migration is nucleophilic enough to attack an ester moiety in an intramolecular fashion to afford a cyclic ketone. Thus, an internal alkyne equipped with ester groups at a specific place was subjected to the rhodium-catalyzed hydroarylation conditions (Scheme 21). Indeed, the desired ketone was obtained in an 89% yield. Not only methyl esters can serve as acylation agents ethyl esters and isopropyl esters are also suitable substrates. 

Fig. 8 Direct separation of the enantiomers of methyl, ethyl and isopropyl esters of lactic acid by LC on CSP of Pirkle" type (racemic mixtures).

Aminolysis of methyl, " ethyl, propyl, " and isopropyl " phenylphosphonochlori-dothioates (RO)PhP(=S)Cl with substituted anilines XC5H4NH2 and deuterated anilines XC5H4ND2 at 333 K in MeCN yielded large inverse deuterium isotope effects of = 0.63-0.99 for the isopropyl ester consistent with backside attack in a concerted pathway and corresponding values of 1.02-1.48 for the methyl and propyl esters consistent with a predominant frontside attack, again in a concerted process. 

A study has been made of the diastereotopy of the methylene and methyl protons in ethyl and isopropyl esters of purine nucleoside uronates. Anisochromism of the diastereotopic protons was conditional upon both close spatial proximity and fixed orientation of the heterocycle vis-a-vis the alkoxycarbonyl group. 139 Unsaturated nucleoside uronates such as (89) have been prepared by p-elimination and their chirooptical properties investigated, i ... 

Biodiesel fuels (composed typically of methyl, ethyl, and isopropyl esters) generally reduce particulate matter (PM), HC, and carbon monoxide (CO) emissions, because of greater oxygen content in the fuel. The reduction in PM and HC emissions makes diesel less toxic. emissions, however, are observed to increase. 

Composition. Rosin is primarily a complex mixture of monocarboxyUc acids of alkylated hydrophenanthrene nuclei. These constituents, known as resin acids, represent about 90% of rosin. The resin acids are subdivided into two types, based on their skeletal stmcture. The abietic-type acids contain an isopropyl group pendent from the carbon numbered 13. The pimaric-type acids have a methyl and vinyl group pendent from the same carbon atom. Figure 1 shows the stmcture of typical resin acids abietic acid, C2QH2QO2 (1) is predominant. The remaining 10% of commercial rosin consists of neutral materials that are either hydrocarbons or saponifiable esters. These materials are derived from resin acids by decarboxylation or esterification. 

The acetates of most alcohols are also commercially available and have diverse uses. Because of their high solvent power, ethyl, isopropyl, butyl, isobutyl, amyl, and isoamyl acetates are used in ceUulose nitrate and other lacquer-type coatings (see Cellulose, esters). Butyl and hexyl acetates are exceUent solvents for polyurethane coating systems (see Coatings Urethane polymers). Ethyl, isobutyl, amyl, and isoamyl acetates are frequentiy used as components in flavoring (see Flavors and spices), and isopropyl, benzyl, octyl, geranyl, linalyl, and methyl acetates are important additives in perfumes (qv). 

Dimetilan (dimethylcarbamic acid l-[(dimethylamino)carbonyl]-5-methyl-l//-pyrazol-3-yl ester 724) and isolan (dimethylcarbamic acid 3-methyl-l-isopropyl-l//-pyrazol-5-yl... 

The intermediates 74 and 76 can now lose OR to give the acid (not shown in the equations given), or they can lose OH to regenerate the carboxylic ester. If 74 goes back to ester, the ester will still be labeled, but if 76 reverts to ester, the 0 will be lost. A test of the two possible mechanisms is to stop the reaction before completion and to analyze the recovered ester for 0. This is just what was done by Bender, who found that in alkaline hydrolysis of methyl, ethyl, and isopropyl benzoates, the esters had lost 0. A similar experiment carried out for acid-Catalyzed hydrolysis of ethyl benzoate showed that here too the ester lost However, alkaline hydrolysis of substimted benzyl benzoates showed no loss. This result does not necessarily mean that no tetrahedral intermediate is involved in this case. If 74 and 76 do not revert to ester, but go entirely to acid, no loss will be found even with a tetrahedral intermediate. In the case of benzyl benzoates this may very well be happening, because formation of the acid relieves steric strain. Another possibility is that 74 loses OR before it can become protonated to 75. Even the experiments that do show loss do not prove the existence of the tetrahedral intermediate, since it is possible that is lost by some independent process not leading to ester hydrolysis. To deal with this possibility. Bender and Heck measured the rate of loss in the hydrolysis of ethyl trifluorothioloacetate- 0 ... 

In 12 the upper face is shielded by the isopropyl group, whereas in 13 the lower face is shielded by the methyl and phenyl groups. As a result, alkylation of the two derivatives gives products of the opposite configuration. The initial alkylation product ratios are typically 95 5 in favor of the major isomer. Since these products are diastereomeric mixtures, they can be separated and purified. Subsequent hydrolysis or alcoholysis provides acids or esters in enantiomerically enriched form. Alternatively, the acyl imides can be reduced to alcohols or aldehydes. The final products can often be obtained in greater than 99% enantiomeric purity. 

The ability of NB-355 to stimulate locomotor activity and induce dyskinesia in MPTP-treated squirrel monkeys was studied (MPTP induces parkinsonism) [9], NB-355 was similar to L-dopa in stimulating locomotor activity. Furthermore, NB-355 induced less severe dyskinesia than was seen with L-dopa. Some other prodrugs of L-dopa include short-chain alkyl esters (methyl, ethyl, isopropyl, butyl, hydroxypropyl, and hydroxybutyl) intended for rectal absorption [10], These esters of L-dopa have high water solubility (>600 mg/mL). Initial bioavailability studies indicated that all of these esters, with the exception of the hydroxypropyl ester, resulted in significantly greater bioavailability than that obtained with L-dopa itself. However, given the high level of esterase activity in the small intestine, the use of these compounds is limited to rectal administration. 

---