Ester selective

Scheme 8.40. Synthesis of ABO esters selected examples. Mechanism of formation of ABO esters and tetrahydrofurans.

Compared to the well-known OBO esters [81], the ABO esters are more stable towards protic acids but less stable towards the Lewis acid Cp2ZrCl2/AgC104, which isomerizes the ABO orthoester to a tetrahydrofuranyl ester. Selective deprotection is therefore possible (Scheme 8.41). 

Magnesium iodide reacts with a,p-epoxy esters to form p-iodo-a-hydroxy esters selectively (200 1). The regioselectivity is attributed to favorable chelation of the iodohydrin. These products are reduced by Bu,SnH to hydroxy esters in 75-95% overall yields with retention of the original configuration at the a-position. 

Selective reduction of esters.1 Sodium borohydride does not usually reduce esters, but it can reduce normal esters slowly in CH3OH. This reaction has been used to reduce a normal ester selectively in the presence of a vinylogous urethane. Thus the methylene group of the enamine (E)-l, formed from dimethyl acetone-... 

Other Carboxylic Esters. Selective 2-0-acylation of sucrose has been achieved by way of the 2-oxyanion compound. Treatment of sucrose in DMF with 3-lauryl-, 3-stearyl-, 3-hydrocinnamoyl-, and 3-(4-phenylbutyryl)-thiazolidine-2-thione derivatives and sodium hydride produced the corresponding 2-0-acyl derivatives in good yield (69). Syntheses of 6-O-acylsucroses were also achieved by acylation with 3-acylthiazolidine-2-thione and 3-acyl-5-methyl-1,3,4-thiadiazole-2(3ff)-thione derivatives in the presence of sodium hydride in DMF, followed by acyl migration using l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) or aqueous triethyl amine. 6-0-Acylsucroses were obtained directly when onlyDBU was used (70). 

Catalytic systems to afford linear esters selectively are scant.306,309 A report in 1995 disclosed that palladium complexes based on l,l -bis(diphenylphosphine)fer-rocene showed excellent regioselectivity for the formation of linear a,p-unsaturated esters.309 The results with phenylacetylene are remarkable because this compound is known to exhibit a distinct preference for the formation of the branched products on palladium-catalyzed carboxylations. Mechanistic studies indicate that the alkoxycarbonylation of alkynes proceeds via the protonation of a Pd(0)-alkyne species to give a Pd-vinyl complex, followed by CO insertion and alcoholysis.310... 

The influence of the ester selectivity is even more pronounced when a phenyl group is present on the /3 carbon selectivity decreases further... 

To conclude, palladium on charcoal is only a precursor for soluble palladium species which are active in the carbonylation of allyl ethers. The formation of these species requires the presence of chloride and proceeds jointly with the generation of dimethyl carbonate. This catalyst could be reused but we have been unable so far to recover the ester selectivities achieved with fresh catalysts. The observation of a higher content of oligomeric materials in the reaction mixture resulting from the use of the recovered catalyst led us to suggest that an increase of the surface acidity of the carrier is presumably responsible for this increase in the oligomer content. 

Ester and amide yields, ester selectivity at 24 hours over typical zeolites... 

The influence of the zeolite catalyst is clearly shown by the change in ester selectivity with the nitrile conversion (Fig. 4) at a 25% conversion, the percentage ester selectivity varies from 80% for the HY to 50% for the He... 

Fig. 4. Ester selectivity as a function of nitrile conversion over o HY2 x H014, + HMg.

Hints The basic conditions are used for the cleavage of esters. Selectively, only one ester group is removed. 

Imada, Y. Yuasa, M. Nakamura, I. Muraha-shi, S.-I. Copper(I)-catalyzed amination of pro-pargyl esters. Selective synthesis of propargyl-amines, l-alken-3-ylamines, and (Z)-allyl-amines./. Org. Chem. 1994, 59, 2282-2284. 

Diisobutylaluminum hydride, formula ( -Bu)2A1H. Used to reduce nitriles and esters selectively to aldehydes, (p. 834)... 

Reduction of methoxyethoxymethyl MEM) esters. MEM esters are more easily reduced by lithium triethylborohydride than other esters. Selective reduction of a less hindered MEM ester is also possible. ... 

The solvated ion pair [(C8Hi7)3NMe] [RhCl4]", formed from aqueous rhodium trichloride and Aliquat-336 in a two-phase liquid system, hydrogenates a,p-unsaturated ketones and esters selectively at the C==C double bond. The reduction of benzylideneacetone follows first-order kinetics in substrate below 0.2 M, and approaches second-order in hydrogen at partial pressures below 0.12 atm (1 atm = 101.3 kPa). The catalysis also depends on the nature of the solvent, the phase transfer catalyst and stirring rates. 

Esters. Aliphatic acids including a-amino acids and citric acid form methyl esters selectively (in the presence of aromatic acids) and in excellent yields when treated with... 

B4. Baum, G., Lyim, M., and Ward, F. B., Polymer membrane electrodes. Part 1. A choline ester-selective electrode. Anal. Chim. Acta 65, 385-391 (1973). 

A choline ester-selective electrode has been developed by Baum and his... 

Examination of the results in Tab. 1 indicates that several enzymes are effident and seledive in the hydrolysis of the (R,S)-4, however most lipases preferentially hydrolyzed the (S)-ester while several proteases exhibited the desired (R)-ester selectivity. Based on these results, we focused all further efforts on the screening and optimization of inexpensive alkaline proteases. The corresponding butyl ester was also screened as a substrate. While several selective hydrolases were identified, the rate of hydrolysis was about three-fold less than that of ethyl ester (see Experimental), presumably due to the decrease in water solubility of the butyl ester. 

Dioxolan-2-ylium cations (7), derived from aldehyde ethylene acetals by hydride abstraction, react with silyl ketene acetals to give P-keto esters, selectively monoprotected at the ketone carbonyl (equation 14). 2... 

The variation in the esterification activity with different aleohols is shown in Fig. 2. It is clear from the figure that the activity is relatively low with iso-propanol and sec-butanol at all the temperatures studied. Fig. 3 shows the ester selectivity over HP zeolite in the temperature range 100°C to 150 C at 2 ml h" feed. High ester selectivities are observed with primary alcohols and ester selectivity decreased with increasing temperature in the case of secondary alcohols due to alcohol side reaction. 

Pancreatic enzyme hydrolyzes L-methionine esters selectively.—E DL-methionine isopropylate with pancreatic enzyme in water allowed to stand at room temp, for 2 days, with initial stirring, and worked up via... 

Z)-3-Alkenoic esters. Selective 1,4-hydrogenation of 2,4-alkadienoic esters to give the deconjugated (Z)-alkenoic esters is readily accomplished using Cr(CO)6 as catalyst. However, high temperatures and pressures are required. For purification [to obtain 99% (Z)-isomers] the products are hydrolyzed in the presence of porcine pancreatic lipase. 

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