Selective cleavage esters

Zn, THF-H2O, pH 4.2, 25°, 4 h. The authors suggest that selective cleavage should be possible by this method since at pH 4.2, 25°, 2,2,2-trichlo-roethyl esters are cleaved in 10 min, 2,2,2-trichloroethyl carbamates are cleaved in 30 min, and the 2,2,2-trichloroethyl carbonate of estrone, formed in 87% yield from estrone and the acid chloride, is cleaved in 4 h (97% yield). 

NO BF )" have also been used to cleave amides. Since only tertiary amides are cleaved by potassium r-butoxide (eq. 6), this method can be used to effect selective cleavage of tertiary amides in the presence of primary or secondary amides. " (Esters, however, are cleaved by similar conditions.) Photolytic cleavage of nitro amides (eq. 7) is discussed in a review. ... 

Vinylboronates are generally less reactive than vinylzirconocenes towards various electrophiles and hence selective reactions of the latter should be possible. It was found that selective cleavage of the carbon—zirconium bond in 45 by N-halosuccinimides provides (a-haloalkenyl)boronic esters 53 in excellent chemical yields and with complete re-gioselectivity (Scheme 7.17) [54], An X-ray crystal structure determination of 45 confirmed the configuration of the four-coordinate Zr complex, with two cyclopentadienyl rings, Cl, and C(sp2) as the four ligands (Fig. 7.5) [54,126]. 

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. 

E Guibe-J ampel, M Wakselman. Selective cleavage of p-nitrobenzyl ester with sodium dithionite. Syn Commun 12, 219, 1982. 

Recently, Yoneda et al. [52] have reported an interesting electrochemical fluori-native a-cleavage reaction of cyclic ketones as a route to fluorocarboxylic acid esters. The reaction is carried out in the presence of Et3N 5HF as electrolyte, when the 2,2-disubstituted cyclic ketone is subject to selective cleavage of the C-C bond between the carbonyl carbon and the substituted a-carbon. 

Bromotrimethylsilane has proven to be useful for a wide variety of applications most of them being reviewed.5 8 Other recent applications are mild cleavage of oxiranes,9 the synthesis of glycosyl bromides,10 the selective cleavage of tetrahydro-2,5-dimethoxyfuran and tetrahydro-2,6-dimethoxy-pyran,11 the cleavage of esters and ethers,12 and the synthesis of benzyl bromides.13... 

The alternative selective cleavage of the allyl ester was achieved by Pd(0)-catalyzed allyl transfer. Because the Fmoc group is sensitive to morpholine, a very weak base must be used as the allyl-trapping nucleophile. IV-Methyl aniline was favorable, and its use as a scavenger nucleophile gave rise to the acid 29 in a high yield. 

Scheme 28 Selective Cleavage of a Phosphonate Methyl Ester with Bromotrimethylsilane 66 ...

Sulfonic acid esters of carbohydrates have been extensively studied. No attempt will be made to review the entire literature.s9a Only those investigations which make a significant contribution in illustrating the selective nature of sulfonylation as well as those in which the selective cleavage of sulfonic esters is concerned, will be considered. 

Step 1 Selective cleavage of the -Bu ester (analogous to Boc deprotection). 

Selective cleavage of 2,3-epoxy esters.1 The highly toxic reagent i -Pr2(C2H5)-NHN3 (l)2, obtained by combination of hydrogen azide and the amine, cleaves 2,3-... 

A preparatively satisfactory means of selective enol ester cleavage consists in the treatment with hydroxylamine, which not only induces hydroxylaminolysis of the more reactive enol ester group to form the respective hydroxamic acid, but captures the keto group thus liberated in the form of its stable oxime (29) ... 

This chapter consequently focuses on the application of enzymes for the selective cleavage of esters, amides and nitriles [2], Out of all the reported industrial applications of enzymes these type of hydrolyses constitute more than 40% [3], Enzymatic hydrolyses are often performed because of the enantioselectivity of enzymes, and in particular of the lipases that are used for the production of enantiopure fine chemicals. 

Cleavage of MEM ethers. 2-Methoxyethoxymethyl (MEM) ethers are cleaved by NaI/ClSi(CH,)3 in CHjCN at —20 or 25° in moderate to high yield, lodotrimethylsilane (commercial) is less effective. Fairly selective cleavage of MEM ethers is possible in the presence of lactones, methyl or benzyl ethers, and methyl esters. 

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