T-butyl esters

Thiacyclotrideca-2,4,10,12-tetraene-6,8-diyne 1 -oxide, 5,10-dimethyl- HNMR, 7, 717 (75JA640) 4-Thia-2,6-diazabicyclo[3.2.0]heptane-2-carboxylic acid, 7-OXO-, t-butyl ester X-ray, 7, 349 (B-72M151201) 5H-2aA -Thia-2,3-diazacyclopent[cd]indene, 2,3-dimethyl-6,7-dihydro-X-ray, 6, 1054 (72ACS343)... 

Mc2NH, MeOH, 0°, 75% yield. t-Butyl esters could be cleaved with HCl without affectipg the Peoc group. 

Me2Sn(SMe)2, BF3-Et20, PhCH3, 0°, 3-24 Ji AcOH, 75-100% yield." An ethyl ester can be hydrolyzed in the presence of an MEC ester with 1 N aqueous NaOH-DMSO (1 1), and MEC esters can be cleaved in the presence of ethyl, benzyl, cinnamyl, and t-butyl esters as well as the acetate, TBDMS and MEM ethers. 

The dibenzosuberyl ester is prepared from dibenzosubeiyl chloride (which is also used to protect —OH, —NH, and —SH groups) and a carboxylic acid (Et N, reflux, 4 h, 45% yield). It can be cleaved by hydrogenolysis and, like t-butyl esters, by acidic hydrolysis (aq. HCl/THF, 20°, 30 min, 98% yield). ... 

The change of mechanism with tertiary alkyl esters is valuable in synthetic methodology because it permits certain esters to be hydrolyzed very selectively. The usual situation involves the use of t-butyl esters, which can be cleaved to carboxylic acids by action of acids such as p-toluenesulfonic acid or trifluoroacetic acid under anhydrous conditions where other esters are stable. 

The carboxamidomethyl ester was prepared for use in peptide synthesis. It is formed from the cesium salt of an A-protected amino acid and o -chloroacet-amide (60-85% yield). It is cleaved with 0.5 M NaOH or NaHC03 in DMF/H2O. It is stable to the conditions required to remove BOC, Cbz, Fmoc, and t-butyl esters. It cannot be selectively cleaved in the presence of a benzyl ester of aspartic acid. ... 

The Pet ester is stable to the acidic conditions required to remove the BOC and t-butyl ester groups, to the basic conditions required to remove the Fmoc and Fm groups, and to hydrogenolysis. It is not recommended for use in peptides that contain methionine or histidine, since these are susceptible to alkylation with methyl iodide. 

Note that a cumyl ester can be selectively cleaved in the presence of a t-butyl ester and a /3-lactam. ... 

TsOH, benzene, reflux, 30 min, 76% yield." A t-butyl ester is stable to the conditions needed to convert an a,/3-unsaturated ketone to a dioxolane (HOCH2CH2OH, TsOH, benzene, reflux). ... 

TMSOTf, TEA, 53-90% yield. t-Butyl esters are cleaved in preference to t-butyl ethers. ... 

TiCl4, CH2CI2, —10° to 0°, 54-91% yield. These conditions were developed for use with cephalosporin t-butyl esters." ... 

M HCl, EtOAc, 25°, 30 min, 96% yield. With MeOH as the solvent, a diphenylmethyl ester is not affected. The combination of HCl/EtOAc leaves TBDMS and TBDPS ethers and t-butyl esters and nonphenolic ethers intact during BOC cleavage, but 5 -BOC derivatives are cleaved. 

Mg(C104)2, CH3CN, 99% yield.These conditions do not cleave a t-butyl ester or r-butyl carbamate. 

In order to enhance the oral bioavailability of oximonam (104), a prodrug has been made by esterification of the carboxyl group with the t-butyl ester of hydroxyacetic acid (105). The product is prodrug gloximonam (106) [31], Gloximonam is efficiently converted to oximonam in the body by metabolic processes. 

The progress of the reaction is followed by HPLC (Zorbax RX-C8 column, 1.5 mL/min, gradient elution of 20 80 to 70 30 CH3CN 0.01 M H3P04 in water over 10 min, held for 20 min, room temperature, detection at 200 nm, retention times P-keto t-butyl ester 2 8.31 min, hydroxy fcrt-butyl ester 3 7.10 min, hydroxy methyl ester 4.45 min, P-keto methyl ester 5.47 min). 

Further rinsing with methylene chloride yielded trace quantities of additional product, which was less pure by HPLC analysis. The main impurity was 2 (see Discussion) produced by cleavage of the t-butyl ester by the Amberlyst 15 resin. 

Boron enolates can be obtained from esters40,41 and amides42 by methods that are similar to those used for ketones. Various combinations of borylating reagents and amines have been used and the E.Z ratios are dependent on the reagents and conditions. In most cases esters give Z-enolates, which lead to syn adducts, but there are exceptions. Use of branched-chain alcohols increases the amount of anti enolate, and with t-butyl esters the product ratio is higher than 97 3. 

Benzyl, methyl, and t-butyl esters are rapidly cleaved, but secondary esters react more slowly. In the case of t-butyl esters, the initial silylation is followed by a rapid ionization to the t-butyl cation. 

If only the O-H, as opposed to the carbonyl, of a carboxyl group has to be masked, it can be readily accomplished by esterification. Alkaline hydrolysis is the usual way for regenerating the acid. t-Butyl esters, which are readily cleaved by acid, can be used if alkaline conditions must be avoided. 2,2,2-Trichloroethyl esters, which can be reductively cleaved with zinc, are another possibility.267 Some esters can be cleaved by treatment with anhydrous TBAF. These reactions proceed best for esters of relatively acidic alcohols, such as 4-nitrobenzyl, 2,2,2-trichloroethyl, and cyanoethyl.268... 

In the past, ionomers have generally consisted of 10-12 mole percent of ions and it is our intention to be consistent with the corresponding random ionomers previously discussed in the literature. In addition to gel permeation chromatography (GPC), H and 3C NMR can readily be utilized to verify the relative amount of monomer successfully incorporated into the block copolymer. For example, the composition of a PMMA-PTBMA diblock can be verified by H NMR ratioing the methyl ester integration (3.5 ppm) to the t-butyl ester integration (1.36 ppm). Figure 1 depicts the t-butyl ster chemical shift which appears reproducibly at 1.J6 ppm. C or FTIR can be utilized in certain instances when H NMR chemical shifts overlap. For... 

We next investigated the dealkylation of S-b-tBM with TMSI. Unlike the reaction with S-b-MM, it required only 4 hr at room temperature to completely cleave the t-butyl ester. Work-up under acidic conditions gave S-b-MA which was virtually identical by NMR, IR, GPC, and titration with that just described above. Likewise, neutralization with KOH resulted in quantitative conversion to S-b-MA.K. Although the initially formed product of the reaction of alkyl esters with TMSI is presumably the trimethylsilyl ester (1 7 ), we were not able to isolate or characterize this copolymer. It is known that trimethysilyl methacrylate and its polymers spontaneously hydrolyze even in moist air (19). Any traces of water in the methanol used to precipitate the reaction mixture would thus preclude isolation of the intermediate trimethylsislyl ester. 

By means of nickel-catalyzed couplings of vinyl zirconocenes, Schwaebe et al. succeeded in obtaining fluorinated materials using a-bromo esters as electrophiles (Scheme 4.62) [113]. The yields achieved, albeit modest (24—65%), were far better using Ni(0) than those obtained in experiments based on several palladium(O) sources (no product observed). Isopropyl esters appear to be crucial, as competing 1,2-addition occurs with both ethyl and n-butyl analogues. Curiously, t-butyl esters were found to completely inhibit both modes of reaction of the zirconocene. 

D-a-Hydroxy carboxylic acids.1 These optically active acids can be prepared by a Sn2 reaction between the t-butyl esters of L-2-halo carboxylic acids and cesium p-nitrobenzoate, which proceeds with complete inversion. 

R Roeske. Preparation of t-butyl esters of free amino acids. J Org Chem 28, 1251, 1963. 

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