Esters protecting group

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, 

Regioselective acylations can also be achieved by the stannylene procedure but, in general, selectivities are not as high as for alkylations. For example, treatment of methyl P-D-galactoside with dibutyltin oxide followed by reaction with benzoyl chloride results in the formation of a mixture of methyl 6-O-benzoyl-P-D-galactoside (53%) and methyl 3,6-O-di-benzoyl- P-D-galactoside (21 %).40 

Esters are usually cleaved by bases such as NaOMe, KOH or NH3 in methanol but they can also be hydrolysed by acid-catalysed solvolysis (MeOH/HCl).44 However, they are relatively stable to acid treatment in the absence of water or alcohols. The relative order of base stability of the commonly employed ester groups is as follows  

Several mild reagents have been reported to cleave the chloroacetyl group, including 2-mercaptoethylamine (H2NCH2CH2SH), thiourea 

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Show the steps involved in the synthesis of Ala-Leu from alanine and leucine using benzyloxycarbonyl and benzyl ester protecting groups and DCCI-promoted peptide bond formation. 

Exposure of compound 16, a substance that can be obtained in a straightforward manner from glycine, to sodium tert-butoxide furnishes an enolate that undergoes conversion to 8 upon treatment with terf-butyl formate. It was anticipated that the phthalimido and tert-butyl ester protecting groups in 8 could be removed easily and selectively under anhydrous conditions at a later stage in the synthesis. 

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

The benzyl ester protecting group and oxazolidinone ring were hydrogeno-lyzed on 5% Pd/C in MeOH for 16 hours (Scheme 4.16).34... 

During the preparation of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, the benzyl ester protecting group was removed by catalytic hydrogenolysis (Scheme 4.42). 

PREPARATION OFACETONIDE PROTECTED [G2J-C00H [12] AND GENERAL PROCEDURE FOR THE REMOVAL OF THE BENZYL ESTER PROTECTING GROUP... 

Peptide 21, synthesized using the C-terminal enzymatic-cleavable choline ester (Scheme 11), was also synthesized using the C-terminal allyl ester protecting group (Scheme 12). The selective Pd(0)-catalyzed... 

Sequence inversion and racemization have been associated with uncatalyzed formation of the cyclic dipeptides and has been shown to greatly complicate the kinetics of formation. Cyclic dipeptide formation, by uncatalyzed processes, is rapid enough to pose an apparent threat to the stability of proteins and a possible rationale for the posttranslational N-acetylation of proteins that have been observed in higher organisms. The rate of DKP formation will also depend on the carbonyl ester protecting groups or the structures of the peptide-resin linkage in the solid-phase mode. Furthermore, cyclization is a concentration-independent reaction and demands the use of dilute solutions. ... 

The dimethoxytrityl ester protecting group is now removed by treatment with mild acid (CCI3CO2H), which is insufficiently reactive to hydrolyse the amide protection of bases, or the cyanoethyl protection of the phosphate. The coupling cycle can now be repeated using a phosphoramidite derivative of the next appropriate nucleoside. The sequences will be continued as necessary until the desired oligonucleotide is obtained. 

The oxime 71d has several advantages over other resins, since the protected peptide segment can be cleaved by aminolysis under conditions which do not affect benzyl ester protecting groups. Moreover, the whole procedure is compatible with the Boc group employed for a-amino protection. The synthesis of several peptides using 71d has been... 

Carboxylic acids can also be protected as ortho esters. Ortho esters derived from simple alcohols are very easily hydrolyzed, and a more useful ortho ester protecting group is the 4-methyl-2,6,7-trioxabicyclo[2.2.2]octane structure. These bicyclic orthoesters can be prepared by exchange with other ortho esters, by reaction with iminoethers, or by rearrangement of the ester derived from 3-hydroxymethyl-3-methyloxetane. 

Compound 13 was the first arsenosugar to be synthesized (Fig. 4) (56). The orthoester 26 was transesterified with the alcohol 27 to give an intermediate orthoester, which rearranged on treatment with mer-cury(II) bromide to the riboside 28. Removal of the ester protecting groups yielded the triol 29, which was converted to the isopropylidene derivative and then the chloride 30. The key intermediate 31 was then prepared by treating the chloride 30 with dimethylarsinosodium after the method of Feltham et al. (62). Attempts to purify this arsine proved difficult. Instead, it was oxidized without purification, giving 32, which... 

M. Ciommer and H. Kunz, Synthesis of glycopeptides with partial structure of human glycophorin using the fluorenylmethoxycaibonyl/allyl ester protecting group combination, Synleti p. 593 (1991). 

H. Kunz and 1. Miirz, Synthesis of glycopeptides with Lewis antigen side chain and HIV peptide T sequence using the trichloroethoxycaibonal/allyl ester protecting group combination, Synlett p. 591 (1992). 

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