Benzyl ester type link

Easy access to peptide methyl esters is offered by the transesterification of peptides linked to benzyl alcohol type resins such as the HMBA resin.t l Tertiary amines generally serve as catalysts,although even better results are obtained with potassium cyanideor lithium bromide/DBUt as base catalysts. Note that Asp or Glu side-chain esters are likewise transformed under these conditions. 

Peptide hydrazides are readily obtained by hydrazinolysis of suitably protected peptides linked to the resin via a benzyl ester.Alternative to the hydroxymethyl ester linkage of the Merrifield resins, l-methyl-2-oxo-phenylethyl ester,2-nitrobenzyl ester O or 4-meth-oxybenzyl ester type linkages are also utilized.b - Recently, fuUy protected hydrazides were obtained in good yields on 2-methoxy-4-alkoxybenzyl alcohol resin (Sasrin ) by hydrazinolysis with 20% anhydrous hydrazine in DMF using DMA as solvent the reaction was significantly slower, but with less side products. 

A new type of latent-active glycosyl donors, 2 -carboxybenxyl glycosides and their benzyl esters, have been recently introduced [62-64]. The active carboxylic acids are readily available from the latent benzyl esters by catalytic hydrogenation in the presence of ammonium acetate. In an example [62] shown in Scheme 5.24 the latent mannoside (136) was glycosylated with the active donor (135) to provide exclusively the p-linked disaccharide (137) in excellent yield. The latent... 

After having discussed these principal aspects, some modifications on polystyrene are described in the following to punctuate the limitations mentioned above. N-protected amino acids, attached onto chloromethylated polystyrene, form links of the benzyl ester [73] type well known in conventional peptide synthesis (for details see the next section). 

The tetrakis-sulphoxides (67) have been reported as a new type of PT catalyst in a solid-liquid mode, for example in 5n reactions of 1-bromo-octane. Nucleophilic substitutions of 1-bromo-octane and benzyl bromide are again the test reactions in a report on the use of the sucrose-ethylene oxide adducts (68a) as PT catalysts in both solid-liquid and liquid-liquid modes. The methacrylate ester derivative (68b) has been polymerized to a cross-linked gel that acts in a TC capacity for the same reactions. In a related approach some modified dextran anion exchangers carrying lipophilic substituents, such as the modified hydroxypropylated dextran gel shown in (69), have been synthesized and shown to catalyse displacement reactions including the alkyl bromide to iodide transformation under TC conditions. ... 

Organomagnesiums frequently prove superior also in other types of reactions. They may facilitate the oxidation of a carbon-metal to a carbon-oxygen bond, secure clean monoaddition of an acetylide to an activated ester (a critical issue in a monensin synthesis X favor in the presence of a copper catalyst 1,4-addition onto a conjugated enone over 1,2-addition, reorient the attack of formaldehyde on a benzylic entitiy from the a- to the or /to-position, and provide diastereoselectivity in nucleophilic additions onto aldehydes. Furthermore organomagnesiums combine under carbon-carbon linking with a variety of organic halides, tosylates, and acetates if the process is mediated by transition elements such as palladium(O) copper(I), nickel(II) or iron(II) Organoalkalis are often less fit to enter such catalytic cycles. 

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