Allyl esters carboxy-protecting groups

In a formal sense, alkyl ester type carboxy protecting groups, e.g. benzyl, allyl, 4-picolyl, and (9,10-dioxo-2-anthryl)methyl esters, can be viewed at as modified methyl esters. However, because of their specific properties and cleavage conditions these esters will be discussed separately. In this section a series of acetal, phthalimidomethyl, various phenacyl, acetonyl, 9-anthryhnethyl, and cyclopropyhnethyl esters will be presented. 

As has already been detailed for the allyloxycarbonyl (Aloe) moiety, allyl esters also proved to be very versatile and useful carboxy-protecting groups. They can be easily constructed by azeotropic esterification or nucleophilic displacement on allylic halides. For the cleavage of these esters lithium di-methylcuprate can be used. However, a much milder method is found in the Rh -catalyzed isomerization of the allyl moiety to a propenyl ester which immediately hydrolyzes under the reaction conditions (Scheme 67). Even milder is the Pd°-catalyzed allyl transfer to morpholine as an accepting nucleophile. The removal of allyl ester protection has earlier been used in particular in -lactam anti-... 

H Kunz, H Waldmann, C Unverzagt. Allyl ester as temporary protecting group for the (S-carboxy function of aspartic acid. Int J Pept Prot Res 26, 493, 1985. 

Allyl (Al) esters are suitable for the temporary protection of carboxy groups in peptide and glycopeptide synthesisP " and also in the synthesis of cyclic peptides.In the latter case these esters allow, for example, on-resin head-to-taU cyclizations of peptides that are attached via the side chains of aspartic acid and glutannic acid residues to the solid support.t 1 The application of aUylic protecting groups in peptide synthesis has been comprehensively reviewed by Guibe.f ... 

Allyl protection of terminal a-carboxy groups has been used in convergent syntheses of N-linked glycopeptides on a solid support.PI For instance, the polymer-bound trisaccharide pentapeptide allyl ester 12 (Scheme 7) is built up by condensation of the polymer-linked trisaccharide with Z-Ala-Ile-Asp-Ile-Ser(Bzl)-OAl. Further conversion into the polymer-bound trisaccharide octapeptide derivative 14 is achieved by cleavage of the allyl ester with tetrakis(triphenylphosphine)palladium and A,A-dimethylbarbituric acid to give 13, followed by condensation with H-Asp(OMob)-Leu-Thr(Bzl)-OAl. The presence of the allyl ester group at the C-terminus of the octapeptide provides the opportunity for further chain elongation. 

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