4-Picolyl esters cleavage

A more useful application of the electron-attracting effect of aryl subtituents is found in the cleavage of 4-picolyl derivatives of protected cysteine and tyrosine in peptide synthesis (Scheme 14).Catalytic cleavage of these derivatives is usually unattractive for SProtecting groups removable also include 3- and 4-picolyl esters. ... 

The first examples required a 2-picolyl ester for efficient cleavage because of its chelation of copper. By introducing a Cu -binding group in the linker between the PNA strand and the target ester, it was shown that (in principle) any ester could be... 

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. 

A similar mechanism could also be proposed for the 9-methylcarbazole-sensitized C—O bond cleavage of the picolinium esters (142-144) (Scheme 21)251. The formation of Af-methyl-4-picolyl radical 145 is verified by the observation of IV-methylpicolinium ion (146) in the presence of 1,4-cyclohexadiene, a good H-atom donor. Other anilines such ALALW. /V -tetramethyl benzidine and triphenylamine are also efficient sensitizers in this reaction. 

---