Chloroacetyl esters cleavage

Numerous researchers have employed thiols as weak bases in the thioalkylation reaction to ligate unprotected peptides with a haloacetyl group to form thioethers at pH 7 8.5[90 91 131-133 or thioesters at acidic to basic conditions. 108"110 Of these two reactions, thioether formation is often the choice because thioesters suffer from instability in aqueous basic conditions. Haloacetyl derivatives, either as carboxylic acids or active esters, can be attached to either the N-terminal or side-chain amines during the stepwise solid-phase synthesis of either the peptide or the core and are stable to either HF or TFA cleavage conditions. Capping an amino group with a chloroacetyl group is compatible with Fmoc chemistry when used at a terminal step. 

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