Peptide alkyl thioesters are versatile reagents in various synthetic applications, commonly generated from peptide hydrazides and thiols. However, a notable limitation is the need for a substantial excess of the thiol reagent, restricting the usage to simple thiols.[1] This limitation confines these synthetic strategies to the production of thioesters from economical and thus relatively simple thiol nucleophiles.

Here, we introduce an adapted procedure that significantly enhances thioester production with just a minimal thiol excess, facilitating the use of advanced thiol nucleophiles.[2] Indeed, following a procedure reported by Liu and colleagues, the hydrazide initially undergoes activation with sodium nitrite to yield a peptide azide. Subsequently, an excess of aryl thiol is added to the peptide azide solution under neutral pH conditions, resulting in the formation of an intermediate aryl thioester.

In our method, an additional step involves a thiol-thioester exchange between the in situ formed aryl thioester and an alkyl thiol added in quasi-stoichiometric amounts to the reaction, under conditions enabling to concomitantly remove the excess of aryl thiol.

This was conveniently achieved by using p-hydroxythiophenol, a thiol extractable under neutral pH conditions. Our method has proven highly effective in accessing peptide thioesters from a variety of alkyl thiols.