Backbone amide linker (BAL) strategy for N-alpha-9-fluorenylmethoxycarbonyl (Fmoc) solid-phase synthesis of unprotected peptide p-nitroanilides and thioesters

A novel and general backbone amide linker (BAL) strategy has been devised f or preparation of C-terminal modified peptides containing hindered, unreact ive, and/or sensitive moieties, in concert with N-alpha-9-fluorenylmethoxyc arbonyl (Fmoc) solid-phase synthesis protocols. This strategy comprises ii) start of peptide synthesis by anchoring the penultimate residue, with its carboxyl group orthogonally protected, through the backbone nitrogen,(ii) c ontinuation with standard protocols for peptide chain elongation in the C - -> N direction, (iii) selective orthogonal removal of the carboxyl protecti ng group, (iv) solid-phase activation of the pendant carboxyl and coupling with the desired C-terminal residue, and (v) final cleavage/deprotection to release the free peptide product into solution. To illustrate this approac h, several model peptide p-nitroanilides and thioesters have been prepared in excellent yields and purities, with minimal racemization. Such compounds are very difficult to prepare by standard Fmoc chemistry, including the BA L strategy as originally envisaged.