A general strategy for the preparation of C-terminal peptide α-ketoacids by solid phase peptide synthesis

Abstract

A new cyanosulfur-ylide based linker makes possible the synthesis of C-terminal peptide α-ketoacids by solid phase synthesis. The preparation of the requisite linker and its application to a variety of C-terminal peptide α-ketoacids with unprotected side chains is reported.As part of the intense interest in the development of new methods for native amide bond formation via chemoselective ligation reactions, 1,2 our group has identified a novel and unexpectedly simple amide-forming ligation reaction that enables the coupling of the peptide fragments by decarboxylative condensation between α-ketoacids and N-alkyl hydroxylamines. 3 This reaction offers great promise as a general method for the chemoselective ligation of two unprotected peptide fragments, but is currently limited by the lack of practical methods for the preparation of ligation partners bearing the requisite Cterminal α-ketoacids 4 and N-terminal hydroxylamines. 5 In addressing this, we have recently reported a robust, chemos-elective method to produce peptide α-ketoacids with minimal epimerization via oxidation of cyanosulfur ylides. 6 This method is operationally friendly, high yielding and provides peptide α-ketoacids in high enantiopurity. It is also compatible with all unprotected amino acid side chains save cysteine and methionine. To extend this strategy further in preparing larger peptide derived ketoacids, we now report the synthesis and utility of a solid-supported cyanosulfur ylide that makes possible the preparation of C-terminal α-ketoacids using standard Fmoc-based solid phase peptide synthesis (Scheme 1). 7 Inspired by Rademann et al.'s work, 8 which performed Wasserman et al.'s elegant phosphorus ylide 9 chemistry on a solid support, we hoped to enable the acylation of a cyanosulfur ylide moiety and elongation of the peptide chain on a standard solid support. We were encouraged by our previous results that the cyanosulfur ylides could tolerate the standard procedures employed for Fmoc-based N-terminal deprotection and extension of the peptide chain in the C to N direction. 6 This finding anticipated an alkyl sulfide-containing linker as a precursor to peptide cyanosulfur ylides for synthesis on a suitable solid support.Tetrahydrothiophene-derived linker 5 was prepared in five steps from commercially available tetrahydrothiophen-3-one (Scheme 2). It contains a free acid for loading onto an amine or alcohol derivatized solid support, and a four atom-spacer from the acid to the tetrahydrothiophene core. Horner-Wadsworth-Emmons reaction 10 provided α,β-unsaturated † Electronic supplementary information (ESI) ester 1, which was subjected to 1,4-conjugate reduction to afford 2. All attempts at metal catalyzed hydrogenation (Pd, Pt, Rh) failed due to catalyst poisoning by the sulfur. 11 We found that the conjugated ester could be reduced by in situ generated nickel boride, 12 accompanied with a small amount of desulfurization as a side reaction. Subsequent reduction of the ester with LiAlH 4 provided alcohol 3. 13 Elaboration ...

New Methods for Chemical Protein Synthesis

“…C-terminal peptide α-ketoacids are also easily obtained by SPPS with the use of a recently developed cyanosulfur-ylide linker and Rink amide MBHA resin. Cleavage of the linker from the resin gives a free cyanosulfur-ylide which is readily and selectively oxidized to the α-ketoacid upon treatment with oxone [162]. After ligation, the 5-oxaproling ring is opened to form a homoserine residue.…”

Chemical Protein Synthesis with the KAHA Ligation

“…A solid supported reagent would render the preparation of the peptide sulfurylides in a transparent process simply by using a suitable resin at the beginning of the synthesis. To achieve this, a suitable linker with a carboxylic acid was attached to the thiolane ring and immobilized on Rink amide polystyrene resin by regular amide coupling (Scheme 10) [35]. After alkylation on a solid phase a solid supported version of the sulfonium salt is obtained, ready for coupling with standard Fmoc-amino acids (Fmoc ¼ 9-fluorenylmethyl carbamate) to give the corresponding solid supported sulfur ylides, which are generally stable and storable.…”

Chemical Biology: What is Its Role in Drug Discovery?

“…A method for the solid phase preparation of C-terminal peptide -ketoacids has been described. This functional group can be reacted further using the -ketoacid-hydroxylamine ligation reaction to afford peptides (Ju & Bode, 2009). A novel method for the synthesis of peptide alcohols has been developed (A. R. .…”