Backbone Amide Linker (BAL) Strategy for Solid-Phase Synthesis of C-Terminal-Modified and Cyclic Peptides^1,2,3

Abstract

Peptide targets for synthesis are often desired with C-terminal end groups other than the more usual acid and amide functionalities. Relatively few routes exist for synthesis of C-terminal-modified peptidesincluding cyclic peptidesby either solution or solid-phase methods, and known routes are often limited in terms of ease and generality. We describe here a novel Backbone Amide Linker (BAL) approach, whereby the growing peptide is anchored through a backbone nitrogen, thus allowing considerable flexibility in management of the termini. Initial efforts on BAL have adapted the chemistry of the tris(alkoxy)benzylamide system exploited previously with PAL anchors. Aldehyde precursors to PAL, e.g. 5-(4-formyl-3,5-dimethoxyphenoxy)valeric acid, were reductively coupled to the α-amine of the prospective C-terminal amino acid, which was blocked as a tert-butyl, allyl, or methyl ester, or to the appropriately protected C-terminal-modified amino acid derivative. These reductive aminations were carried out either in solution or on the solid phase, and occurred without racemization. The secondary amine intermediates resulting from solution amination were converted to the 9-fluorenylmethoxycarbonyl (Fmoc)-protected preformed handle derivatives, which were then attached to poly(ethylene glycol)−polystyrene (PEG-PS) graft or copoly(styrene−1% divinylbenzene) (PS) supports and used to assemble peptides by standard Fmoc solid-phase chemistry. Alternatively, BAL anchors formed by on-resin reductive amination were applied directly. Conditions were optimized to achieve near-quantitative acylation at the difficult step to introduce the penultimate residue, and a side reaction involving diketopiperazine formation under some circumstances was prevented by a modified protocol for Nα-protection of the second residue/introduction of the third residue. Examples are provided for the syntheses in high yields and purities of representative peptide acids, alcohols, N,N-dialkylamides, aldehydes, esters, and head-to-tail cyclic peptides. These methodologies avoid postsynthetic solution-phase transformations and are ripe for further extension.

Amino Acid-Protecting Groups

“…38 However, an advantage of Ddz-over Trt-amino acids is that their incorporation is easier, which is a crucial factor when the corresponding amino acids are to be incorporated on hindered amines. 38 -2-(4-Biphenyl)isopropoxycarbonyl (Bpoc).…”

Efficient Solid-Phase Synthesis of 2-Substituted-3-Hydroxy-4(1H)-Quinolinone-7-Carboxamides with Two Diversity Positions

“…Preparation of N-derivatized carboxamides a a Reagents: (i) amine, 10% AcOH/DMF, overnight, NaBH(OAc) 3 , 5 h; (ii) 1-methyl-2-aminoterephtalate, DIC, HOBt, DCM, DMF, rt,overnight; (iii) TMSOK, THF, rt, 7 h; (iv) 2-bromo-4′-methyl-acetophenone, TEA, DMF, rt, 2 h; (v) 50% TFA, DCM, 30 min; (vi) TFA, 80 °C, 2 h. Six amines for the first diversity position. Bromoketones used for the synthesis of 2-substituted-3-hydroxy-4(1H)-quinolinone-7-carboxylic acid propylamides (12).…”

Solid-Phase Methods for the Preparation of Epoxysuccinate-Based Inhibitors of Cysteine Proteases

“…For this synthetic method, the desired R 1 amines were loaded onto the BAL resin using sodium cyanoborohydride, as previously reported. 13 Next, the P2 amino acid was coupled to the resin-bound secondary amine in high yields using HATU/DIEA in DMF. After Fmoc deprotection, activated epoxysuccinate 5 was coupled to the free amine, and the ethyl ester was hydrolyzed with KOH as previously described.…”