Optimized Ring Closing Metathesis Reaction Conditions To Suppress Desallyl Side Products in the Solid-Phase Synthesis of Cyclic Peptides Involving Tyrosine(O-allyl)

Abstract

We are exploring constraining aromatic residues in the kappa opioid receptor selective antagonist arodyn (Ac[Phe 1,2,3 ,Arg 4 ,D-Ala 8 ]dynorphin A(1-11)-NH 2 ) by ring closing metathesis (RCM) involving tyrosine(O-allyl) (Tyr(All)), but desallyl products limited the yields of the desired cyclic peptide. The model dipeptide Fmoc-Tyr(All)-Tyr(All) was used to explore different reaction conditions, including the use of isomerization suppressants, to minimize formation of the desallyl products and enhance formation of the desired RCM product. Reaction conditions were identified that enhanced the RCM product yield while suppressing desallyl products using both second-generation Grubbs and second-generation Hoveyda−Grubbs catalysts. These optimized reaction conditions were then applied to the cyclization of a tripeptide and an arodyn analog resulting in ≥70% conversion to the desired cyclic peptides. These strategies should be applicable to RCM involving Tyr(All) and similar residues in peptide and peptidomimetic cyclizations performed on solid phase.

Conformational Constraint between Aromatic Residue Side Chains in the “Message” Sequence of the Peptide Arodyn Using Ring Closing Metathesis Results in a Potent and Selective Kappa Opioid Receptor Antagonist

“…However, initial cyclization of resin-bound [Tyr­(All) 2,3 ,Ile 8 ]­arodyn 12 (Table ) using Grubbs second generation catalyst (G II) resulted in desallyl products, which complicated purification and decreased the product yield . In a model study, low catalyst concentration (0.3 mM) and phenol, added as an isomerization suppressant, were found to suppress these side products and enhance RCM product yield . The initial series of cyclic arodyn analogs 4 – 8 (Figure and Scheme ) were synthesized by conventional heating using the conditions developed for the model dipeptide.…”

“…Therefore, we performed docking studies on regioisomers of the peptides where the cyclizations involved m -Tyr­(All) as well as Tyr­(All) (Figure ). These peptides were synthesized using optimized cyclization conditions , and evaluated for their opioid receptor affinities along with KOR antagonist activity of the most selective analogs. Herein, we describe the results of these studies and the structure–activity relationships for the cyclic arodyn analog regioisomers.…”

A Peptide Stapling Strategy with Built‐In Fluorescence by Direct Late‐Stage C(sp2)−H Olefination of Tryptophan

“…Up to now,afew stapling methods including lactamization, intramolecular disulfide bridging/thioether formation, lactonization, ring-closing metathesis, and click chemistry have been successfully developed. [11,[29][30][31][32][33][34][35][36][37] Recently,C ÀHa ctivation,a na tomic economic and direct functionalization methodology,h as been employed for efficient and direct stapling of linear peptidep recursors on the side chains of amino acid residues,s uch as alanine( Ala) [10,38] phenylalanine( Phe), [39] and unnatural amino acids. [40,41] Particularly,t ryptophan has received much attentionb ecause it is an important amino acid to maintain peptide and protein function.…”

“…Up to now, a few stapling methods including lactamization, intramolecular disulfide bridging/thioether formation, lactonization, ring‐closing metathesis, and click chemistry have been successfully developed [11, 29–37] …”

Macrocyclization strategies for cyclic peptides and peptidomimetics

“…224 The application of RCM to peptides has been reviewed previously, 209,219,225 including a perspective discussing all-hydrocarbon-stapled α-helical peptides in general. 208 Alkenes for peptide stapling have been introduced as modified sidechains on carbon 13,204,[226][227][228][229][230] as well as on the α-N, [231][232][233][234] side-chain aliphatic alcohols 206,229 and phenols, 235,236 C-terminal or side-chain acids, 46,237,238 N-terminal carbamates, 237 and cysteine thiol groups. 239 Often, when the alkene is introduced as a modified C-bound chain, α-methyl-αalkenyl sidechains are used for additional helix stabilisation.…”