Facile and E-Selective Intramolecular Ring-Closing Metathesis Reactions in 3₁₀-Helical Peptides:  A 3D Structural Study

Abstract

Synthesis of Octapeptide 3Boc-Ser(Al)-OH 1 : NaH (2.44 g of a 60% oil dispersion, 1.46 g, 0.061 mol) was suspended in 20 mL of DMF and cooled to 0 °C in a flask fitted with a dropping funnel. Boc-Ser-OH (5.0 g, 0.024 mol) was dissolved in 40 mL of DMF and the resulting solution was transferred to the dropping funnel. This solution was added drop-wise to the NaH solution over a period of 30 minutes. Allyl bromide (2.0 mL, 0.024 mol) was added and the flask contents warmed to room temperature and stirring was continued for 3 hours. The reaction was quenched by the slow addition of 12 mL of water. The solvents were then removed on the rotary evaporator and the residue dissolved in water (40 mL), which was washed with two 20 mL portions of ethyl acetate. The aqueous phase was acidified to pH 2 with the addition of 6 M HCl and extracted with two 40 mL portions of ethyl acetate. The organic phases were combined and dried over anhydrous magnesium sulfate. After filtration, the solvent was evaporated to yield a viscous oil (4.8 g, 82%) which was found to be of sufficient purity to be used in subsequent reactions.1 H NMR (400 MHz, CDCl 3 ): δ 9.1 (1H br s), 5.85 (1H, m), 5.41 (1H, d, J = 8.4 Hz), 5.25 (1H, dq, J = 17.2, 1.5 Hz), 5.19 (1H, dq, J = 10.4, 1.5 Hz), 4.45 (1H, d, J = 8.5 Hz), 4.00 (2H, dt, J = 5.7, 1.5 Hz), 3.90 (1H, dd, J = 9.5, 3.0 Hz), 3.67 (1H, dd, J = 9.5, 3.6 Hz), 1.45 (9H, s).13 C NMR (100 MHz, CDCl 3 ): δ 175. 96, 156.15, 134.25, 118.21, 80.77, 72.80, 69.96, 54.16, 28.70

Synthesis and High‐Resolution NMR Structure of a β3‐Octapeptide with and without a Tether Introduced by Olefin Metathesis

“…. Furthermore, as mentioned above 6 ), a helixtypical CD spectrum is observed with b-peptides, which cannot possibly fold to a 3 14 -helix. For these reasons we have always considered CD spectra of b-peptides as a mere qualitative kind of fingerprint.…”

“…5, A and B. Within the same subset of the calculated structures, the terminal H-bond from HN 6 to CO 8 is observed eight times for 1 and four times for 4. The H-bond from HN 1 to CO 3 is absent in the ten lowest-energy structures of 4 and observed only twice within the ten structures of 1 shown in Fig.…”

“…Significantly, this enhanced effect is also apparent in H 2 O with maximum negative mean molar ellipticities of À 5000 and À 15000 observed for 1 and 4, respectively. Based on the assumption that a correlation exists between intensity of the negative Cotton effect near 215 nm and the 3 14 -helical propensity of a b 3 -peptide 6 ), we conclude that 4 displays an enhanced helical propensity in MeOH, and especially in H 2 O. The observed effect of (CH 2 ) 4 -tethering on the CD spectra compares favorably with that of the b-peptides tethered with an amide linker [1d] [13].…”

Stapled α−helical peptide drug development: A potent dual inhibitor of MDM2 and MDMX for p53-dependent cancer therapy

“…Stapled peptides were first described as a novel approach to creating macrocyclic α-helical peptides through the addition of an all-hydrocarbon cross-link between two α−methyl-substituted amino acids having terminal olefin side-chains to enable ringclosing metathesis using Grubbs ruthenium catalyst (17,18). Historically, it is important to recognize the precedence macrocyclization of the O-allyl-modified peptides by the Grubbs research group (19,20) as well as intrinsic helical-inducing properties of linear peptides incorporating α−methyl-substituted amino acids by the Toniolo research group (21,22). Relative to a previously published stapled peptide, SAH-p53-8 (3,15), the limited biological potency determined in our cellular assays using more stringent physiologic conditions (i.e., in the presence of 10% serum) inspired us to tackle this highly important cancer target and advance a more rigorously tested preclinical lead compound relative to both in vitro and in vivo efficacy.…”

Metathesis in Peptides and Peptidomimetics

“…[39,40] More recently, Grubbs and OLeary reported RCM in a 3 10 -helix system where the olefin moieties were positioned at the i and i + 3 rather than at the i and i + 4 or i+ 7 positions as was previously established. [41] As a result, a facile and E-selective (> 20:1) intramolecular RCM was achieved, contrary to previous system where E/Z mixtures are the usual products of RCM. The high selectivity could be attributed to the high content of a-aminoisobutyric acids (Aib) in the peptide sequence, thereby imposing conformational restrictions on the peptides secondary structure and affecting the E/Z ratio.…”