Total Synthesis of Nocardiotide A by Using a Combination of Solid‐ and Solution‐Phase Methods

Abstract

Nocardiotide A has been successfully synthesized using a combination of solid‐ and solution‐phase synthesis. Peptides 1 and 2 of nocardiotide A, with different C and N termini, were synthesized on 2‐chlorotrityl chloride resin. A combination of N,N,N′,N′‐tetramethyl‐O‐(1H‐benzotriazol‐1‐yl)uronium hexafluorophosphate and hydroxybenzotriazole (HBTU/HOBt) was employed as coupling reagent and the release of the linear peptides from the resin was undertaken using AcOH and trifluoroethanol (TFE) reagents resulting in linear hexapeptides with protected side chains in good yields (83.4 % and 76 %, respectively). Peptide 1, which has Ala as C terminus and Trp as N terminus, was the only peptide that was successfully cyclized without epimerization under a dilute condition (1.25×10−3 M) using HBTU (3 equiv.) and 1 % N,N‐diisopropylethylamine (DIPEA) in dichloromethane. A combination of a small residue at the C terminus and a larger residue at the N terminus was found to be effective for the cyclization. The protecting group of the crude cyclic product was then deprotected by 95 % TFA in water and the crude was then purified to give white solid product with 16 % overall yield. The peptide was then characterized by HR‐ToFMS, 1H‐ and 13C‐NMR spectroscopy. In order to ensure the absence of epimerization during cyclization, the D‐analogue of nocardiotide A was synthesized using similar synthetic protocol. The NMR data of D‐analogue was found to have a larger difference compared to the synthetic and natural product, showing there is no epimerization at C‐terminus during cyclization.

Total synthesis of a reversed cyclopurpuracin using a combination of solid and solution phase methods

“…The 13 C NMR spectra also showed 93 signals, including the presence of carbonyls, carbon quarternaries, aromatic carbons, oxygenated carbons, α-carbons, aliphatic methine carbon, aliphatic methylene carbon, and aliphatic methyl carbon. 1 H and 13 C NMR spectra showed excessive signals. The NMR spectra of reversed cyclopurpuracin 2 showed the presence of two conformers (Table 1), whereas this profile was not found in the NMR spectra of the natural cyclopurpuracin 7 due to the difference in the sequence.…”

“…C 37 H 54 N 8 O 9 755.4089), representing the correct molecular ion peak of the desired 2 (Figure S5). Cyclic peptide 2 was further characterized by 1 H NMR, 13 C NMR, and 2D NMR (HMQC, HMBC, COSY, NOESY).…”

“…The linear peptide was analyzed by analytical RP-HPLC (5%-50% acetonitrile in water for 40 min, flow rate 1 ml/min, λ 240 nm) and the crude peptide was purified by semi-preparative RP-HPLC using 5%-80% ACN eluent for 40 min. The target fractions (white powder, 11.1 mg) were collected and characterized by ToF-ESI-MS, 1 H-NMR (500 MHz, DMSO-d6), and 13 C-NMR (125 MHz, DMSO-d6).…”

Cyclic Peptides for the Treatment of Cancers: A Review

“…This compound has also been successfully synthesized by the peptide synthesis method through combinations between solid and liquid phase peptide synthesis techniques, with a yield of 16%. However, no anticancer activity was recorded on the cell lines tested, based on the low IC 50 values obtained [ 67 ] ( Table 2 ).…”

Genus Nocardiopsis: A Prolific Producer of Natural Products

“…collected from the Red Sea in 2018, resulted in the purification of one new cyclic hexapeptide, nocardiotide A (35) (Figure 8) [64]. Compound 35 was synthesized by combining solid-and solution-phase synthesis methods in 2021 [65]. Compound 35 exhibited obvious cytotoxic activities against HeLa, CT26 (murine colon carcinoma), and MM.1S (human multiple myeloma) cell lines with IC 50 values ranging from 8 to 12 µM [64].…”