Synthesis of New Triazolopyrazine Antimalarial Compounds

Abstract

A radical approach to late-stage functionalization using photoredox and Diversinate™ chemistry on the Open Source Malaria (OSM) triazolopyrazine scaffold (Series 4) resulted in the synthesis of 12 new analogues, which were characterized by NMR, UV, and MS data analysis. The structures of four triazolopyrazines were confirmed by X-ray crystal structure analysis. Several minor and unexpected side products were generated during these studies, including two resulting from a possible disproportionation reaction. All compounds were tested for their ability to inhibit the growth of the malaria parasite Plasmodium falciparum (3D7 and Dd2 strains) and for cytotoxicity against a human embryonic kidney (HEK293) cell line. Moderate antimalarial activity was observed for some of the compounds, with IC50 values ranging from 0.3 to >20 µM; none of the compounds displayed any toxicity against HEK293 at 80 µM.

Synthesis and characterisation of new antimalarial fluorinated triazolopyrazine compounds

“…In addition, consistent with reported SAR data [14,16,21], the ether-linked compounds 4-6 exhibited strong activity with IC 50 values of 0.2-1.2 µM, whereas compounds 7-9 with the removal of the phenyl ring from the ether methylene group resulted in a loss of potency at the tested concentrations. For fluorinated compounds, previous studies [14] reported that the introduction of CF 3 and CF 2 Me groups at the C-8 position of scaffold 2 improved the antimalarial activity. In particular, compounds with a CF 2 Me moiety showed a 7.3-fold improvement in potency (IC 50 = 1.7 µM) compared to the parent scaffold 2 (IC 50 = 12.6 µM) [14].…”

“…Previous structure–activity relationship (SAR) studies reported that any substitution at the C8 position of Series 4 triazolopyrazines can lower the potency for P. falciparum [ 14 – 16 ]. However, a recent preliminary SAR study identified that substitution at the C8 position with trifluoromethane and difluoroethane moieties using Diversinate™ chemistry increased the potency of the parent scaffold (compound 2 ), suggesting the potential of these fluoroalkyl groups for improving the potency of other promising leads within the OSM project [ 14 ]. Fluorine-containing compounds have exhibited wide applications in pharmaceuticals and agrochemicals – approximately 20% of marketed drugs are fluoro-pharmaceuticals, while for agrochemicals, 53% are fluoro-compounds [ 17 – 18 ].…”

“…Three selected Series 4 triazolopyrazine scaffolds (1-3) were converted to three known (4-6) and three new ether-linked derivatives (7)(8)(9). H-8 of the known OSM leads 4-6 was subsequently substituted with three different fluoroalkyl moieties using Diversinate™ chemistry that resulted in the synthesis of nine new fluorinated triazolopyrazines (10)(11)(12)(13)(14)(15)(16)(17)(18)(19). The antimalarial data indicated that substitution of H-8 of ether-linked triazolopyrazines with fluoroalkyl moieties led to a reduction or loss of activity at the tested concentrations.…”

“…General procedure for Diversinate™ derivatisations on triazolopyrazine scaffolds. In a manner analogous to [14], the scaffold (0.1 mmol) was dissolved in DMSO/CH 2 Cl 2 (1:1, 250 µL:250 µL) before the addition of Diversinate™ (0.2 mmol, 2 equiv), TFA (40 μL, 0.5 mmol, 5 equiv) and filtered H 2 O (100 μL). The reaction mixture was stirred for 30 min at room temperature, cooled to 4 °C then 70% TBHP solution (41 μL, 0.3 mmol, 3 equiv) was slowly added over a 3D7 = P. falciparum (chloroquine-sensitive strain); b Dd2 = P. falciparum (chloroquine, pyrimethamine and mefloquine drug-resistant strain); c All compounds 1-18 and controls tested for cytotoxicity against human embryonic kidney cells (HEK293) in order to determine selectivity index (SI) using the formula: SI = HEK293 IC 50 /parasite IC 50 , all compounds were inactive towards HEK293 at the top dose of 80 µM; d Estimated IC 50 as a plateau of inhibition was not reached; SD = standard deviation.…”

“…In our previous work on OSM Series 4 scaffolds [ 14 ], we had undertaken some preliminary investigations into the use of commercially available Diversinate™ reagents and showed the bicyclic nitrogen-rich core of Series 4 was amenable to this chemistry, with radical sulfinate substitution occurring with high-selectivity at C-8 and in respectable yields. This paper reports additional and more thorough Diversinate™ studies on three phenethyl ether substituted triazolopyrazine scaffolds, with a particular focus on incorporating the fluoro fragments -CF 3 , -CF 2 H and -CF 2 Me into the OSM Series 4 structures via LSF chemistry.…”