Green Synthesis of Tetrahydropyrazino[2,1-a:5,4-a']diisoquinolines as SARS-CoV-2 Entry Inhibitors

Abstract

A class of tetrahydropyrazino[2,1-a:5,4-a']diisoquinoline derivatives were synthesized under environmentally friendly conditions using water as the solvent. The 3-D structures of some synthesized compounds were determined by X-ray diffraction. Since naturally occurring isoquinoline alkaloids have significant antiviral activities against a wide range of viruses, including coronaviruses, the synthesized compounds were assayed for their inhibitory activities against SARS-CoV-2. Our results showed that the active compounds 50 and 96 blocked the delta SARS-CoV-2 entry into VeroE6 cells to display EC₅₀ of 26.5 ± 6.9 and 17.0 ± 3.7 μM, respectively, by inhibiting the interaction between SARS-CoV-2 Spike's receptor binding domain (RBD) and human receptor angiotensin-converting enzyme 2 (ACE2), and CC₅₀ greater than 100 μM. This study provides a green synthesis method of tetrahydropyrazinodiisoquinoline for antiviral or other applications.

Figure 1

Chemical structures of some biologically active isoquinoline derivatives. These include emetine and its OCH₃ group reduced analogue cephaline that shows lower toxicity, palmatine, and berberine containing a nitrogen cation, as well as berbamine, and cepharanthine in cyclic forms.

Scheme 1. Synthesis of Tetrahydropyrazino[2,1-a:5,4-a′]diisoquinoline Derivatives from Isoquinoline or Bromoisoquinoline and Various Phenacyl Bromides under the Optimized Condition

Figure 2

3-D structures of (A) 98, (B) 103Br, (C) 68Br, (D) 122Br, and (E) 115Br determined from the diffraction data are summarized in Tables S2–S6. Their chemical structures are shown in Tables 2 and 3.

Figure 3

EC₅₀ measurements of the antiviral activities of active compounds. (A, B) Inhibitor dose-dependent virus inhibition curves for measurements of EC₅₀ values of the inhibitors, 50 and 96, against the delta variant of SARS-CoV-2 infecting VeroE6 cells, are 26.5 ± 6.9 and 17.0 ± 3.7 μM, respectively. These curves were generated according to the plaque reduction assay data. Their CC₅₀ values derived from the plots were >100 μM. All of the measurements were performed in triplicate to yield the averaged EC₅₀ and standard deviations.

Figure 4

Inhibition of RBD:ACE2 by the compounds. (A, B) Percentages of RBD:ACE2 binding inhibition by increasing compound concentrations were measured with RBD from delta SARS-CoV-2 to yield IC₅₀ values of 32.7 ± 8.9 and 10.4 ± 2.1 μM for compounds 50 and 96, respectively. All of the measurements were performed in triplicate to yield the averaged IC₅₀ values and standard deviations.

Figure 5

Binding mode of active antiviral compound 50 with RBD. Compound 50 was docked into the RBD of the delta-strain Spike protein (PDB: 7w92). Compound 50 is colored purple, and the residues involved in hydrogen bonding are highlighted in cyan, while those involved in van der Waals interactions are in orange. The trimeric Spike protein is depicted in yellow and the RBD is in green. In this model, compound 50 interacts with residues at the RBD:ACE2 interface, potentially interfering with the binding of the RBD to ACE2.