Design, Synthesis, Anti-TMV Activity, and Structure–Activity\nRelationships of <i>Seco</i>-pregnane C<sub>21</sub> Steroids\nand Their Derivatives

<i>seco</i>-pregnane C<sub>21</sub> steroids exhibit\nhigh antiviral activity against the tobacco mosaic virus (TMV). However,\nthe structural modification of <i>seco</i>-pregnane C<sub>21</sub> steroids and the structure–activity relationship\n(SAR) of the modified compounds remain unevaluated. Hence, the present\nstudy investigated how variations in the original skeletons of natural <i>seco</i>-pregnane C<sub>21</sub> steroids affect their antiviral\nactivity. A series of glaucogenin C and A derivatives were designed\nand synthesized for the first time, and their anti-TMV activity was\nevaluated. Bioassay results showed that most of the newly designed\nderivatives exhibited good to excellent antiviral activity; among\nthese derivatives, <b>5g</b>, <b>5j</b>, and <b>5l</b> with higher antiviral activity than that of ningnanmycin emerged\nas new antiviral candidates. Reverse transcription-polymerase chain\nreaction and Western blotting assay revealed reduced levels of TMV\ncoat protein (TMV-CP) gene transcription and TMV-CP protein expression,\nwhich confirmed the antiviral activity of these derivatives. These\ncompounds also downregulated the expression of <i>NtHsp70-1</i> and <i>NtHsp70-061</i>. Computational simulations indicated\nthat <b>5l</b> displayed strong van der Waals energy and electrostatic\nwith the TMV coat protein, affording a lower binding energy (Δ<sub>Gbind</sub> = −56.2 kcal/mol) compared with Ribavirin (Δ<sub>Gbind</sub> = −47.6 kcal/mol). The SAR of these compounds\nwas also evaluated, which demonstrated for the first time that substitutions\nat C-3 and double bonds of C-5/C-6 and C-13/C-18 are crucial for maintaining\nhigh anti-TMV activity.