Design, Synthesis, Anti-TMV Activity, and Structure-Activity Relationships of Seco-pregnane C21 Steroids and Their Derivatives

seco-pregnane C₂₁ Steroids exhibit high Antiviral activity against the tobacco mosaic virus (TMV). However, the structural modification of seco-pregnane C₂₁ Steroids and the structure-activity relationship (SAR) of the modified compounds remain unevaluated. Hence, the present study investigated how variations in the original skeletons of natural seco-pregnane C₂₁ Steroids affect their Antiviral activity. A series of glaucogenin C and A derivatives were designed and synthesized for the first time, and their anti-TMV activity was evaluated. Bioassay results showed that most of the newly designed derivatives exhibited good to excellent Antiviral activity; among these derivatives, 5g, 5j, and 5l with higher Antiviral activity than that of ningnanmycin emerged as new Antiviral candidates. Reverse transcription-polymerase chain reaction and Western blotting assay revealed reduced levels of TMV coat protein (TMV-CP) gene transcription and TMV-CP protein expression, which confirmed the Antiviral activity of these derivatives. These compounds also downregulated the expression of NtHsp70-1 and NtHsp70-061. Computational simulations indicated that 5l displayed strong van der Waals energy and electrostatic with the TMV coat protein, affording a lower binding energy (Δ_Gbind = -56.2 kcal/mol) compared with Ribavirin (Δ_Gbind = -47.6 kcal/mol). The SAR of these compounds was also evaluated, which demonstrated for the first time that substitutions at C-3 and double bonds of C-5/C-6 and C-13/C-18 are crucial for maintaining high anti-TMV activity.