Skeletal modifications of β-carboline alkaloids and their antiviral activity profile

To study the effect of the variation of fused ring size and substitution on the antiviral activity of [Formula: see text]-carboline alkaloids, four types of structurally novel [Formula: see text]-carboline alkaloids analogues, with indole-fused six- to nine-membered-rings motifs, were designed, synthesized, and evaluated for the inhibition of tobacco mosaic virus (TMV). Bioassay results indicated that most of these analogues had significant anti-TMV activity; especially I-14 (54 [Formula: see text] 3 % at 500 [Formula: see text]g/mL in vitro; 51 [Formula: see text] 2, 45 [Formula: see text] 2, and 42 [Formula: see text] 1 % at 500 [Formula: see text]g/mL in vivo), II-4 (53 [Formula: see text] 1 % at 500 [Formula: see text]g/mL in vitro; 49 [Formula: see text] 2, 57 [Formula: see text] 2, and 48 [Formula: see text] 1 % at 500 [Formula: see text]g/mL in vivo), and II-8 (48 [Formula: see text] 1 % at 500 [Formula: see text]g/mL in vitro; 53 [Formula: see text] 2 %, 56 [Formula: see text] 2 %, and 46 [Formula: see text] 1 % at 500 [Formula: see text]g/mL in vivo), which were more potent vs. TMV than was ribavirin (36 [Formula: see text] 1 % at 500 [Formula: see text]g/mL in vitro; 37 [Formula: see text] 2, 41 [Formula: see text] 2, and 38 [Formula: see text] 1 % at 500 [Formula: see text]g/mL in vivo). The size of the fused ring has important effects on anti-TMV potency, which may be ascribed to conformational differences. The X-ray structures of I-1, I-6, II-8, and III show differing conformational preferences. The most potent compounds can be used as leads for further optimization as antiphytoviral agents.