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Sanguinarine (1) and chelerythrine (2) are two quaternary benzocphenanthridine alkaloids (QBAs). Eighteen derivatives of 1 and 2 were synthesized by modification of C=N(+) bond and evaluated for their in vitro acaricidal activity against Psoroptes cuniculi, a mange mite. A new method was developed to prepare 6-alkoxy dihydro derivatives of 1 and 2 (1a-e, 2a-e). Among all the compounds, only 6-alkoxy dihydrosanguinarines (1a-e) showed significant acaricidal activity at 5.0 mg/mL and 1a possessed the strongest activity (50% lethal concentrations (LC(50))=339.70±0.75 mg/L, 50% lethal time (LT(50))=6.53±0.04 h), comparable with a standard drug ivermectin (LC(50)=168.19±11.79 mg/L, LT(50)=16.54±0.11 h). The iminium moiety in 1 and 2 was proven to be the determinant for their acaricidal properties. 6-Alkoxy dihydro derivatives (1a-e, 2a-e) were prodrugs of 1 and 2. Compared with 7,8-dimethoxy groups, 7,8-methylenedioxy group was able to significantly improve the bioactivity. The present results suggested that QBAs are promising candidates or lead compounds for the development of new isoquinoline acaricidal agents.
Miao et al. (Sun,) studied this question.