Concentration-dependent mortality effects were observed for three pure synthetic natural products (alkannin, shikonin, and shikalkin) and three acetylated derivatives of shikonin against Culex pipiens (Culicidae: Diptera) for the first time. The larvicidal properties of all naphthoquinones were evaluated under laboratory conditions against the larvae of the mosquito species C. pipiens biotype molestus, the anthropophilic biotype of the C. pipiens mosquito species. Experimental data of the tested toxicity of quinones revealed generally high efficacy where shikonin (3.9 mg/L) was the most active followed by shikalkin (8.73 mg/L) and alkannin (12.35 mg/L). The insecticidal performance of shikonin-acetylated derivatives was also investigated, aiming at the same time in the establishment of the relationships between the structure and the activity of shikonin-type compounds with larvicidal activity against C. pipiens. Results indicated that naphthoquinones, compared with other natural compounds with larvicidal activity, are very toxic against mosquito larvae and could be a potential source of natural larvicidal substances. Finally, bioassays with shikonin derivatives also revealed that although hydroxylic groups seem to play a secondary role in efficacy, the quinone moiety is essential.

No abstract

[reaction: see text] Stereospecific synthesis of the pair of natural macrolides, trans- and cis-resorcylide, was performed using ring-closing metathesis on dienes 3 and 4, which lack or feature an intramolecular H-bond, respectively. An effective Stille carbonylative coupling of benzyl chlorides 11 and 15 was employed for their preparation. The influence of intramolecular H-bonding on the interconversions of resorcylides was also studied.