Insecticidal pyrido[1,2-a]azepine alkaloids and related derivatives from Stemona species.
Eight new alkaloids, the pyrido[1,2-a]azepines stemokerrin, methoxystemokerrin-N-oxide, oxystemokerrin, oxystemokerrin-N-oxide, and pyridostemin, along with the pyrrolo[1,2-a]azepines dehydroprotostemonine, oxyprotostemonine, and stemocochinin were isolated from four Stemona species together with the known compounds protostemonine, stemofoline, 2'-hydroxystemofoline, and parvistemonine. Their structures were elucidated by 1H and 13C NMR including 2D methods and two key compounds additionally by X-ray diffraction. Besides the formation of a six membered piperidine ring, additional oxygen bridges and N-oxides contributed to structural diversity. The co-occurrence of pyrrolo- and pyridoazepines suggested biosynthetic connections starting from more widespread protostemonine type precursors. Bioassays with lipophilic crude extracts against Spodoptera littoralis displayed very strong insecticidal activity for the roots of S. curtisii and S. cochinchinensis, moderate activity for S. kerrii, but only weak effects for the unidentified species HG 915. The insect toxicity was mainly caused by the accumulation of stemofoline, oxystemokerrin, and dehydroprotostemonine displaying two different modes of action. Based on the various insecticidal activities of 13 derivatives structure-activity relationships became apparent.
Kaltenegger
Elisabeth
E
Comparative & Ecological Phytochemistry Department, Institute of Botany, University of Vienna, Rennweg 14, A-1030 Vienna, Austria.
Brem
Brigitte
B
Mereiter
Kurt
K
Kalchhauser
Hermann
H
Kählig
Hanspeter
H
Hofer
Otmar
O
Vajrodaya
Srumya
S
Greger
Harald
H
D000470:Alkaloids / Q000737:chemistry* / Q000494:pharmacology*; D000818:Animals; D007306:Insecticides / Q000302:isolation & purification*; D007928:Lethal Dose 50; D009682:Magnetic Resonance Spectroscopy; D019684:Magnoliopsida / Q000737:chemistry* / Q000145:classification; D015394:Molecular Structure; D018515:Plant Leaves / Q000737:chemistry; D018517:Plant Roots / Q000737:chemistry; D018411:Spodoptera / Q000187:drug effects; D013329:Structure-Activity Relationship
D000470:Alkaloids; D007306:Insecticides