Abstract
Alectrol was first isolated from root exudates of cowpea (Vigna unguiculata), a genuine host of a root parasitic weed Striga gesnerioides, as a germination stimulant for seeds of the parasite. The proposed structure, an isomer of strigol, was disproven by chemical synthesis. Recently, another structure, namely orobanchyl acetate, was proposed. Surprisingly, however, the synthetic compound having this proposed structure for alectrol was not active in inducing germination of S. gesnerioides seeds although it was active toward seeds of other root parasitic weeds such as S. hermonthica and Orobanche minor. Detailed studies on 1H NMR, mass and CD spectra of naturally occurring alectrol, re-isolated from cowpea root exudates, revealed that the genuine structure of the germination stimulant is not orobanchyl acetate but its stereoisomer ent-2′-epi-orobanchyl acetate. Accordingly, the structure of natural orobanchol was revised to ent-2′-epi-orobanchol 12 years after a tentative structure of orobanchol was proposed. Strict stereochemical requirements of strigolactones for germination induction of S. gesnerioides seeds, authentic samples of synthetic strigolactones and advanced analytical instruments made the structural assignment possible, thus ending a 20 years controversy concerning the true structure of alectrol.
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Acknowledgments
We thank Emeritus professor Kenji Mori (The University of Tokyo, Japan) for the generous gifts of the synthetic orobanchol isomers, orobanchol (9), 2′-epi-orobanchol (2′-epi-9), 4-epi-orobanchol (4-epi-9) and 4,2′-bisepi-orobanchol (4,2′-bisepi-9). The work was supported, in part, by grants from the Asia Africa Science Platform Program of the Japan Society for the Promotion of Science, JST/JICA, Science and Technology Research Partnership for Sustainable Development (SATREPS), and Grants-in-Aid for Scientific Research from the Ministry of Education, Science, and Culture of Japan (Nos. 11J01777, 23405023, 24658111 and 25292065).
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Ueno, K., Sugimoto, Y. & Zwanenburg, B. The genuine structure of alectrol: end of a long controversy. Phytochem Rev 14, 835–847 (2015). https://doi.org/10.1007/s11101-014-9380-2
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DOI: https://doi.org/10.1007/s11101-014-9380-2