The naturally occurring sesquiterpene sorgolactone (2) belongs to the class of “strigolactones”, which are highly potent germination stimulants for seeds of the parasitic weeds Striga and Orobanche. The aim of the present work was to synthesize all eight stereoisomers of sorgolactone and to evaluate their activities in the stimulation of germination of S. hermontica and O. crenata. Two racemic diastereomers of the ABC part of sorgolactone, rac.10a and rac.10b respectively, were prepared and coupled with homochiral latent D-ring synthons 12 and ent.12. In this manner, four mixtures of two separable (protected) sorgolactone diastereomers were obtained. Deprotection gave all eight target compounds as single isomers. Bioassays revealed that only those isomers possessing the same stereochemistry as natural sorgolactone at two adjacent chiral centers exhibit high biological activities.

Striga and Orobanche seeds germinate in response to a host-derived germination stimulant. Dose-response curves of the synthetic strigolactone analogues GR 24 and Nijmegen 1 were determined, and their activities were compared to that of the naturally occurring stimulant sorgolactone. Typical sigmoidal curves were obtained. ED(50) values for GR 24 were in the order of 10(-)(9)-10(-)(8) mol/L; for Nijmegen 1 these values were 3 orders of magnitude higher. Both synthetic stimulants are appreciably active at low concentrations and merit investigation as agents for the suicidal germination approach (i.e., treatment of the soil with stimulant in the absence of a host).

The seeds of the parasitic weed Striga germinate in response to stimulants exuded by the roots of host plants and some nonhost plants. Literature data are summarized that support the view that strigolactones induce germination of parasitic weed seeds via a receptor-mediated mechanism. The suggestion by Lynn et al. that the strigol D-ring is solely responsible for germinating Striga seeds via a redox reaction was based on hypothesized structural similarities between the natural compound dihydrosorgoleone (SXSg) and the strigol D-ring. Experiments have shown that the mechanistic connection between SXSg and the strigol D-ring is not valid, and therefore the proposed redox mechanism for the induction of Striga seed germination by strigolactones does not hold.

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Strigolactones are highly potent germination stimulants for seeds of the parasitic weeds Striga and Orobanche spp. The induction of seed germination is thought to proceed via a receptor-mediated mechanism. Isolation and purification of the strigolactone receptor by affinity chromatography using immobilized avidin or streptavidin requires a biotin labelled strigolactone analogue. For this purpose biotin has been attached, directly as well as indirectly, via a hydrophilic linker to the amino function of optically active amino-GR24. Using the same amino substituted synthetic stimulant GR24, labelled stimulants have been prepared which may be suitable for the identification of the receptor by means of fluorescence correlation spectroscopy, scanning force microscopy or photoaffinity techniques. Bioassays of the labelled stimulants reveal that the germination activity on seeds of Striga hermonthica is retained. Crystal data for the diastereoisomer (+)-8 are reported.