Witchweed, germination

Another useful reagent for alcohols is Pirkle s (-)-(/ )-l-(l-naphthalenyl)ethyl isocyanate (14)161. For example, it was used to separate the two diastereomeric carbamates obtained from rue-13. The carbamate shown by X-ray analysis to have structure 15 was cleaved162 to furnish (+ )-strigol (13), the witchweed germination factor, thus confirming the absolute configuration of 13163. 

Fischer, N.H., Weidenhamer, J.D., Bradow, J.M. Dihydroparthenolide and other sesquiterpene lactones stimulate witchweed germination. Phytochemistry 1989 28 2315-2317. 

Dihydroparthenolide and other sesquiterpene lactones stimulate witchweed germination. Phytochemistry 28, 2315-2317... 

Menelaou, M. A. 1990. Stimulation of witchweed germination by sesquiterpene lactones a structure-activity study. Phytochemistry 29, 2479-2483... 

Egley, G. H. and Dale, J. E. 1970. Ethylene, 2-chloroethyl phosphonic acid, and witchweed germination. Weed Sci. 18, 586-589... 

In addition, 5 has the -OH at the 4-position and a methyl group at the 1-position of the A-ring. Kendall s compound has more structural similarity to strigol than 3, yet its activity is only about 22 that of strigol for witchweed germination. 

The ability of viable witchweed seed to remain dormant in the soil for many years, only to germinate when favorable conditions prevail, makes eradication difficult (2). Usually, seed will not germinate unless pretreated in a warm, moist environment for several days before exposure to a chemical exuded from the roots of a host plant or some non-host plants (4J. An active chemical in the root... 

The compound (I) was given the trivial name strigol and it has been shown to be a very potent witchweed seed germination stimulant. 

The synthetic studies which have been described have resulted in significant improvements in the preparation of racemic strigol and have also provided access to several analogs which will subsequently be tested for seed germination activity in order to elucidate key structure-activity relationships. These results and further investigations will hopefully lead to effective synthetic compounds for the control of witchweed and related parasitic plants. 

Strigol is a natural product with unusually strong properties as a germination stimulant for parasitic weeds of the genera Striga (witchweed) and Orobanche (broomrape). 

The germination stimulant or stimulants from host plants have not yet been identified, but research on isolation and identification of these allelopathic compounds continues. Other nonhost plants, such as cotton, also release chemicals which stimulate the germination of witchweed seed and these crops can replace the cereal crops in witchweed-infected fields. If no acceptable host is present, the witchweed plant is unable to mature and produce seed. The importance of cereal crops as a staple food in underdeveloped countries makes growth of nonhost crops only partially acceptable, and there are numerous wild hosts that allow the witchweed to germinate, mature, and produce more seed (several thousand seeds can be produced by a single plant). Nevertheless, application of either natural or synthetic stimulants in the absence of a host plant is an effective way of reducing and eventually eliminating the witchweed problem. 

Most of the work on correlation of molecular structure with bioactivity in witchweed seed germination has been produced by two groups (7-10). Johnson, et al. (7-9) prepared and evaluated a large number of strigol analogs and many approached the activity of strigol. In many studies by others, the results of the bioassays are presented, but the compounds from Johnson are described only by GR-number. GR-7 and -24, probably the more promising of these compounds, have been used in extensive field studies, and their structures are known. 

R. Brown and coworkers observed that seeds of Striga, known as witchweed, a semi-parasitic plant of grasses and corn, germinated only in the presence of root exudates from the living host. They isolated the sirupy exudate, [VJ > + 20° (water), resembling a pentose, but did not specifically identify the active substance. A study of many carbohydrates, mostly pentoses, showed that only D-ihreo-pentulose, C 3) — 32° (water), had the required activity L-[Pg.416]

Hsiao, A. I., Warshem, A. D., and Moreland, D. E. 1988. Effect of chemicals often regarded as germination stimulants on seed conditioning and germination of witchweed (Striga asiatica). Ann. Bot. 62, 17-24... 

Eplee, R.E. 1975. Ethylene a witchweed seed germination stimulant. Weed Sci. 23, 433-436... 

Formation of the 3j8-acetoxyeti-5-enic esters has been used to obtain optically pure samples of (+)- and (—)-tran5-verbenoP and to resolve an alcohol intermediate in the synthesis of the witchweed seed germination stimulant (+)-strigol. A general synthesis of thiol esters from carboxylic acids, exemplified by the formation of the n-propylthio-, isopropylthio-, and t-butylthio-esters of cholic acid, comprises reaction with diethyl chlorophosphate-triethylamine, followed by the thallium(i) salt of the appropriate thiol. 

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