Synthesis and Herbicidal Activity of Modified Monoterpenes Structurally Similar to Cinmethylini
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Weed Science. 1996. V61ume 44:7-1 I Synthesis and Herbicidal Activity of Modified Monoterpenes Structurally Similar to CinmethylinI STEVEN F. VAUGHN and GAYLAND F. SPENCER2 Abstract. The preemergence herbicide cinmethylin is a benzyl use as herbicides. A group of natural products most successfully ether derivative of the monoterpene l,4-cineole. Other oxy adapted for use have been terpenoids (4, 5).. The soil-applied genated monoterpenes (carvone, citronellol, fenchone, gera preemergence herbicide cinmethylin is a benzyl ether derivative niol, and pulegone) were previously found to inhibit the ofthe monoterpene 1A-cineole (Figure 1). Cinmethylin is active germination and growth ofseveral weed species while exhib primarily against annual grass weeds and some small-seeded iting low phytotoxicity to soybean. Benzyl ether derivatives broadleaf weeds in several crops (9). Although a specific mode of these oxygenated monoterpenes were synthesized and ex of action for the compound has not been elucidated, the mecha amined for preemergence and postemergence activity to nism of action of cinmethylin appears to result from the inhibi wards corn, soybean, wheat, and velvetleaf. Benzyl pulegyl tion of mitosis in meristematic regions of susceptible plants (6, etherexhibited the most preemergence activity when applied 9). directly to the soil, completely inhibiting wheat and velvetleaf Monoterpenes containing several different functional groups emergence at 1.0 kg ha-1 while reducing soybean emergence were shown to inhibit the growth ofthe green alga ChIarella (8), 80%. Several of the benzyl ethers were more inhibitory to potato (Solanum tuberosum L.) tuber sprouting (15) and crop and velvetleaf radicle elongation relative to chmethylin but less weed seed germination (1, 2, 3, 7,10,11,14,16). We found that inhibitory to corn and wheat radicle elongation in petri dish several monoterpenes cpntaining oxygen functions were espe bioassays. Several of the benzyl ethers exhibited limited cially inhibitory to crop and weed species. These included postemergence activity when applied at 1.0 kg ha-1 to seed monoterpenes with ketone (carvone, fenchone, pulegone) and lings ofthe testspecies 10 d afteremergence. The benzyl ether alcohol (citronellol, geraniol) functional groups (16; Figure 1). derivatives demonstrated altered selectivity and sensitivity The germination and growth ofseveral weed species were inhib as compared to the parent compounds and cinmethylin. ited by these monoterpenes, while soybean was not. However, Nomenclature. Cinmethylin, exo-l-methyl-4-(1-methyl these compounds are highly volatile and likely would be rapidly ethyl)-2-[(2-methylphenyl)methoxy]-7-oxabicyclo[2.2.l] lost unless immediately mixed into the soil by tillage or irriga heptane; carvone, 2-methyl-5-(1-methylethenyl)-2-cyclo tion. Because cinmethylin is a benzyl ether derivative of 1,4 hexene-l-one; citronellol, 3,7-dimethyl-6-octen-l-ol; cineole and has a volatility several orders ofmagnitude less than fenchone, 1,3,3-trimethylbicyclo[2.2.l]heptan-2-one; gera its parent compound, the conversion of the parent monoterpenes niol, 3,7-dimethyl-2,6-octadien-l-ol; pulegone, 5-methyl-2 to their respective benzyl ethers should decrease volatility. The (l-methylethylidene)cyclohexanone; benzyl carvyl ether, parent monoterpenes are readily available commercially by ex 1-(phenylmethoxy)-2-methyl-5(1-methylethenyl)-2-cyclo traction from plant sources or chemical synthesis, and generally hexene; benzyl citronellyl ether, 1-(phenylmethoxy)-3,7-di have low mammalian toxicities. To assess the potential of these methyl-6-octene; benzyl fenchyl ether, 2-(phenylmethoxy) 1,3,3-trimethylbicyclo[2.2.l]heptane; benzyl geranyl ether, l-(phenylmethoxy)-3,7-dimethyl-2,6-octadiene; benzyl pu legyl ether, 1-(phenylmethoxy)-5-methyl-2-(1-methyl ethylidene)cyclohexane; velvetIeaf, Ablltiloll theophrasti Medicus #3 ABUTH; corn, Zea mays L. 'Dekalb IL645-786'; 0 soybeans, Glycille max (L.) Merr. 'Williams'; wheat, Triticu11l aestivll11l L. 'Cardinal.' Additional index words. Terpenoids, growth inhibition. t:9: Geraniol Carvone Fenchone INTRODUCTION Plants produce a myriad ofchemicals that have only recently begun to be closely explored for use as pesticides, especially for I Received for publication January 4. 1995, and in revised form May 30, 1995. 2Plant PhysioI. and former Res. Chern. (retired), respectively, Bioactive Constituents Res., USDA-ARS. Nat. Ctr. for Agric. Utilization Res., 1815 N. Cltronellol Pulegone University St., Peoria. IL 61604. Cinmethylln 3Letters following this symbol are a WSSA-approved computer code from Composite List of Weeds. Revised, 1989. Available from WSSA. 1508 West Figure I. Chemical structures ofcinmethylin and parent monoterpenes ofbenzyl University Ave., Champaign. IL 61801-3133. ethers. 7 VAUGHN AND SPENCER: \lODIFIED MONOTERPENES STRUCTURALLY SIMILAR TO CINMETHYLlN compounds as herbicides. we synthesized benzyl ether analogues 5890 gas chromatograph equipped with a flame ionization de of the compounds found to be most inhibitory in our previous tector and a 15 m DB-II column. temperature programmed from work (16). i.e.. carvone. citronella!. fenchone. geraniol. and 100 to 200 Cat 5°/min. I Hand I3C NMR spectra \vere measured pulegone. In this paper we describe their synthesis and report on in CDCh by a Brucker WM-300 spectrometer. Samples were their herbicidal activity. introduced through a Hewlett-Packard 5890 gas chromatograph and mass spectra \vere produced on a Hewlett-Packard 5971 Mass Selective Detector. MATERIALS AND METHODS Bioassays. Soybean. corn, wheat, and velvetleaf were used as Chemicals. Because our synthesis procedure required alcohols. bioassay species. To assess preemergence, soil-applied activity. the monoterpene ketones were converted to their corresponding five seeds of com and soybean or ten seeds of wheat and alcohols (i.e.• carvone/carveol: fenchonelfenchyl alcohol: pule velvetleaf were placed in 170 ml cups filled with 50 g of a gonelpulegyl alcohol). All compounds used were technical grade mixture of 20% coarse sand and 80% Redi-Earth"" (supple and were obtained from a commercial source-+ except for pulegyI mented with Osmocote"" and Micromax"")8 and watered to field alcohol, which was produced by the reduction of pulegone as capacity. The test compounds were added by thoroughly mixing follows: pulegone (50 g) was dissolved in 300 ml of MeOH. an additional 20 g of the soil mixture with 5 ml of solutions NaBH-+ (25 g) was added slowly to the mixture while stirring in containing the test compounds and 0.1 % (v/v) Tween-20 (neces an ice bath. The solution was allowed to come to room tempera sary as an emulsifying agent due to the low water solubilities of ture and stirring was continued overnight. The bath was re the ethers and cinmethylin) at rates equivalent to either 0.1 or 1.0 charged with ice and 6 N HCl was added very slowly to the kg ai ha-I (cinmethylin has recommended rates of 0.56-1.68 kg solution (this prevented the solution from overheating as the ai ha-I (17». This treated soil mixture was placed in the cups and reaction was highly exothermic) until it \vas acidic. Distilled 10 ml of water was added to activate the compounds. The cups water (500 ml) was then added to the solution. and the resultant were placed in a growth chamber with a 25/17 C (day/night) mixture containing pulegyl alcohol was extracted three times temperature regime with a 16-h day and 95% R.H. The high with ethyl ether (200 ml each). The ether extract was concen humidity prevented the soil mixture from drying. Emerged seed trated under N2 and purified by column chromatography over lings (~ I cm above the soil surface) were counted after 10 d and silica gel-+ (70 to 230 mesh) with hexane or hexane:ethyl acetate expressed as a percentage of nontreated controls. (25: 1) as the eluting solvents, with approximately 80% yield The effect of the ethers on radicle elongation was studied (reduction of pulegone to pulegyl alcohol). Each monoterpene using a standard petri dish bioassay. Seeds were surface sterilized alcohol used for synthesis was analyzed at> 99% purity by gas with 0.5% NaOCI for 15 min, then rinsed twice with distilled chromatography-mass spectroscopy (GC-MS)5 and nuclear water. Seeds were wrapped in sterilized paper towels saturated magnetic resonance spectroscopy (NMR). with 10-4 M CaCI . After 24 (wheat) or 48 (velvetleaf, corn, Preparation of benzyl ethers. Benzyl ethers of monoterpene 2 soybean) h, five (com. soybean) or 10 (wheat, velvetleaf) seeds alcohols were prepared as follows: a monoterpene alcohol (50 g) were placed in 9 cm petri dishes containing a Whatman No. I and powdered KOH (100 g) were added to 200 ml of benzyl filter paper disk containing 2.0 ml of 0.1 % Tween-20 (controls) chloride and heated (70 C) with stirring for 3 h. Distilled water or solutions ofthe emulsified ethers (0.01 and 0.1 %, v/v) in 0.1 % (500 ml) was added, and the organic phase of the resulting (v/v) Tween-20. Plates were placed in darkness in a growth bipartite mixture was extracted three times in a separatory funnel chamber at 25 C for 4 d, at which time radicle lengths were with ethyl acetate (200 ml aliquots). The ethyl acetate fraction measured. was separated from the aqueous phase and concentrated by To examine postemergence activity, the four test species were rotoevaporation6, and the resultant ethers (> 80% of the total fraction) were purified (> 99%) by column chromatography on grown in the same growth medium as in the preemergence (a) Sephadex LH-20 eluted with methanol and then on (b) silica studies in a growth chamber with a 16-h photoperiod of 350 I-lE 2 gel (70 to 200 mesh) eluted with hexane:ethyl acetate (25: I). m- S-I PPFD. The temperature was held constant at 25 C with a Progress of the chromatography (i.e., isolation and purification relative humidity of 80%, with plants watered as needed.