Natural Products and Their Potential in Agriculture - American

Chapter 1

Natural Products and Their Potential in Agriculture A Personal Overview

Downloaded by GEORGETOWN UNIV on September 17, 2013 | http://pubs.acs.org Publication Date: November 28, 1988 | doi: 10.1021/bk-1988-0380.ch001

Horace G. Cutler Richard B. Russell Research Center, Agricultural Research Service, U.S. Department of Agriculture, Athens, GA 30613

Biologically active natural products are derived from three major sources: the fermentation of microorganisms, higher plants, and insects. However, compounds derived from these sources may act within each or all of these domains. That is, compounds derived from microorganisms and higher plants may affect insects and vice versa. Often, natural products are obtained in limited quantities and small yields do not lend themselves to extensive testing. During the past three years there has been increased synthesis of natural product templates and their analogs for evaluation in biological systems. Relative to these approaches, assorted natural products from microorganisms, including oligopeptides, acyclic polyketols and some relatively simple molecules are examined. The brassinosteroids and photodynamic herbicides, from higher plants, are discussed. Finally, compounds that are produced by insects, or which affect insects, are reviewed. In common with my primal ancestors, whose l i f e began i n a garden, my f i r s t recollections were not so much of people but of trees, flowers, the sun, rain, clouds, blue sky, and the sound of the cuckoo. Especially imprinted on my senses was the peppery smell of lupin at the early age of two, and there followed the scent of roses and English lavender: dire warnings about foxglove and deadly nightshade were issued as I wandered about gardens. An introduction to the world of secondary metabolites had started early i n l i f e and subsequently led to my f i r s t s c i e n t i f i c job, i n the mid 1950*s, at the Boyce Thompson Institute for Plant Research when i t was located i n Yonkers, New York. While at the Institute, I came under the i n t e l l e c t u a l guidance of Lawrence J . King, who was something of a genius, and that led me into the area of plant growth regulators, especially the natural product of both This chapter not subject to U.S. copyright Published 1988 American Chemical Society

In Biologically Active Natural Products; Cutler, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


2

BIOLOGICALLY ACTIVE NATURAL PRODUCTS

animal and plant o r i g i n , indole-3-acetic acid. There followed another period, at the same location, with A. J . "Chuck" V l i t o s who was investigating flowering i n plants, indole chemistry, and the growth promoting effects of long chain fatty alcohols i n collaboration with Donald G. Crosby (a topic that was quite revolutionary at the time). This pioneering atmosphere was further enlivened by I.D.J. "Dai" P h i l l i p s who was an exchange pre-doctoral student from P.F. Wareing's laboratory i n Aberystwyth, Wales. His exuberance for l i f e , and natural products of growth regulatory i l k , was contagious. He was hot on the t r a i l of the then structurally unknown substance from Acer pseudoplatanus (and cotton b o l l s ) , abscisic acid. Contemporaneously, the team of P.W. Zimmerman and A.A. Hitchcock had synthesized the herbicide 2,4-dichlorophenoxyacetic acid at Boyce Thompson and, as they stated, that synthesis was the result of their examining the model of indole-3-acetic acid. The former being a substituted phenyl, the l a t t e r a phenyl-pyrrole. I also married one of their technicians and, I suppose, my thoughts concerning natural products for synthetic templates became subconsciously fixed as I came to know them better. A three year s t i n t i n the West Indies (Trinidad) led me into the world of bush medicine (the use of t r o p i c a l plants for medicinals) and took me down that curious path of ethnobotany. The range of useful plants was extraordinary. There were plants i n the genus Verbena that induced increased l a c tation i n nursing mothers, grasses that broke fevers, and the bark of a tree which, when steeped i n b o i l i n g water, gave r i s e to a tea that caused erections of some duration i n males. The trees yielding this compound were obvious even to the untrained eye because near the l o c a l v i l l a g e s the bark was generally stripped from ground level to the height which an adult could reach standing on the seat of a bicycle. There are several yarns surrounding the sexual efficacy of the bark, some apocryphal, but a l l amusing. The t h i r d part of the triology, insofar as the science i s concerned, involved my training i n the biochemistry of nematodes with Lorin Krusberg, at the University of Maryland. I t happened that one of the nematodes under scrutiny, Ditylenchus t r i f o r m i s , could be cultured on the fungus Pyrenochaeta t e r r e s t r i s . Three flasks of that fungal substrate yielded more indole-3-acetic acid than I had seen from the extraction of one-quarter acre of sugarcane, and convinced me that fungi would be excellent sources of b i o l o g i c a l l y active compounds. My psychological compass was, once again, fixed on a natural product course. Hence, my interests and training cut across several f i e l d s and at some point the thought c r y s t a l l i z e d that apparently divergent d i s c i p l i n e s were inter-related. There are several temptations that confront the author of a chapter of this nature. One such i s the propensity to i n t e l l e c t u a l l y bludgeon anyone who dares to insinuate that natural products have no r e a l use i n agriculture either as agrochemicals, or as templates for the further synthesis of biodegradable agrochemicals, by quoting a series of products


1. CUTLER

3

Natural Products in Agriculture

that have been used successfully. Among them would be indole butyric acid, a homolog of the o r i g i n a l compound, indole-3acetic acid, that f i r s t aroused the curiosity of plant physiologists because i t was shown to be responsible for the phototropic response of plants. And indole-3-butyric acid has been used by amateur gardeners and h o r t i c u l t u r a l i s t s to induce rooting i n plant cuttings. Another well known natural product i s g i b b e r e l l i c acid (GA ) which, again, has high specific a c t i v i t y and limited use i n grapes and celery where the responses are quite dramat i c . We tend to forget that t h i r t y - f i v e years ago i t was a common h o r t i c u l t u r a l practice to g i r d l e grapes by cutting i n to the bark of the vines j u s t above ground level i n order to increase y i e l d s . This was a delicate procedure and often resulted i n the death of the mature vines. With the advent of GA- the art of g i r d l i n g disappeared and was replaced by spraying vines at flowering time with 100 ppm GA~ solutions to increase yields 250%! The natural product ethylene, found i n fungi and higher plants, has been used i n both the pure form and as a derivatized chemical (2-chloroethylphosphonic acid)to ripen a variety of crops from bananas to cherries, to oranges. The chemistry of the insecticide pyrethrin, the natural product of the Pyrethrum daisy consisting of two viscous l i q u i d esters 2i 28°3' 22 28°5' known and i t was precisely these mSlecules that l a i d the foundation for the synthesis of the highly successful pyrethroids. Various natural lures (pheromones) have also played a decisive part i n controlling insects. Some of the most interesting i n s e c t i c i d a l and antiparasit i c natural products to enter the f i e l d have been the avermect i n s . Their history i s unique because of their complexity, the fact that the fermentation products go through a synthet i c sequence to arrive at the f i n a l products and their duality as both agrochemicals and pharmaceuticals. Certainly, the average Board of Directors would l i k e l y be i n i t i a l l y negative to developing such elaborate molecules. Another temptation would be to attempt to convince the reader that i n the three areas covered by the symposium — natural products from microorganisms, secondary metabolites from higher plants, and natural products from insects or that affect insects — there are myriad examples of secondary metabolites that have been isolated that have b i o l o g i c a l a c t i v i t y and considerable potential as agrochemicals. However, space does not allow for that discussion and further elaboration would also detract from the contributions made by the other authors i n t h i s volume. Instead, I have chosen a different approach, which i s to discuss compounds that have been isolated i n each of these three categories, have been shown to possess unique b i o l o g i c a l a c t i v i t y , and have then been synthesized. The reason for this approach i s that i n these days the burden of proof for a structure no longer necessarily requires that a compound be synthesized. The advent of sophisticated NMR techniques and X-ray c r y s t a l l o -


3

C

H

o r

C

H

i s

a

l

s

o

w e l J



4


graphy have v i r t u a l l y eliminated that necessity. Generally, the need for synthesis arises because, i n many cases, there i s i n s u f f i c i e n t quantity of the metabolite to make extensive examination of i t s properties i n b i o l o g i c a l systems. With respect to products from microbes, f o r example, the speed with which a fermentation system can be cranked up to produce large quantities of a derived metabolite i s d i r e c t l y proportional to the number of substrates on which the organism can be grown, the number of people available to do the work, and the access of a high yielding strain. For those who have forgotten, the history of the discovery and production of penic i l l i n bears r e c a l l i n g . And the recent synthesis of avermect i n A. by Danishefsky et a l . , (1) lends credence to this point of view even though, at present, the avermectins are produced by fermentation of Streptomyces a v e r m i t i l i s . F i n a l l y , doing chemistry for the sake of doing chemistry has been relegated to limited situations. Funding for such ventures i s not r e a l l y feasible or available i n today's management climate so that when funds are forthcoming they are made so for very s p e c i f i c reasons, and commercial ones at that. The examples given are neither a l l that exist, nor do they necessarily represent those that w i l l eventually find their way into the marketplace. They are, however, diverse structures with specific a c t i v i t y and, i n some instances, their synthesis has been improved upon. Most of the examples represent work that has taken place during the past three years, a time during which the synthesis of natural products seems to be going through an exponential stage. Some have a certain dichotomy or polychotomy i n that they are active i n more than one system. That appears to be the character of b i o l o g i c a l l y active natural products and, consequently, the reader may not be i n t o t a l agreement as to their categorization. Synthesis of compounds based on templates from microorganisms E a r l i e r , we reported that the c y c l i c oligopeptides were compounds that had attracted much attention from synthetic chemi s t s because of their high specific a c t i v i t y and their assortment of unnatural amino acids, both L and D (2,3,4). Their usefulness as agrochemicals ranges from direct to indirect. For example, the i t u r i n s , which are c y c l i c octapeptides, may be used to control soft rot, Monilinia f r u c t i c o l a , i n stored peaches (5); the fragments of tentoxins, a c y c l i c tetrapeptide, show both plant growth promotory and herbicidal propert i e s (see Edwards, et a l . ; Lax, et a l . , i n this volume). The AM toxins I, I I , III from Alternaria mali, a pathogen of apple trees, of which AM toxins I and III are particularly potent and produce interveinal necrosis i n the susceptible c u l t i v a r "Indo" within 18 hours following treatments with 0.1 ppb (6,7), are p a r t i c u l a r l y useful tools for determining the mechanisms whereby these compounds act. Hence, the question might be posed: why are apple varieties l i k e "Indo" highly susceptible to A. mali toxins while resistant varieties l i k e "Jonathan" require 1 ppm of AM toxin I and 10 ppm of AM toxin III to induce necrosis? Once the s i t e , or s i t e s , of a c t i v i t y



1. CUTLER


5

are known i n the susceptible cultivars there exists the poss i b i l i t y of protecting these vulnerable l o c i from the action of the toxin(s). With regard to the s e l e c t i v i t y of these peptide toxins, much structure-activity work has been done with AM toxin I (Figure 1). F i r s t , i t has been determined that the toxin response i s d i r e c t l y dependent upon the backbone conformation of the amino acids, which are a l l L, and that the presence of L-a-hydroxyisovaleric acid, a-aminoa c r y l i c acid, L-a-amino-8 -(p-methoxyphenyl)-valeric acid are important, while the presence of L-alanine i s not c r i t i cal (8,9,10,11,12). Furthermore, synthesis of the retroenantio - AM toxin I i n which the peptide sequence was reversed and, thereby, the configuration of each residue, revealed, upon bioassay with "Indo" apple leaves, that a l l b i o l o g i c a l a c t i v i t y was deleted. In an attempt to ascertain whether there was a s p e c i f i c receptor i n apple leaves the enantio - AM toxin I, an antipode of that toxin, was synthesized. That i s , the amino acid sequence was identical to the o r i g i n a l toxin but they were a l l D amino acids. I t was postulated that i f the enantio-toxin was active i n the bioassay the interaction between AM toxin I and the membrane of the apple leaf c e l l i s not a biological one, but rather a physicochemical one between, say, a peptide and a l i p i d . However, no b i o l o g i c a l responses were noted with concentrations up to 100 ppm. This i s several orders of magnitude above the threshold amount necessary to produce a b i o l o g i c a l response with AM toxin I. Therefore, i t appears that the enantiotoxin did not interact with the receptor s i t e and i t suggests that the recptor recognizes the c h i r a l i t y of the molecule and that the receptor may be a protein. No doubt the characterization of the active s i t e and the subsequent manipulations that must follow w i l l have a major impact on the apple growing industry. Certainly, the oligopeptides obtained from microorganisms offer a wealth of active products both i n their o r i g i n a l and degraded states. There i s a wide range of synthetic permutations and the offering of templates on which to base the production of novel herbicides. Two examples are proffered. The f i r s t i s the well known compound glyphosate (N-[phosphonomethyl] glycine), an eminently successful herbicide. The second involves some research carried out by the Tanabe Seiyaku Co., Ltd., Japan, which involves both their Applied Biochemistry and Microbiological Research Laboratories. They i n i t i a l l y found that amino acid analogs, the ot-isocyanoacetic acid derivatives, were potent seed germination i n h i b i tors (13,14), and had similar properties i n assays as 2,4dicholophenoxyacetic acid. I t i s significant that the authors state, "We had great interest i n peptide compounds containing the isocyano group". But of the six homologs of the N-(a-isocyanoacetic) amino acid methyl esters synthesized, none were as active as N-isocyanoacetyl-L-valine methyl ester (Figure 2) which inhibited the stems and roots of germinating cucumber seed 80-100% at 100 and 10 ppm, and inhibited the shoot germination of r i c e , but not the roots,



6


at 100 ppm. There was no effect on radish germination, indicating s e l e c t i v i t y . It should be noted that D or L - v a l i n e i s an amino acid that i s often found i n peptidic, phytoactive natural products. Small molecules isolated from natural sources nearly always appeal to the synthetic chemist. The pyrenochaetic acids A, B, and C (Figure 3) are i n t h i s class and have been shown to be phytotoxins. They originate i n the pathogen Pyrenochaeta t e r r e s t r i s which i s responsible for onion pink root and are readily produced by fermentation. The most phytotoxic of t h i s t r i o i s pyrenochaetic acid A which completely inhibited the root growth of onion, at 250 ppm, and lettuce at 500 ppm. Because of the r e l a t i v e l y greater phytot o x i c i t y , pyrenochaetic acid A was synthesized and proved to be as active as the natural product (15). In addition, the regio- isomer of pyrenochaetic acid A was produced (Figure 4) and i t inhibited lettuce root growth 80% at 500 ppm. No data were given for the effects on onion root growth and comparison of the data for b i o l o g i c a l a c t i v i t y versus structure were tantalizingly b r i e f . However, the synthesis of pyrenochaetic acid A led to the synthesis of the regio-isomer and t h i s , i n turn, may lead to the production of other b i o l o g i c a l l y active molecules. Indeed, the functional groups are present to make some pertinent derivatives. Nothing i s more frustrating to the natural products chemi s t than to read of the synthesis of a b i o l o g i c a l l y active compound that i s i d e n t i c a l to the o r i g i n a l natural metabolite and to then find that neither the precursors, or f i n a l product, or derivatives of the product have been tested i n b i o l o g i c a l systems. Such i s the case with the synthesis of p y r i c u l o l (Figure 5) isolated from the culture broth of P y r i cularia oryzae, the organism responsible for r i c e blast. Pyriculol i n h i b i t s the growth of r i c e seedlings and induces necrotic lesions on the leaves. Four stereoisomers of p y r i c u l o l were synthesized, one of them was i d e n t i c a l to the natural product, that i s , 3'R, 4'S (16). Another synthesis that yielded derivatives that would predictably possess biol o g i c a l a c t i v i t y was that of (S)-(-)-vertinolide (Figure 6), a tetronic acid derivative obtained from the culture broth of VerticiIlium intertextum, a fungus isolated from wilted Japanese maple trees (17). While the o r i g i n a l chemical structure was solved by X-ray crystallography (18), the absolute configuration had to await confirmation by synthesis. But again, b i o l o g i c a l data are lacking. Similar events surround the synthesis of gregatin B (Figure 7), a phytotoxic metabolite of Cephalosporium gregatum and Aspergillus panamensis (19). In contrast, pyrenolide B (Figure 8), a ten-membered lactone ring, was o r i g i n a l l y isolated from the culture broth of the phytopathogen, Pyrenophora teres i n conjunction with pyrenolides A and C (20,21). The preparation of (±)-pyrenolide B involved several synthetic intermediates and, of these, seven were tested against the fungi Aspergillus niger Cochliobolus miyabeanus and the yeast, Saccharomyces cerevesiae (22). Both A. niger and C. miyabeanus were s i g n i f i c a n t l y inhibited by f


CUTLER


HC

CH

3

3

CH


H CH 0-< 3

>-

v

°*

C

yvy°

CH -CH -CH m»C^H 2

2

H-C»'i»CH

2

o * > c - C H

Figure

1. H3C

II

3

H

O

AM Toxin 1. ^/CH

3

H CH I I CNCHCONHCHCOOCH3

Figure

2.

N-isocyanoacetyl-L-valine methyl ester. O CH O a

R HOOC

Figure

3.

Pyrenochaetic acid.

COOH

Figure

4.

Pyrenochaetic acid, regio-isomer.


8



OH

Figure

6.

Vertinolide. O

Figure

7.

Gregatin B.

O Figure

8.

Pyrenolide B.



1. CUTLER


9

(±) -pyrenolide B at 100jig/disk and a l l microorganisms were s l i g h t l y inhibited at 50ng/disk. But only one of the intermediates s l i g h t l y inhibited the fungi at 50*ig/disk and that intermediate was i d e n t i c a l to (_+)-pyrenolide B except for p a r t i a l saturation of the ring and the double bond remaining intact i n the dienone system. Also, (±)-pyrenolide B inhibited the growth of r i c e roots 4 0 % , and shoot 3 0 % , when applied at 100 ppm. Other toxins have also been synthesized and include race T toxin, a corn s p e c i f i c toxin o r i g i n a l l y isolated from the phytopathogen Helminthosporium maydis, which i s composed of an a c y c l i c S-polyketol with 3 5 - 4 5 carbons. To date, four constituents of C or C chain lengths which make up 70 to 90% of the natural toxin have been elucidated and each constituent has v i r t u a l l y the same b i o l o g i c a l a c t i v i t y as the native material. While shorter chain lengths of C-,- to C have been synthesized ( 2 3 , 2 4 ) and indicate thai the presence of a 8,S'-dioxooxy group and linking of two sets of ketol groups by a (CH ) bridge are essential f o r t o x i c i t y , as i n race T toxin, the increase i n chain length of carbon chain or increase i n the number of ketol groups had not been examined. The C~ chain length was the most effective of the set synthesized yet i t was ten times less active than the natural toxin. Consequently, a C stereoisomeric mixture of ( + ) - 8 , 1 6 , 2 6 , 3 4 - t e t r a h y d r o x y - 6 , 1 0 , 1 4 , 1 8 , 2 4 , 2 8 , 3 2 , 36-octaoxohentetracontane, containing four B-ketol groups spaced by ( 2 ^ 3 ^ ' 2 ^ 5 bridges (which occur i n race T toxin) was prepared. Also, a C analog with two of the 6-ketol groups spaced by a trimethylene bridge was made, that i s , (nh)-8,16-dihydroxy-6,1 0,14,18-tetraoxotricosane. When each synthetic product was bioassayed i n leaves of susceptible corn c u l t i v a r s i t was shown that the material, i n fact a mixture of 10 stereoisomers, stimulated NADH oxidation by mitochondria and inhibited dark C 0 f i x a t i o n as e f f e c t i v e l y as the native toxin. The C ^ product was approximately ten times less active than the native toxin i n both these systems, and was as active as the C product discussed e a r l i e r ( 2 5 ) . The same research group has also synthesized a stereoisomeric mixture of (_+)-PM toxin B o r i g i n a l l y found i n the plant pathogen, Phyllosticta maydis, which destroys corn that has Texas-male s t e r i l e cytoplasm. The toxin which i s primarily composed of PM-toxin B (6,14,22,30,32-pentahydroxy-8,16,24-trioxotritriacontane) i s toxic to the Texas-male s t e r i l e l i n e at 1 0 " to 10 M while corn with normal f e r t i l e cytoplasm i s unaffected with treatments of 10"" M. The mixture of stereoisomers comp r i s i n g PM-toxins B and having the syn-1 ,3-hydroxy configuration at C~Q and C~ was as s p e c i f i c i n toxic action to corn as the native toxin ( 2 6 ) . Two p o s s i b i l i t i e s exist for each set of compounds. F i r s t , they may have application i n other crops as herbicides but, as a minimum, the elucidation of the molecules that act as toxins i n corn suggests that blocking agents may be found to protect susceptible cultivars. 4 1

3 g

2 6

2

5

5

4 1

C H

a n c

C H

2 3

2

2 5

2



10


One argument posed by c r i t i c s for using natural products in agriculture concerns abscisic acid, the compound f i r s t isolated from cotton b o l l s , Gossypium hirsutum L., (27) and dormant Sycamore buds, Acer pseudoplatanus (28), which i s a growth inhibitor i n certain systems. To date, the compound has not been successfully used i n agriculture even though there i s ample evidence to support the fact that i t mediates several important events i n plant growth and development. I t has been synthesized and i s readily available from chemical supply houses. The 2-cis(+) form, which i s the b i o l o g i c a l l y active isomer, has been isolated from the fungi Cercospora cruenta (29), and Botrytis cinerea that has been irradiated with UV (30). Both the (2Z) and (2E)-deoxy-abscisic acid isomers were prepared (Figure 9) and tested against r i c e seedlings and lettuce seed. The (2Z) acid inhibited the growth of the seed leaf sheath of r i c e approximately 80% at 10_ M, but lettuce seed germination was inhibited 100% by 10" M solutions. The_12E) acid inhibited r i c e second leaf sheaths only 20% at 10 _M and lettuce seed germination was inhibited 100% at 5 X 10 M (31)• It would appear that, eventually, a u t i l i t a r i a n homolog of abscisic acid w i l l be found for use i n agriculture. 4

Synthesis of compounds based on templates from higher plants. I f i r s t saw movies of J.W. Mitchell's work with the brassinosteroids i n the late winter of 1963. At that time, i t had been observed that an elongation response could be obtained in the bean second internode test upon treatment with extracts of rape pollen (Brassica napus L.) and some years later a formal report followed (32). The Japanese had independently observed novel growth regulatory responses with extracts of Distylium racemosum Seib. et Zucc. and these were reported as Distylium factors A and B, which produced a dramatic response i n the r i c e lamina i n c l i n a t i o n test (33). However, HPLC developments had not been perfected and the work was temporarily set aside. The f i n a l characterization of the brassinosteroid structure appeared i n 1979, s p e c i f i c a l l y that of brassinolide from B. napus as (22R,23R)-2a-3a,22,23 -tetrahydroxy-24S-methyl-S-homo-7-oxa-5a-cholestan-6one (34) (Figure 10). But, as i s usually the case with b i o l o g i c a l l y active natural products, much work was accomplished between the i n i t i a l discovery of brassinolide and the f i n a l structure determination concerning i t s b i o l o g i c a l properties. "Brassins" from rape pollen enhanced the production of vegetation and f r u i t when applied to bean plants (35). Later, brassins were shown to accelerate the growth of barley plants i n greenhouse and growth chamber experiments when seed had been treated prior to sowing (36); and bean plants treated with brassinosteroids were shown to be affected by l i g h t quality which influenced the growth regulatory response (37). Further, i t was demonstrated that brassinolide affects very s p e c i f i c tissues that are sensitive to indole-3-acetic acid-induced growth and tissues that are gravi-perceptive (38). f



1. CUTLER


While many of the l a t e r studies appear to be a search for the mechanism of action of the brassinosteroids, the Japanese have intensively approached the problem from three angles. The i s o l a t i o n of new brassinosteroids, the synthesis of brassinosteroids and their analogs, and the p r a c t i c a l application of these materials for agrochemical use. Insofar as the i s o l a t i o n of new brassinosteroids i s concerned, two recent reviews already cover this topic i n d e t a i l (39, 40) the former complete with references, unfortunately, i n untranslated Japanese. Indeed, there presently e x i s t twenty-two new brassinosteroids and one glucosidic conjugate (39). What i s important i s the p r a c t i c a l application of these materials to crops. Both brassinolide and 24-epibrassinolide (Figure 11) have been used i n f i e l d t r i a l s to promote the growth of young plants, according to seven Japanese references, and crop yields have been improved (41) i n corn, cucumber, r i c e and sweet potato. Other applications have shown an increase i n cold resistance i n corn, cucumber, egg plant, and r i c e (42) . And decreased injury by the herbicide simetryn, butachlor, and p r e t i l a c h l o r has been noted i n treated r i c e , while wheat treated with simazine was more tolerant to that herbicide after brassinolide pre-treatment (42). Rice has been made less susceptible to salt following treatment (42). Another, as yet puzzling e f f e c t i s the a b i l i t y of brassinosteroids to enhance disease resistance i n Chinese cabbage to soft rot and i n r i c e to sheath blight (42). Some pictures of these effects were shown at the Joint Plant Growth Regulator Society of America - Japan Society for the Chemical Regulation of Plants by S. Marumo (39) i n 1987 and the f u l l impact of what these compounds may mean to agriculture became clear. The most remarkable s l i d e was that of a brassinolide treated ear of corn (Zea mays L.) i n which a l l the kernels had completely f i l l e d out to the extreme t i p . The control, on the other hand, exhibited the usual 1-2 inches of t o t a l l y immature kernels at the t i p . This potential t i p productivity represents as much as 5% of the t o t a l . Thus the potential economic impact i s readily apparent and the accelerated e f f o r t by the Japanese i n the area of synthesis i s easily understood! A l l brassinolide syntheses require as starting materials, natural sterols and their degraded products, stigmasterol, brassicasterol, pregnenolone, and dinorcholenic acid, (39). Improvements have been made on the synthesis of brassinolide (43) and other derivatives have been produced. In 1984, the synthesis of hexanor-brassinolide 22-esters was accomplished by German workers (44) and some were active i n the bean second internode bioassay including the methyl, ethyl, and n-propyl ethers. The t-butyl and ethyl methyl ethers were inactive (Figure 12) and none of these derivatives contained the C , C ~, or C~ asymmetric centers. Homodolichol i d e and homodolicnosterone (Figures 13 and 14), o r i g i n a l l y isolated from the immature seed of Dolichos lablab, are brassinosteroids that possess plant growth promoting properties. These, too, have been made (45) i n a series of short-step 2 2

2

4


11


COOH

(2Z)


Figure

9.

(2E) 2Z and 2E-deoxy-abscisic

Figure 11.

Figure 12.

acid.

24-epibrassinolide .

Hexanor-brassinolide

22-esters.



1.

CUTLER


13

syntheses. Recently, the f a c i l e synthesis of brassinolide, castasterone (Figure 15), teasterone (Figure 16), and typhasterol (Figure 17) has been accomplished using the key intermediate (22R,23R,24S)-3a,5-cyclo-22-23-diacetoxy5cc-ergostan-6-one (46)• F a i r l y complex syntheses have been used to modify the 7 membered B ring p f (22S,23S) and (22R,23R)- homobrassinolide to effect the production of the 7-aza, 7-thia, and 6-deoxo compounds (47) each of which exhibited weak biological a c t i v i t y i n the rice-lamina i n c l i n a t i o n assay. From the frenetic a c t i v i t y surrounding the brassinosteroids i t seems that a patented agrochemical may well be on the market within a few years and that product w i l l most probably be a Japanese one that has broad spectrum application. Another recent advance that has significance for the development of natural product agrochemicals are the photodynamic herbicides. While a recent comprehensive review has covered the topic (48), reference i s given i n this overview because of the implications that this new approach o f f e r s . Simply, the concept of photodynamic herbicides revolves around two principles. F i r s t , one of the vulnerable processes i s that of greening during plant growth and development. Second, 8aminolevulinic acid (Figure 18) i s a natural amino acid that can supply a l l the atoms necessary to build protoporphyrin. S p e c i f i c a l l y , two molecules of 8-aminolevulinic acid may condense to form porphobilinogen and four molecules of this contribute to porphyrin synthesis. Chlorophyll i s a magnesium-porphyrin derivative, a tetrapyrrole. I t was postulated that i f green plants could be chemically treated to induce large amounts of chlorophyll precursors, that i s , tetrapyrroles, then a mechanism for the production of biodegradable herbicides existed. The theory being that magnesiumtetrapyrroles are type II photosensitizers (49,50,51) and absorb l i g h t energy to photosensitize the formation of singlet oxygen. This, i n turn leads to a free radical chain reaction that destroys membranes, proteins, and nucleic acids (51). Preliminary experiments were conducted with 8-aminolev u l i n i c acid sprayed onto cucumber seedlings (Cucumis sativus L.) at rates of 525 gram/acre (48). The plants were covered with aluminum f o i l so that dark conversion of the 8-aminol e v u l i n i c acid could take place. Indeed, the experiments progressed as expected i n that 8-aminolevulinic acid was converted to magnesium-protoporphyrins and protochlorophyllides. Upon exposure to l i g h t , treated plants rapidly degraded and died within a few hours, whereas those kept i n the dark survived. It transpired that the choice of cucumber as a test plant was most serendipitous. When 8-aminolevul i n i c acid was tested against monocotyledonous plants such as barley, corn, oat, and wheat, the effects were negligible. Depending upon the species of plant treated, a Type I, I I , or III response was obtained. Type I plants were those that died rapidly after treatment, as did cucumber. Type II plants, such as soybean, accumulated tetrapyrroles i n leaves,






1.

15


CUTLER

but not cotyledons and stems and, upon exposure to l i g h t , there was photo-dynamic damage i n the leaves only and plants soon recovered. Type III plants, including barley, oat, corn, and wheat, accumulated high levels of tetrapyrroles but photodynamic damage was minimal and these plants survived. Further examination revealed that there was a multibranched chlorophyll a biosynthetic pathway consisting of six routes and the species-dependent photodynamic herbicide a c t i v i t y was d i r e c t l y dependent upon monocarboxylic routes 2 and 3, and 4 and 5 (48). I t became apparent that certain compounds may act as photodynamic herbicide modulators and, depending upon the modulator used, the action of 8-aminolevulic acid could be enhanced. The search for modulators uncovered 13 compounds that acted i n concert with 8-aminolevulinic acid and served to control the biosynthesis of chlorophyll a. These f e l l into three groups. The f i r s t group enhanced 8-aminolevulinic acid transformation to tetrapyrroles and were 2-pyridine aldoxime; 2-pyridine aldehyde; p i c o l i n i c acid; 2,2 -dipyridyl d i s u l f i d e , 2,2 -dipyridyl amine; 4,4'd i p y r i d y l , and phenanthridine. The second group were inducers of 8-aminolevulinic acid biosynthesis and subsequent tetrapyrrole accumulation and were comprised of 2,2 dipyridyl and 1,1O-phenanthroline. The f i n a l group inhibited monovinyl protochlorophyllide accumulation and these compounds were 2,3-dipyridyl; 2,4-dipyridyl; 1,7-phenanthroline; and 4,7-phenanthroline (48). In a fine piece of s c i e n t i f i c sleuthing, the authors put their findings to a p r a c t i c a l test i n a Kentucky bluegrass lawn that they had d i f f i c u l t y establishing because of the precocious growth of creeping charlie (Glechoma hederacea), common yellow wood sorrel (Oxalis s t r i c t a L.), blackseed plantain (Plantago r u g e l i i Dene), dandelion (Taraxacum o f f i c i n a l e Weber), v i o l e t (Viola adunea) and musk t h i s t l e (Carduus nutans). After placement of each species into i t s type, i t was decided that the best strategy would be to spray the plots with 8-aminolevulinic acid plus 2,2 -dipyridyl (524 gram of 8-aminolevulinic acid plus 403 gram 2,2'dipyridyl/acre). With the exception of v i o l e t the control of a l l broadleaved weeds was excellent and, of course, the dynamics of the v i o l e t plant are presently under scrutiny (48). The lawn i s becoming well established and the environment appears to be clean of any residual herbicides. 1

1

f

1

Synthesis of compounds based on templates from insects, or natural products that affect insects. Those compounds that have been isolated from insects, which possess b i o l o g i c a l a c t i v i t y and have subsequently been copied by chemical synthesis are, for the most part, the insect pheromones. Because this i s the topic of another recent symposium honoring J . Tumlinson on the occasion of the Burdick and Jackson Award, i t i s not covered here. Neither i s pyrethrin discussed even though i t has been used as a model upon which the highly successful pyrethroids have been developed. Instead, some recent development of compounds that affect insects are illustrated.



16


Much has been written about the effects of black pepper (Piper nigrum L . ) extracts on house f l i e s (52,53,54) especia l l y the compounds piperine, p e l l i t o r i n e , and pipericide. In a p a r t i c u l a r l y imaginative piece of work, Nair et a l . , (55) isolated Z and E-fadyenolide (Figure 19) from Piper fadyenii with a view to accomplishing two goals. To open the fadyenol i d e ring at the oxygen bridge, and to replace the exocyclic carbonyl with a nitrogen thereby making a piperine analog. To produce the pyrethrin analog: that i s , the structure would be almost i d e n t i c a l to the piperine analog except that adjacent to the methyoxyl group o r i g i n a l l y present on the ring of fadyenolide there would now be a cyclopropane function. In fact, during the syntheses three hybrid compounds were produced which were amides of 3,5-dimethyloxy-4-oxo-5-phenylpent-2-enoic acid (Figure 20). Tests were conducted on adult cockroaches (Blatella germanica L . ) and within 3 hours a l l the amides paralyzed the insects and within a day they were dead. Flour beetles (Tribolium confusum) were also challenged with amides A, B, and C and the LC was established as 30, 28, and 27 ppm, respectively. None of the products were active against cattle ticks (Boophilis microplus), but egg production appeared to be inhibited 50-70% with a p p l i cations of 0.5 and 1 .Ojig/tick. One major drawback to the use of these compounds seems to be their photodegradation, though depending upon the circumstances this characteristic may be considered an attribute. Other compounds of Piper species have served as templates for b i o l o g i c a l l y active derivatives. For example (56), using the parent compound (2E,4E)-N-isobutyl-6-phenyl-hexa-2,4-dienamide, seven derivatives were synthesized that showed biol o g i c a l a c t i v i t y (Figure 21). Each of these analogs was made because of the relationship to tetrahydroanacyclin, a compound having weak b i o l o g i c a l a c t i v i t y and i n which the acyc l i c diene system was seen as equivalent to a phenyl ring; this strategy p a r a l l e l i n g that used i n the synthesis of pyrethriods (57). A l l compounds (Figure 21) were up to four times more effective against super-kdr strains of adult houseflies (Musca domestica L . ) than the susceptible strain. Thus, the results obtained with the N-alkylamides suggest that they may combat resistance conferred by the super-kdr genetic background which delays the onset of knockdown and k i l l by DDT, i t s analogs, and by the pyrethroids (58). Even esoteric compounds, i n the sense that they appear to be highly specific i n a c t i v i t y against a r e l a t i v e l y small insect population, have been synthesized. These include osmundalactone, the aglycone of osmundalin from the fern Osmunda j aponia Thunberg. Osmundalactone i s a congener of phomalactone, acetylphomalactone, asperlin, olguin, and phomopsolide: a l l oligoketides and fungal products. Also, osmundalactone i s a feeding inhibitor for the larvae of the butterfly Eurema hecabe mandarina (59). Synthesis of osmundalactone, (5R,6S)- 5,6-dihydro-5-hydroxy-2H-pyran-2-one and i t s diasterosisomers (5S,6S)-5,6-dihydro-2H-pyan-2-one (Figure 22) was from 3-triethylsiloxy-1-propyne and (S)-1-


1. CUTLER



O

Figure 17.

Typhasterol.

NH H C-C-CH -CH -COOH ii ' O 2

2

2

2

8 -aminolevulinic acid.

Figure 18.

:0 CH 0 3

Figure 19.

Z + E Fadyenolide .

C. R

Figure 20.

= N \

O /

Amides of 3,5-dimethoxy-4-oxo-5-phenylpent-2-enoic acid.


17

18



Ri

R

1. PHENYL 2. 3,5-DIFLUOROPHENYL 3. DIBENZOFURAN-3-YL 4. DIBENZOFURAN-3-YL 5. 5-BROMONAPHTH-2-YL 6. 5-BROMONAPHTH-2-YL 7. 7-FLUORONAPHTH-2-YL

H H H CH H CH CH

R

2

3

3 3

3

H CH CH H CH H H

3 3

3

Figure 21. (2E,4E)-N-isobutyl-6-phenyl-hexa-2,4-dienamide, and derivates .

(5R,6S)-5,6-DIHYDRO-5-HYOROXY-2H-PYRAN-2-ONE

(5S,6S)-5,6-DIHYDRO-5-HYDROXY-2H-PYRAN-2-ONE

Figure 22.

(5R 6S)-5,6-Dihydro-5-hydroxy-2H-pyran-2-one, (5S,6S)-5,6-Dihydro-5-hydroxy-2H-pyran-2-one. r

Figure 23. Ancistrofuran .

Figure 24.

4-Dodecanolide .



1. CUTLER


19

methyl-2-oxoethylbenzoate (60) to yield 60 mg of the former and 78 mg of the latter. Defense secretions have also been the subject of isolation and synthesis. Two are given as recent examples. Ancistrofuran (Figure 23) is the major chemical secreted by the West African termite, Ancistrotermes cavithorax, soldier, (61). While previously somewhat complex syntheses have been reported, a simple procedure has been discovered in which 9-hydroxydendrolasin is cyclized to produce ancistrofuran and its C2 epimer (62). Another defensive secretion has been isolated from the pygidial glands of the rove beetles, Bredius mandebularis and B. spectabilis (63), and identified as 4-dodecanolide (Figure 24). Oddly enough, this compound has also been isolated from assorted fruits (64,65,66) and butterfat (67,68). Synthesis of enantiomers of 4-dodecanolide was achieved from (S)- and (R)-2-aminodecanoic acid in gram quantities (69). Conclusion. The wide array of secondary metabolites that possess biological activity is striking. While there is a certain degree of crossover, for example, Piper sp. natural products that affect insects, one is particularly struck by the relative lack of inter-disciplinary cooperation. But, as the complete synthesis of natural products becomes more common or genetic engineering gives rise to greater biosynthetic production of useful secondary metabolites, i t is hoped that enough of the materials will be available for testing in several, apparently unrelated, systems. It is further hoped that promising lead chemicals will be synthetically adapted to f u l f i l l specific roles. Literature Cited 1. 2. 3. 4.

5.

6. 7. 8. 9.

Chemical and Engineering News, 1988; p. 26. Cutler, H.G. In "CRC Critical Reviews in Plant Science"; Conger, V . , Ed.; CRC Press: Boca Raton, 1988; 6, 323. Cutler, H.G. Proc. 12th Annual Meeting Plant Growth Regulator Soc. America, 1985; p. 160. Cutler, H.G. In "Alleochemicals: Role in Agriculture and Forestry"; Waller, G.R., Ed.; ACS Symposium Series No. 330, American Chemical Society; Washington, DC 1987; p.23-38. Gueldner, R.C.; Reilly, C.C.; Pusey, P.L.; Costello, C.E.; Arrendale, R.F.; Cox, R.H.; Himmelsbach, D.S.; Crumley, F.G.; Cutler, H.G. J. Agric. Food Chem. 1988; 36, 366. Ueno, T . ; Nakashima, T . ; Hayashi, Y.; Fukami, H. Agric. Biol. Chem. 1975; 39, 2081. Ueno, T . ; Nakashima, T . ; Hayashi, Y . ; Fukami, H. Agric. Biol. Chem. 1975; 39, 1115. Aoyagi, H.; Mihara, H.; Lee, S.; Kato, T . ; Ueno, T . ; Izumiya, N. Int. J. Pept. Protein Res. 1984; 25, 144. Shimohigashi, Y.; Lee, S.; Kato, T . ; Izumiya, N. Bull. Chem. Soc. Japan 1978; 51, 584.


20 10. 11. 12. 13.


14 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.

BIOLOGICALLY ACTIVE NATURAL PRODUCTS Mihara, H. Aoyagi, H.; Lee, S.; Waki, M.; Kato, T.; Izumiya, N. Int. J. Pept. Protein Res. 1984; 23, 447. Mihara, H.; Ikesue, K.; Lee, S.;Aoyagi, H.; Kato, T.; Ueno, T.; Izumiya, N. Int. J. Pept. Protein Res. 1986; 28, 141. Hagashijima, T.; Shimohigashi, Y.; Kato, T.; Izumiya, N.; Ueno, T.; Miyazawa, T. Biopolymers. 1983; 22, 1167. Matsumoto, K.; Suzuki, M.; Nunami, K.; Yoneda, N.; Takguchi, K. Bull. Inst. Chem. Res. Kyoto University 1983,; 61, 79. Nunami, K.; Suzuki, M.; Matsumoto, K.; Yoneda, N.; Takiguchi, K. Agric. Biol. Chem. 1984; 48, 1073. Ichihara, A.; Murakami, K.; Sakamura, S.; Agric. Biol. Chem. 1984; 48, 833. Suzuki, M.; Sugiyama, T.; Watanabe, M.; Murayama, T.; Yamashita, K. Agric. Biol. Chem. 1987; 51, 2161. Takaiwa, A.; Yamashita, K. Agric. Biol. Chem. 1984; 48, 961. Trifonov, L.; Bieri, J.H.; Prewo, R.; Dreiding, A.S. Tetrahedron. 1982; 38, 397. Takaiwa, A.; Yamashita, K. Agric. Biol. Chem. 1984; 48, 2061. Nukima, M.; Sassa, T.; Ikeda, M. Tetrahedron Lett. 1980; 21, 301. Nukima, M.; Ikeda, M.; Sassa, T. Agric. Biol. Chem. 1980; 44, 2761. Suzuki, S.; Tanaka, A.; Yamashita, K. Agric. Biol. Chem. 1987; 51, 3095. Suzuki, Y.; Knoche, H.W.; Daly, J.M. Bioorg. Chem. 1982; 11, 300. Suzuki, Y.; Tegtmeier, K.J.; Daly, J.M.; Knoche, H.W. Bioorg. Chem. 1982; 11, 313. Suzuki, Y.; Danko, S.J.; Kono, Y.; Takeuchi, S.; Daly, J.M.; Knoche, H.W. Agric. Biol. Chem. 1984; 48, 2321. Suzuki, Y.; Danko, S.J.; Kono, Y.; Daly, J.M.; Takeuchi, S. Agric. Biol. Chem. 1985. 49, 149. Addicott, F.T. In "Abscisic Acid", Praeger Pubs. (Holt, Reinhart and Winston): New York, 1983. Cornforth, J.W.; Milborrow, B.V.; Ryback, G.; Wareing, P.F. Nature 1965; 205, 1269. Oritani, T.; Ichimura, M.; Yamashita, K.; Agric. Biol. Chem. 1982, 46, 1959. Marumo, S.; Kohno, E.; Natsume, M.; Kanoh, K. Proc. 14th Annual Meeting Plant Growth Regulator Soc. America, 1987; p. 146. Takahashi, S.; Oritani, T.; Yamashita, K. Agric. Biol. Chem. 1986; 50, 3205. Mitchell, J.W.; Mandava, N.; Worley, J.F.; Plimmer, J.R.; Smith, M.V. Nature 1970; 225, 1065. Marumo, S.; Hattori, H.; Abe, H.; Nonoyama, Y.; Munakata, K. Agric. Biol. Chem. 1968; 32, 528. Grove, M.D.; Spencer, G.F.; Rohwedder, W.K.; Mandava, N.; Worley, J.F.; Warthen, J.D., Jr.; Steffens, G.L.; Flippen-Anderson, J.L.; Cook, J.C., Jr. Nature 1979; 281, 216.


1. CUTLER 35. 36. 37. 38.

39.


40. 41. 42. 43. 44. 45. 46. 47. 48.

49. 50. 51. 52. 53. 54.

55. 56. 57. 58.


21

Mitchell, J.W.; Gregory, L.E. Nature New Biology 1972; 239, 253. Gregory, L.E. Amer. J. Bot. 1981; 68, 586. Krizek, D.T.; Mandava, N.B. Physiol. Plant. 1983; 57, 317. Meudt, W.J. In "Ecology and Metabolism of Plant Lipids"; Fuller, G.; Nes, W.D.; Eds.; ACS Symposium Series No. 325, American Chemical Society: Washington, DC. 1987; p. 53-75. Marumo, S. Proc. 14th Annual Meeting Plant Growth Regulator Soc. America, 1987; p. 174. Hamada, K. In "Food & Fert. Tech. Ctr. Book Series No. 34, Plant Growth Regulators in Agriculture"; 1986; p. 190-196. Luo, B.; Kumura, A . ; Ishii, R.; Wada, Y. Proc. of the Crop Science of Japan; 1984; 53, 164. Hamada, K.; Nishi, S.; Uezono, T . ; Fujita, S.; Nakazawa, Y. Abstracts of the 12th IPGSA (Heidelberg). 1985; p. 43. Sakakibara, M.; Mori, K. Agric. Biol. Chem. 1983; 47, 663. Kerb; U . ; Eder, U; Krähmer, H. Agric. Biol. Chem. 1986; 50, 1359. Sakakibara, M.; Mori, K. Agric. Biol. Chem. 1984; 48, 745. Aburatani, M.; Takauchi, T.; Mori, K. Agric. Biol. Chem. 1987; 51, 1909. Kishi, T.; Wada, K.; Marumo, S.; Mori, K. Agric. Biol. Chem. 1986; 50, 1821. Rebeiz, C.A.; Montazer-Zouhoor, A . ; Mayasichi, J.M.; Tripathy, B.C.; Wu, S-M.; Rebeiz, C.C. In "CRC Critical Reviews in Plant Science"; Conger, V . ; Ed.; CRC Press: Boca Raton, 1988; 6, 385. Rebeiz, C.A; Montazer-Zouhoor, A . ; Hopen, H.J.; Wu, S-M. Enzyme Microbiol. Technol. 1984; 6, 390. Hopf, F.R.; Whitten, D.G. In "The Porphyrins" vol. 2. Dolphin D.; Ed.; Academic Press: New York, 1978; 161. Foote, C.S. In "Porphyrin Localization and Treatment in Tumors". Alan R. Liss; New York, 1984; 3. Harvill, E.K.; Hartzell, A . ; Arthur, J.M. Contrib. Boyce Thompson Inst. 1943; 13, 87. Jacobson, M. J. Am. Chem. Soc. 1953; 75, 2584. Miyakado, M.; Nakayama, I.; Ohno, N.; Yoshioka, H. In "Natural Products for Innovative Pest Management" vol 2., Whitehead, L . ; Bowers, W.S.; Eds.; Pergamon Press: Oxford, 1983; 369. Nair, M.G.; Mansingh, A.P.; Burke, B.A. Agric. Biol. Chem. 1986; 50, 3053. E l l i o t t , M.; Farnham, A.W.; Janes, N.F.; Johnson, D.M.; Pulman, D.A.; Sawiki, R.M. Agric. Biol. Chem. 1986; 50, 1347. E l l i o t t , M.; Janes, N.F. Chem. Soc. Rev. 1977; 7, 473. Sawiki, R.M. Nature 1978; 875, 4443.


22



59.

Numata, A . ; Hokimoto, K.; Takemura, T.; Fukui, S. Appl. Ent. Zool. 1983; 18, 129. 60. Murayama, T . ; Sugiyama, T.; Yamashita, K. Agric. Biol. Chem. 1986; 50, 2347. 61. Baker, R.; Briner, P.H.; Evans, D.A. J. Chem. Soc. Chem. Commun. 1978; 410. 62. Saito, A . ; Matsushita, H.; Kaneko, H. Agric. Biol. Chem. 1986; 50, 1309. 63. Wheeler, J.W.; Happ, G.M.; Araujo, J.; Pasteels, J.M. Tetrahedron Lett. 1972; 4635. 64. Broderick, J.J. Am. Perfumer Cosmet. 1966; 81, 43. 65. Willhelm, B.; Palluy, E . ; Winter, M. Helv. Chim. Acta. 1966; 49, 65. 66. Tang, C.S.; Jennings, W.G. J. Agric. Food Chem. 1968; 16, 252. 67. Jurriens, G.; Oele, J.M. J. Am. Oil Chemist's Soc. 1965; 42, 857. 68. Forss, P.A.; Urbach, G.; Stark, W. Int. Dairy Cong. Proc. (Munich) 1966; 3, 211. 69. Sugai, T.; Mori, K. Agric. Biol. Chem. 1984. 48, 2497. RECEIVED June 10, 1988


Natural Products and Their Potential in Agriculture - American

Recommend Documents