Chapter 9
Ryanoid Chemistry and Action Phillip R. Jefferies and John E. Casida
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Pesticide Chemistry and Toxicology Laboratory, Department of Entomological Sciences, University of California, Berkeley, CA 94720
Ryanodine and 9,21-denydroryanodine are the principal insecticidal components and toxicants among 11 identified ryanoids in the botanical insecticide ryania. Their biological activity is attributed to block of the calcium release channel, which is conveniently measured as inhibition of [ H]ryanodine binding in muscle and nerve preparations. Structureactivity relationships based on 10 natural ryanoids and 45 of their derivatives help define the conformation of the ryanodine binding site and the structural requirements for insecticidal activity and selective toxicity. 3
Ryania the Botanical Insecticide Ryania was recognized about 50 years ago as an insecticide (1) but was known much earlier as a toxicant for mammals (2,3). The current commercial insecticide is the ground stem-wood of Ryania speciosa Vahl (Flacourtiaceae) from Trinidad supplied by Agrisystems International (Windgap, PA). The roots are even more insecticidal but for reasons of convenience, conservation and regrowth the stem-wood is used. This genus is also found in the Amazon basin and adjacent parts of South America (3). Ryania is effective against corn borer, codling moth, other lepidopterous insects, and pests of stored food (4). The extent and scope of use is limited by cost and supply. Ryania acts best in the field by ingestion causing flaccid paralysis and can be effective at a few grams of contained ryanoids per acre (5). Although the toxicity of ryania is enhanced by methylenedioxyphenyl compounds (6) it is used without addition of a synergist. Ryania is of moderate to low toxicity to mammals on ingestion (oral L D 150-2500 mg/kg) and it has low persistence (3,4). However, its chronic toxicity is only partially defined (4) and its detailed metabolism and environmental fate are unknown. 50
0097-6156/94/0551-0130$06.00/0 © 1994 American Chemical Society In Natural and Engineered Pest Management Agents; Hedin, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
9. JEFFERIES AND CASIDA
Ryanoid Chemistry and Action
131
Natural Ryanoids and Their Degradation Products (Figure 1)
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The natural ryanoids are conveniently isolated from ryania by wet chloroform extraction and rotary chromatography on silica gel with chloroform/methanol/ aqueous methylamine followed by reverse phase HPLC with aqueous methanol (2,7,8). Ryanodine (1) and dehydroryanodine (2) are the major ryanoids, each making up 480-700 ppm (w/w) relative to the stem-wood, with seven other ryanoids contributing 10-64 ppm (w/w) each (8).
Ryanodine Series. Ryanodine (1) is an ester of ryanodol (6), a complex bridged diterpene heptol. More specifically, 1 is ryanodyl 3-(pyrrole-2-carboxylate). It was isolated in 1948 (2) and identified in 1967 (9,10). Structure elucidation involved a brilliant series of degradations based on ryanodol (6) formed from 1 with base, anhydroryanodine (13) with acid, and 4.12-seco.4.12-dioxorvanodine (15) with periodate (9,11). The structure of 6 was confirmed (except for the stereochemistry at C-2) by X-ray crystallography of the mono-p.-bromobenzyl ether (which was found surprisingly at the 4-position) (10). Synthesis of 6 has been achieved by a multistage sequence proceeding through reductive cyclization of the 1,2-epoxide of the alcohol component of 13 with interesting use of carbonates, orthoacetates and orthocarbonates as protecting groups (12). The C-3 hydroxyl of 6 is very hindered so that esterification to 1 could not be effected. 3-Deoxyryanodol (7) is also a natural product discussed later. Dehydroryanodine (2), reported in 1984, usually exceeds 1 in amount but was overlooked in early studies because it is more soluble in the recrystallization solvents and was apparently lost in the purification steps (13-15). Minor compounds in the ryanodine series are 18-hydroxy-l (3) (14), 8-oxo-10-deoxy9,10-dehydro-l (4) (7), and ryanodyl 3-(pyridine-3-carboxylate) (5) (16). 8 - or 9 -Hydroxy-10-epiryanodine Series. Compounds in the 8^-hydroxy series have the following substituents: 10-(O-methyl) (8) (7,17), 18-hydroxy-10-(Q-methyl) (9) (17), 10-(O-methyl)-9,21-dehydro (10) (7,17) and 9,21-dehydro (Π) (17,18). Another minor 10-epiryanoid is 9 -hydroxy compound 12 (7,17). njc
M
ax
Anhydroryanodine and 4.12-Diketo Series. The ryania constituent 9hydroxyanhydroryanodine (14) is closely related to 13, the major acid-degradation product of 1 referred to above (7). Diketo compounds 15 and jUj are oxidation products of 1 and 2, respectively, used in characterization and derivatization studies. Cinncassiol (3-DeoxyryanodoD Series. Nonester ryanoids from genera of Lauraceae include 6 from Persea indica where it occurs together with cinnzeylanol (7) (19,20). Alcohol 7 and its 10-acetate occur naturally in Cinnamomum zeylanicum Nees (21) and CL cassia (Chinese cinnamon) (22) in the latter case along with sets of ryanoids with skeletons corresponding to either the anhydro series 13 (22) or the diketo type
In Natural and Engineered Pest Management Agents; Hedin, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
132
NATURAL A N D ENGINEERED PEST M A N A G E M E N T AGENTS
Ryanodine series 1 2 2 4
ryanodine (480 ppm) 9,21-dehydroryanodine (700 ppm) 18-HO-l (trace) 8-oxo-10-deoxy-9,10dehydro-^ (trace) (B)
17 19 HO Me Me 2; 1
Me is Me
,HO H Q
1 0 1 U
OH '
2 1
M
5 R = pyridine-3-carboxylate (58 ppm) (Q) 6 ryanodol R=OH 7 cinnzevlanol R=H
e
OH HO Me Me^W\
" S - ^ ^4 rr?^ ^ Me ^
20 R
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Me
H O H( Li O H
Η
Ο 2
Η
Η
^
4
HO H ^ H
Η
26 8.,- or 9..-Hvdroxv-10-epirvanodine series (C-ring modification of η 8 (33 ppm) (A) £ 18-HO-S (10 ppm) (Ε) MeO
10 R = Me (32 ppm) (β) Π R = Η (64 ppm) (F)
Me OH
R
j>ZJ-
H
Q
H OH
HO
101 > J
H
8
12 (17 ppm) (Ci)
5L
9
Me
H-70H
Anhydroryanodine and 4.12-diketo series
