Monocrotalines

Monocrotaline on alkaline hydrolysis yields retronecine and monocrotic acid, CjHijOg, b.p. 145-6°/18 mm., [a]p 0°, which forms a p-bromo-phenacylester, m.p. 78°,and a methyl ester, b.p. 94-6°/18 mm., characterised by a 2 4-dinitrophenylhydrazone, m.p. 95-6° see below). The acid gives the iodoform reaction and is oxidised by sodium hypobromite to a mixture of dl- and mcso-aa -dimethylsuccinic acids (I). These and other reactions show that monocrotic acid is a -dimethyllaevulic acid (II) and this has been eonfirmed by comparison with a synthetic specimen of the acid. The methyl ester of the synthetie acid forms a mixture of 2 4-dinitrophenylhydrazones, m.p. 108-9° and 121-2°, into which the analogous produet, m.p. 95-6°, first made from methyl monocrotate see above), has also been separated. 

When monocrotaline is hydrogenolysed the acid scission product is monocrotalic acid, CgHigOj, m.p. 181-2°, [a]p ° — 5-33° (HgO), which provides a methyl ester, m.p. 79-80°, [ ]d°° — 16-2° (EtOH), containing one active H atom and a p-bromophenacyl ester, m.p. 162-3°. It is a lactonic acid, which on boiling with sodium hydroxide solution loses carbon dioxide and produces a/3-dimethyllaevulic acid (monocrotic acid, II). 

Some male arctiid moths produce their courtship pheromone from dietary pyrrolizidine alkaloids acquired during feeding by the larvae [ 126]. Conversion of monocrotaline to hydroxydanaidal by males is accomplished by aromatiza-tion, ester hydrolysis and oxidation of an alcohol to the aldehyde [7]. In the case of Utetheisa ornatirx the stereo-configuration at C7 of the dietary alkaloid is the same as the pheromone released (R). In contrast, another arctiid, Creatono-tos transiens, can convert a dietary precursor alkaloid with the (S) configuration at C7 (heliotrine) to (l )-hydroxydanaidal. The biosynthesis occurs by first oxidation-reduction at C7 to convert the stereochemistry and then proceeds through aromatization, hydrolysis, and oxidation [7]. 

Monocrotaline (170) has been the subject of extensive metabolic study with mammalian and microbiological systems. Pyrrolizidine alkaloids such as monocrotaline require metabolic activation to the corresponding pyrrole derivatives or dehydro alkaloids before they are capable of forming covalent bonds with critical macromolecules within the cell. The X-ray structure of dehydromonocrotaline has recently been determined (226), and the ability of dihydroretronicine derived from monocrotaline to react with deoxyguanosine has been demonstrated in vitro (225). 

We knew Utetheisa to feed on poisonous plants as a larva (Figure 1B). The plants, of the genus Crotalaria (family Leguminosae), were known to contain pyrrolizidine alkaloids (henceforth abbreviated as PAs), intensely bitter compounds potently hepatotoxic to mammals (7). Other species of Utetheisa were known to sequester PAs (8). We found this to be true for U. ornatrix as well. Adult Utetheisa raised on Crotalaria spectabilis, one of the principal foodplants available to the moth in the United States, contain on average about 700 p,g of monocrotaline (1), the principal PA in that plant (9, 10). 

In the laboratory, we succeeded in raising Utetheisa on two alternative artificial diets, one made up with Crotalaria seeds and containing the PA monocrotaline (CS diet), the other based on pinto beans and devoid of PA (PB diet). On the assumption that the PB diet-raised moths, which we proved to be PA-free, would be vulnerable to predation, we took moths... 

A further finding concerns the stereochemistry of HD and its derivation from PA. Both HD and the primary PAs (monocrotaline, usaramine) that we know to be available to Utetheisa in the field are of the same (7R) stereochemical configuration. It was therefore not surprising to find that Utetheisa is unable to convert a PA of opposite (7S) stereochemistry (heliotrine) into HD. However, we found another arctiid moth, the Asian species Creatonotus transiens, which also produces HD in its coremata, to be able to use 7R and 7S PAs interchangeably for HD production (31). We are tempted to conclude that Creatonotus, unlike Utetheisa, has dietary access to PAs of both stereochemical configurations in its environment. 

As with many A7(sp3)-azaheterocyclcs, monocrotaline 80 undergoes regioselective ring opening leading to 81 when treated with 2,2,2-trichloroethylchloroformate (Troc-Cl) in presence of potassium iodide (Scheme 8) <1999JA2951>. 

At overwintering sites of the monarch butterfly [Danaus plexippus) in Mexico, only one of the three local mouse species, Peromyscus melanotis, actually feeds on the butterflies. The monarchs contain cardiac glycosides (CG) and pyrrolizidine alkaloids (PA). All three species of mice have similarly low avoidance thresholds to PA (specifically, monocrotaline). But P. melanotis is less sensitive to CG (specifically, digitoxin) than the other two, Reithrodontomys sumichrasti and Peromyscus aztecus. Laboratory tests indicate that PA is toxic to young mice. 

Glendinning, J. F., Brower, L. P., and Montgomery, C. A. (1990). Responses of three mouse species to deterrent chemicals in the Monarch butterfly. I. Taste and toxicity tests using artificial diets laced with digitoxin or monocrotaline. Chemoecology 1,114-123. 

Bober, M. A., Kurth, M. J., Milco, L. A., Roseman, D. M., Miller, R. B. and Segal, H. J. 1991. A pyrrolizidine alkaloid-enzyme-linked immunosorbent-assay detection strategy. ACS Symposium Series, 451 176-183 and, Bober, M. A., Milco, L. A., Miller, R. B., Mount, M., Wicks, B. and Kurth, M. J. 1989. A competitive enzyme-linked immunosorbent-assay (ELISA) to detect retronecine and monocrotaline in vitro. Toxicon, 27(9) 1059-1064. 

Lasiocarpine Monocrotaline Retrorsine Riddelliine Senecipliylline Senkirkine)... 

Crotalaria sessiliflora L. Ye Bai He (Narrow-leaved rattlebox) (whole plant) Monocrotalines.33 This herb is toxic. Anticancer. 

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