Rhizoctonia leguminicola

A trail pheromone of the Pharaoh ant was identified as 3-butyl-5-methylindolizidine of the stereochemistry given in (219) (75JHC289). Slaframine (220) is a metabolite isolated from Rhizoctonia leguminicola responsible for producing excessive salivation in cattle. A stereoselective synthesis was reported (73JOC3848). Swainsonine (221), a potent inhibitor of a-mannosidase, has been isolated from Swainsona caneszens. The stereochemistry was derived from the NMR spectrum (79AJC2257). 

Slaframine.—Slaframine (37) is produced by the phytopathogen Rhizoctonia leguminicola. It has been known for some time that (37) derives in part from lysine via pipecolic acid (33), which is incorporated intact the earliest bicyclic intermediate identified is (38) (c/. Vol. 5, p. 9 and ref. 2). New results have shown that the two skeletal carbons in (37), and also in the metabolite (36), not accounted for by pipecolic acid, derive from malonate (and acetate).13 The labelling of (37) by, in particular, [2-2H2]acetate was deduced to be of C-2 on the basis of mass spectral evidence (which is not entirely convincing). The acyl-CoA derivative (34) has been suggested as an intermediate in the biosynthesis of (37) and also of (36). It is to be noted that condensation between malonyl-CoA and pipecolic acid (33) to give (34) must be simultaneous with decarboxylation of malonyl-CoA, since two deuterium atoms of acetate are retained at C-2 in (37) (later intermediates with a double-bond to C-2 are also excluded by these results). 

Swain sonine (26) Swamsona canescens (Leguminosae) leaves [36] Astragalus spp. (Leguminosae) leaves/stems [37] Oxytropis spp. (Leguminosae) leaves/stems [37] Ipomoea sp. affi calobra (Convolvulaceae) seeds [38] Rhizoctonia leguminicola (Basidiomycetes) [39] Metarhizium anisopliae (Deuteromycetes) [40]... 

The biosynthesis of swainsonine was studied in the fungus Rhizoctonia leguminicola The piperidine ring was shown to be derived from lysine via pipecolic acid. A noteworthy... 

Pyrrolizidines compounds have been produced in connection with syntheses of other types of natural products. For example, Gensler and Hu prepared the dioxopyrrolizidine ester (46) as an intermediate in the synthesis of ( + )-slaframine, an indolizidine alkaloid obtained from cultures of Rhizoctonia leguminicola. The pyrrolidone ester (47), prepared from l-glutamic acid [Eq. (14)], was optically active, but the cyclized product, formed in quantitative yield from 47, was completely racemized. The synthesis of 2-acetyl-1,3-dioxopyrrolizidine (48) was carried out by Kruger and Arndt to assist with their investigations on model compounds aimed toward the total synthesis of a-cyclopiazonic acid, the main toxic principle of Penicillium cyclopium The spectra of the product (48) obtained in 30% overall yield was typical of an intramolecularly H-bonded enolized )S-... 

Harris et al. postulated that (liS, 8aS)-( + )-l-hydroxyindolizidine (109) and (lf ,8aS)-l-hydroxyindolizidine (110) were pivotal intermediates in the biosynthesis of slafi-amine (3) and swainsonine (cf. Section lU.C) in the fungus Rhizoctonia leguminicola (74). Compound 110 was subsequently isolated as e acetate from extracts of the diablo locoweed. Astragalus oxyphysus (75). 

Rhizoctonia leguminicola (2-ep/-lentiginosine), 344 Rhizoctonia leguminicola (slaframine), 349... 

Slaframine.—Further details are now available on the biosynthesis of slaframine (39), a toxin produced by the mould Rhizoctonia leguminicola. Both dl-[1- C]-and DL-[6- " C]-lysine afforded labelled slaframine, indicating intact incorporation of all the carbons of the amino-acid. Addition of inactive pipecolic acid (34) to the cultures diluted the lysine label in the derived slaframine, and pipecolic acid was labelled by radioactive lysine. Further carboxyl- and ring-labelled pipecolic acids [as (34)] were both well incorporated into slaframine. A clear indication is thus obtained of the biosynthetic sequence lysine —> pipecolic acid (34) — slaframine (39) 2-hydroxymethylpiperidine is not a precursor. Attention is drawn to the discovery of a similar pathway to a metabolite of similar structure also produced by R. leguminicola. ... 

The biosynthesis of slaframine (38) and swainsonine (35) in Rhizoctonia leguminicola involves lysine and proceeds via the intermediacy of pipecolic acid. Carbon atoms 2 and 3 have been demonstrated to come from acetate via malonate (Fig. 30.14). The biosynthesis of swainsonine and related compounds in plants does not appear to have been investigated (Elbein and Molyneux, 1987 Harris et al, 1987 Howard and Michael, 1986). 

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