Stereochemical Assignments of the Chlorinated Residues in Victorin C

Abstract

Victorins are cyclic pentapeptide natural products isolated from the fungus Cochliobus victoriae. Using a combination of synthesis and spectroscopic methods we have assigned the geometry of the vinyl chloride residue and determined the stereochemistry of the 5,5-dichloroleucine moiety.Victoria blight is a disease of oats caused by the fungus Cochliobolus victoriae (also known as Helminthosporium victoriae) which only affects oat crops that have been derived from the oat variety Victoria. Five fractions displaying host-selective activity were isolated from the culture filtrate of C. victoriae and collectively these are known as 'victorin', 'the victorins' or HV-toxins. 1 Approximately 1-2 mg of the major metabolite, victorin C 2, was isolated from 1 L of culture broth and the NMR spectrum was complicated by the presence of different conformers. 1a,bThe cyclic pentapeptide structures of the victorins 1-5 ( Figure 1) were elucidated by extensive spectroscopic studies combined with degradation of the natural products. 1,2 The L-configuration of each amino acid residue was determined from the ORD shifts of their 2,4-DNP derivatives. 2 In addition the stereochemistry of erythro-bhydroxyleucine and threo-b-hydroxylysine residues were assigned by correlation with synthetic materials. 1,2 However, the unequivocal assignment of the stereochemistry of the two chlorinated moieties, 5,5-dichloroleucine and the a-amino-b-chloroacrylic acid requires further investigation. Herein we describe our synthetic and spectroscopic studies leading to elucidation of the geometry of the vinyl chloride moiety and stereochemistry of 5,5-dichloroleucine. Assignment of the Geometry of the a-Amino-b-chloroacrylic Acid ResidueThe 1 H NMR spectrum of the major metabolite from cultures of C. victoriae, victorin C (2) displayed a signal at d = 7.52, which was attributed to the vinylic proton, but the geometry of the vinyl chloride (highlighted in Figure 1) was not assigned by Macko, Arigoni and coworkers. 1a-c Subsequently Kinoshita, Daly, and co-workers isolated a further metabolite, victorin M (or HV-toxin M) (5), which differs from the other victorins in that the glyoxylic acid portion (R 3 , Figure 1) is replaced by glycine. 2 Following acetylation of victorin M (5) and methylation with diazomethane, they isolated methyl ester 6 as the major product as well as traces of methyl enol ethers 7 and 8, formed by displacement of the vinyl chloride (Scheme 1).Kinoshita, Daly, and co-workers 2 suggested that the geometry of the vinyl chloride in victorin M (5) was Z by comparison of the chemical shift (d = 7.68) of the olefinic proton in derivative 6 to that of (E) and (Z)-chloroacrylic acids 9 and 10 ( Figure 2). In the E-isomer 9, the b-proton is cis to the carboxylic acid and resonates at d = 7.32, similar to that observed for the signal assigned to the vinyl chloride in 6, whereas in the Z-isomer 10 the signal is upfield at d = 6.85. However, this assignment does not take into account the effect of the nitrogen of the peptide bond on the chemical s...

Insight into the Structure of Victorin, the Host-Selective Toxin from the Oat PathogenCochliobolus victoriae. Studies of the Unique Dehydroamino Acid β-Chlorodehydroalanine

“…The most significant difference (0.39 ppm in DMSO- d 6 ) is for the Boc-Gly-ΔAla­(βCl)-OMe compound, in which the C-terminus is flanked by a methyl ester. The vinyl proton of the isomer E resonates upfield as compared to the isomer Z (in DMSO- d 6 ); however, this is opposite to that established for Ac-ΔAla­(βCl)-OMe, where the isomer E resonates downfield (in CDCl 3 ) . Furthermore, for the derivative of victorin M the vinyl proton of N -methylated ΔAla­(βCl) resonates at δ = 7.68 ppm in CD 3 OD and for the product of hydrolysis of victorin C at δ = 7.52 ppm in D 2 O .…”

“…Determination of the geometric isomer of the chlorodehydroalanine residue in victorin structures was undertaken by comparing the chemical shift of the vinyl proton of the methylated victorin M derivative to the chemical shifts of selected model chemical compounds. Initially, the geometry of the CC double bond was established as Z by the comparison of the vinyl protons of both isomers of N -methylated β-chlorodehydroalanine Δ­(Me)­Ala­(βCl). , Later, more detailed attempts for the stereochemical assignments of the chlorinated residues in victorin C were undertaken . The selected ΔAla­(βCl) esters were taken as a pattern, and the geometric isomers were assigned by comparing the coupling constant C H with the calculated values.…”

“…In the mentioned example of phytotoxic phomalide, produced by the fungus Leptosphaeria maculans and responsible for leaf spot and stem cancer (blackleg), a disease of oilseed Brassicas (e.g., canola), the isomer E of the dehydrobutyrine residue with a C-terminal ester is present . In contrast, the isophomalide with the isomer Z is biologically inactive . In another example, tentoxin, a selective weed killer that causes chlorosis of higher plants, the isomer Z of N -methyl­dehydro­phenyl­alanine is present.…”

“…The oat, genus Avena sativa , is a commercially cultivated plant of considerable economic importance. − The necrotrophic fungal pathogen Cochliobolus victoriae , producing the host-selective toxins victorins, causes disease of some oat varieties. , Victorins are a family of peptides − in which victorin C is the predominant compound (Figure ).…”

“…The β-chlorodehydroalanine residue, ΔAla­(βCl), is marked, with flanking peptide bonds. The geometry of the CαCβ double bond is debatable ( E or Z ).…”