O
HO
HO
OH
H
2
C
OH
H
N
NH
2
OH NH
2
COOH
COOH
3
4
5
6
2
1
1'
2'
3'
4'
5'
6'
7
TETRAHEDRON
LETTERS
Tetrahedron Letters 42 (2001) 8611–8613 Pergamon
Stereochemical studies on ascaulitoxin: a J -based NMR
configurational analysis of a nitrogen substituted system
Carla Bassarello,
a
Giuseppe Bifulco,
a
Antonio Evidente,
b
Raffaele Riccio
a
and
Luigi Gomez-Paloma
a,
*
a
Dipartimento di Scienze Farmaceutiche, Universita ` di Salerno, via Ponte don Melillo, Fisciano (SA) 84084, Italy
b
Dipartimento di Scienze Chimico -Agrarie, Universita ` di Napoli Federico II, via Universita ` 100, Portici (NA) 80055, Italy
Received 26 July 2001; accepted 25 September 2001
Abstract—The J -based NMR configurational analysis was applied to the determination of relative stereochemistry of the
ascaulitoxin molecule (1), a phytotoxic metabolite with herbicidal activity against Chenopodium album. This method is particularly
suitable for acyclic structures containing hydroxy(alkoxy) groups, because in this kind of system, besides
3
J
H–H
and
3
J
C–H
,
2
J
C–H
values can also be used to extract additional angular information. Ascaulitoxin (1), with its bis-amino acid side chain containing
four stereocenters, two of which with nitrogen substituents, required a more careful analysis of heteronuclear J -couplings, also in
comparison with theoretical values obtained by ab initio methods. © 2001 Elsevier Science Ltd. All rights reserved.
In the last decades many efforts were made to develop
methods for the biological control of the infestant
plants by using their natural antagonists. Among these,
fungi appear to be the most appropriate agents as
mycoherbicides and some of them have also been com-
mercialised.
1
The phytotoxins produced by pathogenic
weed fungi can also be used as pure compounds to
develop new natural and safe herbicides.
2
These include
the phytotoxic metabolites produced in vitro by
Ascochyta caulina (P. Karst) v.d. Aa and v. Kest, which
has been proposed as a mycoherbicide against
Chenopodium album, a common world-wide weed of
many arable crops such as sugar beet and maize.
3
From
the in vitro fungal culture filtrates, three new phytotox-
ins were recently isolated and characterised as non-
proteinogenic amino acids.
4–6
The main toxin was
named ascaulitoxin (1) and characterised as the -D-
glucopyranoside of the unusual bis-amino acid 2,4,7-tri-
amino-5-hydroxyoctandioic acid. As the configuration
of the naturally occurring toxin was still undetermined,
the relative configurational analysis of its four stereo-
centers (C-2, C-4, C-5 and C-7) appears to be of
interest and a necessary step for establishing its abso-
lute configuration and to realise its stereoselective
synthesis.
Herein, we describe a stereochemical and conforma-
tional study on ascaulitoxin (1) based on extensive use
of heteronuclear
2,3
J
CH
values (in combination with
ROESY responses), a methodology recently described
as J -based NMR approach.
7–11
The method has proved
particularly useful for the configurational analysis of
polyoxygenated/polymethylated frameworks, typically
found in polyketides of natural origin. Conversely, the
method (which relies also on
2
J
CH
couplings) has not
been extended so far to the stereochemical analysis of
nitrogen-substituted chains, such as the ascaulitoxin
molecule (1). However, we envisaged that the J -based
approach was likely to display more breadth of applica-
tion, since the need of oxygen (hydroxy, methoxy, etc.)
substituents stemmed just on the need of suitable corre-
lations between the values
2
J
CH
couplings and the dihe-
dral angles between the proton and the heteroatom
attached to the carbon coupled to the proton. Indeed,
in oxygenated (and also dioxygenated) C
2
-fragments
small
2
J
CH
couplings (in absolute value, e.g. +2-1
Hz) are commonly observed for anti -like arrangements
Keywords : heteronuclear coupling constants; phytotoxins; bioherbi-
cides; ascaulitoxin; relative stereochemistry.
* Corresponding author. Tel.: +39-089-962811; fax: +39-089-962828;
e-mail: gomez@unisa.it
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII:S0040-4039(01)01815-9