First synthesis of an O-glycosylated glucosinolate isolated from Moringa oleifera

Article abstract of DOI:10.1016/S0040-4039(00)01466-0

Starting from L-rhamnose, the first synthesis of the major glucosinolate (1) isolated from Moringa oleifera seeds was effected in seven steps. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01466-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8307–8309
First synthesis of an O-glycosylated glucosinolate isolated
from Moringa oleifera
a
b
b
b
a,
D. Gueyrard, J. Barillari, R. Iori, S. Palmieri and P. Rollin *
a
Institut de Chimie Organique et Analytique, Universit e´ d’Orl e´ ans, BP 6759, F-45067 Orl e´ ans Cedex 2, France
b
Istituto Sperimentale per le Colture Industriali, via di Corticella 133, I-40129 Bologna, Italy
Received 11 July 2000; accepted 1 September 2000
Abstract
Starting from
L
-rhamnose, the first synthesis of the major glucosinolate (1) isolated from Moringa
oleifera seeds was effected in seven steps. © 2000 Elsevier Science Ltd. All rights reserved.
Glucosinolates constitute a homogeneous class of naturally occurring thiosaccharidic com-
pounds mainly found in the botanical order Brassicales: they can be hydrolyzed by myrosinase
(
thioglucoside glucohydrolase E.C. 3.2.3.1.) to produce
D
-glucose and various degradation
1
products—particularly isothiocyanates—depending on the aglycon part.
Moringa oleifera (Moringaceae) is a multipurpose tree widespread throughout most of the
2
tropics, the seeds of which contain 8–10% of a structurally unusual glucosinolate 1—an
3
O-rhamnosylated form of glucosinalbin 2.
Compound 1 is a representative member in a group of glucosinolates whose aglycon bears an
O-glycosylated hydroxyl function. Other examples arise from the family Resedaceae: glucosino4-
late 3—an O-rhamnosylated form of isoglucosinalbin 4—was identified in the genus Reseda,
whereas 5—an O-arabinosylated derivative of glucobarbarin 6—was shown to be present in the
5
genus Sesamoides.
*
Corresponding author. E-mail: patrick.rollin@univ-orleans.fr
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01466-0

8308
Unlike glucosinolates 3 and 5—which were isolated and spectroscopically characterized,
compound 1 has previously only been postulated as the likely precursor to bioactive molecules
6
such as isothiocyanate 7 and thionocarbamate 8 (Scheme 1).
Scheme 1.
Extraction and purification of glucosinolate 1 from M. oleifera seeds was performed according
7
to a well established protocol; after purity assessment by reversed phase HPLC according to the
1
13
ISO 9167-1 method, compound 1 was fully characterized by H and C NMR and mass
8
spectrometry.
In former years, our group has been strongly involved in synthesizing both natural and
9
artificial glucosinolates. The elaboration of a synthetic sequence to produce O-glycosylated
glucosinolates is challenging in several aspects: we thus decided to undertake the synthesis of 1
with a view to producing final evidence of structure.
The synthetic pathways to glucosinolates which have been developed within recent decades
are constantly based on a key coupling step between 2,3,4,6-tetra-O-acetyl-1-thio-b-
D
-glucopy-
ranose (‘thiol’) and a highly reactive hydroximoyl chloride, furnishing stereospecifically a
1
0
(
Z)-thiohydroximate precursor.
Starting from -rhamnose, the previously described aryl a-
reasonable yield through a convenient modification of Kunz’ approach —i.e. N-iodosuccinimide
1
1
L
L
-rhamnoside 10 was obtained in
6
1
2
activation of the corresponding phenyl thioglycoside 9. The p-O-rhamnosylated E-nitrostyrene
13
1
1 was produced from compound 10 under the mildly basic conditions prescribed by Lappin,
14
then 11 was converted into the corresponding hydroximoyl chloride to be condensed right
15
away with the protected thiol following the methodology recently developed in our laboratory.
The precursor 12 was obtained from 10 in 54% overall yield (Scheme 2).
Scheme 2.

8309
The hydroximino group of 12 was O-sulfated using a SO –pyridine complex, to yield after
3
aqueous KHCO neutralization the peracetylated form of the glucosinolate. Standard transester-
3
ification conditions finally afforded the target molecule 1, whose spectral data (MS, NMR) were
found to be identical to those measured on a natural sample (Scheme 3).
Scheme 3.
By using the synthetic sequence described above, the structure of the major glucosinolate
extracted from M. oleifera seeds has been unequivocally ascribed. Our methodology appears
versatile enough to consider a possible extension to the building up of many analogues of 1
16
either diversely O-glycosylated or modified on the thio-sugar moiety. This is being currently
investigated in our group.
Acknowledgements
A grant (D.G.) from the French MENRT is acknowledged. We also thank Dr. J. C. Jacquinet
for helpful discussions, Dr. A. Quinsac (CETIOM) and S. Cassel for analytical assistance.
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