Chemical and enzymatic synthesis of buprestin A and B-bitter acylglucosides from Australian jewel beetles (Coleoptera: Buprestidae)

Article abstract of DOI:10.1016/j.tetlet.2006.08.119

A chemical and enzymatic synthesis was developed for buprestin A and B originally isolated from Australian jewel beetles (Coleoptera: Buprestidae). The common motif of both acylglucosides is a β-d-glucopyranose-1,2-bis(pyrrole-2-carboxylate). Starting fro

Full text of DOI:10.1016/j.tetlet.2006.08.119

Tetrahedron Letters 47 (2006) 7741–7743
Chemical and enzymatic synthesis of buprestin A
and B—bitter acylglucosides from Australian jewel beetles
(Coleoptera: Buprestidae)
Sabine Schramm,a Konrad Dettnerb and Carlo Unverzagta,*
aBioorganische Chemie, Geba¨ude NWI, Universita¨t Bayreuth, 95440 Bayreuth, Germany
bTiero¨kologie II, Geba¨ude NWI, Universita¨t Bayreuth, 95440 Bayreuth, Germany
Received 21 July 2006; revised 21 August 2006; accepted 25 August 2006
Abstract—A chemical and enzymatic synthesis was developed for buprestin A and B originally isolated from Australian jewel beetles
(Coleoptera: Buprestidae). The common motif of both acylglucosides is a b-D-glucopyranose-1,2-bis(pyrrole-2-carboxylate). Start-
ing from 1,3,4,6-tetra-O-acetyl-a-D-glucose, the first pyrrole-2-carboxylate was introduced by DCC–DMAP mediated esterification.
After conversion to a trichloroacetimidate the anomeric pyrrole-2-carboxylate was installed. Selective removal of the acetates was
accomplished using immobilized Candida antarctica lipase. The resulting triol was converted to Buprestin A or B via a Mitsunobu
reaction.
Ó 2006 Elsevier Ltd. All rights reserved.
Two publications describe the isolation and structural
characterization of two acylglucosides containing
multiple pyrrole-2-carboxylic acid residues in Australian
jewel beetles (Buprestidae).1 The compounds named
buprestin A and B (Fig. 1) were purified and structurally
characterized by NMR. The compounds were found to
act as deterrents for ants, suggesting a biological role
of the buprestins.2 To further investigate the biological
properties of the buprestins and related compounds,
we developed a synthetic approach based on chemical
and enzymatic methods. This approach should provide
sufficient amounts of the reference compounds for struc-
ture confirmation and biological assays. Since both
buprestins differ only in the residue attached to O-6,
the use of a common intermediate was desirable. We
herein describe the synthesis of buprestin A and B from
a common precursor.
idues at O-6, which may be present in minor buprestins.
The desired precursor (triol 6) contains a pyrrole-2-car-
boxylic acid moiety at O-2 and at the anomeric center.
In order to exploit the anticipated neighboring group
activity of the 2-O-pyrrole-2-carboxylic acid group, this
residue should be installed first, thus leading to the glu-
cose-tetraacetate 1,3 which is available in a one-pot reac-
tion starting from glucose. It was first examined how to
esterify pyrrole-2-carboxylic acid with OH-2 of com-
pound 1. Initial attempts to generate the ester at O-2
using pyrrole-2-trichloromethylketone4 and compound
1 activated with DBU or NaH gave low yields due to
the instability of 1. A suitable method was found using
dicyclohexylcarbodiimide (DCC) and N,N-dimethyl-
aminopyridine (DMAP) giving the desired ester 2 in
93% yield. To avoid the separation of anomers resulting
from the acylation of anomeric hydroxyl groups,5 the b-
selective introduction of the anomeric pyrrole-2-carbox-
ylic acid was investigated using the trichloroacetimidate6
method. As a model compound 2,3,4,6-tetra-O-acetyl-a-
D-glucosyltrichloroacetimidate6 was reacted with pyr-
role-2-carboxylic acid in methylene chloride. Glycosyl-
trichloroacetimidates react with benzoic acid without
activation,7 whereas pyrrole-2-carboxylic acid was only
incorporated when BF3–Et2O was added as an activator
to give the desired 1-O-pyrrole-2-carboxylated b-gluco-
side in 72% yield (data not shown). According to this
Retrosynthetic analysis (Fig. 1) suggested the introduc-
tion of the variable acyl residue at O-6 in the last step.
This would also allow the attachment of alternative res-
Keywords: Acylglucoside; Buprestidae; Lipase; Carbohydrate; Pyrrole-
2-carboxylic acid.
*
Corresponding author. Tel.: +49 921 552670; fax: +49 921 555365;
e-mail: carlo.unverzagt@uni-bayreuth.de
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.08.119

7742
S. Schramm et al. / Tetrahedron Letters 47 (2006) 7741–7743
O
O
R
HN
O
HN
HO
AcO
AcO
O
O
O
O
HO
HO
HO
O
O
AcO
HO
O
O
HO
OAc
O
O
N
N
1
6
H
H
A (Buprestin A) R =
B (Buprestin B) R =
N
H
HO
Figure 1. Retrosynthetic analysis of buprestin A and B leading to the common precursor 6.
approach, compound 2 was first converted to hemiacetal
3 by removal of the anomeric acetate with hydrazine
acetate8 (73%) followed by the addition of trichloro-
acetonitrile in the presence of 1,8-diazabicyclo[5.4.0]
undec-7-ene (DBU) (81%). When reacting imidate 4
with pyrrole-2-carboxylic acid in the presence of BF3–
Et2O, acylglucoside 5 was obtained in 89% yield
(Fig. 2). The pyrrole-2-carboxylate moiety at O-2 gave
only the b-product resulting from neighboring group
participation. With compound 5 in hands, removal of
the three acetates to the envisioned universal precursor
6 was investigated. It was found by TLC analysis that
the reaction of 5 with solid K2CO3 in CH2Cl2/MeOH
(2:1) led to numerous deacylation products. After 30%
conversion, the desired compound 6 was present in
approximately 15%. Prolonged reaction time gave rise
to hemiacetals and glucose. When attempting the
deacetylation using catalytic NaOMe in MeOH, the
yield of 6 could be increased to 30–40% (TLC). The
yields remained unsatisfactory due to a comparable
reactivity of the acetates and the anomeric acyl group.
Thus, a selective deprotection9 under neutral conditions
was investigated using hydrolytic enzymes. A panel of
twelve commercially available lipases was tested with 5
as a substrate. Using LC-MS analysis, only the lipase
from Candida antarctica (CAL B; Novozym 435)
showed sufficient activity.10 With this enzyme, the condi-
tions of deacetylation were improved stepwise yielding
the desired triol 6 in an optimized yield of 86% after
flash chromatography. A side product resulting from
acyl group migration11 onto O-3 was identified as com-
pound 7 (8% yield).
Compound 6 was used to install the third acyl group at
O-6. Initial attempts to selectively obtain an O-6 acyl-
ated product by reacting activated pyrrole-2-carboxylic
acid (DCC–DMAP) gave only low yields (19%) along
with an O-4-regioisomer and products carrying multiple
acylations. Selective introduction at O-6 was success-
fully accomplished using the Mitsunobu reaction12 (tri-
phenylphosphine, diethylazodicarboxylate (DEAD))
1
giving buprestin A (A) in 72% yield. The H and 13C
NMR data of the synthetic material13 are in accordance
with the published data, thus confirming the structure of
buprestin A proposed by Moore and co-workers.1
The Mitsunobu reaction was also applied to introduce
p-hydroxybenzoic acid directly, however, the yield was
only 22%. Thus, p-acetoxybenzoic acid was used giving
the O-acetylated buprestin
B
derivative
8
(75%)
AcO
AcO
AcO
AcO
AcO
AcO
O
AcO
AcO
AcO
AcO
O
O
O
O
AcO
a
b
c
AcO
OH
HO
O
O
CCl3
OAc
O
OAc
O
O
O
NH
N
N
N
H
1
2
4
3
H
H
HN
HN
O
HN
O
AcO
AcO
HO
HO
HO
HO
O
O
O
O
O
d
e
O
O
O
O
AcO
HO
O
OH
O
O
O
N
N
HN
5
6
7
H
H
Figure 2. (a) Pyrrole-2-carboxylic acid, DCC, DMAP, CH2Cl2, (93%); (b) hydrazine acetate, DMF (73%); (c) Cl3CCN, DBU, CH2Cl2, 0 °C (81%);
˚
(d) pyrrole-2-carboxylic acid, BF3–OEt2, molecular sieves 4 A, CH2Cl2, 0 °C (89%); (e) Candidia antarctica lipase B, immobilized (Novozym 435),
0.5 M NH4OAc/CH3CN (9:1), 40 °C, (6: 86%), (7: 8%).

S. Schramm et al. / Tetrahedron Letters 47 (2006) 7741–7743
7743
O
O
O
O
RO
N
H
HN
HN
O
a or b
c
O
O
O
O
HO
HO
HO
HO
6
O
O
d
O
O
N
H
O
N
H
A
R = Ac
R = H
8
e
B
Figure 3. (a) Pyrrole-2-carboxylic acid, DCC, DMAP, THF, (19%); (b) pyrrole-2-carboxylic acid, PPh3, DEAD, THF, (72%); (c) p-hydroxybenzoic
acid, PPh3, DEAD, THF, (22%); (d) p-acetoxybenzoic acid, PPh3, DEAD, THF, (75%); (e) Candidia antarctica lipase B, immobilized (Novozym
435), 0.5 M NH4OAc/CH3CN (9:1), 40 °C, (90%).
(Fig. 3). Deprotection10 of the phenolic acetate was
achieved with immobilized CAL B leading to the desired
9. Kadereit, D.; Waldmann, H. Chem. Rev. 2001, 101, 3367–
3396.
10. Lambusta, D.; Nicolosi, G.; Patti, A.; Sanfilippo, C. J.
Mol. Catal. B: Enz. 2003, 22, 271–277.
11. Tsuda, Y.; Yoshimoto, K. Yuki Gosei Kagaku Kyokaishi
1984, 42, 479–492.
12. Mitsunobu, O. Synthesis 1981, 1–28.
buprestin B (B) in 90% yield after 30 min of reaction
time. The NMR data13 of the synthetic buprestin B
are in accordance with the data published for the iso-
lated compound.
13. Analytical data for buprestin A (A): ESI-MS (CH3CN/
In summary, a short synthesis was developed for bupr-
estin A (six steps, 30% total yield) and for buprestin B
(seven steps, 28% total yield). The analytical data for
both compounds proved identical with the isolated
material, thus confirming the structural assignment.
With several hundred milligrams of the synthetic com-
pounds in hand, biological assays will be conducted to
further investigate the role of buprestins as chemical
defense molecules in detail.
H2O, 0.1% formic acid): C21H21N3O9 Mr (calcd) 459.13,
Mr (found) 482.25 (M+Na)+, Rf = 0.27 (cyclohexane/
23
ethyl acetate 1:2); ½aꢀD ꢁ65.0 (0.5, MeOH); 1H NMR
(270 MHz, DMSO-d6): d = 11.98 (s, 1H, NH), 11.90 (s,
1H, NH), 11.87 (s, 1H, NH), 7.06–7.01 (m, 2H, Ar), 6.98
(m, 1H, Ar), 6.80 (m, 1H, Ar), 6.75 (m, 1H, Ar), 6.71 (m,
1H, Ar), 6.18 (m, 1H, Ar), 6.14–6.10 (m, 2H, Ar), 5.85 (d,
J1,2 = 8.5 Hz, 1H, H-1), 5.59–5.53 (m, 2H, OH-3, OH-4),
5.04 (dd, J1,2 = 8.5 Hz, J2,3 = 8.9 Hz, 1H, H-2), 4.52 (dd,
Jgem = 11.7 Hz, J5,6a <1 Hz, 1H, H-6a), 4.29 (dd,
Jgem = 11.7 Hz, J5,6b = 5.3 Hz, 1H, H-6b), 3.82–3.65 (m,
2H, H-5, H-3), 3.52 (m, 1H, H-4), 13C NMR (67.5 MHz,
DMSO-d6): d = 160.2, 159.3, 158.4 (C@O Pyrr), 125.6
(Ar), 124.2 (2C, Ar), 121.6, 121.5, 120.4 (Ci–Ar), 116.4
(Ar), 115.3 (2C, Ar), 109.8 (Ar), 109.5 (2C, Ar), 91.9 (C-1),
74.9 (C-5), 73.6 (C-3), 72.2 (C-2), 69.7 (C-4), 62.6 (C-6).
Analytical data for buprestin B (B): ESI-MS (CH3CN/
H2O, 0.1% formic acid): C23H22N2O10 Mr (calcd) 486.13,
Mr (found) 509.29 (M+Na)+, Rf = 0.42 (dichlorometh-
Acknowledgements
We are grateful to the Deutsche Forschungsgemeins-
chaft (DFG) Graduate College 678 and the Fonds der
Chemischen Industrie for financial support.
23
References and notes
ane/methanol 1:2); ½aꢀD ꢁ64.7 (0.5, MeOH); 1H NMR
(270 MHz, DMSO-d6): d = 11.97 (s, 1H, NH), 11.87 (s,
1H, NH), 10.35 (s, 1H, OH–Ph), 7.81 (d, 3J = 8.7 Hz, 2H,
Ph-2/6), 7.02 (m, 1H, Pyr), 6.97 (m, 1H, Pyr), 6.86 (d,
3J = 8.7 Hz, 2H, Ph-3/5), 6.74 (m, 1H, Pyr), 6.70 (m, 1H,
Pyr), 6.14–6.08 (m, 2H, Pyr), 5.85 (d, J1,2 = 8.4 Hz, 1H,
H-1), 5.60 (d, JOH,4 = 5.7 Hz, 1H, OH-4), 5.55 (d,
JOH,3 = 5.7 Hz, 1H, OH-3), 5.00 (dd, J1,2 = 8.4 Hz,
J2,3 = 9.1 Hz, 1H, H-2), 4.50 (dd, Jgem = 11.9 Hz,
J5,6a <1 Hz, 1H, H-6a), 4.31 (dd, Jgem = 11.9 Hz,
J5,6b = 5.1 Hz, 1H, H-6b), 3.79 (m, 1H, H-5), 3.69 (m,
1H, H-3), 3.49 (m, 1H, H-4), 13C NMR (67.5 MHz,
DMSO-d6): d = 165.4 (C@O Ph), 162.0 (Ph-4), 159.3,
158.4 (C@O Pyr), 131.5 (Ph-2/6), 125.5 (Pyr), 124.2 (Pyr),
121.5, 120.4 (Ci–Pyr), 120.2 (Ph-1), 116.4 (Pyr), 115.3 (3C,
Pyr, Ph-3/5), 109.8 (Pyr), 109.4 (Pyr), 91.9 (C-1), 74.8 (C-
5), 73.6 (C-3), 72.2 (C-2), 69.7 (C-4), 63.1 (C-6).
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