Total synthesis of (+)-epopromycin B and its analogues - Studies on the inhibition of cellulose biosynthesis

Article abstract of DOI:10.1016/S0040-4039(00)01944-4

The described inhibition of the cellulose biosynthesis by epopromycin B prompted us to establish a short and efficient synthesis of the natural product, suitable for accessing a broad range of unnatural analogues in a multiparallel fashion. During the course of our synthesis the absolute configuration of (+)-epopromycin B could be determined.

Full text of DOI:10.1016/S0040-4039(00)01944-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 215–218
Total synthesis of (+)-epopromycin B and its analogues—studies
on the inhibition of cellulose biosynthesis
Markus R. Dobler*
Novartis Crop Protection AG, Lead Discovery, WRO-1060.310, 4002 Basel, Switzerland
Received 27 October 2000; accepted 2 November 2000
Abstract—The described inhibition of the cellulose biosynthesis by epopromycin B prompted us to establish a short and efficient
synthesis of the natural product, suitable for accessing a broad range of unnatural analogues in a multiparallel fashion. During
the course of our synthesis the absolute configuration of (+)-epopromycin B could be determined. © 2000 Elsevier Science Ltd.
All rights reserved.
Epopromycin A (1) and B (2) (Fig. 1) are fermenta-
tion products that were isolated from a Streptomycete
by Tatsuta and co-workers.1 The metabolites were
found on the basis of their biological activity, the
inhibition of the cell-wall synthesis of plant proto-
plasts.1 To confirm this interesting herbicidal activity
and to explore its mode of action, we aimed to estab-
lish a total synthesis of epopromycin B (2), as well as
suitable derivatives and analogues.
The most intriguing structural feature of 1 and 2 is the
b-hydroxy-a-epoxyketone terminus, which is also
found in the proteasome inhibitors TMC-86 and
TMC-96,2 as well as in the angiostatic natural product
eponemycin.3 We proposed that this common struc-
tural motif may represent the toxophore, and decided
to conserve it, but to investigate the activity of both
epoxy epimers during the course of our SAR studies.
This would also allow us to establish the C-2 stereo-
O
O
H
N
H
N
O
O
OH
OH
N
H
N
H
O
O
O
O
OH
OH
1
2
core
left side
right side
(toxophore)
Figure 1. Epopromycin A (1) and B (2).
(i)
+
H
94%
+
Br
TritHN
TritHN
TritHN
OH
O
OH OH
OH OH
1 : 1
3
4
6
5
Scheme 1. Reagents: (i) tBuLi, THF, −78°C, 2 h.
* Tel.: +41 (061) 6978896; fax: +41 (061) 6978726; e-mail: markus.dobler@syngenta.com
0040-4039/01/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01944-4

216
M. R. Dobler / Tetrahedron Letters 42 (2001) 215–218
chemistry of the natural product 2, which is so far
derivate 9. Epoxidation with mCPBA led to a nearly
1:1 mixture of oxiranes 10 and 11, column chromato-
graphic separation and deprotection under mild condi-
tions afforded the desired epoxy diols 12 and 13 in high
overall yield (Scheme 3).
unknown.
Our synthetic approach was influenced by the previ-
ously reported strategies for the synthesis of epone-
mycin.4–6 We envisaged a multiparallel synthesis using
stable and easily accessible building blocks and without
chromatographic purification after the final coupling
step. The results are outlined in the following schemes.
In order to verify the C-2 configuration of 13, its
precursor 10 was synthetically linked to the known
compound 144 (Scheme 3). Correlation of the spectro-
scopic data of 14 with the published ones established
the C-2 stereochemistry of 10 and 11, respectively.
The most challenging task was to establish a high
yielding access to the right side building block in both
C-2-epimeric forms and with assigned stereochemistry.
The chosen sequence starts with the addition of a vinyl
lithium species derived from 2-bromo-1-hydroxy-2-
propene7,8 (4) to aldehyde (3), providing an easily sepa-
rable mixture of the epimeric alcohols 5 and 6 in 93%
yield (Scheme 1).9
We then turned our attention towards the preparation
of the left side fragment. The required isononanic acid12
(15) was synthesized by Wittig reaction and hydrogena-
tion over Pd/C starting from 5-bromo-methylpen-
tanoate (16) (Scheme 4).
DCC mediated coupling of 15 with serine methylester
(18), followed by silylation and mild saponification
gave the desired ‘left side-core’-fragment 19 of epo-
promycin B (2) (Scheme 5).
Regioselective silylation of the terminal hydroxyl group
of 5 was followed by deprotection of the amino
group.10 The configuration of the obtained amino alco-
hol 7 was determined via its oxazolidinone derivative 8
1
by analyzing its H NMR shifts and coupling constants
(Scheme 2).11
The final coupling of 19 with each of the amino epox-
ides 12 or 13, was achieved using DCC and N-hydroxy-
succinimide. IBX oxidation and deprotection of the
The amino alcohol 7 was reprotected as its FMOC-
(i), (ii)
(iii)
TritHN
H2N
HN
78%
97%
OH OH
OH OTBS
O
OTBS
8
7
5
O
Scheme 2. Reagents: (i) TBSCl, DMF, imidazole, rt; (ii) AcOH, rt; (iii) (imidazole)2CO, NaH, DMF, rt.
(ii)
(i)
+
O
O
95%
95%
H2N
FMOCHN
FMOCHN
FMOCHN
OH OTBS
OH OTBS
OH OTBS
OH OTBS
7
9
10
(iii) quant.
11
1:1
(iii)
(iv), (v)
quant.
64%
O
O
O
H2N
H2N
H2N
O
OTBS
OH OTBS
OH OTBS
14
13
12
Scheme 3. Reagents: (i) FMOCCl, DMF, Hu¨nig’s base; (ii) mCPBA, CH2Cl2, rt; (iii) triethylamine, CH2Cl2, rt; (iv) IBX, DMSO,
rt; (v) triethylamine, CH2Cl2, −20°C.
Br
OCH3
OH
OCH3
(iii), (iv)
quant.
(i), (ii)
65%
O
O
O
16
17
15
Scheme 4. Reagents: (i) Ph3P, CH3CN; (ii) NaH, iPrCHO; (iii) K2CO3, MeOH; (iv) H2, Pd/C, EtOH.

M. R. Dobler / Tetrahedron Letters 42 (2001) 215–218
217
O
O
O
H
H
N
(iii), (iv)
N
(i), (ii)
53%
HO
OMe
OH
OTBS
OMe
quant.
NH3
O
Cl
O
OH
20
19
18
Scheme 5. Reagents: (i) NH3, CH2Cl2; (ii) DCC, CH2Cl2, 15; (iii) TBSCl, DMF, imidazole; (iv) LiOH, H2O, MeOH.
O
O
H
N
H
N
O
O
(S)
(R)
O
(S)
(i) - (iii)
68%
(iv), (ii), (iii)
72%
N
H
N
H
19
O
O
O
OH
OH
OH
OH
2
21
Scheme 6. Reagents: (i) DCC, N-hydroxysuccinimide, 13, CH2Cl2, −20°C, 4 h; (ii) IBX, DMSO, rt, 8 h; (iii) HF, CH3CN, 0°C,
2 h; (iv) DCC, N-hydroxysuccinimide, 12, CH2Cl2, −20°C, 4 h.
crude products led after a liquid–liquid extraction to
pure epopromycin B (2) and its C-2 epimer (21) in
about 70% yield (Scheme 6).
5. Schmidt, U.; Schmidt, J. J. Chem. Soc., Chem. Commun.
1992, 7, 529–530.
6. Hoshi, H.; Onnuma, T.; Aburaki, S.; Konishi, M.; Oki,
T. Tetrahedron Lett. 1993, 34 (6), 1047–1050.
7. Hatch, L. F.; Alexander, H. E.; Randolph, J. D. J. Org.
Chem. 1950, 15, 654–658.
8. Al Dulayymi, A. R.; Al Dulayymi, J. R.; Baird, M. S.;
Gerrard, M. E.; Koza, G.; Harkins, S. D.; Roberts, E.
Tetrahedron 1996, 52 (10), 3409–3424.
9. Other common N-protecting groups used to block the
amino substituent, led to the formation of alkyne side
products (30–50%) via elimination of the vinyl bromide
prior to the addition.
In addition, following the above protocol more then 15
different epopromycin B analogues were synthesized in
both C-2-epimeric forms. None of them showed any
significant inhibition of the biosynthesis of cellulose13
except for the natural product itself, for which an I50 of
7.6 mmol l−1 was measured. The C-2-epimer 21 was also
found to be inactive. However, some of the analogues
expressed pronounced antifungal activity against vari-
ous phytopathogens.
10. In the following only the reactions for 5 are shown.
However, the demonstrated chemistry works as well for
6. Both epimers can be used, since the distinguishing
stereochemical center collapses during the IBX oxidation.
11. Kano, S.; Yokomatsu, T.; Shibuya, S. J. Org. Chem.
1989, 54, 513–515. The 4-H and 5-H signals (1H, 400
MHz, CDCl3) of 8 (cis) and its trans isomer are as
follows: 8 (cis): l 3.81 (ddd, J=11.3, 7.9, 3.8 Hz, 4-H),
5.18 (d, J=7.9 Hz, 5-H); (trans): l 3.72 (ddd, J=9.0,
5.8, 4.9 Hz, 4-H), 4.71 (d, J=5.8 Hz, 5-H).
In conclusion a total synthesis of (+)-epopromycin B
(2), its C-2-epimer 21, and more then 30 unnatural
analogues has been established.14 Since the approach is
based on the utilization of the (2S)- and (2R)-epoxide
intermediates (13 and 12) the absolute configuration of
the natural and the unnatural metabolite was deter-
mined as shown in Scheme 6.
Acknowledgements
12. Davidson, B. S.; Schumacher, R. W. Tetrahedron 1993,
49 (30), 6569–6574.
13. The assay is based on the incorporation of 14C from
[14C]glucose into acid-insoluble cellulose in a tomato cell
suspension culture.
The author thanks Dr. K. Kreuz (Novartis Crop Pro-
tection AG, Basel) for the measurement of the cellulose
biosynthesis inhibition of the prepared compounds.
14. All new compounds were completely characterized and
gave satisfactory spectral and analytical data. Selected
data:
References
Compound 7: 1H NMR l 5.23 (s, 1H), 5.15 (s, 1H),
4.19–4.30 (m, 2H), 3.98 (d, J=5.5 Hz, 1H), 2.98 (m,
1H), 2.1 (sbr, 2H), 1.75 (m, 1H), 1.40 (m, 1H), 1.20 (m,
1H), 0.97 (d, J=6.4 Hz, 3H), 0.95 (s, 9H), 0,91 (d,
J=5.3 Hz, 3H), 0.12 (s, 6); ES-MS m/z 288 (M+1).
1. Tsuchiya, K.; Kobayashi, S.; Nishikiori, T.; Nakagawa,
T.; Tatsuta, K. J. Antibiot. 1997, 50 (3), 261–263.
2. Koguchi, Y.; Kohno, J.; Suzuki, S.-I.; Nishio, M.; Taka-
hashi, K.; Ohnuki, T.; Komatsubara, S. J. Antibiot. 1999,
52 (12), 1069–1076.
1
Compound 9: H NMR l 7.79 (d, J=7.8 Hz, 2H), 7.61
(d, J=7.3 Hz, 2H), 7.42 (tm, J=7.3 Hz, 2H), 7.32 (tm,
J=7.4 Hz, 2H), 5.19 (s, 1H), 5.12 (s, 1H), 4.45 (m, 2H),
4.18–4.32 (m, 4H), 3.92 (m, 1H), 2.91 (d, J=5.5 Hz,
1H), 1.60 (m, 1H), 1.34–1.40 (m, 2H), 0.94 (m, 15H),
0.13 (s, 3H), 0.12 (s, 3H); ES-MS m/z 510 (M+1).
3. Sugawara, K.; Hatori, M.; Nishiyama, Y.; Tomita, K.;
Kamei, H.; Konishi, M.; Oki, T. J. Antibiot. 1990, 43 (1),
8–18.
4. Sin, N.; Meng, L.; Auth, H.; Crews, C. M. Bioorg. Med.
Chem. 1998, 6, 1209–1217.

218
M. R. Dobler / Tetrahedron Letters 42 (2001) 215–218
Compound 12: 1H NMR l 3.96 (d, J=11.1 Hz, 1H),
0.82 (d, J=6.7 Hz, 3H), 0.82 (m, 12H), 0.0 (s, 3H),
−0.02 (s, 3H); ES-MS m/z 304 (M+1).
3.52 (d, J=11.1 Hz, 1H), 3.17 (d, J=6.9 Hz, 1H), 2.91
(ddd, J=10.0, 6.9, 2.7 Hz, 1H), 2.77 (d, J=4.7 Hz, 1H),
2.74 (d, J=4.7 Hz, 1H), 1.68 (m, 1H), 1.37 (m, 1H), 1.15
(m, 1H), 0.81 (m, 15H), 0.0 (s, 3H), −0.02 (s, 3H);
ES-MS m/z 304 (M+1).
Compound 21: 1H NMR l 7.31 (d, J=7.2 Hz, NH), 6.62
(d, J=7.4 Hz, NH), 4.62 (dd, J=11.8, 6.1 Hz, 1H), 4.41
(m, 1H), 4.23 (d, J=12.2 Hz, 1H), 3.94 (dd, J=11.0, 3.5
Hz, 1H), 3.57 (dd, J=11.0, 6.1 Hz, 1H), 3.51 (d, J=12.2
Hz, 1H), 3.37 (m, 1H), 2.96 (J=d, 4.7 Hz, 1H), 2.91 (d,
J=4.7 Hz, 1H), 2.17 (m, 2H), 1.86 (m, 1H), 1.08–1.64
(m, 12 H), 0.84 (s, 3H), 0.82 (s, 3H), 0.78 (s, 3H), 0.77 (s,
3H); ES-MS m/z 415 (M+1), 437 (M+Na).
Compound 13: 1H NMR l 3.82 (d, J=11.6 Hz, 1H),
3.60 (d, J=11.6 Hz, 1H), 3.60 (m, 1H), 2.91 (ddd,
J=9.8, 6.7, 2.8 Hz, 1H), 2.82 (d, J=4.7 Hz, 1H), 2.71
(d, J=4.7 Hz, 1H), 1.71 (m, 1H), 1.10–1.35 (m, 1H),
.
.