Total synthesis of nafuredin-γ, a γ-lactone related to nafuredin with selective inhibitory activity against NADH-fumarate reductase

Article abstract of DOI:10.1016/S0040-4039(03)01583-1

Nafuredin-γ (2) converted from nafuredin (1) under mild basic conditions showed the same inhibitory activity and selectivity against NADH-fumarate reductase as 1. Total synthesis of 2, a proposed active form of 1, has been accomplished by a convergent app

Full text of DOI:10.1016/S0040-4039(03)01583-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6441–6444
Total synthesis of nafuredin-g, a g-lactone related to nafuredin
with selective inhibitory activity against NADH-fumarate
reductase
Tohru Nagamitsu,a Daisuke Takano,a Kazuro Shiomi,a Hideaki Ui,a Yuichi Yamaguchi,b
b,
Rokuro Masuma,b Yoshihiro Harigaya,a Isao Kuwajimab,c and Satoshi Omura *
(
aSchool of Pharmaceutical Science, Kitasato University, 5-9-1 Shirokane, Minato-ku, Tokyo 108-8641, Japan
bKitasato Institute for Life Sciences, Kitasato University, 5-9-1 Shirokane, Minato-ku, Tokyo 108-8641, Japan
cCREST, The Japan Science and Technology Corporation (JST), 5-9-1 Shirokane, Minato-ku, Tokyo 108-8641, Japan
Received 7 June 2003; revised 20 June 2003; accepted 25 June 2003
Abstract—Nafuredin-g (2) converted from nafuredin (1) under mild basic conditions showed the same inhibitory activity and
selectivity against NADH-fumarate reductase as 1. Total synthesis of 2, a proposed active form of 1, has been accomplished by
a convergent approach using Stille coupling.
© 2003 Elsevier Ltd. All rights reserved.
Nafuredin (1)1,2 was isolated from the fermentation
broth of a fungal strain, Aspergillus niger FT-0554, in
the course of our screening for NADH-fumarate reduc-
tase (NFRD) inhibitors. 1 inhibited NFRD of Ascaris
suum with an IC50 value of 12 nM and showed selective
inhibition of its target enzyme complex I in helminth
mitochondria. In addition, 1 exerted anthelmintic activ-
ity against Haemonchus contortus in in vivo trials with
sheep.1 These useful biological activities of 1 attracted
our attention, and we previously reported the elucida-
tion of the absolute configuration3 and the total
synthesis4 of 1.
During the course of the total synthetic studies of 1, we
discovered that 1 was converted to a novel g-lactone
derivative (2) under mild basic conditions (K2CO3 in
MeOH) as shown in Scheme 1, which existed as a
mixture of keto-enol tautomers.5 We named the g-lac-
tone derivative (2) nafuredin-g. Since nafuredin-g (2) is
not detected in the above fermentation broth6 at all, 2
is not produced directly by the above strain and is
formed only from 1 under the basic conditions via
enolization, opening of the epoxide, and formation of
the g-lactone. We expected 2 to have inhibitory activity
against NFRD as well as 1 because the conversion of 1,
Scheme 1.
* Corresponding author. Tel.: +81-3-5791-6101; fax: +81-3-3444-8360; e-mail: omura-s@kitasato.or.jp
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01583-1

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T. Nagamitsu et al. / Tetrahedron Letters 44 (2003) 6441–6444
derivatives with modification at the side chain moiety.
The enol lactone 3 could be constructed by Hornor–
Emmons reaction of aldehyde 5 with a known phos-
phonate 6 followed by hydrostannylation.
We started from the known diol 7 (97% e.e.)8 to lead to
the aldehyde 5 as shown in Scheme 2. Although acetal-
ization of 7 under usual conditions (cyclopentanone
and a catalytic amount of TsOH in benzene) caused
serious racemization,9 treatment with cyclopentanone
dimethylacetal in the presence of Sc(OTf)3 catalyst10
gave 8 almost quantitatively. Cleavage of the p-
methoxybenzoyl ester with sodium methoxide in
methanol produced 9 in 97% yield. Oxidation of 9 with
Dess–Martin periodinane11 followed by Wittig reaction
with methyl (triphenylphosphoranylidene)acetate fur-
nished 10 in 75% yield in two steps. The resulting ester
was reduced with DIBAL to give allyl alcohol 11 in
94% yield. Sharpless asymmetric epoxidation12 of 11
using (+)-diethyl tartrate provided the corresponding
epoxide (94% d.e.), which was further converted to
chloride 12 by treatment with NCS and Ph3P in the
Figure 1.
presence of propylene oxide as
a chloride ion
possessing the labile b,g-epoxycarbonyl unit, to 2 is
likely to occur under the assay conditions for NFRD.
We tested the inhibitory activity against NFRD of 2.
Our test of the inhibitory activity against NFRD
proved 2 shows the same inhibitory activity and selec-
tivity as 1, which suggests the possibility that 2 may be
an active form of 1 and may permit a structural sim-
plification of the lactone moiety in the structure–activ-
ity relationship studies of 1. We report herein the total
synthesis of nafuredin-g (2).
scavenger13 in 94% yield in two steps. Base-induced
elimination14 of 12 with n-BuLi furnished 13 in 99%
yield. Silyl ether protection of 13 with TBSOTf and
2,6-lutidine followed by hydrolysis of cyclopentylidene
acetal with 70% aq. AcOH led to diol 14 in 87% yield
in two steps. Subsequent oxidation of the primary
alcohol in 14 with 2,2,6,6-tetramethylpiperidinyloxy
(TEMPO) and trichloroisocyanuric acid15 gave the
desired aldehyde 5 in 77% yield, while DMSO oxidation
(Swern conditions and Parikh–Doering conditions),
Dess–Martin oxidation and TEMPO–NaClO–KBr oxi-
dation lowered the product yield accompanied with
decomposition of the product. The use of trichloroiso-
cyanuric acid as co-oxidant in TEMPO oxidation of 14
was crucial for high yield and reproducibility.
As outlined in Figure 1, our synthetic strategy of
nafuredin-g (2) is convergent and involves the assembly
of C1ꢀC7 segment 3 and C8ꢀC18 segment 47 by Stille
coupling at the final step, which will give us a simple
and efficient way to synthesize various nafuredin-g
Scheme 2.

T. Nagamitsu et al. / Tetrahedron Letters 44 (2003) 6441–6444
6443
18 was obtained in 52% yield along with its regioisomer
19 (23%). Subsequent Stille coupling of 18 with 4 could
not be effected so well under the same conditions, but
addition of Ph2P(O)O−Bu4N+20 improved the yield of
the coupling product, which afforded nafuredin-g (2) in
85% yield by removal of the TBS ether group with
HF·pyridine. But these moderate yields in hydrostannyl-
ation and Stille coupling were still far from satisfactory.
Next, we focused on the high regioselectivity of a
palladium-catalyzed hydrostannylation of alkynyl bro-
mides reported by Guibe´ et al.21 Although bromination
of the terminal alkynes 15 and 16 led to decomposition
of substrates, TBS protected enol ether 20, which was
constructed by Hornor–Emmons reaction of 5 with 6
and LHMDS in 75% yield, afforded alkynyl bromide
22
21 quantitatively by treatment with NBS and AgNO3
(Scheme 4). Removal of the TBS ether group with
HF·pyridine gave enol 22 quantitatively. Subsequent
palladium-catalyzed hydrostannylation of 22 provided
the desired E-alkenylstannane 3 in 71% yield with high
regioselectivity (3:17=15:1), which was finally con-
verted to nafuredin-g (2) in good yield as mentioned
above. Synthetic nafuredin-g (2) was identical with that
derived from natural nafuredin (1) in all respects23
1
([h]D, H and 13C NMR, IR, FAB-MS and inhibitory
activity against NFRD).
Scheme 3.
In conclusion, we have discovered nafuredin-g (2), a
proposed active form of nafuredin (1), and achieved its
first total synthesis. Further investigation of the struc-
ture-activity relationship and biological studies of 1 and
2 are currently in progress.
Hornor–Emmons reaction of 5 with the known phos-
phonate 616 in the presence of DBU and LiCl17
afforded g-lactone 15 in 75% yield, which was treated
with HF·pyridine to give 16 in 91% yield (Scheme 3).
Subsequent palladium-catalyzed hydrostannylation18 of
16 with Bu3SnH and PdCl2(PPh3)2 proceeded with non-
regioselectivity, yielding the desired E-alkenylstannane
3 (39%) and its regioisomer 17 (30%). After separation
by silica gel chromatography, Stille coupling19 between
3 and vinyl iodide 4 in the presence of PdCl2(MeCN)2
gave nafuredin-g (2) in 72% yield. Although the total
synthesis of 2 was achieved, we needed a more effective
hydrostannylation procedure with higher regioselectiv-
ity for large-scale and analog synthesis of 2. We also
examined hydrostannylation of the TBS ether 15, and
Acknowledgements
This study was supported in part by the Grand of 21st
Century COE Program, Ministry of Education, Cul-
ture, Sports, Science and Technology, Japan. T. N.
acknowledges a Kitasato University research grant for
young researchers.
Scheme 4.

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T. Nagamitsu et al. / Tetrahedron Letters 44 (2003) 6441–6444
15. (a) Anelli, P. L.; Biffi, C.; Montanari, F.; Quici, S. J. Org.
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[h]2D5=+4.0 (c 0.1, CHCl3): IR (KBr) 3374, 2960, 2925,
2858, 1754, 1737, 1660, 1261, 1101 cm−1: HRMS [FAB,
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1
H 361.2379: H NMR (270 MHz, CDCl3) enol l 0.86 (t,
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0.99 (d, 3H, J=6.6 Hz, 16-Me), 1.32 (m, 2H, H-17), 1.46
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J=7.3 Hz, H-5), 5.46 (dd, 1H, J=14.8, 7.6 Hz, H-15),
5.50 (dd, 1H, J=15.5, 7.3 Hz, H-6), 5.70 (dd, 1H, J=
15.5, 7.3 Hz, H-9), 5.76 (d, 1H, J=10.9 Hz, H-13), 6.00
(dd, 1H, J=15.5, 10.2 Hz, H-8), 6.15 (s, 1H, H-3), 6.18
(dd, 1H, J=14.8, 10.9 Hz, H-14), 6.30 (dd, 1H, J=15.5,
10.2 Hz, H-7), ketone l 0.86 (t, 3H, J=7.6 Hz, H-18),
0.97 (d, 3H, J=6.7 Hz, 10-Me), 0.99 (d, 3H, J=6.6 Hz,
16-Me), 1.32 (m, 2H, H-17), 1.54 (s, 3H, 4-Me), 1.70 (s,
3H, 12-Me), 1.91–2.12 (m, 3H, H-11 and H-16), 2.32 (d,
1H, J=18.8 Hz, 1/2 H-3), 2.36–2.43 (m, 1H, H-10), 3.03
(d, 1H, J=18.8 Hz, 1/2 H-3), 4.21 (m, 1H, H-5), 5.46 (dd,
1H, J=14.8, 7.6 Hz, H-15), 5.50 (dd, 1H, J=15.5, 7.3
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1H, J=10.9 Hz, H-13), 6.00 (dd, 1H, J=15.5, 10.2 Hz,
H-8), 6.18 (dd, 1H, J=14.8, 10.9 Hz, H-14), 6.30 (dd,
1H, J=15.5, 10.2 Hz, H-7).
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