Total synthesis of (+)-Sch 725680: Inhibitor of mammalian A-, B-, and Y-family DNA polymerases

Article abstract of DOI:10.1021/ol301865u

The total synthesis of (+)-Sch 725680, a member of the hydrogenated azaphilone family, has been accomplished. The synthesis confirmed the absolute configuration and biological activities of the natural product. A key reaction to construct a hydrogenated azaphilone core skeleton is a Ti-mediated aldol reaction.

Full text of DOI:10.1021/ol301865u

ORGANIC
LETTERS
2012
Vol. 14, No. 17
4303–4305
Total Synthesis of (þ)-Sch 725680:
Inhibitor of Mammalian Aꢀ, Bꢀ, and
YꢀFamily DNA Polymerases
Toshifumi Takeuchi,† Yoshiyuki Mizushina,‡ Satoshi Takaichi,† Natsuki Inoue,§
Kouji Kuramochi, Satomi Shimura,† Yusuke Myobatake,† Yuri Katayama,†
Kenji Takemoto,† Shogo Endo,† Shinji Kamisuki,† and Fumio Sugawara*,†
Department of Applied Biological Science, Faculty of Science and Technology,
Tokyo University of Science (RIKADAI), Noda-shi, Chiba 278-8510, Japan,
Laboratory of Food & Nutritional Sciences, Department of Nutritional Science,
Kobe-Gakuin University, Nishi-ku, Kobe, Hyogo 651-2180, Japan,
Department of Radiation Biosciences, Faculty of Pharmaceutical Sciences,
Tokyo University of Science (RIKADAI), Noda-shi, Chiba 278-8510, Japan, and
Graduate School of Life and Environmental Sciences, Kyoto Prefectural University,
Sakyo-ku, Kyoto 606-8522, Japan
sugawara@rs.noda.tus.ac.jp
Received July 9, 2012
ABSTRACT
The total synthesis of (þ)-Sch 725680, a member of the hydrogenated azaphilone family, has been accomplished. The synthesis confirmed the
absolute configuration and biological activities of the natural product. A key reaction to construct a hydrogenated azaphilone core skeleton is a
Ti-mediated aldol reaction.
Sch 725680, 1, was originally isolatedbyYangetal. from
a culture of an Aspergillus sp. and was shown to display
growth inhibitory activity against Saccharomyces cerevi-
siae PM503 and Candida albicans C43 with MICs of 8 and
64 μg/mL, respectively (Figure 1).1 Recently, Stierle et al.
reisolated (þ)-1, named berkazaphilone C, together with
berkazaphilone B, 2, from a culture of Penicillium rubrum
as caspase 1 inhibitors with an IC100 of 25 μM.2 They also
reported that 1 selectively inhibits cell proliferation against
the SR cell line in the National Cancer Institute (NCI) 60
cell line assay with a GI50 of 0.38 μM. In our natural
product isolation program for the construction of a small
molecule library,3 we also independently reisolated (þ)-1
together with 2, named pinophilin A, and pinophilin B, 3,
from a culture of Penicillium pinophilum Hedgcok as
inhibitors of the mammalian A-, B-, and Y-family of
DNA polymerases.3g Using the excitation chirality method,
we also determined that the absolute configurations are
7S,8S,8aS.
† Department of Applied Biological Science, Faculty of Science and
Technology, Tokyo University of Science.
‡ Laboratory of Food & Nutritional Sciences, Department of Nutritional
Science, Kobe-Gakuin University.
§ Department of Radiation Biosciences, Faculty of Pharmaceutical
Sciences, Tokyo University of Science.
Graduate School of Life and Environmental Sciences, Kyoto Prefectural
University.
(1) Yang, S.-W.; Chan, T.-M.; Terracciano, J.; Patel, R.; Patel, M.;
Gullo, V.; Chu, M. J. Antibiot. 2006, 59, 720–723.
(2) Stierle, A. A.; Stierle, D. B.; Girtsman, T. J. Nat. Prod. 2012, 75,
344–350.
Compounds 1, 2, and 3 belong to a class of natural
products called hydrogenated azaphilones, a subclass of
azaphilones, that share a highly oxygenated bicyclic core, a
r
10.1021/ol301865u
2012 American Chemical Society
Published on Web 08/28/2012

available imide 76 and aldehyde 87 using the Ti-mediated
aldol reaction developed by Kobayashi’s group.8
Scheme 1. Retrosynthetic Analysis of Sch 725680
Figure 1. Structure of Sch 725680, Berkazaphilone B, and
Pinophilin B.
chiral tertiary hydroxyl group, and a trienone moiety.4
Although a number of synthetic studies of azaphilones,
such as mitorubrinic acid,5 have been reported, no syn-
thetic study of hydrogenated azaphilones has been dis-
closed. A flexible and scalable synthetic route for
structureꢀactivity relationship studies is required for
further biological studies of hydrogenated azaphilones.
Herein, we report the first total synthesis of (þ)-1, as well
as confirmation of its absolute configuration and inhibi-
tory activity against the mammalian A-, B-, and Y-family
of DNA polymerases and the proliferation of human
cancer cell lines.
Our retrosynthesis of (þ)-Sch 725680 is depicted in
Scheme 1. Sch 725680, 1, should be derived from hydro-
genated azaphilone core structure 4 by the attachment of
an orsellinate fragment. The hydrogenated azaphilone
core skeleton should be prepared from diketone 5 using
intramolecular aldol cyclization and 1,6-oxy-Michael cy-
clization. Diketone 5 should be derived from lactone 6,
which should be stereoselectively obtained from readily
The Ti-mediated aldol reaction of imide 7 and aldehyde
8 (LDA, ClTi(O-i-Pr)3, THF, 81%) gave anti-1,2-diol 9,9
followed by the deprotection of isopropylidene acetal and
cleavage of the chiral auxiliary group (Zn(NO3)2 6H2O,
3
MeCN, 89%) to afford lactone 10 as a diastereomeric
mixture of the hydroxy methyl group (R/β = 4:1). Iso-
propylidene acetalprotection of 1,3-diol10((MeO)2CMe2,
10-camphorsulfonic acid (CSA), CH2Cl2) gave lactone 6r
and 6β in yields of 70% and 18%, respectively. After both
lactone 6r and 6β were converted to Weinreb amides
(MeO(Me)NH HCl, i-PrMgBr, THF), oxidation of alco-
3
hols (PCC, MS4A, CH2Cl2) gave corresponding aldehydes
11β and 11β in yields of 65% and 68% over two steps,
respectively. The stereochemistry of the C8a formyl group
in aldehyde 11β was inverted under basic conditions
(DBU, THF, 96%) to afford aldehyde 11r, as shown in
Scheme 2.
(3) (a) Ogawa, A.; Murakami, C.; Kamisuki, S.; Kuriyama, I.;
Yoshida, H.; Sugawara, F.; Mizushina, Y. Bioorg. Med. Chem. Lett.
2004, 14, 3539–3543. (b) Kamisuki, S.; Ishimaru, C.; Onoda, K.;
Kuriyama, I.; Ida, N.; Sugawara, F.; Yoshida, H.; Mizushina, Y. Bioorg.
Med. Chem. 2007, 15, 3109–3114. (c) Naganuma, M.; Nishida, M.;
Kuramochi, K.; Sugawara, F.; Yoshida, H.; Mizushina, Y. Bioorg. Med.
Chem. 2008, 16, 2939–2944. (d) Nishida, M.; Ida, N.; Horio, M.;
Takeuchi, T.; Kamisuki, S.; Murata, H.; Kuramochi, K.; Sugawara,
F.; Yoshida, H.; Mizushina, Y. Bioorg. Med. Chem. 2008, 16, 5115–
5122. (e) Kimura, T.; Nishida, M.; Kuramochi, K.; Sugawara, F.;
Yoshida, H.; Mizushina, Y. Bioorg. Med. Chem. 2008, 16, 4594–4599.
(f) Kimura, T.; Takeuchi, T.; Kumamoto-Yonezawa, Y.; Ohashi, E.;
Ohmori, H.; Masutani, C.; Hanaoka, F.; Sugawara, F.; Yoshida, H.;
Mizushina, Y. Bioorg. Med. Chem. 2009, 17, 1811–1816. (f) Yamaguchi,
Y.; Manita, D.; Takeuchi, T.; Kuramochi, K.; Kuriyama, I.; Sugawara,
F.; Yoshida, H.; Mizushina, Y. Biosci. Biotechnol. Biochem. 2010, 74,
793–801. (g) Myobatake, Y.; Takeuchi, T.; Kuramochi, K.; Kuriyama,
K.; Ishido, T.; Hirano, K.; Sugawara, F.; Yoshida, H.; Mizushina, Y.
J. Nat. Prod. 2012, 75, 135–141.
Freshly prepared dibromoalkene 1210 was treated with
n-BuLi (THF, ꢀ78 °C to rt, 2 h in the dark) to give alkynyl
lithium 13; the resulting solution was immediately used in
the next step (Scheme 3). The treatment of aldehyde 11r
with alkynyl lithium 13 followed by treatment with MeLi
and oxidation of the resultant alcohol gave diketone 5.
Aldol cyclization11 of diketone 5 (TBAF, THF, 0 °C, 2 h)
gave β-hydroxy ketone 14 and R,β-unsaturated ketone 15
(8) (a) Kamino, T.; Murata, Y.; Kawai, N.; Hosokawa, S.; Kobayashi,
S. Tetrahedron Lett. 2001, 42, 5249–5252. (b) Murata, Y.; Kamino, K.;
Hosokawa, S.; Kobayashi, S. Tetrahedron Lett. 2002, 43, 8121–8123.
(9) The absolute configuration of secondary alcohol was determined
as S by modified Mosher’s method using (S)- and (R)-MTPA esters of 9.
The relative configuration of 1,2-diol was determined as the anti con-
figuration by NMR studies of acetonide derivatives 6r and 6β. See
Supporting Information.
(4) Osmanova, N.; Schultze, W.; Ayoub, N. Phytochem. Rev. 2010, 9,
315–342.
(5) (a) Chong, R.; Gray, R. W.; King, R. R.; Whalley, W. B. J. Chem.
Soc., Chem. Commun. 1970, 101. (b) Whalley, W. B.; Chong, R.; Gray,
R. W.; King, R. R. J. Chem. Soc. C 1971, 3571–3575. (c) Marsini, M. A.;
Gowin, K. M.; Pettus, T. R. R. Org. Lett. 2006, 8, 3481–3483. (d) Zhu, J.;
Porco, A., Jr. Org. Lett. 2006, 8, 5169–5171.
(6) (a) Murata, Y.; Kamino, T.; Aoki, T.; Hosokawa, S.; Kobayashi,
S. Angew. Chem., Int. Ed. 2004, 43, 3175–3177. (b) Onodera, Y.; Suzuki,
T.; Kobayashi, S. Org. Lett. 2011, 13, 50–53.
(7) Bates, H. A.; Farina, J.; Tong, M. J. Org. Chem. 1986, 51, 2637–
2641.
(10) (a) Bestmann, H. J.; Frey, H. Liebigs Ann. Chem. 1980, 12, 2061–
2071. (b) Bialy, L.; Waldmann, H. Chem.;Eur. J. 2004, 10, 2759–2780.
(11) Mischne, M. Tetrahedron Lett. 2003, 44, 5823–5826.
4304
Org. Lett., Vol. 14, No. 17, 2012

agreement with those reported for 11,2,3g ([R]D þ103
22
(c 0.27, MeOH)3g).
Scheme 2. Preparation of Aldehyde 11r
Scheme 3. Completion of the Total Synthesis of (þ)-Sch 725680
at yields of 69% and 11% asca. 10:1 and ca. 3:1 mixtures of
E/Z isomers, respectively. Although isomerization of
E-olefin to Z-olefin occurred,12 we thought Z-olefin should
be isomerized to E-olefin under 1,6-oxy-Michael cycliza-
tion conditions. Dehydration of alcohol 14 (Burgess re-
agent, toluene, 100 °C) gave enone 15 in 94% yield as a ca.
3:1 mixture of E/Z isomers. Isopropylidene acetal depro-
tection gave alcohol 16 (CSA, MeOH, 77% as a ca. 0.8:1
mixture of E/Zisomers) to cause further E/Z-isomerization.
1,6-Oxy-Michael cyclization of alcohol 16 was achieved
using silver trifluoromethanesulfonate13 in CH2Cl2 to
give hydrogenated azaphilone core skeleton 4 with only an
E-olefin configuration, as expected. After benzyl depro-
tection (BCl3, CH2Cl2, 0 °C, 67%) and the introduc-
tion of protected orsellinate (4-benzyloxy-2-methoxy-
6-methylbenzoyl chloride, DMAP, i-Pr2NEt, CH2Cl2,
53%),5d both benzyl and methyl deprotection (BCl3,
CH2Cl2, 63%)5d gave hydrogenated azaphilone Sch
725680, 1. The characterization data (1H and 13C NMR,
IR, MS, UV, CD, inhibitory activity of mammalian DNA
polymerases and of the proliferation of human cancer and
In summary, we have accomplished the first total synthe-
sis of hydrogenated azaphilone Sch 725680, thereby, con-
firming its absolute configuration and biological activities.
The natural product was obtained in a 13-step sequence
with an overall yield of 6.0%, starting from known alde-
hyde 8. Key steps to establish the hydrogenaged azaphi-
lone core skeleton were a Ti-mediated aldol reaction with
controlled introduction of the C7- and C8-stereochemistry
and a silver-catalyzed 1,6-oxy-Michael cyclization.
Acknowledgment. We are grateful to Prof. Susumu
Kobayashi of Tokyo University of Science for his helpful
advice.
normal cells14) for 1 ([R]D þ103 (c 0.1, MeOH)) was in
22
Supporting Information Available. Detailed experimen-
tal procedures, characterization data, 1H and 13C NMR
spectra, and biological activities of both synthetic and
natural Sch 725680. This material is available free of
charge via the Internet at http://pubs.acs.org.
(12) Ramage, R.; Sattar, A. Chem. Commun. 1970, 173–175.
(13) (a) Wang, C.; Forsyth, C. J. Org. Lett. 2006, 8, 2997–3000.
(b) Nicolaou, K. C.; Cole, K. P.; Frederick, M. O.; Aversa, R. J.;
Denton, R. M. Angew. Chem., Int. Ed. 2007, 46, 8875–8879.
(14) Synthetic Sch 725680’s ability to inhibit the activity of DNA
polymerases and proliferation of five human cancer cell lines was
investigated and compared to that of the natural substance. Synthetic
Sch 725680 showed identical biological activity. See Supporting
Information.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 17, 2012
4305