The first total synthesis of moracin O and moracin P, and establishment of the absolute configuration of moracin O

Article abstract of DOI:10.1039/b823340c

The first total synthesis of the naturally occurring benzofurans, moracins O and P was achieved using a Sonogashira cross coupling reaction followed by in situ cyclization, and the absolute configuration of natural moracin O was established.

Full text of DOI:10.1039/b823340c

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COMMUNICATION
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The first total synthesis of moracin O and moracin P, and establishment
of the absolute configuration of moracin Ow
a
a
Navneet Kaur,z Yan Xia,z Yinglan Jin,a Nguyen Tien Dat,a Kondaji Gajulapati,b
Yongseok Choi,b Young-Soo Hong,a Jung Joon Leea and Kyeong Lee*a
Received (in College Park, MD, USA) 5th January 2009, Accepted 10th February 2009
First published as an Advance Article on the web 4th March 2009
DOI: 10.1039/b823340c
The first total synthesis of the naturally occurring benzofurans,
moracins O and P was achieved using a Sonogashira cross
coupling reaction followed by in situ cyclization, and the
absolute configuration of natural moracin O was established.
Mori Cortex Radicis, the root bark of some Morus species, has
been used in oriental medicine as an antidiabetic, a diuretic,
an expectorant and a laxative agent.1 Various prenylated
flavonoids, benzofurans and other phenolic compounds2 with
biological activities, including cytotoxicity,3 COX-1 and 2
inhibition,4 and NO production,5 have been isolated from
Fig. 1 Natural moracins.
these species.
In search of small molecule inhibitors against hypoxia-
inducible factor (HIF)-1,6 which is a master regulator of
the adaptation process of cancer cells to tumor hypoxia, a
bioassay-guided fractionation study with a hypoxia response
element (HRE) reporter assay on natural products has
been conducted. Some moracins were found to exhibit
potent inhibitory effects in cell-based HRE assays in human
hepatocarcinoma Hep3B cell lines. (ꢀ)-Moracin O (1; Fig. 1)
exhibited the strongest inhibitory activity (IC50 = 0.14 nM),
while (ꢀ)-moracin P (2) had an IC50 value of 0.65 nM.7
Moracins O and P were first isolated in 1998 from an
acetone extract of cortex and phloem tissues of Morus alba
shoots infected with Fusarium solani f. sp. mori, and their
spectra were reported.8 Interesting biological properties with a
novel biosynthetic pathway made these phytochemicals an
attractive synthetic target. Syntheses using simple strategies
for moracin M9 and an efficient route to moracin C have been
reported.10 To the best of our knowledge, there are no reports
concerning the synthesis of moracins O or P. Emerging interest
in the field of HIF inhibitors, together with the considerable
biological activity of moracins O and P, have provided the
motivation to complete an adaptable and scalable total
synthesis of these compounds. However, the absolute
configuration of moracin O has still not been confirmed.
In this communication, we first report efficient synthesis
of (ꢁ)-moracins O and P via ortho-prenylated phenolic
Scheme 1 Retrosynthetic approach.
intermediate 6 as a common precursor. The asymmetric
synthesis of (R)- and (S)-moracin O will be introduced, and
the absolute configuration of bioactive natural moracin O is
also demonstrated.
A retrosynthetic analysis for (ꢁ)-moracins O and P through
the disconnection of the benzo[b]furan nucleus of these two
compounds can be constructed by Sonogashira cross coupling
with acetylene 9 and 14 and subsequent in situ cyclization
(Scheme 1). The substituted acetylene, 1,3-bis-(tert-butyldi-
methylsilanyloxy)-5-ethynylbenzene (9) can be derived from
commercially available 3,5-dihydroxybenzaldehyde (15).11 It
was also envisioned that nucleus 14 (a benzohydropyran in
the case of moracin P; a benzohydrofuran for moracin O)
could be synthesized from the common ortho-prenylated
phenolic intermediate, 6, which in turn can be derived from
2,4-dihydroxybenzaldehyde (3).
a Molecular Cancer Research Center, Korea Research Institute of
Bioscience and Biotechnology (KRIBB), 52 Eoeun-dong,
Yuseong-gu, Daejeon 305-806, Korea. E-mail: kaylee@kribb.re.kr;
Fax: +82 42 860 4595; Tel: +82 42 860 4382
b School of Life sciences and Biotechnology, Korea University,
Anam-dong, Seongbuk-gu, Seoul 136-713, Korea
w Electronic supplementary information (ESI) available: Experimental
details for synthesis and biological procedures, characterization data
and NMR spectra. See DOI: 10.1039/b823340c
z These authors contributed equally to this work.
Our synthesis started with the iodination of 3 in the
presence of iodine monochloride to produce 2,4-dihydroxy-5-
iodobenzaldehyde (4; Scheme 2). In this reaction, however, we
ꢂc
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Scheme 2 Reagents and conditions: (a) ICl, AcOH, rt; (b) Boc2O, K2CO3,
49% over steps; (c) (i) NaBH4, THF, H2O; (ii) (2-methylprop-1-enyl)-
magnesium bromide, THF, ꢀ78 1C to rt.
obtained the undesired regioisomer 2,4-dihydroxy-3-iodobenz-
aldehyde as a side product in an inseparable 1 : 2 ratio. This
mixture was confirmed by the 1H NMR spectrum, leading us to
believe that we in fact had two regioisomers. This crude mixture
was protected with a Boc group using Boc2O and K2CO3. At
this stage the two isomers could be easily separated by column
chromatography and the doubly protected iodobenzaldehyde,
5, was obtained in an overall yield of 49% over two steps from
3. The reduction of 5 with NaBH4 in THF–H2O (19 : 1)
afforded an intermediate benzyl alcohol, which, after work
up, but with no additional purification, was allowed to react
with 2-methylpropenyl magnesium bromide to provide the
prenylated derivative, 612 in a 38% yield over two steps. This
compound served as a common intermediate for the synthesis
of both moracin O and moracin P.
Scheme 4 Reagents and conditions: (a) m-CPBA, EtOAc, 0 1C;
(b) LiOH, MeOH, 56.7% over 2 steps; (c) 9, Pd(PPh3)2Cl2, CuI,
Et3N, dioxane, 85 1C, 31%; (d) HF–Py, THF–Py, 0 1C to rt, 74.8%.
After completion of the synthesis of (ꢁ)-moracin P, we
turned our attention to (ꢁ)-moracin O (Scheme 4). The key
step was the construction of benzohydrofuran nucleus 12. This
compound can be distinguished from its isomer, 8, at the
Boc-deprotected stage by thin layer chromatography (TLC)
and was characterized by 1H NMR spectroscopy as the cyclic
CH2 of compound 8 undergoes geminal coupling, while the
CH2 of compound 12 lacks this coupling. In situ epoxidation
of 6, followed by [5-exo-tet]-cyclization using NaHCO3 in
CHCl3, was unsatisfactory for Boc-protected 12, because
it gave
a product that was found to be significantly
For the synthesis of (ꢁ)-moracin P, in situ epoxidation with
m-CPBA, followed by [6-endo-trig]-closure under acidic
conditions (p-TSA), furnished Boc-protected benzohydropyran
7 in a 70% yield (Scheme 3). During initial trials of the
Boc deprotection of 7 under HCl–dioxane conditions, we
observed simultaneous de-iodination,13 but under mild
conditions, i.e. with ZnBr2,14 8 was readily obtained in an
80% yield. The reaction of 8 with alkyne 9 under Sonogashira
cross coupling conditions, followed by in situ cyclization in
dioxane, afforded benzo[b]furan intermediate 10 in a 36%
yield. Early attempts to remove the TBDMS groups of
compound 10 with TBAF, stirred overnight, yielded mixtures
of products, possibly due to the strong basic conditions
and/or the long reaction time, which either permitted group
migration15 or opening of the pyran ring. The same deprotection
reaction with HF–pyridine complex afforded clean removal of
the TBDMS groups and provided the desired racemic moracin
P (2) in a 75% yield.
contaminated with pyran 7. Accordingly, we addressed the
reaction of intermediate epoxide 11 with LiOH, which gave 12
as the sole product in good yield. The reaction of 12 with 9,
employing Sonogashira cross coupling under basic conditions
and in situ cyclization, afforded 13, which upon final deprotec-
tion with HF–pyridine, provided (ꢁ)-moracin O (1) in a 75%
yield. The NMR spectra of synthetic (ꢁ)-1 and (ꢁ)-2 were
identical to the spectra of the corresponding natural products.8b
Furthermore, in order to determine the absolute configuration
of natural moracin O, the asymmetric syntheses of (R)- and
Scheme 5 Reagents and conditions: (a) AD-mix-a, methanesulfonamide,
t-BuOH–H2O for 17a; AD-mix-b, methanesulfonamide, t-BuOH–H2O
for 17b; (b) (i) tosyl chloride, pyridine, (ii) K2CO3, methanol; (c) Pd
black, 1,2-cyclohexadiene, EtOH; (d) ICl, AcOH; (e) 9, Pd(PPh3)2Cl2,
CuI, Et3N, dioxane, 85 1C; (f) HF–Py, THF–Py, 0 1C to rt.
Scheme 3 Reagents and conditions: (a) m-CPBA, p-TSA, rt, 70%;
(b) ZnBr2, DCM, rt, 80%; (c) 9, Pd(PPh3)2Cl2, CuI, Et3N, dioxane,
85 1C, 36.9%; (d) HF–Py, THF–Py 0 1C to rt, 75%.
ꢂc
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Table 1 In vitro inhibition of HIF transcriptional activity
This work was supported by the Korea Science and
Engineering foundation (KOSEF) grant funded by the
Korea Government (Bio-technology 2006-02662 and General
Research Grants RO1-2008-11354-0).
a
Compound
IC50
Cell viability
(ꢁ)-1
6.76 nM
10.7 nM
0.19 nM
6.22 mM
0.14 nM
0.65 mM
430 mM
430 mM
430 mM
430 mM
430 mM
430 mM
(ꢁ)-2
(R)-(ꢀ)-1
(S)-(+)-1
(ꢀ)-17
Notes and references
(ꢀ)-27
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95.
a
Values were obtained from a cell-based HRE luciferase assay
(see the ESI).
(S)-moracin O were conducted as shown in Scheme 5.
Through three steps,16 3 provided benzyl-protected prenylated
derivative 16. The Sharpless asymmetric dihydroxylation of 16
using commercially available catalyst AD-mix-a or b17,18
yielded diols (S)-17a or (R)-17b in 90% and 95% ee,
respectively (determined by chiral HPLC).19 The selective
tosylation of the diols (S)-17a or (R)-17b using tosyl chloride,
followed by basic treatment of the resulting tosylated
compound, led to exclusive formation of the desired epoxides,
(R)-18a or (S)-18b, in a 45% overall yield. Debenzylation of
the requisite enantiomers (R)-18a or (S)-18b using palladium
black and 1,4-cyclohexadiene as a hydrogen donor,20 provided
the corresponding benzofuran compounds, (R)-19a or (S)-19b.
Introduction of the iodo group at the ortho position of
benzofuran derivative (R)-19a or (S)-19b by using iodine
monochloride furnished optically active benzofurans (R)-20a
or (S)-20b in good yields. This key intermediate reacted with
protected ethynyl benzene compound 9 through a Sonogashira
reaction by using palladium catalyst to afford chiral moracin
O precursors (R)-21a or (S)-21b in moderate yields. Finally,
deprotection with hydrogen fluoride in pyridine produced
target compounds (R)- and (S)-moracin O in good yields.
The spectroscopic data of (R)-moracin O were in accordance
with that reported for the natural moracin O.19b The specific
3 Y. Q. Shi, T. Fukai, H. Sakagami, W. J. Chan, P. Q. Yang,
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Fitoterapia, 1998, 69, 554; (c) L. Cui, M. Na, H. Oh, E. Y. Bae,
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rotation of (R)-moracin O ([a]28 = ꢀ4.45 (c 0.05, MeOH))
D
was matched to that of natural moracin O ([a]25 = ꢀ4.02
D
(c 0.04, MeOH)), thereby validating the full configurational
assignment of natural moracin O.
The synthetically generated racemic compounds (ꢁ)-1 and
(ꢁ)-2, as well as (R)-(ꢀ)-1 and (S)-(+)-1, were evaluated for
their effects on hypoxia-induced HIF activation by a cell-based
HRE-reporter assay in Hep3B cell lines. (ꢁ)-1, (ꢁ)-2 and
(R)-(ꢀ)-1 exhibited potent inhibition, with IC50 values of
6.76, 10.7 and 0.19 nM, respectively, without cytotoxicity
(Table 1).
18 G. A. Crispino, K.-S. Jeong, H. C. Kolb, Z.-M. Wang, D. Xu and
K. B. Sharpless, J. Org. Chem., 1993, 58, 3785–3786.
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F. Delle Monache, R. S. Compagnone, A. I. Suarez and
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16, 759.
In summary, we have developed a simple and efficient
protocol for the first total syntheses of (ꢁ)-moracin
O (1) and (ꢁ)-moracin P (2) in overall yields of 2.21% and
2.27%, respectively. In addition, the enantioselective syntheses
of (R)- and (S)-moracin
O have been achieved from
2,4-dihydroxybenzaldehyde. The absolute stereochemistry
was introduced by employing Sharpless’ AD reaction.
20 A. M. Felix, E. P. Heimer, T. J. Lambros, C. Tzougraki and
J. Meienhofer, J. Org. Chem., 1978, 43, 4194.
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Chem. Commun., 2009, 1879–1881 | 1881