Total synthesis of (+)-angelmarin

Article abstract of DOI:10.1021/jo900613u

(Chemical Equation Presented) An efficient 8-step enantioselective total synthesis of (+)-angelmarin, starting from commercially available umbelliferone, has been achieved. Key reactions include olefin cross-metathesis and a Shi epoxidation-cyclization sequence.

Full text of DOI:10.1021/jo900613u

SCHEME 1. Retrosynthetic Analysis for (+)-Angelmarin
Total Synthesis of (+)-Angelmarin
Jakob Magolan and Mark J. Coster*
Eskitis Institute for Cell and Molecular Therapies, Griffith
UniVersity, Nathan 4111, Queensland, Australia
m.coster@griffith.edu.au
ReceiVed March 25, 2009
nutrient-deprived medium while showing no activity in nutrient-
sufficient medium.5 Two of these compounds, kigamicin D5a
and arctigenin,5c were further demonstrated to suppress tumor
growth of pancreatic cancer cell lines in nude mice.
An efficient 8-step enantioselective total synthesis of (+)-
angelmarin, starting from commercially available umbellif-
erone, has been achieved. Key reactions include olefin cross-
metathesis and a Shi epoxidation-cyclization sequence.
In an effort to identify new antiausterity natural products,
Kadota and co-workers reported the structure of (+)-angelmarin
(1, Scheme 1), isolated by bioassay-guided fractionation of the
CH2Cl2-extract of Angelica pubescens.5b Angelmarin (1) shows
100% preferential cytotoxicity (PC100) against PANC-1 cells at
0.01 µg/mL. The absolute configuration of this new natural
product was assigned through analysis of its circular dichroism
spectrum and comparison of specific rotation values for its
saponification product to that reported for columbianetin (2), a
compound identified in 1964 from hydrolysis of two related
natural products from Lomatium columbianum.6
The potent bioactivity of 1 in this unique therapeutic area
motivated us to develop an efficient and scalable total synthesis
that would allow ready access to structural analogues for
biological evaluation and the elucidation of structure-activity
relationships (SAR). Our retrosynthetic approach relies on initial
disconnection of 1 across the ester to give columbianetin (2)
and p-hydroxycinnamic acid (Scheme 1). In turn, 2 would be
derived from epoxide 3, via base-mediated 5-exo-tet cyclization,
and the latter compound would be obtained by asymmetric
epoxidation of osthenol (4), using the Shi protocol.7
Cancer cells within rapidly growing tumors are often subject
to low oxygen and nutrient supply,1 and show remarkable
tolerance to such starvation conditions.2 Pancreatic cancer is
the most deadly of human malignancies, with the lowest 5-year
survival rates of all cancers (generally <5%). It is unresponsive
to most current chemotherapeutic agents3 and displays an
astonishing tolerance to extreme nutrient-deprivation over
prolonged periods of time.2a
An anti-austerity therapeutic strategy, targeting this metabolic
adaptation of cancer cells, was proposed in 2000 by Esumi and
co-workers.2a Through the development of an assay method for
antiausterity activity,4 several natural products have been
identified with preferential cytotoxicity to PANC-1 cells in
(1) (a) Vaupel, P.; Kallinowski, F.; Okunieff, P. Cancer Res. 1989, 49, 6449.
(b) Helmlinger, G.; Yuan, F.; Dellian, M.; Jain, R. K. Nat. Med. 1997, 3, 177.
(2) (a) Izuishi, K.; Kato, K.; Ogura, T.; Kinoshita, T.; Esumi, H. Cancer
Res. 2000, 60, 6201. (b) Esumi, H.; Izuishi, K.; Kato, K.; Hashimoto, K.;
Kurashima, Y.; Kishimoto, A.; Ogura, T.; Ozawa, T. J. Biol. Chem. 2002, 277,
32791.
(3) Li, D.; Xie, K.; Wolff, R.; Abbruzzese, J. L. Lancet 2004, 363, 1049.
(4) Esumi, H.; Lu, J.; Kurashima, Y.; Hanaoka, T. Cancer Sci. 2004, 95,
685.
(5) (a) Lu, J.; Kunimoto, S.; Yamazaki, Y.; Kaminishi, M.; Esumi, H. Cancer
Sci. 2004, 95, 547. (b) Awale, S.; Nakashima, E. M. N.; Kalauni, S. K.; Tezuka,
Y.; Kurashima, Y.; Lu, J.; Esumi, H.; Kadota, S. Bioorg. Med. Chem. Lett. 2006,
16, 581. (c) Awale, S.; Lu, J.; Kalauni, S. K.; Kurashima, Y.; Tezuka, Y.; Kadota,
S.; Esumi, H. Cancer Res. 2006, 66, 1751. (d) Zaidi, S. F. H.; Awale, S.; Kalauni,
S. K.; Tezuka, Y.; Esumi, H.; Kadota, S. Planta Med. 2006, 72, 1231. (e) Win,
N. N.; Awale, S.; Esumi, H.; Tezuka, Y.; Kadota, S. J. Nat. Prod. 2007, 70,
1582. (f) Awale, S.; Li, F.; Onozuka, H.; Esumi, H.; Tezuka, Y.; Kadota, S.
Bioorg. Med. Chem. 2008, 16, 181. (g) Win, N. N.; Awale, S.; Esumi, H.; Tezuka,
Y.; Kadota, S. Bioorg. Med. Chem. Lett. 2008, 18, 4688. (h) Win, N. N.; Awale,
S.; Esumi, H.; Tezuka, Y.; Kadota, S. Bioorg. Med. Chem. 2008, 16, 8653. (i)
Win, N. N.; Awale, S.; Esumi, H.; Tezuka, Y.; Kadota, S. Chem. Pharm. Bull.
2008, 56, 491.
Racemic columbianetin has previously been prepared by
Shipchandler (1970),8 Steck (1971),9 and Franke (1971).10 The
syntheses of Steck and Franke utilized a base-mediated 5-exo-
tet cyclization of a phenolic epoxide (3) to achieve the
dihydrobenzofuran framework. Franke’s synthesis of rac-
columbianetin was the most efficient to date, providing 2 in 5
steps from commercially available substrates, cf. Shipchandler
(6) Willette, R. E.; Soine, T. O. J. Pharm. Sci. 1964, 275.
(7) For a recent review, see: Wong, O. A.; Shi, Y. Chem. ReV. 2008, 108,
3958.
(8) Shipchandler, M.; Soine, T. O.; Gupta, P. K. J. Pharm. Sci. 1970, 59,
67.
(9) Steck, W. Can. J. Chem. 1971, 49, 1197.
(10) Bohlmann, F.; Franke, H. Chem. Ber. 1971, 104, 3229.
10.1021/jo900613u CCC: $40.75  2009 American Chemical Society
Published on Web 05/21/2009
J. Org. Chem. 2009, 74, 5083–5086 5083

SCHEME 2. Franke’s Synthesis of Racemic Columbianetin
FIGURE 1. Anticipated selectivity for the Shi epoxidation.
columbianetin, via Shi epoxidation allows enantiodivergent
access through the use of either enantiomer of the chiral ketone
catalyst.
Our synthetic sequence began with allylation of umbellifer-
one, followed by Claisen rearrangement of allyl ether 8 in
refluxing N,N-diethylaniline to yield 9 (Scheme 3). These steps
utilized precedented, highly scalable procedures,15 requiring no
chromatographic purification steps. Cross-metathesis of terminal
alkene 9 with 2-methyl-2-butene, in the presence of Grubbs’
second generation catalyst16 was successful on small scales
(<200 mg of substrate) under typical reaction conditions (15
mol % of catalyst, 0.015 M in 1:1 v/v 2-methyl-2-
butene-CH2Cl2).17 Upon scaling up (>1 g), where the use of
large quantities of 2-methyl-2-butene became uneconomical, we
observed the formation of significant quantities of the insoluble
homodimerization product 10 when the reaction was performed
at higher concentration and/or with lower quantities of 2-methyl-
2-butene, with concomitant low yields of 4.18 The formation of
10 was circumvented, and the quantity of 2-methyl-2-butene
minimized, by slow addition of a dilute solution of 9 (0.02 M
in CH2Cl2) to a refluxing solution of Grubbs II (5 mol %) in
1:1 v/v 2-methyl-2-butene-CH2Cl2 (see the Experimental
Section for details).
SCHEME 3. Synthesis of Osthenol via Cross-Metathesis
We next sought to exploit the Shi asymmetric epoxidation
procedure7 for the enantioselective conversion of osthenol (4)
to (+)-columbianetin, via epoxide 3. Applying the model offered
by Shi to our system (Figure 1), we anticipated approach of
the dioxirane catalyst from the bottom face of osthenol (as
shown). Following olefin epoxidation, 5-exo-tet cyclization of
the corresponding phenoxide onto the resulting epoxide was
thought likely to occur in situ under the basic conditions, to
yield (S)-columbianetin, corresponding to the natural (+)-
enantiomer.
Epoxidation of osthenol with m-CPBA in the presence of
K2CO3 readily yielded rac-columbianetin (2) in 82% yield
(Table 1, entry 1). Application of commonly used Shi epoxi-
dation conditions,19 which involve simultaneous syringe pump
addition of aqueous K2CO3 and Oxone solutions to a buffered,
biphasic reaction mixture of substrate and D-fructose-derived
catalyst (11) in DMM-CH3CN-H2O, resulted in very slow
epoxidation of osthenol (4) providing low yields of 2, with
negligible enantioselectivity (Table 1, entries 2 and 3).20
Attempts to optimize this reaction by varying concentrations
and ratios of reagents were not successful. Similarly, little
(ca. 2% over 10 steps) and Steck (ca. 1% over 4 steps). As
illustrated in Scheme 2, Franke converted commercially avail-
able umbelliferone (5) to osthenol (4),11 via phenol alkylation
with 3-chloro-3-methyl-1-butyne (6), alkyne reduction using
Lindlar’s catalyst, and a notably regioselective Claisen rear-
rangement of 7, according to a three-step procedure previously
developed by Taylor.12,13 Epoxidation of osthenol (4), followed
by cyclization in the presence of sodium carbonate, provided
rac-columbianetin (2) in ca. 46% yield over the two steps.
We noted an opportunity to modify the Franke-Taylor
strategy by avoiding the use of relatively expensive 3-chloro-
3-methyl-1-butyne (6),14 in favor of a simpler allyl ether Claisen
rearrangement, followed by olefin cross-metathesis to convert
the allyl substituent to the desired isoprenyl moiety. This
approach is cost-effective, highly scalable, and has the advantage
that analogues of the natural product may be readily accessed
through the use of different cross-metathesis partners, greatly
facilitating the future extension of this work to explore
structure-activity relationships for 1. Furthermore, compared
to previous work, our approach to the key intermediate,
(15) Clarke, D. J.; Robinson, R. S. Tetrahedron 2002, 58, 2831.
(16) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1,
953.
(17) Chatterjee, A. K.; Sanders, D. P.; Grubbs, R. H. Org. Lett. 2002, 4,
1939.
(11) Spa¨th, E.; Bru¨ch, J. Ber. Dtsch. Chem. Ges. 1937, 70B, 1023.
(12) (a) Hlubucek, J. R.; Ritchie, E.; Taylor, W. C. Tetrahedron Lett. 1969,
10, 1369. (b) Hlubucek, J. R.; Ritchie, E.; Taylor, W. C. Chem. Ind. (London,
UK) 1969, 1780.
(13) The synthesis of osthenol by the same approach was also published by
Murray et al. in 1970, wherein the Claisen rearrangement of neat 7 gave osthenol
(4) and its 6-isoprenyl isomer, 7-demethylsuberosin, in 74% and 14% yield,
respectively. (a) Murray, R. D. H.; Ballantyne, M. M.; Mathai, K. P. Tetrahedron
Lett. 1970, 11, 243. (b) Murray, R. D. H.; Ballantyne, M. M.; Mathai, K. P.
Tetrahedron 1971, 27, 1247.
(18) Olefin 10 precipitated from the reaction mixture was isolated by filtration
and identified by NMR analysis in d6-DMSO.
(19) Wang, Z.-X.; Tu, Y.; Frohn, M.; Zhang, J.-R.; Shi, Y. J. Am. Chem.
Soc. 1997, 119, 11224.
(20) The enantiomeric excess of 2 could not be determined by using chiral
HPLC under standard conditions; however, polarimetry proved satisfactory, due
to the large specific rotation reported for (+)-columbianetin, see ref 5b.
(14) 3-Chloro-3-methyl-1-butyne (6) is available for $96.20 for 5 g.
5084 J. Org. Chem. Vol. 74, No. 14, 2009

TABLE 1. Shi Epoxidation Study: Synthesis of (+)-Columbianetin
SCHEME 4. Completion of the Total Synthesis
entry substrate
method and conditionsd
yield,a
%
ee,b
%
1
2
3
4
5
4
4
4
4
m-CPBA, K2CO3, CH2Cl2, rt
A (0.2 equiv of 11, rt)
A (1 equiv of 11, rt)
B (1 equiv of 11, 0 °C)
A (0.2 equiv of 11, rt), then
TBAF, THF, rt, 2 h
A (0.2 equiv of 11, rt), then
TBAF, THF, rt, 2 h
A (1 equiv of 11, rt), then
TBAF, THF, rt, 2 h
A (0.2 equiv of 11, 0 °C), then
TBAF, THF, rt, 2 h
B (1 equiv of 11, rt), then
TBAF, THF, rt, 2 h
A (1 equiv of 11, rt), then
TBAF, THF, 50 °C, 16 h
82
35
39
42
49
5
5
9
70
this in mind, we prepared tert-butyldimethylsilyl (TBS) ether
derivative 12a. Gratifyingly, Shi epoxidation of 12a using
standard conditions, followed by treatment of the crude product
with TBAF in THF, yielded (+)-columbianetin (2) in 49% yield
and 70% ee (Table 1, entry 5).
12a
6
12b
12b
12b
12b
12c
65
66
68
31
71
68
70
We noted the TBS ether was somewhat labile under the basic
epoxidation conditions, as a small amount of columbianetin was
7
71 (75)c
59
1
8
observable by H NMR prior to TBAF treatment. Thus, the
significantly more base-stable triisopropylsilyl (TIPS) ether 12b
was prepared. Shi epoxidation of 12b, followed by fluoride
treatment, resulted in considerably improved yields but similar
ee values (Table 1, entries 6-8). Unfortunately, although
columbianetin (2) could be recrystallized from EtOAc/hexane,
this process yielded no improvement in ee beyond 75%.
Application of the alternative, H2O2-mediated, Shi epoxidation
conditions led to decreased yield and ee (Table 1, entry 9). On
the basis of the transition state model illustrated in Figure 1,
we considered the possibility that the silyl protection of the
phenol led to unfavorable steric interaction between this bulky
group and the upper acetonide. With this in mind, we prepared
a more flexible, and potentially less sterically demanding,
derivative 12c bearing a 2-(tert-butyldiphenylsilyl)ethyl (TB-
DPSE) protecting group.23 Epoxidation of 12c and deprotection/
cyclization of the crude product yielded (+)-columbianetin with
a slight improvement in yield and enantioselectivity (Table 1,
entry 10).
To complete the total synthesis of angelmarin, we sought to
esterify the tertiary alcohol of (+)-columbianetin (2) directly
with coumeric acid (13, Scheme 4). Not surprisingly, the
presence of the nucleophilic phenol of coumeric acid made
standard esterification procedures untenable. Furthermore, our
efforts to achieve acid-promoted esterification of columbianetin,
via a tertiary carbocation, were hampered by lack of reactivity
under mild conditions,24 while more forceful acidic conditions,
e.g., p-toluenesulfonic acid (TsOH) in refluxing toluene, resulted
in elimination/isomerization to give the corresponding benzo-
furan. A more lucrative, and ultimately successful, approach
involved treatment of (+)-columbianetin with Meldrum’s acid
(14) to afford carboxylic acid 15. Crude 15, when heated with
4-hydroxybenzaldehyde (16) in pyridine in the presence of
piperidine, underwent a Doebner-Knoevenagel condensation
to yield (+)-angelmarin (1) in 82% over the 2 steps. Synthetic
1 provided spectroscopic data in full accordance with that
9
10
75
a Isolated yields. b Approximate ee values were determined by
polarimetric analysis in comparison to literature specific rotation values
for (+)-2.5b The correlation between optical purity and ee for 2 was
validated with material of 75% ee, see below and ref 25. c Product from
entry 8 was recrystallized from EtOAc/hexanes to afford material with
slightly improved 75% ee. d Method A: Ketone 11 (0.2-1 equiv), TBAS
(0.2 equiv), Oxone (1.8 equiv), K2CO3 (7.2 equiv), MeCN-DMM-0.05
M Na2B4O7 in 0.004 M aq Na2EDTA (1:2:2 v/v). Method B: Ketone 11
(1 equiv) and H2O2 (4.0 equiv) in CH3CN-2.0 M K2CO3 in 0.0040 M
Na2EDTA (1:1 v/v), 0 °C.
improvement in ee was observed when Oxone was replaced
with H2O2 as described in an alternative procedure published
by Shu and Shi21 (Table 1, entry 4). We reasoned that the
observed lack of enantioselectivity in these epoxidations is due
to the presence of the free phenol functionality in the substrate.
To the best of our knowledge, there are no reported examples
of Shi epoxidations in the presence of a phenol. Low enanti-
oselectivities have previously been reported for some aliphatic
alcohol substrates (at pH <10),22 with epoxidation by Oxone
itself being implicated. However, unlike these reported ex-
amples, careful control of pH did not improve selectivity in
our system. We propose that the low enantioselectivities we
observed are due to significant water solubility of the phenol
substrate at the high pH (ca. 10) required for efficient oxidation
of 11 by Oxone, resulting in direct epoxidation by Oxone
representing the major oxidation pathway.
In an effort to improve the enantioselectivity of the above
procedure, we next sought to mask the phenol of osthenol, prior
to epoxidation. Our choice of protecting group was restricted
by both the alkaline epoxidation conditions and the knowledge
that subsequent deprotection conditions would need to be
nonacidic, in order to avoid an undesired, acid-promoted,
6-endo-tet cyclization of the phenol onto the epoxide 3. With
(23) Gerstenberger, B. S.; Konopelski, J. P. J. Org. Chem. 2005, 70, 1467.
(24) Srinivas, K. V. N. S.; Mahender, I.; Das, B. Synthesis 2003, 2479.
(25) Synthetic 1 was obtained from 2 of 75% ee by polarimetry (Table 1,
entry 8). The specific rotation for synthetic 1 is consistent with 75% ee and this
value was confirmed by chiral HPLC (see the Supporting Information).
(21) Shu, L.; Shi, Y. Tetrahedron 2001, 57, 5213.
(22) Wang, Z.-X.; Shi, Y. J. Org. Chem. 1998, 63, 3099.
J. Org. Chem. Vol. 74, No. 14, 2009 5085

reported for the natural product.5b The measured specific rotation
tion of starting material. The reaction mixture was cooled to room
temperature and the solvent was removed in vacuo to yield crude
carboxylic acid 15 as a yellow oil that was immediately dissolved
in pyridine (5 mL). Piperidine (4 drops) and p-hydroxybenzaldehyde
(16, 56.1 mg, 0.501 mmol) were added and the reaction mixture
was heated at 70 °C for 16 h. After being cooled to room
temperature, the reaction mixture was diluted with ethyl acetate
and washed with 5% HCl, water, and brine, then dried over MgSO4
and the solvent was removed in vacuo. The crude product was
purified by flash column chromatography (gradient elution with
ethyl acetate in petroleum spirits) to yield (+)-angelmarin (1, 134
mg, 82% yield). Rf 0.21 (30% ethyl acetate in petroleum spirits);
[R]25 +163 (c 0.2, CHCl3) [cf. +218.7 (c 0.025, CHCl3)5b]
D
matched well in both sign and magnitude, validating the absolute
configuration assigned for the natural product.25
In conclusion, we have completed the first total synthesis of
the antiausterity agent (+)-angelmarin, in 8 steps with an overall
yield of 37%. Furthermore, our sequence is well suited to the
generation of structural analogues, e.g., by substitution of
4-hydroxybenzaldehyde (16) with alternate aldehydes in the final
step of the synthesis. The synthesis and biological evaluation
of analogues of 1, providing valuable SAR data, will be reported
in due course.
[R]25 + 163 (c 0.2, CHCl3), 75% ee by chiral HPLC, see the
D
Supporting Information. IR (neat) ν 3284, 1733, 1706, 1616, 1606,
1586, 1514, 1489, 1457, 1387, 1330, 1263, 1169, 1136, 1008, 980,
Experimental Section
1
833 cm-1; H NMR (500 MHz, CDCl3) δ 7.67 (d, J ) 9.5 Hz,
1H), 7.34 (d, J ) 16.0 Hz, 1H), 7.29 (d, J ) 8.5 Hz, 1H), 7.26 (d,
J ) 8.5 Hz, 2H), 6.83 (d, J ) 8.5 Hz, 2H), 6.78 (d, J ) 8.5 Hz,
1H), 6.19 (d, J ) 9.5 Hz, 1H), 6.12 (d, J ) 16.0 Hz, 1H), 5.19
(dd, J ) 9.5, 7.5 Hz, 1H), 3.32-3.42 (m, 2H), 1.64 (s, 3H), 1.60
(s, 3H); 13C NMR (125 MHz, CDCl3) δ 166.5, 164.1, 161.6, 158.4,
151.2, 144.6, 144.3, 129.9, 128.9, 126.6, 116.1, 115.9, 113.6, 113.1,
112.0, 106.9, 89.2, 82.2, 27.5, 22.1, 21.2; HRMS (ESI-TOF) calcd
for C23H20O6Na [M + Na]+ 415.1152, found 415.1171.
Osthenol (4). A solution of 2-methyl-2-butene (50 mL) and
dichloromethane (50 mL) was briefly deoxygenated by purging with
argon, before Grubbs’ second generation catalyst (0.367 g, 0.433
mmol) was added. The reaction mixture was heated to reflux (oil
bath temperature of 45 °C) and a warm (40 °C) solution of 9 (1.75
g, 8.66 mmol) in deoxygenated CH2Cl2 (400 mL) was added via
cannula over 2 h. The reaction was heated for an additional 2 h,
then cooled to room temperature, absorbed onto silica, and purified
via column chromatography (gradient elution with ethyl acetate in
petroleum spirits) to yield osthenol (4) (1.79 g, 7.80 mmol, 90%)
as a pale green solid. Mp 89-91 °C; Rf 0.57 (50% ethyl acetate in
petroleum spirits); IR (neat) ν 3270, 1694, 1603, 1572, 1308, 1248,
Acknowledgment. We thank Professor Shigetoshi Kadota
and Dr. Suresh Awale (University of Toyama) for copies of
authentic spectra for 1, the Australian Research Council (ARC)
for funding (DP0986795) and a fellowship to M.J.C., and Drs.
Joshua Fischer and Hoan Vu (Griffith University) for preparation
of t-BuPh2Si(CH2)2OH and mass spectrometry, respectively.
1
832 cm-1; H NMR (500 MHz, CDCl3) δ 7.62 (d, J ) 9.5 Hz,
1H), 7.22 (d, J ) 8.5 Hz, 1H), 6.81 (d, J ) 8.5 Hz, 1H), 6.30 (s,
1H), 6.24 (d, J ) 9.5 Hz, 1H), 5.27 (t, J ) 7.0 Hz, 1H), 3.62 (d,
J ) 7.0 Hz, 2H), 1.86 (s, 3H), 1.75 (s, 3H); 13C NMR (125 MHz,
CDCl3) δ 161.8, 158.4, 153.1, 144.3, 135.7, 126.6, 120.3, 114.9,
113.2, 112.7, 112.3, 25.8, 22.1, 18.0; HRMS (ESI-TOF) calcd for
C14H14O3Na [M + Na]+ 253.0835, found 253.0830.
(+)-Angelmarin (1). To a solution of (+)-columbianetin (2, 103
mg, 0.418 mmol) in toluene (5 mL) was added Meldrum’s acid
(14, 67 mg, 0.465 mmol). The resulting solution was heated to reflux
for 7 h, at which point TLC analysis indicated complete consump-
Supporting Information Available: General experimental,
experimental procedures, characterization data for compounds
8, 9, 12, 13, 14, (+)-2, and rac-2, and 1H and 13C NMR spectra
for all new compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
JO900613U
5086 J. Org. Chem. Vol. 74, No. 14, 2009