First total synthesis of malvone A and formal syntheses of boryquinone and hybocarpone using a concise strategy for construction of unsymmetrical naphthoquinones

Article abstract of DOI:10.1055/s-0028-1088134

The first total synthesis of malvone A (1) is presented in seven steps along with a nine-step formal synthesis of boryquinone (2) and an eleven-step formal synthesis of hybocarpone (3). The overall strategy, which accesses compounds 7-12 for the first tim

Full text of DOI:10.1055/s-0028-1088134

LETTER
1273
First Total Synthesis of Malvone A and Formal Syntheses of Boryquinone and
Hybocarpone Using a Concise Strategy for Construction of Unsymmetrical
Naphthoquinones
S
ynthesis of Malvo
u
ne
A
n-Liang Wu, Elysia P. M. T. Cohen, Yaodong Huang, Thomas R. R. Pettus*
Department of Chemistry and Biochemistry, University of California at Santa Barbara, Santa Barbara, CA, 93106-9510, USA
Fax +1(805)8935690; E-mail: pettus@chem.ucsb.edu
Received 15 December 2008
O
O
OH
OH
OH
O
O
Abstract: The first total synthesis of malvone A (1) is presented in
seven steps along with a nine-step formal synthesis of boryquinone
(2) and an eleven-step formal synthesis of hybocarpone (3). The
overall strategy, which accesses compounds 7–12 for the first time,
employs an easily constructed a-tetralone, and it has significant im-
plications for synthesis of the naphthoquinone portion of b-rubro-
mycin (4) as well as a host of other unsymmetrical and differentially
protected naphthoquinones.
MeO
Me
OH
HO
Me
OH
Et
malvone A (1)
boryquinone (2)
O
O
OH
HO
OH
HO
HO
O
Key words: naphthoquinone, o-quinone, natural product synthesis,
total synthesis
OH
Et
Et
O
O
Me
Me
HO
OH
hybocarpone (3)
2-Hydroxynaphthalene-1,4-diones and their derivatives
comprise many natural products1–4 (Figure 1). We needed
a straightforward route to access this class of pharmaco-
logically important compounds, because of our interest in
a related naphthoquinone b-rubromycin (4).5 The usual
strategy for reaching this motif entails the synthesis of a
naphthazarin that is eventually oxidized to afford the de-
sired naphthoquinone.6 Elix has utilized a similar strategy
with naphthazarin intermediates in a concise synthesis of
boryquinone (2) and hybocarpone (3).7 Boryquinone (2) is
also known as 6-methylchristazarin. We needed a route
that in principle could distinguish alkoxy substituents.
Herein, we show our strategy as it applies in the syntheses
of malvone A (1), boryquinone (2), and hybocarpone (3).
OH
O
O
MeO
HO
O
O
O
H
CO2Me
MeO
O
β–rubromycin (4)
Figure 1 Some naphthoquinones and their derivatives
slowly for our 3-methyl derivative 5.13 To circumvent this
problem, we replaced succinic anhydride with the acid
chloride 6. Submission to the similar reaction conditions
followed by in situ saponification of the methyl ester with
potassium hydroxide affords the desired acid 7 in a 92%
isolated yield. Reductive erasure of the aryl ketone func-
tionality in 7 with triethylsilane and trifluoroacetic acid
proceeds as expected and provides the acid 8 in an 85%
yield.
We found our inspiration from the work of Thomson, who
had reported that oxygen oxidation of 5,6,8-trimethoxy-a-
tetralone afforded a 16% yield of the corresponding naph-
thoquinone.8 This result suggested that a-tetralone could
serve for a useful naphthoquinone precursor. Bekaert im-
proved the yield of the Thomson process by initial seleni-
um dioxide oxidation of a-tetralone followed by a second
potassium superoxide oxidation.9 Anufriev subsequently
reported a method employing an admixture of manganese
dioxide in concentrated sulfuric acid.10
Upon treatment with polyphosphoric acid the acid under-
goes smooth intramolecular cyclization and affords the a-
tetralone 9a in an 81% yield. It should be noted that the
methoxy group nearest to the carbonyl group in com-
pound 9a and its 3-H analogue 9b could be selectively
cleaved and differentially protected, if required, as would
be the case in the anticipated syntheses of b-rubromycin
analogues.
For our construction of 1 and 2 we chose to begin with
compound 5, which we prepared in 95% yield in one step
from commercially available 1,2,4-trimethoxybenzene as
previously demonstrated by Carreño11 (Scheme 1). Next,
we examined a Friedel–Crafts acylation of 5 with succinic
anhydride.12 Much to our surprise, the reaction proceeds
The stage was now set for the oxidative formation of the
ortho-naphthoquinone from the 5,6,8-trimethoxy-a-
tetralone. In our hands, the Thomson, Ameer, and Bekaert
procedures proved untenable; none afforded satisfactory
yields in our hands.8,9,12 Inspired by some recent work of
Jacobsen,14 we opted to employ the oxidation of the a-
tetralone 9a with selenium dioxide, providing the o-
SYNLETT 2009, No. 8, pp 1273–1276
x
x.
x
x.
2
0
0
9
Advanced online publication: 08.04.2009
DOI: 10.1055/s-0028-1088134; Art ID: S12708ST
© Georg Thieme Verlag Stuttgart · New York

1274
K.-L. Wu et al.
LETTER
O
OMe
OMe
OMe
Cl
CO2Me
MeO
Me
CO2H
MeO
Me
CO2H
MeO
Me
Et3SiH
6
AlCl3,CH2Cl2;
KOH
TFA
85%
OMe
O
OMe
OMe
92%
5
7
8
OR
O
OMe
O
MeO
A
O
SeO2
PPA
81%
AcOH
73%
Me
Me
OMe
OMe
9a R = Me
BBr3
82%
10
9b R = H
Scheme 1 Synthesis of the o-naphthoquinone 10 from 3-methyl-1,2,4-trimethoxybenzene (5)
quinone 10 in a 73% yield (Scheme 2). Further reduction naphthoquinone 11 in a 40% yield. Compound 11 under-
of this material with sodium borohydride and in situ bisa- goes a Mannich reaction with acetaldehyde and methyl-
cylation with acetyl anhydride produces the bisacyl ester. amine hydrochloride and subsequent reduction of the
The left side of this naphthol undergoes a CAN oxidation resulting vinyl residue with palladium on carbon and hy-
to the corresponding para-naphthoquinone. In situ sapon- drogen to afford the corresponding naphthoquinone 12 in
ification of the acetate residues with sodium methoxide a 61% yield for two steps. This compound proves identi-
produces malvone A (1) in 73% isolated yield displaying cal in every respect with that spectroscopic data previous-
spectroscopic data in accord with those previously report- ly reported by Nicolaou et al.15 for 12 and thereby our
ed for the natural product.4
efforts constitutes formal syntheses of both boryquinone
(2) and hybocarpone (3).
On the other hand, reduction of o-quinone 10 with sodium
borohydride and subsequent oxidation in the presence of In conclusion, the first total synthesis of malvone A (1)
potassium tert-butoxide and oxygen results in the desired has been accomplished in seven steps and an overall yield
of 32% starting from commercially available 1,2,4-tri-
OMe
O
OMe
O
methoxybenzene. This route has been presented along
with a nine-step formal synthesis of boryquinone (2), and
an eleven-step formal synthesis of the hybocarpone (3).
This concise and versatile strategy, which employs an a-
tetralone may have significant implications for the synthe-
sis of differentially protected naphthoquinones in the
future, particularly those resembling b-rubromycin (4).
The details of these subsequent studies will be reported in
due course.
MeO
O
MeO
OH
1) NaBH4
2) O2, t-BuOK
t-BuOH
40%
Me
Me
OMe
OMe
O
10
11
1) NaBH4; Ac2O,
Et3N, DMAP
CH2Cl2
2) CAN; NaOMe
MeOH
1) acetaldehyde
MeNH2⋅HCl
benzene
2) H2, Pd/C
61%
73%
O
O
OH
OH
OMe O
Synthesis of Compound 7
MeO
Me
MeO
Me
OH
Et
To a solution of compound 5 (5.43 g, 29.8 mmol) and acid chloride
6 (4.8 mL, 32 mmol) in dry CH2Cl2 (60 mL) was added anhyd AlCl3
(9.3 g, 70 mmol) in one portion at 0 °C. The resulting solution was
stirred and warmed to r.t. for 3 h. The mixture was poured into ice
water (200 mL) and then extracted three times with CH2Cl2 (200
mL). The combined organic layers were washed with brine and
dried over MgSO4, and the solvent was removed under reduced
pressure. To a solution of remaining mixture in MeOH (55 mL) and
H2O (18 mL) was added KOH (5.6 g, 100 mmol) in one portion. The
resulting solution was stirred at r.t. for 3 h. The organic solvent was
removed under reduced pressure. The mixture was acidified with aq
HCl (3 M) and extracted three times with CH2Cl2 (200 mL). The
combined organic layers were washed with brine and dried over
MgSO4, filtered, and the solvent was removed under reduced pres-
sure. The residue obtained upon workup was chromatographed on
SiO2 (eluent: hexane–EtOAc, 4:6), providing 7.75 g of compound 7
as white crystals (92% yield from compound 5); mp 83–84 °C. 1H
NMR (400 MHz, CDCl3): d = 2.23 (s, 3 H), 2.77 (t, J = 7.2 Hz, 2 H),
OMe
O
O
O
malvone A (1)
12
3 steps
ref. 15
BBr3
ref. 15
O
O
OH
HO
OH
OH
HO
O
HO
Me
OH
Et
OH
HO
Et
Et
O
O
Me
Me
OH
HO
OH
boryquinone (2)
hybocarpone (3)
Scheme 2 Total synthesis of malvone A (1) and formal syntheses of
boryquinone (2), and hybocarpone (3) from the o-naphthoquinone 10
Synlett 2009, No. 8, 1273–1276 © Thieme Stuttgart · New York

LETTER
Synthesis of Malvone A
1275
3.36 (t, J = 7.2 Hz, 2 H), 3.73 (s, 3 H), 3.86 (s, 6 H), 7.13 (s, 1 H).
13C NMR (125 MHz, CDCl3): d = 199.2, 178.4, 152.7, 151.4, 148.8,
126.5, 125.8, 109.7, 61.7, 59.9, 55.5, 36.9, 28.2, 9.2. IR (thin film):
2939, 2835, 1713, 1670, 1485, 1404, 1138, 1088 cm–1. HRMS (EI):
m/z calcd for C14H19O6: 282.1103; found: 282.1111.
3.85 (s, 3 H), 3.91 (s, 3 H), 4.08 (s, 3 H), 5.60 (s, 1 H), 7.16 (d,
J = 8.8 Hz, 1 H), 7.48 (d, J = 8.8 Hz, 1 H), 9.33 (s, 1 H). 13C NMR
(125 MHz, CDCl3): d = 150.9, 147.8, 142.4, 140.0, 137.3, 121.6,
120.5, 116.7, 116.5, 113.6, 61.7, 61.2, 60.5, 9.5. IR (thin film):
3348, 2935, 2843, 1736, 1636, 1612, 1462, 1358, 1238, 1049, 814
cm–1. To a solution of the crude residue in dry t-BuOH (20 mL) was
added KOt-Bu (363 mg, 3.24 mmol) in one portion under oxygen (1
atm) at r.t. for 20 min. The mixture was acidified with aq HCl (3 M)
and extracted three times with EtOAc (50 mL). The combined or-
ganic layers were extracted with twice 10% of aq NaOH. The aque-
ous layers were acidified with aq HCl (3 M) and extracted three
times with EtOAc (50 mL). The combined organic layers were
washed with brine and dried over MgSO4, and the solvent was re-
moved under reduced pressure. The residue obtained upon workup
was chromatographed on SiO2 (eluent: hexane–EtOAc, 5:5), pro-
viding 90 mg of compound 11 as a yellow solid (40% yield from
compound 10). 1H NMR (400 MHz, CDCl3): d = 2.30 (s, 3 H), 3.85
Synthesis of Compound 8
To a solution of compound 7 (5.43 g, 27.3 mmol) in 13 mL of dry
TFA was added Et3SiH (13 mL, 81.4 mmol) in one portion at 0 °C.
The resulting solution was stirred and warmed to r.t. for 4 h. The
mixture was poured into ice water (200 mL) and then extracted
three times with CH2Cl2 (200 mL). The combined organic layers
were extracted twice with 10% of aq NaOH. The aqueous layers
were acidified with aq HCl (3 M) and extracted three times with
CH2Cl2 (200 mL). The combined organic layers were washed with
brine and dried over MgSO4, filtered, and the solvent was removed
under reduced pressure. The residue obtained upon workup was
chromatographed on SiO2 (eluent: hexane–EtOAc, 3:7), providing
6.2 g of compound 8 as yellow oil (85% yield from compound 7).
1H NMR (400 MHz, CDCl3): d = 1.95 (p, J = 7.6 Hz, 2 H), 2.22 (s,
3 H), 2.42 (t, J = 7.6 Hz, 2 H), 2.65 (t, J = 7.6 Hz, 2 H), 3.68 (s, 3
H), 3.79 (s, 3 H), 3.83 (s, 3 H), 6.57 (s, 1 H). 13C NMR (125 MHz,
CDCl3): d = 179.8, 150.4, 149.1, 146.1, 129.0, 125.5, 110.5, 60.7,
60.2, 55.9, 33.5, 29.1, 25.7, 9.6. IR (thin film): 3140, 2939, 2870,
2839, 1709, 1485, 1450, 1412, 1231, 1088 cm–1. HRMS (EI): m/z
calcd for C14H20O5: 268.1311; found: 268.1310.
(s, 3 H), 3.92 (s, 3 H), 3.95 (s, 3 H), 6.20 (s, 1 H), 7.43 (s, 1 H). 13
C
NMR (125 MHz, CDCl3): d = 184.1, 180.8, 157.3, 156.3, 154.9,
151.1, 137.6, 121.4, 120.9, 110.7, 61.3, 60.9, 10.1. IR (thin film):
2943, 1663, 1605, 1558, 1462, 1385, 1327, 1277, 1053, 991 cm–1.
HRMS (EI): m/z calcd for C14H14O6: 278.0790; found: 278.0797.
Synthesis of Compound 12
To a solution of compound 11 (6.2 mg, 0.022 mmol) in benzene (1
mL), MeNH2·HCl (4 mg, 0.06 mmol), and PTSA (13 mg, 0.06
mmol) was added acetaldehyde (13 mL) in one portion. The solution
was heated at 80 °C for 1 h. The mixture was extracted three times
with CH2Cl2 (5 mL). The combined organic layers were dried over
MgSO4, filtered, and the solvent was removed under reduced pres-
sure. To a solution of the crude residue of naphthoquinone in dry
EtOAc (1 mL) was added Pd/C (6 mg) under H2 (1 atm) at r.t. for
18 h. After filtration over Celite and solvent removal, the residue
obtained was chromatographed on SiO2 (eluent: hexane–EtOAc,
8:2), providing 4.1 mg of compound 12 as oil (61% yield from com-
pound 11); mp 106–107 °C. 1H NMR (400 MHz, CDCl3): d = 1.13
(t, J = 8.0 Hz, 3 H), 2.28 (s, 3 H), 2.58 (q, J = 8.0 Hz, 2 H), 3.83 (s,
3 H), 3.90 (s, 3 H), 3.92 (s, 3 H), 7.39 (s, 1 H). 13C NMR (125 MHz,
CDCl3): d = 184.1, 180.8, 157.6, 156.6, 152.0, 151.1, 137.3, 125.6,
121.9, 121.6, 61.6, 61.5, 61.2, 17.2, 13.0, 10.3.
Synthesis of Compound 9a
A mixture of polyphosphoric acid (62 g) and compound 8 (6.2 g,
23.1 mmol) was stirred and heated at 60 °C for 6 h. The resulting
mixture was poured into ice water (1 L) and then extracted three
times with CH2Cl2 (500 mL). The combined organic layers were
washed with brine and dried over MgSO4, filtered, and the solvent
was removed under reduced pressure. The residue obtained upon
workup was chromatographed on SiO2 (eluent: hexane–EtOAc,
7:3), providing 4.68 g of compound 9a as brown oil (81% yield
1
from compound 8). H NMR (400 MHz, CDCl3): d = 2.04 (p,
J = 6.4 Hz, 2 H), 2.26 (s, 3 H), 2.61 (t, J = 6.4 Hz, 2 H), 2.91 (t,
J = 6.4 Hz, 2 H), 3.71 (s, 3 H), 3.85 (s, 6 H). 13C NMR (125 MHz,
CDCl3): d = 197.2, 151.6, 151.2, 150.2, 133.6, 131.9, 125.2, 61.1,
60.5, 60.1, 40.6, 23.7, 22.5, 10.0. IR (thin film): 2935, 1686, 1458,
1400, 1315, 1281, 1115, 1080 cm–1. HRMS (EI): m/z calcd for
C14H18O4: 250.1205; found: 250.1197.
Synthesis of Malvone A (1)
To a solution of compound 10 (71 mg, 0.27 mmol) in dry THF (1
mL) was added NaBH4 (11 mg, 0.27 mmol) in one portion. The so-
lution was stirred for 15 min until the color of the solution was dis-
appeared. The mixture was quenched with aq HCl (3 M) and
extracted three times with CH2Cl2 (10 mL). The combined organic
layers were dried over MgSO4, and the solvent was removed under
reduced pressure. To a solution of residue in CH2Cl2 (2.5 mL) was
added Et3N (0.083 mL, 0.082 mmol), acetyl anhydride (0.084 mL,
0.083 mmol), and DMAP (3.5 mg, 0.029 mmol). The solution was
stirred for 1 h. The mixture was quenched with H2O and extracted
three times with 10 mL of CH2Cl2. The combined organic layers
were dried over MgSO4, and the solvent was removed under re-
Synthesis of Compound 10
To a solution of compound 9a (3.6 g, 14.4 mmol) in AcOH (40 mL)
was added SeO2 (3.34 g, 29.0 mmol) in one portion. The solution
was stirred and heated at 60 °C for 4 h. After filtration over Celite
and solvent removal, the residue obtained was chromatographed on
SiO2 (eluent: hexane–EtOAc, 6:4), providing 2.75 g of compound
10 as purple crystals (73% yield from compound 9); mp 109–110
1
°C. H NMR (400 MHz, CDCl3): d = 2.28 (s, 3 H), 3.80 (s, 3 H),
3.91 (s, 3 H), 3.93 (s, 3 H), 6.36 (d, J = 10.4 Hz, 1 H), 7.78 (d,
J = 10.4 Hz, 1 H). 13C NMR (125 MHz, CDCl3): d = 181.1, 177.8,
155.9, 154.2, 153.7, 140.1, 135.2, 125.6, 123.7, 123.1, 62.6, 61.2,
60.9, 10.3. IR (thin film): 2943, 1663, 1605, 1558, 1462, 1385,
1327, 1277, 1053, 991 cm–1. ESI-HRMS: m/z calcd for
C14H14O5Na23: 285.0739; found: 285.0731.
1
duced pressure, providing bisacylated ester. H NMR (400 MHz,
CDCl3): d = 2.34 (s, 3 H), 2.36 (s, 3 H), 2.40 (s, 3 H), 3.86 (s, 6 H),
3.92 (s, 3 H), 7.21 (d, J = 8.8 Hz, 1 H), 7.96 (d, J = 8.8 Hz, 1 H). 13
C
NMR (125 MHz, CDCl3): d = 168.7, 151.0, 150.4, 143.2, 140.1,
136.3, 125.3, 123.8, 121.8, 120.9, 120.3, 61.4, 60.6, 20.6, 20.4, 9.8.
IR (thin film): 2935, 2847, 1771, 1447, 1393, 1369, 1342, 1211,
1196, 1053, 1011 cm–1. To the solution of bisacylated ester in
MeCN (1.2 mL) and H2O (0.2 mL) was added CAN (290 mg, 0.53
mmol) in one portion at r.t. The solution was stirred for 30 min. The
mixture was quenched with H2O and extracted three times with
CH2Cl2 (20 mL). The combined organic layers were dried over
MgSO4, and the solvent was removed under reduced pressure, fil-
tered, providing naphthoquinone. To a solution of the naphtho-
Synthesis of Compound 11
To a solution of compound 10 (212 mg, 0.81 mmol) in dry THF (3
mL) was added NaBH4 (31 mg, 0.81 mmol) in one portion. The so-
lution was stirred for 15 min until the color of the solution was dis-
appeared. The mixture was quenched with aq HCl (3 M) and
extracted three times with CH2Cl2 (30 mL). The combined organic
layers were dried over MgSO4, and the solvent was removed under
1
reduced pressure. H NMR (400 MHz, CDCl3): d = 2.32 (s, 3 H),
Synlett 2009, No. 8, 1273–1276 © Thieme Stuttgart · New York

1276
K.-L. Wu et al.
LETTER
quinone in MeOH (5 mL) was added NaOMe (50 mg) in one portion
at r.t. The solution was stirred for 10 min. The mixture was
quenched with aq HCl (3 M) and extracted three times with EtOAc
(20 mL). The combined organic layers were dried over MgSO4, and
the solvent was removed under reduced pressure, filtered, providing
crude residue of malvone A. The residue obtained upon workup was
chromatographed on SiO2 (eluent: hexane–EtOAc, 5:5), providing
46 mg of malvone A as red crystals (73% yield from compound 10);
mp 171–172 °C. 1H NMR (400 MHz, acetone-d6): d = 1.98 (s, 3 H),
(4) Veshkurova, O.; Golubenko, Z.; Pshenichnov, E.; Arzanova,
I.; Uzbekov, V.; Sultanova, E.; Salikhov, S.; Williams, H.;
Reibenspies, J. H.; Puckhaber, L. S.; Stipanovic, R. D.
Phytochemistry 2006, 67, 2376.
(5) (a) Lindsey, C. C.; Wu, K.-L.; Pettus, T. R. R. Org. Lett.
2006, 8, 2365. (b) Wu, K.-L.; Wilkinson, S.; Reich, N. O.;
Pettus, T. R. R. Org. Lett. 2007, 9, 5537. (c) Marsini, M.
A.; Huang, Y.; Lindsey, C. C.; Wu, K.-L.; Pettus, T. R. R.
Org. Lett. 2008, 10, 1477.
(6) (a) Clive, D. L. J.; Khodabocus, A.; Vernon, P. G.; Angoh,
A. G.; Bordeleau, L.; Middleton, D. S.; Lowe, C.; Kellner,
D. J. Chem. Soc., Perkin Trans. 1 1991, 1433. (b) Brasholz,
M.; Sörgel, S.; Azap, C.; Reissig, H.-U. Eur. J. Org. Chem.
2007, 3801. (c) Sörgel, S.; Azap, C.; Reissig, H.-U. Eur. J.
Org. Chem. 2006, 4405. (d) Xie, X.; Kozlowski, M. Org.
Lett. 2001, 3, 2661. (e) Lowell, A. N.; Fennie, M. W.;
Kozlowski, M. C. J. Org. Chem. 2008, 73, 1911.
(7) (a) Chai, L. L. C.; Elix, J. A.; Moore, F. K. E. Tetrahedron
Lett. 2001, 42, 8915. (b) Chai, L. L. C.; Elix, J. A.; Moore,
F. K. E. J. Org. Chem. 2006, 71, 992.
4.08 (s, 3 H), 7.18 (d, J = 8.4 Hz, 1 H), 7.51 (d, J = 8.4 Hz, 1 H). 13
C
NMR (125 MHz, CDCl3): d = 188.1, 184.7, 158.4, 153.1, 151.0,
135.3, 124.7, 121.4, 121.1, 116.2, 61.8, 10.1.
Acknowledgment
K.-L. Wu is appreciative of a dissertation fellowship granted by the
University of California Tobacco-Related Disease Research Pro-
gram. T. R. R. Pettus thankfully acknowledges the UC CRCC and a
NSF-Career Award for past support of related naphthoquinone pro-
jects, as well as NSF CHE-0806356 for current support of our rese-
arch program. E. P. M. T. Cohn, as an undergraduate researcher,
greatly appreciates a fellowship provided by a generous gift of the
DeWolfe family that supports undergraduate summer research in
organic chemistry at UCSB. The authors also acknowledge Mr.
Shenlin Huang’s efforts to collect some of the data for key com-
pounds.
(8) Baillie, A. C.; Thomson, R. H. J. Chem. Soc. C 1966, 2184.
(9) Bekaert, A.; Andrieux, J.; Plat, M.; Brion, J.-D. Tetrahedron
Lett. 1997, 38, 4219.
(10) Malinovskaya, G. V.; Chizhova, A. Y.; Anufriev, V. P. Russ.
Chem. Bull. 1999, 48, 1010.
(11) Carreño, M. C.; Ruano, J. G.; Toledo, M. A.; Urbano, A.
Tetrahedron: Asymmetry 1997, 8, 913.
(12) Ameer, F.; Giles, R. G. F.; Green, I. R.; Pearce, R. Synth.
Commun. 2004, 34, 1247.
References
(13) Matsumoto, T.; Ichihara, A.; Yanagiya, M.; Yuzawa, T.;
Sannai, A.; Oikawa, H.; Sakamura, S.; Eugster, C. H. Helv.
Chim. Acta 1985, 68, 2324.
(14) Velde, S. L. V.; Jacobsen, E. N. J. Org. Chem. 1995, 60,
5380.
(15) (a) Gray, D. L. F.; Nicolaou, K. C. J. Am. Chem. Soc. 2004,
126, 607. (b) Nicolaou, K. C.; Gray, D. L. F. Angew. Chem.
Int. Ed. 2001, 40, 761.
(1) Ernst-Russell, M. A.; Elix, J. A.; Chai, C. L. L.; Willis,
A. C.; Hamada, N.; Nash, T. H. III. Tetrahedron Lett. 1999,
40, 6321.
(2) (a) Huneck, S.; Ahti, T. Pharmazie 1982, 37, 302.
(b) Yamamoto, Y.; Matsubara, H.; Kinoshita, Y.; Kinoshita,
K.; Koyama, K.; Takahashi, K.; Ahmadjiam, V.; Kurokawa,
T.; Yoshimura, I. Phytochemistry 1996, 43, 1239.
(3) (a) Berg, A.; Görls, H.; Dörfelt, H.; Walther, G.; Schlegel,
B.; Gräfe, U. J. Antibiot. 2000, 53, 1293. (b) Stepanenko,
L. S.; Krivoshchekova, O. E. Phytochemistry 1997, 46, 565.
Synlett 2009, No. 8, 1273–1276 © Thieme Stuttgart · New York

Copyright of Synlett is the property of Georg Thieme Verlag Stuttgart and its content may not
be copied or emailed to multiple sites or posted to a listserv without the copyright holder's
express written permission. However, users may print, download, or email articles for
individual use.