Synthesis of (±)-lotthanongine, a novel natural product with a flavan-indole hybrid structure

Article abstract of DOI:10.1055/s-2005-868485

First total synthesis of lotthanongine (3), a natural product with a flavan-indole composite structure, has been achieved via the Lewis acid-catalyzed C-C bond formation between the catechin and the indole units.

Full text of DOI:10.1055/s-2005-868485

LETTER
1311
Synthesis of ( )-Lotthanongine, a Novel Natural Product with a Flavan–Indole
Hybrid Structure
Synthesis of
(
)-Lo
tthanongin
i
e
suke Hatakeyama, Ken Ohmori, Keisuke Suzuki*
Department of Chemistry, Tokyo Institute of Technology, and SORST-JST Agency, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8551,
Japan
Fax +81(3)57342788; E-mail: ksuzuki@chem.titech.ac.jp
Received 15 March 2005
OR
Abstract: First total synthesis of lotthanongine (3), a natural prod-
RO
O
uct with a flavan–indole composite structure, has been achieved via
the Lewis acid-catalyzed C–C bond formation between the catechin
and the indole units.
OR
RO
Key words: catechin, indole, polyphenol, heterocycle, hybrid
A
OR
H
N
HN
Flavan-type polyphenols are widely distributed in the
plant kingdom, and show various significant biological
activities.1 Recent structural studies have identified fur-
ther structural diversity of polyphenols by hybridization
with other structure motifs, such as acetate2 and terpenes.3
Among these, lotthanongine (3) isolated from the roots of
Trigonostemon reidioides Craib (Euphorbiaceae, Thai
name: Lot-Tha-Nong),4 has a structure, in which a flavan
unit, afzelechin (2), is connected with an indole unit at its
C(4) position (Figure 1).
O
RO
B
Figure 2
Concerning the latter point, we previously noted that the
Lewis acid promoted SN1-type reaction of catechin
acetate 4 showed a remarkable stereochemical dichotomy
depending on the nucleophilic partner (Scheme 1).5
Most nucleophiles showed b-selectivities including hete-
ro-nucleophiles, such as PhSH and Me3SiN3, and carbon
nucleophiles such as ketene silyl acetals, whereas in sharp
contrast the a-selectivity prevailed for the electron-rich
aromatic rings in the phloroglucinol and catechin deri-
vatives.5 Within the synthetic context of lotthanongine
(3), the question was whether or not indoles are the b-se-
lective nucleophiles for establishing the requisite stereo-
chemistry.
OH
HO
OH
O
4
OH
OH
H
HO
O
4
HO
HN
R
N
OH
O
HO
R = OH, catechin (1)
R = H, afzelechin (2)
HO
lotthanongine (3)
Figure 1
OBn
OBn
OBn
OBn
BnO
O
4
BnO
Nucleophile
Lewis acid
O
Attracted by the novel structure as well as the potential bi-
ological activities expectable in each component, we be-
came interested in the total synthesis of 3. We envisioned
that the cation A, generated from catechin derivative (vide
infra), would be susceptible of the nucleophilic attack of
indole B, a process which is most probably involved in the
biosynthesis (Figure 2).
OBn
OBn
BnO
OAc
4
BnO
Nu
α/β
α-selective nucleophiles
BnO OBn
β-selective nucleophiles
PhSH
TMSN3
OEt
BnO
O
Two questions were due to this key coupling, (1) the
reactivity of the indole nucleophile, and (2) the stereo-
selectivity.
OBn
OBn
OTBDMS
BnO
Ph
OBn
OTBDMS
BnO
: reaction site
SYNLETT 2005, No. 8, pp 1311–1315
Advanced online publication: 21.04.2005
DOI: 10.1055/s-2005-868485; Art ID: U07305ST
© Georg Thieme Verlag Stuttgart · New York
1
7.
0
5.
2
0
0
5
Scheme 1 a-Stereoselective nucleophiles and b-stereoselective
acetate 4 showed remarkable stereochemical nucleophiles

1312
K. Hatakeyama et al.
LETTER
Table 1 The Reaction of Catechin Acetate 4 with 3-Substituted Indole 5
OBn
OBn
BnO
O
OBn
2
3
OBn
α/β
HN
CO2Me
BnO
O
promoter, CH2Cl2
temp., time
OBn
BnO
HN
+
4
CO2Me
OBn
BnO
OAc
4
5 (3 equiv)
6
Run
1
Promoter (equiv)
BF3·OEt2 (0.1)
TMSOTf (0.1)
Temp (°C)
–78 → 5
–78 → –40
–78 → 25
0
Time (h)
2.0
Yield (%)
a/b
86
94
84
97
10:90
6:94
2
2.0
3
Cp2ZrCl2 (0.1), AgClO4 (0.2), MS 4Å
TsOH·H2O (0.1)
31
13:87
10:90
4
1.0
Table 1 shows the initial model study on the connectivity respectively. Oxidation of alkene 11 by OsO4 and N-
of indole and catechin derivatives. Catechin acetate 4 was methylmorpholine-N-oxide (t-BuOH, THF, H2O, r.t., 5 h)
allowed to react with indole-3-acetic acid methyl ester (5) gave crude solid diol 12, which was washed with Et2O to
under some sets of acidic conditions.
give colorless powders (single diastereomer, mp 157–
159 °C, 76% yield from 9).
Representative procedure is described by the reaction cat-
alyzed by BF3·OEt2 (run 1). A solution of catechin acetate Protection of diol 12 with t-butyldimethylsilyl (TBDMS)
4 and indole 5 (3 mol equiv) in CH2Cl2 was treated with group [TBDMSCl (6 mol equiv), imidazole (10 mol
10 mol% of BF3·OEt2 at –78 °C, and the temperature was equiv), DMF, r.t., 14 h] afforded the bis-silyl ether 13 as
gradually raised with monitoring of the reaction by TLC. colorless amorphous solids. The relative stereochemistry
After the complete consumption of catechin acetate 4 was of 1310 was assigned as such by 1H NMR (J2,3 = 10.0 Hz,
assured at 5 °C, usual work up and separation by silica gel
TLC gave product 6 in 86% yield. The NMR analysis
showed the product was mainly composed of the b-isomer
(a/b = 10:90), also that the C–C bond formation occurred
at the C(2) for the indole and the C(4) position for the
catechin.6
J3, 4 = 2.4 Hz).
The synthesis of indole unit 21 relied on the Larock
method11 involving the Pd-catalyzed heteroannulation of
an internal alkyne with an o-iodoaniline derivative
(Scheme 3). The requisite o-iodoaniline 15 was prepared
by the reduction of nitroarene 1412 with hydrazine in the
Further experiments showed that other Lewis acids (runs presence of FeCl3 and activated charcoal (MeOH, reflux,
2 and 3) or even a protic acid (run 4) were also effective 38 h).13 The indole skeleton was constructed via the Pd-
to catalyze the reaction with comparable yield and selec- catalyzed cyclization of o-iodoaniline 15 and four mole
tivity, albeit the final reaction temperatures differed. equivalents of silylacetylene 1614 [Pd(OAc) (0.1 equiv),
TMSOTf was the most reactive and showed the highest DMF, 80 °C, 2 h].
stereoselectivity for this particular case.
The trimethylsilyl group in indole 17 was removed by
Encouraged by these results, we proceeded to the synthe- acidic methanol (MeOH, AcCl, 0 °C, 15 min) to afford in-
sis of lotthanongine (3). Given the poor availability of the dole 18 in 73% overall yield from 15. The hydroxy group
flavan portion, afzelechin (2),7 we prepared its racemic in 18 was transformed into an azide group with triphe-
form by adopting the route of catechin synthesis by Clark- nylphosphine (2 mol equiv), diphenylphosphoryl azide (2
Lewis8 and Kawamoto.9a Aldol condensation of acetophe- mol equiv), and diethyl azodicarboxylate (2 mol equiv,
none 79 and aldehyde 8 was effected by using sodium hy- THF, 0 °C to r.t., 40 min) in 88% yield.15 Azide 19 was
dride (DMF, 0 °C, 25 min) to give crude solid products, converted to indole unit 2116 in 62% yield via the
which were washed with Et2O to give pure chalcone 911 as Staudinger reaction with acid 20 using trimethylphos-
yellow powders (mp 136–137 °C) in 87% yield. Reduc- phine (1.2 equiv, toluene, reflux, 1.5 h).17
tion of 9 with NaBH4 (2-methoxyethanol, 90 °C, 5 min)
gave alcohol 10, which was treated with BF3·OEt2
(CH2Cl2, r.t., 35 min) to afford flavan-3-ene 11
(Scheme 2). It should be noted that alcohol 10 and alkene
11 were highly labile to silica gel chromatography, and
thus, the crude materials were used without purification,
Having afzelechin 13 and indole unit 21 in hand, the stage
was set for the key coupling reaction. The question at this
stage was whether or not the silyl ether would be used for
the key coupling reaction. To the best of our knowledge,
siloxy groups had not been used as leaving groups in this
context. Thus, we examined whether the promoters used
Synlett 2005, No. 8, 1311–1315 © Thieme Stuttgart · New York

LETTER
Synthesis of (±)-Lotthanongine
1313
OBn
for the activation of catechin 4 in the model studies (vide
supra) were effective also for the activation of silyl ether.
OBn
OHC
BnO
OH
O
BnO
OH
O
8
Pleasingly, each acid promoter was effective for this pur-
pose (Table 2). It turned out the difference was, however,
that the catalytic conditions led to rather slow reactions
with poor stereoselectivities (runs 1–4).
NaH, DMF, 0 °C
25 min, 87%
BnO
BnO
9
7
So we attempted the stoichiometric use of acid promoters,
and Cp2ZrCl2–AgClO4 gave best results with high b-se-
lectivity (95%, a/b = 13:87, run 7).5b The other promoters
gave lower stereoselectivities so long as the stoichiomet-
ric reactions were concerned (runs 5, 6, and 8).
NaBH4
2-methoxyethanol
90 °C, 5 min
OBn
OBn
BnO
OH
OH
BnO
O
BF3·OEt2
Since separation of the diastereomers of 22 was difficult
by silica gel chromatography, we chose to proceed, with-
out their separation, to the next desilylation stage. Fortu-
nately, upon careful treatment of the a/b mixture of 22 (a/
b = 13:87) with tetrabutylammonium fluoride (TBAF, 1.2
mol equiv) at 0 °C (THF, 25 min), only the b-isomer, 22b,
selectively underwent detachment of the TBDMS group,
while the a-isomer, 22a, remained intact. If the reaction
was further warmed to room temperature, the a-isomer
also underwent desilylation (vide infra). Finally, five ben-
zyl groups in 23b was removed by careful exposure to
BBr3 (10 mol equiv) at –78 °C (CH2Cl2, 2 h) to give
lotthanongine (3) as amorphous solids in 62% yield
(Scheme 4). The synthetic material proved to be identical
with the natural product by comparison of their spectro-
scopic data (1H NMR, 13C NMR, IR) and combustion
analysis.18
CH2Cl2
r.t., 35 min
BnO
BnO
10
11
OsO4, NMO, H2O, t-BuOH
THF, r.t.
5 h, 76% (3 steps)
OBn
OBn
TBDMSCl
imidazole
BnO
O
BnO
O
2
3
4
DMF
0 °C → r.t.
14 h, 84%
OH
OTBDMS
BnO
OH
BnO
OTBDMS
12
13
Scheme 2 Synthesis of afzelechin unit 13
OBn
OTMS
BnO
O
O2N
BnO
I
H2N
BnO
I
TMS
H2NNH2, FeCl3·H2O
16
OBn
OH
activated charcoal
MeOH, reflux
38 h, 91%
Pd(OAc)2, PPh3, LiCl
H
BnO
N
K2CO3, DMF, 80 °C, 2 h
TBAF, THF
HN
14
15
22 (α/β = 13/87)
O
0 °C, 25 min
87%
TMS
(PhO)2P(O)N3
OH
EtO2CN NCO2Et
OTMS
HN
BnO
HN
HN
BnO
23β
AcCl, MeOH
PPh3, THF
0 °C, 15 min
OH
0 °C → r.t., 40 min
73% (2 steps)
BnO
88%
HO
O
17
19
18
BBr3, CH2Cl2
OH
OH
N3
H
ꢀ78 °C, 2 h
HO
HN
N
62%
OBn
O
H
N
PMe3
toluene
HN
BnO
HO
O
HO
lotthanongine (3)
reflux
1.5 h
62%
OBn
BnO
21
Scheme 4 Removal of protecting groups
O
20
Scheme 3 Synthesis of indole unit 21
The minor diastereomer 22a was also converted to 24, the
a-isomer of the natural product 3 (Scheme 5). Thus, the
TBDMS group in 22a was removed by TBAF at room
temperature (THF, 7 h), and removal of the benzyl
protecting groups in a similar way gave the isomer 24.19
Synlett 2005, No. 8, 1311–1315 © Thieme Stuttgart · New York

1314
K. Hatakeyama et al.
LETTER
Table 2 Coupling Reaction between Afzelechin 13 and Indole 21
OBn
BnO
O
OBn
OBn
OBn
OTBDMS
BnO
O
H
H
+
BnO
HN
N
N
HN
promoter, CH2Cl2
temp., time
OTBDMS
O
O
BnO
OTBDMS
BnO
BnO
13
21 (1.2 equiv)
22
Run
1
Promoter (equiv)
BF3·OEt2 (0.1)
TMSOTf (0.1)
Temp (°C)
–45 → 25
–45 → 25
–45 → 25
0 → 25
Time (h)
4.0
Yield (%)
a/b
88
89
77a
81b
90
84
95
83
27:73
29:71
22:78
41:59
28:72
20:80
13:87
43:57
2
4.0
3
Cp2ZrCl2 (0.1), AgClO4 (0.2), MS 4Å
TsOH·H2O (0.1)
21
4
21
5
BF3·OEt2 (1.2)
–45 → –5
–45 → –15
–45 → –30
–45 → 0
1.3
6
TMSOTf (1.2)
1.0
7
Cp2ZrCl2 (0.1), AgClO4 (2.4), MS 4Å
TsOH·H2O (1.2)
0.33
3.0
8
a Recovery: 10%.
b Recovery: 2%.
OR1
References
R1O
O
(1) (a) Bohm, B. A. Introduction to Flavonoids; Harwood
Academic Publishers: Amsterdam, 1998. (b) Hatano, T.;
Yoshida, T.; Hemingway, R. W. Plant Polyphenols 2:
Chemistry, Biology, Pharmacology, Ecology; Kluwer
Academic Plenum: New York, 1999. (c) Ferreira, D.; Li,
X.-C. Nat. Prod. Rep. 2000, 17, 193.
3
OR1
4
OR2
R1O
HN
H
N
O
(2) (a) The Handbook of Natural Flavonoids, Vol. 2; Harborne,
J. B.; Baxter, H., Eds.; Wiley: Chichester, 1999, 499.
(b) Hwang, T.-H.; Kashiwada, Y.; Nonaka, G.; Nishioka, I.
Phytochemistry 1990, 29, 279. For isolation of related
compounds, see: (c) Karl, C.; Pedersen, A. P.; Müller, G. Z.
Naturforsch., C: Biosci. 1981, 36, 607. (d) Tanaka, N.; Orii,
R.; Ogasa, K.; Wada, H.; Murakami, T.; Saiki, Y.; Chen, C.-
M. Chem. Pharm. Bull. 1991, 39, 55. (e) Baek, N.-I.;
Kennelly, E. J.; Kardono, L. B. S.; Tsauri, S.; Padmawinata,
K.; Soejarto, D. D.; Kinghorn, A. D. Phytochemistry 1994,
36, 513.
R1O
1) TBAF, THF, 81%
22α: R1 = Bn, R2 = TBDMS
24: R1 = R2 = H
2) BBr3, CH2Cl2, 56%
Scheme 5 Synthesis of the a-isomer of lotthanongine, 24
1
The H NMR data revealed the stereochemistry of 24
(3) Porzel, A.; Lien, T. P.; Schmidt, J.; Drosihn, S.; Wagner, C.;
Merzweiler, K.; Sung, T. V.; Adam, G. Tetrahedron 2000,
56, 865.
(4) Kanchanapoom, T.; Kasai, R.; Chumsri, P.; Kraisintu, K.;
Yamasaki, K. Tetrahedron Lett. 2002, 43, 2941.
(5) (a) Ohmori, K.; Ushimaru, N.; Suzuki, K. Tetrahedron Lett.
2002, 43, 7753. (b) Ohmori, K.; Hatakeyama, K.; Suzuki, K.
Tetrahedron 2004, 60, 1365. (c) Ohmori, K.; Ushimaru, N.;
Suzuki, K. Proc. Natl. Acad. Sci., U.S.A. 2004, 101, 12002.
(6) The b-stereochemistry of the major isomer was assigned by
the NOE correlations of N–H proton of the indole with H(2)
and H(4) of the flavan nucleus. On the other hand, the a-
stereochemistry of the minor isomer was assigned by the
NOE correlations between H(2) and H(4) of the flavan
(J2,3 = 9.8 Hz, J3,4 = 8.8 Hz), confirming it diastereomeric
to the natural product 3.
In summary, the first synthesis of lotthanongine (3) was
achieved by using the SN1-type reaction of afzelechin 13
with indole 21, and the structure of 3 was reconfirmed by
the synthetic materials. This synthesis opened flexibility
to various flavan–indole hybridized compounds.
Acknowledgment
Partial financial support from 21st Century COE Program (Chem-
istry) is gratefully acknowledged.
Synlett 2005, No. 8, 1311–1315 © Thieme Stuttgart · New York

LETTER
Synthesis of (±)-Lotthanongine
1315
(15) Lal, B.; Pramanik, B. N.; Manhas, M. S.; Bose, A. K.
OBn
OBn
OBn
OBn
Tetrahedron Lett. 1977, 1977.
H
H
(16) Compound 21: colorless powders, mp 234–236 °C. 1H NMR
(400 MHz, DMSO): d = 2.81 (br t, 2 H, J = 7.3 Hz), 3.43 (br
td, 2 H, J = 7.3, 5.6 Hz), 5.09 (s, 2 H), 5.13 (s, 2 H), 6.47 (d,
1 H, J = 16.1 Hz), 6.72 (dd, 1 H, J = 8.5, 2.2 Hz), 6.90 (br d,
1 H, J = 2.2 Hz), 7.00 (br s, 1 H), 7.03 (d, 2 H, J = 8.6 Hz),
7.27–7.52 (m, 14 H), 8.10 (br t, 1 H, J = 5.6 Hz), 10.6 (br s,
1 H). 13C NMR (100 MHz, DMSO): d = 25.4, 39.6, 69.3,
69.5, 96.0, 109.1, 111.8, 115.2, 118.9, 120.0, 121.4, 121.9,
127.5, 127.6, 127.7, 127.9, 128.4, 128.5, 129.0, 136.8,
137.7, 138.1, 154.4, 159.3, 165.1. IR (KBr): 3224, 1645,
1601, 1539, 1510, 1452, 1174, 694 cm–1. Anal. Calcd for
C33H30N2O3: C, 78.86; H, 6.02; N, 5.57. Found: C, 78.94; H,
5.91; N, 5.43.
BnO
O
BnO
O
2
2
OBn
OBn
4
4
H
H
H
H
H
H
BnO
BnO
HN
CO2Me
HN
CO2Me
4
4
H
H
β isomer
α isomer
Figure 3
skeleton. The NOE correlations of the H(4) of the indole and
the methylene protons of the indole side chain assured the
connectivity as indicated, excluding the exchange of the
C(2) and C(3) substituents (Figure 3). Compare: Jackson, A.
H.; Smith, P. J. Chem. Soc., Chem. Commun. 1967, 264.
(7) Afzelechin is available from Apin Chemicals in milligram
quantities.
(17) Garcia, J.; Urpí, F.; Vilarrasa, J. Tetrahedron Lett. 1984, 25,
4841.
(18) Compound 3: 1H NMR (500 MHz, CD3OD): d = 2.98–3.10
(m, 2 H), 3.47–3.55 (m, 1 H), 3.69–3.77 (m, 1 H), 4.15 (dd,
1 H, J = 9.6, 5.7 Hz), 4.67 (d, 1 H, J = 5.7 Hz), 4.95 (d, 1 H,
J = 9.6 Hz), 5.95 (d, 1 H, J = 2.4 Hz), 6.00 (d, 1 H, J = 2.4
Hz), 6.27 (d, 1 H, J = 15.7 Hz), 6.55 (dd, 1 H, J = 8.5, 2.2
Hz), 6.74 (d, 1 H, J = 2.2 Hz), 6.77 (d, 2 H, J = 8.7 Hz), 6.78
(d, 2 H, J = 8.7 Hz), 7.24 (d, 2 H, J = 8.7 Hz), 7.27 (d, 2 H,
J = 8.7 Hz), 7.33 (d, 1 H, J = 8.5 Hz), 7.35 (d, 1 H, J = 15.7
Hz), 9.30 (br s, 1 H). 13C NMR (125 MHz, CD3OD): d =
25.3, 36.2, 41.3, 71.8, 79.6, 95.8, 96.8, 97.8, 102.9, 109.6,
111.7, 116.1, 116.7, 118.7, 119.4, 123.9, 127.9, 130.2,
130.6, 131.7, 133.9, 138.6, 141.5, 153.6, 157.5, 158.1,
158.5, 159.1, 160.4, 169.3. IR (KBr): 3300, 1647, 1514,
1458, 1227, 1146, 829 cm–1. Anal. Calcd for C34H30N2O8: C,
68.68; H, 5.09; N, 4.71. Found: C, 68.41; H, 5.38; N, 4.42.
(19) Compound 24: 1H NMR (400 MHz, CD3OD): d = 2.90–3.05
(m, 1 H), 3.05–3.20 (m, 1 H), 3.46–3.58 (m, 1 H), 3.64–3.78
(m, 1 H), 4.08 (dd, 1 H, J = 9.8, 8.8 Hz), 4.33 (d, 1 H, J = 8.8
Hz), 4.62 (d, 1 H, J = 9.8 Hz), 5.95 (d, 1 H, J = 2.4 Hz), 5.98
(d, 1 H, J = 2.4 Hz), 6.38 (d, 1 H, J = 15.9 Hz), 6.52 (dd, 1
H, J = 8.6, 2.4 Hz), 6.66 (d, 1 H, J = 2.4 Hz), 6.75 (d, 2 H,
J = 8.6 Hz), 6.81 (d, 2 H, J = 8.8 Hz), 7.25 (d, 2 H, J = 8.6
Hz), 7.27 (d, 1 H, J = 8.6 Hz), 7.33 (d, 2 H, J = 8.8 Hz), 7.37
(8) Clark-Lewis, J. W.; Skingle, D. C. Aust. J. Chem. 1967, 20,
2169.
(9) (a) Kawamoto, H.; Nakatsubo, F.; Murakami, K. J. Wood
Chem. Technol. 1989, 9, 35. (b) Kawamoto, H.; Nakatsubo,
F.; Murakami, K. Synth. Commun. 1996, 26, 531.
(10) Compound 13: colorless amorphous solids. 1H NMR (400
MHz, CDCl3): d = –0.52 (s, 3 H), –0.20 (s, 3 H), –0.18 (s, 3
H), –0.10 (s, 3 H), 0.69 (s, 9 H), 0.85 (s, 9 H), 3.80 (dd, 1 H,
J = 10.0, 2.4 Hz), 4.98 (s, 2 H), 5.01 (d, 1 H, J = 2.4 Hz),
5.02 (s, 2 H), 5.09 (s, 2 H), 5.33 (d, 1 H, J = 10.0 Hz), 6.19
(d, 1 H, J = 2.2 Hz), 6.21 (d, 1 H, J = 2.2 Hz), 6.95 (d, 2 H,
J = 8.8 Hz), 7.28–7.44 (m, 17 H). 13C NMR (100 MHz,
CDCl3): d = –5.7, –5.3, –4.59, –4.58, 17.9, 18.4, 25.8, 26.0,
63.2, 70.0, 70.1, 70.2, 73.7, 76.1, 92.8, 94.1, 106.8, 114.6,
127.3, 127.6, 127.77, 127.82, 128.01, 128.04, 128.47,
128.52, 128.6, 129.2, 132.1, 136.6, 136.7, 137.1, 155.9,
157.2, 158.6, 160.5. IR (KBr): 3066, 3033, 2927, 2856,
1616, 1593, 1514, 1377, 1250, 1151, 1076, 883, 837, 775,
673 cm–1. Anal. Calcd for C48H60O6Si2: C, 73.05; H, 7.66.
Found: C, 72.87; H, 7.86.
(d, 1 H, J = 15.9 Hz), 8.02 (br s, 1 H), 9.52 (br s, 1 H). 13
C
NMR (100 MHz, CD3OD): d = 25.3, 41.3, 41.8, 75.1, 83.4,
96.6, 97.7, 97.9, 104.1, 109.3, 110.6, 116.1, 116.7, 118.8,
119.0, 123.6, 127.9, 130.3, 130.6, 131.0, 136.9, 138.6,
141.5, 153.5, 158.4, 158.6, 158.8, 159.0, 160.4, 169.3. IR
(KBr): 3326, 1647, 1604, 1516, 1458, 1234, 1173, 1145,
1068, 831 cm–1. Anal. Calcd for C34H30N2O8: C, 68.68; H,
5.09; N, 4.71. Found: C, 68.43; H, 5.38; N, 4.46.
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Synlett 2005, No. 8, 1311–1315 © Thieme Stuttgart · New York