Synthesis of the marine alkaloid caulersin

Article abstract of DOI:10.1016/j.tet.2003.12.046

A three-step synthesis of caulersin (3) from indole-2-acetic acid methyl ester and indole-2-carbonyl chloride is described. As the spectral data of the synthetic sample differed from those reported for the natural product, the structure was determined by X-ray crystallography.

Full text of DOI:10.1016/j.tet.2003.12.046

Tetrahedron 60 (2004) 2147–2153
Synthesis of the marine alkaloid caulersin
Niklas Wahlstrom,a,b Birgitta Stenslandc and Jan Bergmana,b
¨
,
*
aUnit for Organic Chemistry, CNT, Department of Biosciences at Novum, Karolinska Institute, Novum Research Park,
SE-141 57 Huddinge, Sweden
b
¨
¨
Sodertorn University College, SE-141 04 Huddinge, Sweden
Preformulation and Biopharmaceutics, Solid State Analysis/SBBG B314:3, Astrazeneca PAR & D, SE-151 85 Sodertalje, Sweden
c
¨
¨
Received 8 August 2003; revised 1 December 2003; accepted 18 December 2003
Abstract—A three-step synthesis of caulersin (3) from indole-2-acetic acid methyl ester and indole-2-carbonyl chloride is described. As the
spectral data of the synthetic sample differed from those reported for the natural product, the structure was determined by X-ray
crystallography.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
caulersin (seven steps, 14% reported overall yield) was
published in 1999.4 A structure related to caulersin, the
bisindole caulerpin (4), isolated from several different green
and red algae,5–8 has showed moderate antitumor activity.9
Furthermore, 4 acts as a plant growth regulator10 and has
also been shown to inhibit the multixenobiotic resistance
(MXR) pump in algae, thus enhancing toxicity of
xenobiotics.11 The synthesis of 4 was reported by Maiti
and Thomson who treated indole-2-acetic acid methyl ester
with a Vilsmeier salt to produce a low yield of 4.12,13 In an
oxidative transformation of 4, several oxygenated adducts
were isolated.14 During the analysis of these structures,
caulersin (3) was suggested as one of the stabile inter-
mediates formed in the mass spectrometer. As no natural
ring-transformed adduct from 4 has been isolated, the
possibility of a product–precursor relationship between 3
and 4 does exist. With the objective of subjecting 3 to
further biological studies, large quantities of 3 were needed.
This encouraged the development of a shorter and higher
yielding synthesis of 3, something that now has been
accomplished.
In connection with previous synthetic studies of indolo-
carbazoles,1 the highly potent aryl hydrocarbon receptor
ligand indolo[3,2-b]carbazole-6,12-dione (1) and the corre-
sponding 2,3-b isomer (2) have been synthesized via new
ring closing strategies (Fig. 1).2 Utilizing a similar ring
closing strategy, the related bisindole marine natural
product caulersin (3), was now considered as a synthetic
target. Caulersin was isolated in 1997 from the algae
Caulerpa serrulata,3 and represents the only natural product
isolated so far containing a bisindole structure bridged by
a central troponoid framework. A lengthy synthesis of
2. Results and discussion
Retrosynthetic analysis of 3 leads to the 2,3-diindolyl
ketone 5 by excision of a carbon atom, probably as
formaldehyde, from the seven-membered ring. Further
scission of 5 gives indole-2-acetic acid methyl ester (6a)15
and indole-2-carboxylic acid (7) (Scheme 1).
Figure 1. Indolo[3,2-b]carbazole-6,12-dione (1), indolo[2,3-b]carbazole-
6,13-dione (2), caulersin (3) and caulerpin (4).
In practice, treatment of indole-2-acetic acid methyl ester
(6a) with dimethylaluminium chloride,16,17 followed
by addition of indole-2-carbonyl chloride produced the
3-acylated bisindole 5 in 35% yield together with two other
Keywords: Caulersin; Marine alkaloid; Vilsmeier salt; Indole.
*
Corresponding author. Tel.: þ46-8-608-9204; fax: þ46-8-608-1501;
e-mail address: jabe@biosci.ki.se
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2003.12.046

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N. Wahlstrom et al. / Tetrahedron 60 (2004) 2147–2153
2148
Scheme 1. Retrosynthetic analysis of caulersin (3).
Scheme 3. (a) Chloromethylenemorpholinium chloride 3 equiv., 21–60 8C;
(b) DMSO, H2O, 50 8C.
presence of a chlorine atom incorporated in the structure.
This information, suggests that 10a is the initial product
formed (Scheme 3). Secondly, another Vilsmeier salt
equivalent reacts with 10a and is thus further transformed
into 10b. Hydrolysis of 10b in DMSO/water at 50 8C for
14 h then produced 3 as an orange solid in 70% yield. The
total yield of caulersin in this facile three-step procedure
was 25%.
When the spectral data of 3 were compared with those
recorded for the natural product the proton spectra proved to
be essentially identical. However, the reported melting
points and the IR spectra differed. During the melting point
measurement we noted that the consistency of the powder of
3 changed only marginally at the reported melting point
(269–274 8C),3,4 to a more golden color. The sample finally
melted between 352–355 8C, 70 8C higher than previously
reported. As other rigid and planar bisindoles exemplified
by 1 and 2 have high melting points (.400 8C),2 the higher
melting point recorded in our hands seems reasonable.
Scheme 2. (a) Me2AlCl, CH2Cl2; (b) indole-2-carbonyl chloride.
products arising from the tautomer 6b (Scheme 2). The acid
sensitive indole 6a undergoes partial isomerization to the
enamine 6b which in turn reacts with indole-2-carbonyl
chloride, producing 8 (36%) together with the doubly
acylated product 9 (8%), obviously a secondary product of
8. Initial reaction between 5 and 1 equiv. of chloromethyl-
enemorpholinium chloride,18 in dichloroethane at 60 8C
failed to give 3. Increasing the number of the Vilsmeier salt
equivalents to 3 produced, upon cooling, a red solid. The
chemical shift of one of the aromatic doublets and the
When the crystals were analyzed with IR-spectroscopy a
similar pattern of peaks, but not identical, could be seen as
compared to the material before sublimation. This was
attributed to the different packing of the crystals before and
after sublimation. In comparison with the previously
synthesized material and the isolated natural product,
there were small differences for the two NH functionalities
1
aromatic singlet were shifted downfield in the H NMR
spectrum of this intermediate, and the aliphatic triplet had
disappeared. This sparingly soluble intermediate was
rapidly transformed into 3 in the NMR-tube, no 13C NMR
data could therefore be obtained. However, when the ethyl
ester analogue of 5 was treated with chloromethylene-
morpholinium chloride a red solid formed that proved to be
slightly more stable, something that enabled the isolation of
the ethyl ester derivative of 10b as the corresponding free
base. Mass spectrometry of the free base showed the
1
of the unsublimed sample. H and 13C NMR spectroscopy
showed that in solution, the sublimed crystals were identical
with the unsublimed material. In the 13C spectrum several
differences with the reported data were observed. This is
best exemplified by the absence of the chemical shift
reported for the position of carbon 20 (C-20) at 146.8 ppm.
Instead, a signal at 114.6 ppm was assigned for C-20
(Fig. 2). The 13C spectrum of the isolated 3 obtained from

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2149
workers was repeated.4 o-Azidobenzaldehyde21,22 and
N-methoxymethyl-3-acetyl-2-chloroindole23 was con-
densed to produce the expected chalcone which when
heated in xylenes, formed the corresponding bisindole
ketone. This ketone was treated with methyl vinyl ketone
and boron trifluoride diethyl etherate in nitromethane to
give the Michael adduct 114 and a small amount of 5-nitro-
2-pentanone.24 During this last reaction several portions of
the boron reagent and the methyl vinyl ketone were added to
complete the reaction, in contrast to the single addition
reported. In the subsequent ring closing reaction with
potassium tert-butoxide/tert-butanol (t-BuOK/t-BuOH,
2.5 equiv.) the cyclized seven-ring bridged bisindole 12
could be obtained via a modification of the workup
procedure (Scheme 4). When several equivalents of
t-BuOK were used, the unexpected deacylation of 12
readily took place, to produce the deacylated heptacycle
13. The retention values of 11 and 13 in several different
eluent systems are similar, thus precluding the use of TLC to
monitor the progress of the reaction. In the selective
formation of 13 in 81% yield, multiple equivalents of
t-BuOK were used. Bisindole 13 was dehydrogenated with
DDQ in benzene to give the aromatized molecule 14 in 76%
yield. Subsequent deprotection of 14 was carried out with
6 M HCl in refluxing ethanol to produce the parent
bisindolotropolone 15 in 82% yield. Dehydrogenation of
12 with DDQ produced the aromatized molecule 16 that was
subsequently subjected to treatment with fresh potassium
hypochlorite (KOCl) solution.4,25 The bisindole ester 17
was isolated in 68% using the procedure of Fresneda.25 The
ester 17 was then heated in MeOH with 6 M HCl, to give 3
in 68%, with similar spectroscopic data with material
obtained via Scheme 3.
Figure 2. Crystal structure of caulersin (3) with atom numbering. CCDC
reference number 210210.
Su,19 had a considerably lower signal to noise ratio
compared with the spectrum from our synthetic product.
Furthermore, the signal with the lowest intensity, a singlet at
119.3 ppm was not accounted for. We now undertook a
detailed 2D NMR study of compound 3, which enabled us to
assign all carbon and hydrogen atoms in the molecule. To
further establish the structure, small fine yellow crystals of 3
were obtained by sublimation at 285 8C and 0.35 mbar. The
X-ray diffraction study clearly showed that the structure was
correct (Fig. 2). Worth to mention here is the extremely high
packing coefficient20 (72.6%), a figure which indicates that
the molecules are exceptionally well packed in the crystal.
To verify the structure of the previously reported 3 and the
isolated natural product,3 the synthesis of Fresneda and co-
3. Conclusions
A simple three-step synthesis in 25% total yield of the
marine natural product caulersin (3) has been developed by
cyclisation of the simple ketoester 5 with the Vilsmeier
reagent chloromethylenemorpholinium chloride to yield the
central seven-membered ring ketone after in situ treatment
with water in DMSO. Unambiguous determination of its
structure via an X-ray diffraction study confirmed the
structure of 3 and thus verified the new data obtained in this
work. The samples of 3 prepared via Schemes 3 and 4 are
identical. The main difference between the reported natural
product,3 the previous synthesis of 3,4 and this work, is the
melting point and 13C NMR data presented here.
4. Experimental
4.1. General aspects
NMR spectra were obtained on a Bruker Avance 300 DPX
spectrometer (Bruker, Newark, DE) operating at 300 MHz
and a Jeol Eclipseþ500 FT NMR spectrometer (Jeol Ltd,
Tokyo, Japan) operating at 500 MHz. Spectra were recorded
in acetone-d6, DMSO-d6 or CDCl3, using the solvent as
internal standard at 300 or 500 MHz for 1H and 75 or
125 MHz for 13C at 298 K if not stated otherwise; d values
are given in ppm and coupling constants are reported in Hz.
Scheme 4. (a) 2.5 equiv. t-BuOK/t-BuOH/PhH, reflux; (b) 8.5 equiv.
t-BuOK/t-BuOH/PhH, reflux; (c) DDQ, PhH, reflux; (d) EtOH, 6 M HCl,
reflux; (e) DDQ, PhH, reflux; (f) 3 equiv. KOCl, THF/MeOH, 0–21 8C,
5 h;25 (g) MeOH, 6 M HCl, reflux, 6 h.25

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2150
IR spectra were recorded on a Perkin–Elmer FT-IR 1600 or
on a ThermoNicolet Avatar 330 FT-IR spectrophotometer.
Melting points were determined using the capillary method
on a Bu¨chi B-545, an Electrothermal IA9200 and on a
Heizbank koffler hotbench and are uncorrected. Mass
spectra were recorded using an LC/MS system operating
in the electron spray ionization (ESI) mode at 70 eV. The
elemental analyses were performed by H. Kolbe Micro-
analytisches Laboratorium, Mu¨hlheim an der Ruhr,
Germany. Sublimation was carried out on a Thermal
Gradient Sublimer, Esoteric Chemicals AB, Sweden. All
reagents were of standard quality and used as received from
Lancaster, Aldrich, or Merck. Solvents were purified by
distillation or were of HPLC grade. Dichloromethane,
dichloroethane and tert-butanol were distilled from CaH2
and stored over activated molecular sieves prior to use.
Benzene and n-heptane were stored over sodium.
Chromatographic separations were performed on silica gel
60 (230–400 mesh). Reactions were monitored by thin-
layer chromatography, on silica gel coated plates with a
fluorescent indicator.
7.11–7.03 (2H, m), 6.95 (1H, dd, J¼7.3, 7.2 Hz), 6.45 (1H,
d, J¼0.7 Hz), 6.21 (1H, s), 3.73 (3H, s); 13C NMR (75 MHz;
DMSO-d6) d 184.5 (s), 168.3 (s), 138.4 (s), 136.5 (s), 133.8
(s), 130.9 (s), 127.5 (s), 126.7 (s), 126.3 (d), 122.9 (d), 121.2
(d), 120.6 (d), 119.8 (d), 119.0 (d), 112.8 (d), 111.5 (d),
111.4 (d), 102.0 (d), 53.5 (d), 52.7 (q). MS ESI m/z [Mþ1]þ
333, [M21]2 331. Anal. Calcd for C20H16N2O3: C, 72.3; H,
4.85; N, 8.4. Found C, 72.2; H, 4.9; N, 8.4.
4.2.2. 2-[3-(1H-Indole-2-carbonyl)-1H-indol-2-yl]-3-(1H-
indol-2-yl)-3-oxo-propionic acid methyl ester (9). Yellow
solid. Recrystallized from CH2Cl2/c-hexane. Mp 130 8C
dec.; IR (KBr) 3382, 3061, 2951, 1738, 1646, 1619, 1591,
1521, 1438, 1342, 1174, 1138, 745 cm21
;
1H NMR
(300 MHz; DMSO-d6) d 12.23 (1H, s), 12.00 (1H, s),
11.98 (1H, s), 7.70–7.58 (4H, m), 7.53–7.44 (3H, m),
7.33–7.27 (2H, m), 7.21 (1H, dd, J¼7.7, 7.2 Hz), 7.12–7.04
(4H, m), 6.69 (1H, s), 3.75 (3H, s); 13C NMR (75 MHz;
DMSO-d6) d 183.9 (s), 182.3 (s), 167.7 (s), 138.5 (s), 137.6
(s), 136.8 (s), 136.0 (s), 136.0 (s), 133.8 (s), 126,8 (s), 126.7
(s), 126.4 (d), 125.4 (s), 125.1 (d), 122.9 (d), 122.6 (d),
123.0 (d), 120.6 (2£d), 120.3 (d), 120.1 (d), 114.4 (s), 112.8
(d), 112.6 (d), 112.5 (d), 111.3 (d), 109.9 (d), 52.9 (q), 52.3
(d); MS ESI m/z [Mþ1]þ 476; FABHRMS Calcd for
C29H22N3O4 476.1610 [MþH]þ found 476.1608.
4.2. Synthesis of compounds 5, 7 and 8
1.0 M Me2AlCl in hexanes (15.80 mmol, 15.8 mL) was
added dropwise to a cold (240 8C) solution of indole-2-
acetic acid methyl ester15 (6a) (2.30 g, 12.16 mmol) in dry
CH2Cl2 (40 mL) under nitrogen. The temperature was then
allowed to rise to 210 8C over 45 min before being lowered
to 220 8C whereupon a solution of indole-2-carbonyl
chloride (2.84 g, 15.80 mmol) in CH2Cl2 (20 mL) was
added dropwise at such a rate as to keep the temperature
below 210 8C. The temperature was then gradually
increased to 21 8C over 16 h. The reaction mixture was
poured out on iced water (100 mL) in a 500 mL beaker.
After the initial frothing, the yellow suspension was diluted
with water (200 mL) and stirred for 1 h. The suspension was
extracted with EtOAc (3£100 mL) and filtered through
celite and washed with a little EtOAc. The combined
organic phases were washed with water (100 mL) and brine
(100 mL) before drying over Na2SO4. All water phases
(approximately 500 mL) were extracted with CHCl3
(2£300 mL), the combined CHCl3 phases were washed
with water (200 mL), brine (200 mL) and then dried over
Na2SO4. The dried organic phases were combined and
evaporated to dryness to form an orange solid. This solid
was subjected to column chromatography on silica with a
gradient eluent system of EtOAc/hexane (20–50%) to give
8 in 0.56 g, 9 in 0.08 g and 5 in 1.07 g. Fractions with more
than one substance were combined and evaporated and
subjected to a second column chromatography with the
gradient eluent Et2O/hexane (70–90%) gave 8 in 0.91 g, 9
in 0.40 g, and finally 5 in 0.35 g. Compound 8 was isolated
in a total yield of 1.47 g (36%), and the trimeric product 9 in
0.48 g (8%) and finally the bisindole 5 in 1.42 g (35%).
4.2.3. [3-(1H-Indole-2-carbonyl)-1H-indol-2-yl]acetic
acid methyl ester (5). Yellow solid. Mp 171 8C dec.
(EtOAc/heptane); IR (KBr) 3313, 3175, 1720, 1597, 1435,
1
1342, 749 cm21; H NMR (300 MHz; DMSO-d6) d 12.05
(1H, s), 11.84 (1H, s), 7.72 (1H, d, J¼7.9 Hz), 7.50–7.47
(2H, m), 7.27 (1H, dd, J¼8.0, 7.2 Hz), 7.20 (1H, dd, J¼8.0,
7.1 Hz), 7.13–7.04 (2H, m), 7.02 (1H, d, J¼1.2 Hz), 4.08
(2H, s), 3.62 (3H, s); 13C NMR (75 MHz; DMSO-d6) d
182.1 (s), 169.7 (s), 138.1 (s), 137.4 (s), 136.9 (s), 135.1 (s),
126.8 (s), 126.1 (s), 124.7 (d), 122.4 (d), 122.2 (d), 120.8
(d), 120.1 (d), 120.0 (d), 114.0 (s), 112.5 (d), 111.7 (d),
109.4 (d), 51.9 (q), 33.1 (t); MS ESI m/z [M21]2 331. Anal.
Calcd for C20H16N2O3: C, 72.3; H, 4.85; N, 8.4. Found C,
72.1; H, 5.0; N, 8.2.
4.2.4. 6,11-Dihydro-6-oxo-5H-cyclohepta[1,2-b:4,5-
b0]diindole-12-carboxylic acid methyl ester. (Caulersin)
(3). Solid chloromethylenemorpholinium chloride (427 mg,
2.51 mmol) was added to a suspension of [3-(1H-indole-2-
carbonyl)-1H-indol-2-yl]acetic acid methyl ester (5)
(278 mg, 0.84 mmol) in dry dichloroethane (20 mL) at
21 8C under nitrogen. The resulting suspension was stirred
for 30 min at 30–35 8C, then heated at 50 8C for 4 h and
finally at 60 8C for 90 min. The solvent was removed under
reduced pressure at 50 8C and the resulting dark red solid
suspended in a mixture of DMSO (12 mL) and water
(2 mL). The suspension was stirred at 50 8C for 14 h and
then poured out on water/brine (1:1, 150 mL). The resulting
brown precipitate was collected and washed with water, and
then EtOAc until an almost clear filtrate was obtained. The
solid was dried at 55 8C/0.35 mbar to give caulersin (3)
(202 mg, 70%) as an orange powder. This sample changes
color to golden crystals at 275–280 8C and melts at 352–
355 8C. (lit.3 mp 269–270 8C, lit.4 mp 273–274 8C); IR
(KBr) 3320, 3169, 1689, 1534, 1496, 1422, 1267,
1241 cm21; after sublimation; IR (KBr) 3348, 3241, 1667,
1540, 1492, 1418, 1270, 1240 cm21; for atom numbering
4.2.1. 2,3-Bis-(1H-indol-2-yl)-3-oxo-propionic acid
methyl ester (8). White solid. Mp 179.5–182.5 8C (Et2O/
hexane); IR (KBr) 3346, 3052, 2954, 1721, 1660, 1636,
1518, 1428, 1343, 1276, 1138, 751 cm21
;
1H NMR
(300 MHz; DMSO-d6) d 11.96 (1H, s), 11.17 (1H, s), 7.71
(1H, d, J¼8.0 Hz), 7.61 (1H, d, J¼1.0 Hz), 7.48–7.44 (2H,
m), 7.40 (1H, d, J¼8.1 Hz), 7.31 (1H, dd, J¼7.7, 7.2 Hz),

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N. Wahlstrom et al. / Tetrahedron 60 (2004) 2147–2153
2151
1
see Fig. 2. H NMR (500 MHz; DMSO-d6) d 13.10 (1H, s,
(1H, t, J¼7.5 Hz), 5.63 (2H, s), 3.47–3.43 (2H, m), 3.37–
3.33 (5H, m); 13C NMR (75 MHz; CDCl3) d 179.7 (s), 148.0
(s), 137.2 (s), 136.3 (s), 134.2 (s), 127.6 (s), 127.5 (s), 125.9
(d), 123.8 (d), 123.3 (d), 123.0 (d), 120.8 (d), 120.2 (d),
119.9 (s), 116.1 (s), 112.3 (d), 109.5 (d), 74.2 (t), 56.4 (q),
25.3 (t), 22.0 (t); MS ESI m/z [Mþ1]þ 331. Anal. Calcd for
C21H18N2O2: C, 76.3; H, 5.5; N, 8.5. Found C, 76.2; H, 5.4;
N, 8.6.
NH, N-1), 12.36 (1H, s, NH, N-18), 9.14 (1H, s, CH, C-21),
9.06 (1H, d, CH, C-13, J¼8.2 Hz), 8.36 (1H, d, CH, C-6,
J¼7.8 Hz), 7.95 (1H, d, CH, C-16, J¼7.8 Hz), 7.77 (1H, d,
CH, C-3, J¼7.8 Hz), 7.57 (1H, dd, CH, C-4, J¼7.8, 7.4 Hz),
7.51 (1H, dd, CH, C-15, J¼7.8, 6.9 Hz), 7.43 (1H, dd, CH,
C-5, J¼7.8, 7.3 Hz), 7.40 (1H, dd, CH, C-14, J¼7.8,
7.4 Hz), 4.09 (3H, –OCH3, C-23, s); 13C NMR (125 MHz;
DMSO-d6) d 172.4 (s, C-10), 168.0 (s, C-22), 140.9 (s, C-9),
138.6 (s, C-19), 136.9 (s, C-2), 136.4 (s, C-17), 129.7 (d,
C-21), 126.9 (s, C-7), 126.7 (d, C-4), 126.1 (s, C-12), 125.8
(d, C-15), 123.5 (d, C-13), 122.0 (d, C-5), 121.7 (d, C-14),
120.3 (d, C-6), 119.3 (s, C-11), 114.6 (s, C-20), 114.4 (s,
C-8), 113.2 (d, C-3), 112.4 (d, C-16), 53.0 (q, C-23); MS
ESI m/z [Mþ1]þ 343. Calcd for C21H14N2O3, C, 73.7; H,
4.1; N, 8.2. Found C, 73.8; H, 4.0; N, 8.1.
4.2.8. 6,11-Dihydro-11-(methoxymethyl)-6-oxo-5H-
Cyclohepta[1,2-b:4,5-b0]diindole (14). DDQ (0.666 g,
2.94 mmol) was added portionwise to a suspension of 13
(0.485 g, 1.47 mmol) in benzene (30 mL) at 21 8C. The
resulting suspension was heated at reflux for 80 min, then
cooled to 21 8C and treated with 0.4 M KOH (250 mL), and
extracted with EtOAc (3£100 mL). The combined organic
phases were washed with water (2£100 mL), then brine
(2£100 mL), and dried over MgSO4. Concentration of the
solvents to dryness followed by crystallization from
dichloroethane afforded 14 (362 mg, 75%) in two crops as
a light yellow-greenish solid. Mp 251.5–254.0 8C dec.; IR
(KBr) 3287, 3053, 2996, 2900, 1616, 1599, 1577, 1527,
1505, 1415, 1323, 1234, 1113, 1055, 910, 887, 760, 738,
711 cm21; 1H NMR (300 MHz; DMSO-d6) d 12.65 (1H, s),
9.11 (1H, d, J¼7.8 Hz), 8.41–8.35 (2H, m), 7.95 (1H, d,
J¼8.2 Hz), 7.82 (1H, d, J¼11.4 Hz), 7.73 (1H, d, J¼
8.2 Hz), 7.60–7.50 (2H, m), 7.44 (1H, dd, J¼7.5, 7.4 Hz),
7.36 (1H, dd, J¼7.7, 7.3 Hz), 6.05 (2H, s), 3.30 (3H, s); 13C
NMR (75 MHz; DMSO-d6) d 172.8 (s), 142.2 (s), 139.6 (s),
138.2 (s), 136.8 (s), 126.5 (d), 126.2 (s), 126.0 (s), 126.0 (d),
125.0 (d), 123.8 (d), 122.1 (d), 120.9 (d), 120.8 (d), 119.2
(s), 117.3 (s), 112.8 (d), 112.6 (d), 110.0 (d), 73.6 (t), 55.6
(q); MS ESI m/z [Mþ1]þ 329 [M21]2 327. Anal. Calcd for
C21H16N2O2: C, 76.8; H, 4.9; N, 8.5. Found C, 76.7; H,
4.85; N, 8.4.
The NMR data of 3 after being heated up to its melting point
was identical with the data given above.
4.2.5. Data of Caulersin (3) obtained via Scheme 4.
Yellow solid prepared with the method of Fresneda.25 Mp
349.0–352.5 8C; IR (neat) 3347, 3231, 1667, 1540, 1490,
1417, 1269, 1240 cm21; 1H NMR (500 MHz; DMSO-d6) d
13.10 (1H, s), 12.36 (1H, s), 9.15 (1H, s), 9.06 (1H, d,
J¼8.2 Hz), 8.36 (1H, d, J¼8.2 Hz), 7.95 (1H, d, J¼7.8 Hz),
7.77 (1H, d, J¼7.8 Hz), 7.58 (1H, dd, J¼7.8, 7.4 Hz), 7.51
(1H, dd, J¼7.8, 7.4 Hz), 7.44 (1H, dd, J¼7.8, 7.4 Hz), 7.40
(1H, dd, J¼7.8, 7.4 Hz), 4.09 (3H, s); 13C NMR (125 MHz;
DMSO-d6) d 172.4 (s), 168.0 (s), 140.9 (s), 138.6 (s), 136.8
(s), 136.4 (s), 129.7 (d), 126.9 (s), 126.7 (d), 126.1 (s), 125.8
(d), 123.5 (d), 122.0 (d), 121.7 (d), 120.3 (d), 119.3 (s),
114.6 (s), 114.3 (s), 113.2 (d), 112.3 (d), 53.0 (q);
4.2.6. Compound 10b. An aliquot from the reaction
between 5 and chloromethylenemorpholinium chloride
was withdrawn from reaction mixture (before removal of
solvent) and quickly filtered under nitrogen, washed with
small amounts of dry dichloroethane. The NMR-sample was
prepared immediately before the spectrum was recorded. 1H
NMR (300 MHz; DMSO-d6) d 14.41 (1H, bs), 14.03 (1H,
bs), 10.12 (1H, s), 9.40 (1H, d, J¼8.3 Hz), 8.88 (1H, d,
J¼8.0 Hz), 8.35 (1H, d, J¼8.1 Hz), 8.14–7.96 (3H, m),
7.82–7.73 (2H, m), 4.45 (3H, s).
4.2.9. 6,11-Dihydro-6-oxo-5H-cyclohepta[1,2-b:4,5-b0]-
diindole (15). The bisindole 14 (164 mg, 0.50 mmol) was
suspended in EtOH (15 mL) and 6 M HCl (10 mL) and
heated at reflux for 8.5 h. Upon cooling to 21 8C a yellow
solid precipitated. The reaction mixture was diluted with
water (175 mL) and then filtered. The precipitate filtered off
and washed with an excess of water. The solid was
recrystallized from EtOAc to give the bisindolotropolone
15 (117 mg, 82%) in two crops as a yellow powder. Mp
398.5 8C (dec.) IR (KBr) 3252, 1756, 1598, 1558, 1514,
4.2.7. 5,11,12,13-Tetrahydro-11-(methoxymethyl)-6H-
cyclohepta[1,2-b:4,5-b0]diindol-6-one
1
(13).
t-BuOK
1408, 1216, 740, 707 cm21; H NMR (300 MHz; DMSO-
(1.95 g, 16.53 mmol) in t-BuOH (50 mL) was added to a
solution of 11 (1.69 g, 4.13 mmol) in anhydrous benzene
(80 mL) under a flow of nitrogen. The solution was heated
at reflux for 1.5 h, when additional t-BuOK (2.2 g,
18.65 mmol) was added and the reflux was continued for
another hour. After cooling the reaction mixture was
washed with saturated NH4Cl (50 mL) and then extracted
with EtOAc (3£50 mL). The combined organic phases were
washed with water (50 mL) then brine (75 mL) before
drying over MgSO4. The solvents were evaporated to give a
yellow solid, which was recrystallized from dichloroethane/
heptane to give 13 as golden crystals (1.69 g, 81%). Mp
214–216 8C; IR (KBr) 3283, 3059, 2928, 2823, 1564, 1460,
d6) d 12.54 (1H, s), 12.45 (1H, s), 8.99 (1H, s, J¼7.9 Hz),
8.32–8.26 (2H, m), 7.72 (1H, d, J¼8.2 Hz), 7.62 (1H, d,
J¼8.6 Hz), 7.59 (1H, d, J¼11.1 Hz), 7.52–7.47 (2H, m),
7.40–7.31 (2H, m); 13C NMR (75 MHz; DMSO-d6) d 172.9
(s), 142.6 (s), 139.2 (s), 136.6 (s), 136.5 (s), 126.8 (s), 126.3
(s), 126.1 (d), 125.4 (d), 124.4 (d), 123.5 (d), 121.2 (d),
120.7 (d), 120.6 (d), 117.9 (s), 116.8 (s), 115.0 (d), 112.7
(d), 111.0 (d); MS ESI m/z [Mþ1]þ 285, [M21]2 283.
Anal. Calcd for C19H12N2O: C, 80.3; H, 4.25; N, 9.85.
Found C, 80.35; H, 4.3; N, 9.7.
4.2.10. 4-[2-[[2-Chloro-1-(methoxymethyl)-1H-indol-3-
yl]carbonyl]-1H-indol-3-yl]-2-butanone (11). Methyl
vinylketone (1.32 g, 18.8 mmol) in MeNO2 (15 mL) was
added at 21 8C to a suspension of [2-chloro-1-(methoxy-
methyl)-1H-indole-3-yl]-1H-indol-2-yl methanone4 (2.12 g,
1
1419, 1228, 1102, 1047, 747 cm21; H NMR (300 MHz;
CDCl3) d 9.31 (1H, s), 8.67–8.64 (1H, m), 7.71 (1H, d,
J¼8.1 Hz), 7.51–7.45 (2H, m), 7.38–7.32 (3H, m), 7.18

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N. Wahlstrom et al. / Tetrahedron 60 (2004) 2147–2153
2152
6.26 mmol) in MeNO2 (40 mL) under nitrogen. The
suspension was cooled to 220 8C and BF3·OEt2 (87 mg,
0.61 mmol) in EtOH (1.0 mL) was added dropwise. The
reaction temperature was then allowed to rise to 0 8C and
kept at this temperature for 3 h, before further 92 mg of
BF3·OEt2 in EtOH (0.5 mL) was added. After 15 h at 0 8C
more methyl vinylketone (960 mg) in MeNO2 (1 mL) was
added followed by BF3·OEt2 (140 mg) in EtOH (0.3 mL).
After 2 h, a saturated solution of NaHCO3 (100 mL) was
added, followed by extraction with CH2Cl2 (4£50 mL). The
combined organic phases were washed with water
(100 mL), then brine (100 mL), and dried over MgSO4.
Evaporation produced a reddish residue that was subjected
to column chromatography (eluent Et2O/hexane, 8:2) to
give 5-nitro-2-pentanone (200 mg) and the bisindole 11
(1.69 g, 66%) identical with the product previously
reported.4
J¼8.0 Hz), 8.69 (1H, s), 8.42 (1H, d, J¼8.0 Hz), 7.86
(1H, d, J¼8.2 Hz), 7.80 (1H, d, J¼8.3 Hz), 7.60–7.54 (2H,
m), 7.46–7.40 (2H, m), 5.75 (2H, s), 2.96 (3H, s), 2.90 (3H,
s); 13C NMR (75 MHz; acetone-d6) d 200.4 (s), 174.5 (s),
141.6 (s), 139.6 (s), 139.3 (s), 138.1 (s), 130.6 (s), 128.3 (s),
127.8 (d), 127.6 (s), 127.1 (d), 126.7 (d), 125.1 (d), 123.8
(s), 123.4 (d), 122.7 (d), 121.7 (d), 116.1 (s), 113.9 (d),
111.5 (d), 78.5 (t), 55.9 (q), 29.0 (q); MS ESI m/z [Mþ1]þ
371, [M21]2 369.
4.2.14. 11-(Methoxymethyl)-6-oxo-5H-cyclohepta[1,2-
b:4,5-b0]diindole-12-carboxylic acid methyl ester (17).
Yellow solid prepared with the method of Fresneda.25 Mp
246.5–250.5 8C, (lit.4,25 mp 255–256 8C); IR (neat) 1705,
1
1579, 1245 cm21; H NMR (300 MHz; CDCl3) d 12.14
(1H, s), 9.40 (1H, d, J¼7.8 Hz), 8.08 (1H, s), 8.23 (1H, d,
J¼7.9 Hz), 7.81 (1H, d, J¼8.1 Hz), 7.96–7.59 (2H, m),
7.55–7.49 (2H, m), 7.42 (1H, dd, J¼7.6, 7.3 Hz), 5.78 (2H,
s), 4.07 (3H, s), 2.96 (3H, s); 13C NMR (75 MHz; CDCl3) d
174.2 (s), 169.4 (s), 141.4 (s), 139.9 (s), 138.6 (s), 137.3 (s),
129.6 (d), 127.4 (d), 127.4 (s), 127.0 (s), 126.3 (d), 125.0
(d), 123. 4 (d), 122.3 (d), 120.7 (d), 119.2 (s), 116.4 (s),
113.3 (d), 110.9 (d), 78.5 (t), 56.2 (q), 53.3 (q); MS ESI m/z
[Mþ1]þ 387, [M21]2 385.
4.2.11. 5-Nitro-2-pentanone.24 1H NMR (300 MHz;
CDCl3) d 4.41 (2H, t, J¼6.6 Hz), 2.58 (2H, d, J¼6.8 Hz),
2.21 (2H, m), 2.14 (3H, s); 13C NMR (75 MHz; CDCl3) d
206.6 (s), 74.7 (t), 39.4 (t), 30.1 (q), 21.2 (t)
4.2.12. 12-Acetyl-5,11,12,13-tetrahydro-11-(methoxy-
methyl)-6H-cyclohepta[1,2-b:4,5-b0]diindol-6-one (12).
t-BuOK (412 mg, 3.67 mmol) in t-BuOH (35 mL) was
added to a solution of 11 (600 mg, 1.47 mmol) in benzene
(50 mL) at 21 8C. The mixture was then heated at reflux for
70 min and then cooled (10 8C) and poured into saturated
aqueous NH4Cl (25 mL) and water (50 mL). The phases
were extracted with EtOAc (3£50 mL) and the combined
organic phases were dried over MgSO4, and then evaporated
to give a yellow solid. Titruation with Et2O produced 12
(511 mg) as yellow crystals, with benzene inclined in the
4.3. X-ray crystallography study
CCDC reference number 210210. Copies of the data can be
obtained, free of charge, on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [fax:þ44-1223-336033 or
e-mail: deposit@ccdc.cam.ac.uk].
Acknowledgements
1
crystals in a ratio 1.00:0.39 as determined by H NMR
spectroscopy. Conventional drying of the solid failed to
remove the benzene. The etheral phases were concentrated
to 10 mL and stirred for 1 h at 21 8C to give another crop of
12 (30 mg), free of benzene. In total 502 mg (92%) (without
benzene) was obtained of the diindolyl ketone 12. Crystal-
lization from CH2Cl2/hexane produced crystals with
identical spectroscopic characteristics as with those reported
previously.4
We thank Professor Jing-Yu Su for the spectra of the
authentic caulersin19 and Professor Pilar Martinez Fresneda
for the procedure and the data of the previously synthesized
caulersin.25
References and notes
4.2.13. 12-Acetyl-5,11-dihydro-11-(methoxymethyl)-6H-
cyclohepta[1,2-b:4,5-b0]diindol-6-one (16). DDQ (366 mg,
1.61 mmol) was added in one portion to a suspension of 12
(300 mg, 0.81 mmol) in benzene (125 mL) at 21 8C. The
mixture was refluxed for 90 min and then additional DDQ
(180 mg) was added. The reflux was continued for 4.5 h and
then allowed to cool to 21 8C. 1 M NaOH (200 mL) and
EtOAc (100 mL) was added to the solution. The organic
solvents were separated and the aqueous phase extracted
with EtOAc (2£50 mL). Drying of the organic solvents over
MgSO4 and removal of the solvents with evaporation
produced a yellow solid. This solid was boiled in Et2O
(60 mL), filtered and then dried to give the yellow bisindole
16 (186 mg). The filtrate was then evaporated and the
remaining solid crystallized from EtOAc/heptane to pro-
duce more of 16 (16 mg). The total yield of 16 was 202 mg
(68%). Mp 225–230 8C; IR (KBr) 3426, 3249, 2926, 1663,
¨
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