Regioselective synthesis of ellipticine

Article abstract of DOI:10.1002/hlca.200690001

An eight-step synthesis of the tetracyclic pyridocarbazole alkaloid ellipticine (=5,11-dimethyl-6H-pyrido[4,3-b]carbazole; 1a) in an overall yield of 13% is reported, starting from 4,7-dimethyl-1H-indene. Key steps were iodination, Suzuki coupling, reductive cyclization, DDQ oxidation, and heterocyclization under loss of H₂O.

Full text of DOI:10.1002/hlca.200690001

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Regioselective Synthesis of Ellipticine
by Tse-Lok Ho* and Sheng-Ying Hsieh
Shanghai Institute of Organic Chemistry (c/o Prof. X. L. Hou, Organometallic Division), Chinese
Academy of Sciences, 354,00 Fenglin Road, Shanghai 200032, P. R. China
(e-mail: tselokho@yahoo.com)
An eight-step synthesis of the tetracyclic pyridocarbazole alkaloid ellipticine (=5,11-dimethyl-6H-
pyrido[4,3-b]carbazole; 1a) in an overall yield of 13% is reported, starting from 4,7-dimethyl-1H-indene.
Key steps were iodination, Suzuki coupling, reductive cyclization, DDQ oxidation, and heterocyclization
under loss of H2O.
Introduction. – Originally isolated from Ochrosia elliptica LABILL. (Apocynaceae)
[1], the tetracyclic pyridocarbazole alkaloid ellipticine (1a) has also been found in
Strychnos dinklagei GILG., together with related substances such as 9-hydroxyellipticine
(1b) [2]. The interest in ellipticine was greatly aroused upon the discovery of its potent
antitumor activities [3], and, consequently, numerous chemists have engaged in active
research in the synthesis of this compound and of its congeners [4]. In this paper, we
report a high-yielding, straightforward synthesis of ellipticine (1a).
Results and Discussion. – 1. General Considerations. Ellipticine (1a) offers the
opportunity to test a symmetry-based synthetic design [5]. Firstly, a symmetrical
hydrindane was identified as precursor to attach an indole moiety for attaining a carba-
zole intermediate. A focal point was the subsequent exploitation of a remote N-atom to
differentiate the locally symmetrical hydrindane, and to convert it regioselectively into
a pyridine ring. The anticipated electronic influence to direct this operation is in con-
trast to our previous synthesis of cuparene and herbertene [6], in which nondiscrimina-
tive oxidation of a dihydroisobenzofuran at two available positions was specifically
required. There, the remote substituent had been a cyclopentane, which did not
exert any effect.
We became interested in ellipticine while pursuing a synthesis of cryptolepine and
cryptotackiene [7], since we noticed that all three tetracyclic frameworks are isomeric,
being indole–quinoline-fused systems.
© 2006 Verlag Helvetica Chimica Acta AG, Zürich

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2. Unsuccessful Attempts. Our first synthetic approach started with 4,7-dimethylin-
dane (2a) [8], which is readily prepared by condensation of cyclopentadiene with hex-
ane-2,5-dione [9], followed by hydrogenation. Compound 2a was reacted with N-
hydroxybenzeneamine [10] in the presence of trifluoroacetic acid (TFA) and its anhy-
dride to furnish the aniline 4, but the conversion was low. An alternative preparation of
4 via Grignard reaction between 4,7-dimethyl-5-nitroindane and PhMgBr [11] did not
proceed well. Moreover, because both the photochemical [12] and the Pd-catalyzed
cyclization [13] of 4 to 5a were inefficient, we had to abandon this approach.
Next, a parallel route was investigated, starting from 4,7-dimethylindan-2-one (2b),
which was obtained by oxidation of the corresponding indene [14]. The C=O group of
2b was acetal-protected, and the resulting 2c was subjected to iodination [15] to 3a. The
latter was used in a Suzuki coupling [16] with 2-nitrobenzeneboronic acid to give 6a,
and cyclization on heating in the presence of P(OEt)3 [17] afforded 5c. Acid-catalyzed
hydrolysis of the acetal gave ketone 5b, which was submitted to Schmidt reaction
(NaN3, H2SO4). Unfortunately, the product was a 1:1 mixture of the two inseparable
lactams 7a,b. Because of this unfavorable result and serious solubility problems associ-
ated with 5b, we decided to study the oxidation of 5c. Regioselective oxidation at the
benzylic position para to the N-atom was considered achievable. However, after a
few trials, e.g., with [Pb(OAc)4]/CAN, we found that the reaction with DDQ1) in
THF containing small amounts of H2O [18] served our purpose best, giving rise to
the desired monoprotected diketone 8a. No other isomers were detected in the reaction
mixture; the high regioselectivity is attributable to activation by the N-atom, notwith-
1
)
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone.

Helvetica Chimica Acta – Vol. 89 (2006)
113
standing the uncertainty of whether hydride abstraction took place at the NꢀH or ben-
zylic CꢀH groups before addition of H2O to the 3-imino-6-alkylidenecyclohexa-1,4-
diene intermediate. At this stage, we were quite confident that a regioselective route
to ellipticine (1a) could be realized. Unfortunately, the seemingly trivial cleavage of
the acetal unit in 8a became a stumbling block.
3. Final Synthesis. A satisfactory denouement of the synthesis finally evolved, when
we applied the above reaction sequence to the acetate 3c. After iodination of 2d to 3b
and esterification to 3c, Suzuki coupling of the latter afforded the nitrobiaryl 6b
(Scheme). Compound 6b showed some signal splitting in its NMR spectra owing to
atropisomerism. For example, the H-bearing ortho-C-atom of the indane moiety
gave rise to two atropisomeric signals at d(C) 123.69/123.72, and the fully substituted
ortho-C-atoms showed even more-significant differences in chemical shift (d(C)
132.29/135.93 and 136.41/138.89), corresponding to their wider spatial separation.
There was no change in the NMR spectra of 6b recorded at 508, which indicated a sub-
stantial barrier of rotation.
Scheme
a) NaBH4, CH2Cl2, MeOH; 98.8%. b) I2, CH2Cl2, silfen2); 81.6%. c) Ac2O, pyridine, DMAP, r.t.;
99.4%. d) 2-(NO2)C6H4B(OH)2, [Pd(PPh3)2Cl2], NaHCO3, DME, H2O, 808, 18 h; 79.2%. e) (Et3O)3P,
1508, 5 h; 72.8%. f) 1. THF, H2O, DDQ1), 8 h; 2. aq. K2CO3; 3. LiAlH4, THF, 10 h; 67.5%. g) 1.
NaIO4, t-BuOH, H2O (pH 8); 2. aq. NH4OAc; 86.8%.
Interestingly, when 6b was subjected to cyclization, 5d was detected as the sole
product. Also, as expected, DDQ oxidation of 5d led to the keto acetate 8b, which
was reduced in situ with LiAlH4 to afford a mixture of the diols 8c (cis) and 8d
(trans). These diols could be separated and characterized (see Exper. Part), but for con-
venience, the mixture was directly treated with NaIO4, and the crude product was
worked up with AcONH4, which resulted in ellipticine (1a).
2
)
Prepared by grinding Fe(NO3)3 ·9 H2O with twice its weight of silica gel (SiO2) in an agate mortar to
furnish a pale yellow powder.

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Conclusions. – We have elaborated a synthesis of ellipticine (1a) in eighth steps in
an overall yield of 13% from 4,7-dimethyl-1H-indene, taking advantage of symmetry
considerations. A single product was generated in the iodination step, and the necessary
desymmetrization proceeded as planned by exploitation of a remote substituent. The
spectral differences between compounds 6a and 6b, precursors of carbazole intermedi-
ates, are noteworthy. The simple features exhibited by 6c arose from a pair of enantiom-
ers (atropisomerism), whereas two pairs of diastereoisomers of 6d gave rise to a more-
complex signal pattern.
We thank the National Science Council, Republic of China, for financial support.
Experimental Part
General. All reactions were conducted under N2. Column chromatography (CC): Merck silica gel
(70–230 mesh). M.p.: Laboratory Devices; uncorrected. TLC: Merck silica gel 60-F254 plates. IR Spectra:
Bio-Rad FTS-165 and Digilab FTS-3100; in cmꢀ1. 1H- and 13C-NMR Spectra: Varian Unity-300 and Unity-
500; in CDCl3, unless otherwise indicated; d in ppm, J in Hz. EI-MS: Trio-2000 and Jeol SX-102A; at 70
eV.
2,3-Dihydro-4,7-dimethyl-1H-inden-2-ol (2d). NaBH4 (0.2 g, 5.28 mmol) was slowly added to a stir-
red, ice-cooled soln. of 2b [14] (0.60 g, 3.75 mmol) in CH2Cl2/MeOH 1:1 (20 ml). After 3 h, H2O was
added, and the solvents were evaporated. The residue was extracted with CH2Cl2, the org. layer was
washed with brine, dried (Na2SO4), and evaporated to furnish 2d. Yield: 0.6 g (98.8%). M.p. 58–598.
IR: 3321, 3036, 3009, 2918, 1867, 1743, 1607, 1497, 1417, 1328, 1295, 1266, 1219, 1200, 1020, 948, 832,
1
805. H-NMR: 2.32 (s, 6 H); 2.87 (dd, J=16.5, 3.3, 2 H); 3.16 (dd, J=16.5, 6.6, 2 H); 3.19 (br., 1 H);
4.69 (tt, J=6.6, 3.3, 1 H); 7.00 (s, 2 H). 13C-NMR: 18.64 (q); 41.08 (t); 71.97 (d); 131.03 (s); 139.21 (s).
EI-MS: 162 (44, M+), 144 (20), 133 (100), 129 (12), 119 (17), 91 (19), 77 (19), 65 (14), 51 (21). HR-
MS: 162.1051 (M+, C11H14O+; calc. 162.1045). Anal. calc. for C11H14O: C 81.44, H 8.70; found: C
81.80, H 8.73.
2,3-Dihydro-5-iodo-4,7-dimethyl-1H-inden-2-ol (3b). To a stirred soln. of 2d (6.26 g, 38.6 mmol) in
CH2Cl2 (60 ml) were added I2 (5.60 g, 22 mmol) and silfen2) (24 g). After 12 h at r.t., the mixture was fil-
tered and washed with CH2Cl2. Excess I2 was destroyed by shaking with aq. sodium thiosulfate soln. The
layers were separated, and the CH2Cl2 layer was dried (Na2SO4), and evaporated. The residue was dis-
solved in ice-cooled MeOH (20 ml), acidified with conc. HCl (10 ml), and stirred for 2 h. Then, H2O
(200 ml) was added, the resulting solids were filtered off, and taken up in CH2Cl2. The organic soln.
was washed with H2O, dried (Na2SO4), and evaporated to afford 3b. Yield: 8.97 g (81.6 %). M.p.
85–868. IR: 3305, 2917, 1573, 1459, 1417, 1331, 1299, 1263, 1201, 1177, 1017, 955, 934, 859, 797, 743.
1H-NMR: 2.15 (s, 3 H); 2.94 (s, 3 H); 2.67–2.83 (m, 2 H); 2.89 (br., 1 H); 2.96–3.14 (m, 2 H);
4.52–4.58 (m, 1 H); 7.47 (s, 1 H). 13C-NMR: 18.25 (q); 24.36 (q); 41.16 (t); 42.75 (t); 71.80 (d); 99.06
(s); 133.37 (s); 134.28 (s); 137.77 (d); 139.65 (s); 140.09 (s). EI-MS: 288 (100, M+), 270 (16), 259 (96),
245 (9), 161 (11), 143 (39), 128 (23), 115 (28), 91 (12). HR-MS: 288.0013 (M+, C11H13IO+; calc.
288.0012). Anal. calc. for C11H13IO: C 45.86, H 4.55; found: C 45.82, H 4.78.
2,3-Dihydro-5-iodo-4,7-dimethyl-1H-inden-2-yl Acetate (3c). A soln. of 3b (0.36 g, 1.25 mmol), Ac2O
(0.64 g, 6.27 mmol), pyridine (0.50 g, 6.33 mmol), and 4-(dimethylamino)pyridine (DMAP; 20 mg) in
CH2Cl2 (10 ml) was stirred at r.t. overnight, and then evaporated in vacuo. The residue was subjected
to CC (SiO2) to furnish 3c. Yield: 0.41 g (99.4%). M.p. 63–648. IR: 2942, 2916, 2860, 1732, 1574, 1462,
1373, 1312, 1242, 1199, 1015, 973, 860, 744. 1H-NMR: 2.02 (s, 3 H); 2.16 (s, 3 H); 2.29 (s, 3 H);
2.85–3.00 (m, 2 H); 3.14–3.31 (m, 2 H); 5.46–5.50 (m, 1 H); 7.48 (s, 1 H). 13C-NMR: 18.22 (q); 21.21
(q); 24.34 (q); 38.52 (t); 40.05 (t); 74.41 (d); 99.30 (s); 133.24 (s); 134.25 (s); 138.08 (d); 139.32 (s);
139.70 (s); 170.79 (s). EI-MS: 330 (M+), 286, 270, 259, 244, 210, 195, 178, 160, 143, 128, 115, 105, 91,
77, 65, 51. HR-MS: 330.0115 (M+, C13H15IOþ2 ; calc. 330.0118).

Helvetica Chimica Acta – Vol. 89 (2006)
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2,3-Dihydro-4,7-dimethyl-5-(2-nitrophenyl)-1H-inden-2-yl Acetate (6b). A mixture of 3c (0.41 g, 1.24
mmol), 2-nitrobenzeneboronic acid (0.33 g, 1.98 mmol), NaHCO3 (0.42 g, 5 mmol), and [Pd(PPh3)2Cl2]
(20 mg, 2 mol-%) was placed in a two-neck, round-bottom flask purged with N2. Then, a 1:1 mixture
of 1,2-dimethoxyethane (DME) and H2O (8 ml in total) was added through a rubber septum, and the
mixture was heated at 808 for 18 h. Then, the org. solvents were evaporated, and the aq. residue was
taken up in CH2Cl2. The org. soln. was washed with H2O, dried (Na2SO4), evaporated, and purified by
CC (SiO2) to afford 6b. Yield: 0.32 g (79.2%). IR: 2943, 2920, 1734, 1609, 1571, 1527, 1469, 1437, 1350,
1
1244, 1194, 1021, 977, 873, 854, 786, 758, 708. H-NMR: 1.95 (s, 3 H); 2.04 (d, J=6.9, 3 H); 2.20 (s, 3
H); 2.95–3.02 (m, 2 H); 3.24–3.35 (m, 2 H); 5.54–5.58 (m, 1 H); 6.78 (s, 1 H); 7.27–7.32 (m, 1 H);
13
7.43–7.49 (m, 1 H); 7.56–7.61 (m, 1 H); 7.90–7.31 (m, 1 H). C-NMR3): 16.18/16.19 (q); 18.53/21.12
(q); 38.50/38.61 (t); 38.84/38.98 (t); 74.48/74.62 (d); 123.69/123.72 (d); 127.88 (d); 127.99 (d); 128.70
(s); 130.77 (s); 132.15/132.17 (d); 132.29/135.93 (s); 136.41/138.89 (s); 139.39 (s); 139.42 (s); 149.25 (s);
170.83/170.99 (s). EI-MS: 325 (M+), 295, 282, 265, 248, 234, 218, 203, 178, 165, 152, 146, 115, 101, 91,
77. HR-MS: 325.1316 (M+, C19H19NO4þ ; calc. 325.1315).
1,2,3,5-Tetrahydro-4,10-dimethylcyclopenta[b]carbazol-2-yl Acetate (5d). A mixture of 6b (0.32 g,
0.98 mmol) and P(OEt)3 (5 ml) was heated at 1508 under N2 for 5 h. Excess (OEt)3P and (OEt)3P=O
were distilled off at reduced pressure to leave a brown residue, which was purified by CC (SiO2) to afford
5d. Yield: 0.21 g (72.8%). IR: 3280, 2740, 1707, 1610, 1517, 1368, 1304, 1263, 1194, 1014, 976, 839, 773, 730.
1H-NMR: 1.97 (s, 3 H); 2.33 (s, 3 H); 2.66 (s, 3 H) 3.01–3.11 (m, 2 H); 3.29–3.38 (m, 2 H); 5.51–5.57 (m,
1 H); 7.11–7.16 (m, 1 H); 7.26–7.37 (m, 2 H); 7.88 (br., 1 H); 8.06–8.09 (m, 1 H). 13C-NMR: 13.51 (q);
16.93 (q); 21.38 (q); 38.07 (t); 38.57 (t); 75.44 (d); 110.37 (d); 112.48 (s); 119.15 (d); 120.98 (s); 122.27 (d);
124.51 (d); 125.85 (s); 130.52 (s); 136.73 (s); 138.69 (s); 139.66 (s); 171.33 (s). EI-MS: 293 (15, M+), 234
(24), 233 (100), 218 (48), 117 (6). HR-MS: 293.1421 (M+, C19H19NO2þ ; calc. 293.1417).
cis- (8c) and trans-1,2,3,5-Tetrahydro-4,10-dimethylcyclopenta[b]carbazole-1,2-diol (8d). To a stirred
mixture of 5d (0.30 g, 1 mmol) and H2O (0.15 g, 8.0 mmol) in THF (5 ml), a soln. of DDQ1) (0.93 g, 4.1
mmol) in THF (10 ml) was added dropwise at 08 over 10 min. After 8 h, the mixture was treated with 50%
aq. K2CO3 soln. (10 ml) and stirred for another 1 h. The solvent was evaporated, the residue was
extracted repeatedly with CH2Cl2/MeOH 10 :1, and the combined extracts were washed with sat. aq.
K2CO3 soln., dried (Na2SO4), and evaporated. The residue (0.26 g) was dissolved in anh. THF (10 ml),
and then dropwise added to a stirred suspension of LiAlH4 (0.26 g, 6.8 mmol) in THF (5 ml). After 10
h, excess hydride was carefully destroyed with H2O. The resulting mixture was filtered, and washed
with CH2Cl2. The combined org. soln. was washed with sat. aq. K2CO3 soln., dried (Na2SO4), and evapo-
rated. The residue was purified by CC (SiO2) to afford 8c (0.06 g, 21.9%) and 8d (0.13 g, 47.6 %).
Data of 8c (cis). IR: 3419, 2923, 2085, 1645, 1519, 1456, 1338, 1297, 1235, 1118, 1085, 1002, 740. 1H-
NMR (CD3OD): 2.40 (s, 3 H); 2.82 (s, 3 H); 2.96 (dd, J=15, 8.7, 1 H); 3.19 (dd, J=15, 7.2, 1 H); 4.31
(ddd, J=8.7, 7.2, 5.4, 1 H); 5.09 (d, J=5.4, 1 H); 7.09–7.14 (m, 1 H); 7.27–7.33 (m, 1 H); 7.43–7.46
(m, 1 H); 8.07–8.10 (m, 1 H). 13C-NMR (CD3OD): 13.53 (q); 16.63 (q); 37.37 (t); 74.39 (d); 74.49 (d);
111.59 (d); 113.99 (s); 119.59 (d); 121.94 (s); 123.00 (d); 125.33 (d); 125.57 (s); 129.05 (s); 132.72 (s);
138.25 (s); 141.58 (s); 141.75 (s). EI-MS: 268 (10, [M+1 ]+), 267 (47, M+), 249 (75), 221 (100), 206
(32), 204 (37), 191 (15), 111 (11). HR-MS: 267.1262 (C17H17NOþ2 ; calc. 267.1260).
Data of 8d (trans). IR: 3314, 2918, 1685, 1609, 1559, 1518, 1457, 1381, 1340, 1296, 1250, 993, 731. 1H-
NMR (CD3OD): 2.43 (s, 3 H); 2.82 (m, 1 H); 2.86 (s, 3 H); 3.44 (dd, J=16.8, 5.4, 1 H); 4.40–4.42 (m, 1
H); 5.16 (d, 1 H); 7.09–7.14 (m, 1 H); 7.27–7.32 (m, 1 H); 7.44–7.46 (m, 1 H); 8.09–8.12 (m, 1 H). 13C-
NMR (CD3OD): 13.50 (q); 16.72 (q); 38.73 (t); 80.24 (d); 81.84 (d); 111.58 (d); 114.17 (s); 119.54 (d);
122.20 (s); 123.00 (d); 125.25 (d); 125.58 (s); 129.23 (s); 133.13 (s); 139.48 (s); 141.79 (s); 141.87 (s).
EI-MS: 268 (12, [M+1]+), 267 (55, M+), 249 (77), 235 (19), 234 (21), 221 (100), 218 (14), 217 (11), 206
(31), 204 (35), 194 (11), 191 (13), 165 (11), 111 (10). HR-MS: 267.1259 (M+, C17H17NO2þ ; calc. 267.1260).
Ellipticine (=5,11-Dimethyl-6H-pyrido[4,3-b]carbazole; 1a). To a stirred soln. of a (nonseparated)
mixture of 8c/8d (0.10 g, 0.37 mmol) and aq. phosphate buffer (pH 8; 2 ml) in t-BuOH (6 ml), NaIO4
(0.16 g, 7.5 mmol) was added. After 6 h, NH4OAc (0.29 g, 3.6 mmol) was added, and stirring was contin-
3
)
Signal splitting due to atropisomerism.

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Helvetica Chimica Acta – Vol. 89 (2006)
ued for 1 h. The solvent was evaporated, the residue was extracted with CH2Cl2/MeOH 10 :1, washed
with sat. aq. K2CO3 soln., dried (Na2SO4), and concentrated. The crude residue was purified by CC
(SiO2) to afford 1a. Yield: 0.08 g (86.8 %). 1H-NMR ((D6)DMSO): 2.78 (s, 3 H); 3.25 (s, 3 H);
7.22–7.27 (m, 1 H); 7.49–7.57 (m, 2 H); 7.90 (d, J=6, 1 H); 8.37 (d, J=8.1, 1 H); 8.42 (d, J=6, 1 H);
9.68 (s, 1 H); 11.37 (s, 1 H). 13C-NMR ((D6)DMSO): 11.92 (q); 14.31 (q); 108.00 (s); 110.67 (d); 115.84
(d); 119.15 (d); 121.95 (s); 123.11 (s); 123.37 (s); 123.78 (d); 127.08 (d); 128.01 (s); 132.45 (s); 140.47
(d); 140.53 (s); 142.66 (s); 149.67 (s). HR-MS: 246.1152 (M+, C17H14N2þ ; calc. 246.1157).
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Received September 19, 2005