Concise and efficient synthesis of calothrixin B

Article abstract of DOI:10.1021/jo061270o

A convergent synthesis of the naturally occurring alkaloid Calothrixin B is presented, which used a regioselective hetero-Diels-Alder reaction between a "push-pull" 2-aza-diene and a N-protected 3-bromo-9H-carbazole-1,4- dione to construct the five-ring skeleton of the molecule. Protection of the indole motif with a benzyl group was unattractive for delivery of sufficient target material because the removal of the protecting group had not been high yielding. We therefore elected to temporarily protect the indole motif with a more labile benzyloxycarbonyl group. Accordingly, the synthesis of calothrixin B proceeded in 17% overall yield over 9 steps from the commercially available 1,2,3,9-tetrahydro-4H-carbazol-4-one.

Full text of DOI:10.1021/jo061270o

Concise and Efficient Synthesis of Calothrixin B
Drissa Sissouma, Lucie Maingot, Sylvain Collet,* and Andre´ Guingant*
UniVersite´ de Nantes, Nantes Atlantique UniVersite´s, CNRS, Faculte´ des Sciences et des Techniques,
Laboratoire de Synthe`se Organique (LSO), UMR CNRS 6513, 2, rue de la Houssinie`re-BP 92208-44322
Nantes, Cedex 3, France
sylVain.collet@uniV-nantes.fr; andre.guingant@uniV-nantes.fr
ReceiVed June 21, 2006
A convergent synthesis of the naturally occurring alkaloid Calothrixin B is presented, which used a
regioselective hetero-Diels-Alder reaction between a “push-pull” 2-aza-diene and a N-protected 3-bromo-
9H-carbazole-1,4-dione to construct the five-ring skeleton of the molecule. Protection of the indole motif
with a benzyl group was unattractive for delivery of sufficient target material because the removal of the
protecting group had not been high yielding. We therefore elected to temporarily protect the indole motif
with a more labile benzyloxycarbonyl group. Accordingly, the synthesis of calothrixin B proceeded in
17% overall yield over 9 steps from the commercially available 1,2,3,9-tetrahydro-4H-carbazol-4-one.
Introduction
In 1999, Rickards, Smith, and co-workers1 reported the
discovery of two new carbazole alkaloids named calothrixins
A (1a) and B (1b) from cell extracts of cyanobacterial Calothrix
species. These natural compounds, which display a unique
indolo[3,2-j]phenanthridine ring system,2 exert in vitro growth-
inhibitory effects in antiplasmodial and anticancer assays at
nanomolar concentrations. The mechanism by which these
compounds exert their cytotoxic activity is still largely
unknown.3The structural novelty and the significant biological
properties of the calothrixins render these molecules attractive
targets for chemical synthesis and several groups, including ours,
have achieved their preparation. Kelly et al. reported the first
synthesis of the calothrixins based on ortho-lithiation reactions
to create the C6a-C7 and C12a-C13 bonds.4 Chai et al. prepared
the calothrixins using a Friedel-Crafts reaction and a lithiation-
FIGURE 1. Structures of calothrixin A (1a) and calothrixin B (1b).
cyclization reaction to create the C7-C7a and C12a-C13 bonds,
respectively.5 An allene-mediated electrocyclic reaction with
creation of the C6a-C13a bond was the key feature of the
synthesis reported by Hibino et al.6 The synthesis of calothrixin
B reported by Bennasar et al. is characterized by a regioselective
cyclization of an acyl radical with concomitant formation of
the C13-C13a bond.7 A strategy different from those reported
previously was followed by our group in 2005 to reach
calothrixin B whose basic five-ring structure was forged in one
single operation by recourse to a hetero-Diels-Alder reaction
(simultaneous creation of the C6-C6a and C13a-C13b bonds).8
(1) Rickards, R. W.; Rothschild, J. M.; Willis, A. C.; de Chazal, N. M.;
Kirk, K.; Saliba, K. J.; Smith, G. D. Tetrahedron 1999, 55, 13513.
(2) For a preparation of the 12H-indolo[3,2-j]phenanthridine skeleton
prior to the discovery of the calothrixins, see: (a) Elango, S.; Srinivasan,
P. C. Tetrahedron Lett. 1993, 34, 1347. (b) Mohanakrishnan, A. Z.;
Srinivasan, P. C. J. Org. Chem. 1995, 60, 1939.
(3) (a) Doan, N. T.; Rickards, R. W.; Rothschild, J. M.; Smith, G. D. J.
Appl. Phycol. 2000, 12, 409. (b) Chen, X.; Smith, G. D.; Waring, P. J.
Appl. Phycol. 2003, 15, 269.
(5) (a) Bernardo, P. H.; Chai, C. L. L.; Elix, J. A. Tetrahedron Lett.
2002, 43, 2939. (b) Bernardo, P. H.; Chai, C. L. L. J. Org. Chem. 2003,
68, 8906.
(6) Tohyama, S.; Tominari, C.; Matsumoto, K.; Yamabuki, A.; Ikegata,
K.; Nobuhiro, J.; Hibino, S. Tetrahedron Lett. 2005, 46, 5263.
(7) Bennasar, M. L.; Roca, T.; Ferrando, F. Org. Lett. 2006, 8, 561.
(4) Kelly, T. R.; Zhao, Y.; Cavero, M.; Torneiro, M. Org. Lett. 2000,
2, 3735.
10.1021/jo061270o CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/28/2006
8384
J. Org. Chem. 2006, 71, 8384-8389

Concise and Efficient Synthesis of Calothrixin B
zol-4-one 710 on Pd/C afforded 9H-carbazol-4-ol 8,11 which was
subjected to conditions for bis-protection with the Cbz-group,
using benzyl chloroformate (NaH, Cbz-Cl). Isolated 9 was
chemoselectively monodeprotected by treatment with aqueous
sodium hydroxyde to furnish the N-Cbz-protected 9H-carbazol-
4-ol 10. Conversion of 10 to the 3-bromo-9H-carbazol-4-ol 11
was accomplished in excellent yield by exposure to 1 equiv of
NBS in acetonitrile. Oxidation of 11 with diacetoxyiodobenzene
under acidic conditions completed the synthesis of the desired
dienophile 12. The optimized synthesis of the N-Bn-protected
analogue 168 was achieved following a somewhat different
route. Thus, 1,2,3,9-tetrahydro-4H-carbazol-4-one 7 was first
transformed into its N-benzyl derivative 13, and this later was
dehydrogenated via a two-step procedure to give the 9-benzyl-
9H-carbazol-4-ol 14. This compound was next smoothly and
selectively brominated by treatment with less than 1 equiv of
NBS (0.5 to 0.75 equiv)12,13 in acetonitrile to give the 9-benzyl-
3-bromo-9H-carbazol-4-ol 15 in 90% yield (based on reacting
14). Subsequent oxidation of 15 with diacetoxyiodobenzene
provided the desired dienophile 16 in excellent chemical yield.
SCHEME 1. Retrosynthetic Analysis of Calothrixin B
Herein, we report full details of our total synthesis of calothrixin
B from the commercially available 2,3,4,9-tetrahydro-1H-
carbazole. Note that oxidation of calothrixin B to calothrixin A
was previously reported4 so that a synthesis of 1b also
constitutes a formal synthesis of 1a.
With dienophile 12 in hand, we were thus in a position to
effect the key [4+2] cycloaddition (Scheme 3). As anticipated
on the basis of our earlier studies,9 this reaction was smoothly
effected by admixture of 12 with a slight excess of diene 3 in
acetonitrile and subsequent heating at 40 °C for 4 h. Surpris-
ingly, the major compound formed was not the expected adduct
17 but its deprotected derivative 18. Moreover, we also observed
that the ratio between 17 and 18 could be greatly improved in
favor of 18 (80% isolated yield) after 48 h of heating in
acetonitrile at 40 °C. Because 18 was isolated along with benzyl
dimethylcarbamate, we assumed that the easy cleavage of the
N-Cbz group took place under the action of in situ liberated
dimethylamine hydrobromide. Similarly, dienophile 16 reacted
with diene 3 to give the expected adduct 19 in 80% isolated
yield. At this stage, it is worth mentioning that the change in
the nature of the indole protective group resulted in a dramatic
change of the velocity of the cycloaddition reaction which, in
the same conditions, was complete after 4 h for dienophile 12
instead of 48 h for dienophile 16. This different behavior
undoubtedly reflects the electron-acceptor nature of the Cbz
protecting group which, in lowering the electron density in the
quinone unit of 12, favors its cycloaddition reaction with the
electron-rich diene 3.
Results and Discussion
We, recently, reported a synthesis of 5-aza-analogues of
angucyclinones, e.g., 2, based on a heterocyclic Diels-Alder
reaction and featuring the novel “push-pull” diene 3.9 Con-
sidering the structural similarities displayed by calothrixin B
1b and compound 2, we thought that an identical cycloaddition
strategy could well be applied to assemble the core structure of
1b in one single operation. From a retrosynthetic perspective
(Scheme 1), trione 4 appears as a possible key precursor to
calothrixin B. The forward sequence from 4 to calothrixin B
would involve a dehydrogenation step to form the fully
aromatized A ring and subsequent excision of oxygen at carbon
C-1. Trione 4 could be disconnected at the C13a-C13b and C6-
C6a linkages to give two fragments, diene 3 and dienophile 5.
Dienophile 5 could be derived from an N-protected 9H-carbazol-
4-ol 6, the synthesis of which could be envisaged from the
commercially available 2,3,4,9-tetrahydro-1H-carbazole (Scheme
2). Due to its high convergency, this approach appears well
suited to prepare analogues of the calothrixins in useful
quantities so that structure-activity relationships can be ex-
plored.
With the cycloaddition reaction accomplished, transformation
of compound 18 into calothrixin B could be completed in a
few additional steps, e.g., triflation, dehydrogenation of ring
A, and excision of the superfluous oxygen at C1 (Scheme 4).
Thus, deprotonation of 18 followed by triflation afforded a
mixture of enol triflate 20 and aryl triflate 21 which, without
separation, were subsequently heated in dioxane in the presence
of DDQ to give the sole triflate 21. Subjection of 21 to
conditions for reduction (Pd(0), formic acid) completed the
synthesis of calothrixin B. It is worth noting that this transfor-
At the outset of our work we noted that, in the two previous
syntheses known at that time,4,5 the MOM protecting group had
been chosen to temporarily mask the indole nitrogen atom and
that its removal could be effected only in relatively harsh
conditions and in fair chemical yield. Our intention was thus to
choose a protecting group that could be more easily and
efficiently removed and started from the premise that a
benzyloxycarbonyl (Cbz) group or, optionally, a benzyl (Bn)
group should meet these requirements. According to the plan
laid out earlier this proposed strategy thus required the formation
of the N-protected 9H-carbazol-4-ol 10 and 14 prior to the
preparation of key dienophiles 12 and 16, respectively.
The synthesis of dienophile 12 was accomplished as depicted
in Scheme 2. Dehydrogenation of 1,2,3,9-tetrahydro-4H-carba-
(10) 1,2,3,9-Tetrahydro-4H-carbazol-4-one 7 was prepared by oxidation
of the commercially available 2,3,4,9-tetrahydro-1H-carbazole following
an already reported procedure. See: Oikawa, K.; Yonemitsu, O. J. Org.
Chem. 1977, 42, 1213.
(11) 1,2,3,9-Tetrahydro-4H-carbazol-4-one and 9H-carbazol-4-ol are both
commercially available.
(12) Use of 1 equiv of NBS led to the formation of a secondary compound
resulting from over-bromination of 15.
(13) Poumaroux, A.; Bouaziz, Z.; Domard, M.; Fillion, H. Heterocycles
1997, 43, 585.
(8) Sissouma, D.; Collet, S.; Guingant, A. Synlett 2004, 2612.
(9) (a) Collet, S.; Re´mi, J. F.; Cariou, C.; La¨ıb, S.; Guingant, A.; Vu, N.
Q.; Dujardin, G. Tetrahedron Lett. 2004, 45, 4911. (b) Vu, N. Q.; Dujardin,
G.; Collet, S.; Raiber, E.-A.; Guingant, A.; Evain, M. Tetrahedron Lett.
2005, 46, 7669.
J. Org. Chem, Vol. 71, No. 22, 2006 8385

Sissouma et al.
SCHEME 2. Synthesis of Dienophiles 12 and 16a
a Reagents and conditions: (i) 7, 10% Pd/C, 10:1 PhOPh:1,2,5-trimethylbenzene, 250 °C, 18 h, 90%; (ii) NaH (2.3 equiv), BnCO2Cl (2.3 equiv), rt, 1 h,
91%; (iii) NaOH, 3:1 dioxane:water, 40 °C, 3 h, 90%; (iv) NaH, DMF, BnBr, 0-20 °C, 2 h, 95%; (v) 13, NaH, THF, PhSO2Me, 20 °C then reflux, 2 h, 94%;
(vi) preparation of 11:10, NBS (1 equiv), CH3CN, rt, 0.5 h, 94%; preparation of 15:14, NBS (0.5 to 0.75 equiv), CH3CN, 20 °C, 0.5 h, 90%; (vii) preparation
of 12:11, PhI(OAc)2, 2:3 AcOH:TFA, rt, 0.5 h, 83%; preparation of 16:15, PhI(OAc)2, 2:3 AcOH:TFA, 20-50 °C, 0.5 h, 96%.
SCHEME 3. The Key Cycloaddition Reaction
SCHEME 4. Completion of the Synthesisa
a Reagents and conditions: (i) LiHMDS, 3:2 THF:HMPA, -78 °C then PhNTf2 in THF, 1 h; (ii) DDQ, dioxane, reflux, 2 h, 51% over two steps; (iii)
Pd(PPh3)4, NEt3, HCO2H, dioxane, reflux, 4 h, 75%; (iv) 10% Pd-C, PhOPh, 250 °C, 15 min, 73%; (v) TfOTf, NEt3, CH2Cl2, -78 to 20 °C, 2 h, 94%; (vi)
Pd(PPh3)4, NEt3, HCO2H, dioxane, reflux, 20 min, 95%; (vii) AlCl3 (5 equiv), benzene, reflux, 2 h, 57%.
mation could be effected without the need for indole nitrogen
atom reprotection.
calothrixin B turned out to be far from routine and could be
effected only in a fair yield of 57% by subjection of 24 to an
excess of AlCl3 in benzene at reflux.
As shown in Scheme 4, calothrixin B could also be reached
from the N-benzyl protected adduct 19, although in a less
efficient manner. Transformation of adduct 19 toward N-Bn
protected calothrixin B 24 was achieved without incident.
However, the removal of the N-benzyl protecting group to give
Conclusion
In conclusion, we have described an approach for the
preparation of calothrixin B exploiting a regioselective hetero-
8386 J. Org. Chem., Vol. 71, No. 22, 2006

Concise and Efficient Synthesis of Calothrixin B
chromatography on silica gel (4:1 CH2Cl2:hexanes) to give 2.24 g
of 9 (4.97 mmol, 91%) as a pale brown solid: mp 104 °C (CH2-
Cl2:hexanes). 1H NMR (CDCl3, 300 MHz) δ 5.27 (s, 2H), 5.49 (s,
2H), 7.16-7.21 (m, 2H), 7.31-47 (m, 12H), 7.87 (d, J ) 7.8 Hz,
1H), 8.14 (d, J ) 7.8 Hz, 1H), 8.22 (d, J ) 8.4 Hz, 1H). 13C NMR
(CDCl3, 75 MHz) δ 68.5, 70.3, 113.9, 115.8 (2C), 117.9, 122.0,
123.2, 123.4, 127.1, 127.15, 128.0, 128.3, 128.35, 128.4, 128.5,
134.6, 134.8, 137.8, 139.3, 145.5, 151.6, 153.0 (NB: 23 out of 28
resonance peaks were observed). FT-IR (KBr, cm-1) 2924, 1752,
1724, 1591. MS (EI, 70 eV) m/z (rel intensity) 451 (M+, 3), 407
(5), 272 (11), 91 (100). HRMS (EI) calcd for C28H21NO5 451.1420,
found 451.143 [M+].
Diels-Alder cycloaddition to assemble the core structure of the
molecule. It was shown that the electronic nature of the indole
moiety N-protecting group (i.e., Cbz vs Bn) influenced the
reactivity significantly. On the whole, the route with a N-Cbz
protective group was more rewarding since this group, in
addition to accelerating the cycloaddition process, was easily
removed in the course of the reaction under the action of the
liberated dimethylamine hydrobromide. Also of particular note
is the fact that recourse to reprotection of the indole nitrogen
atom was unnecessary to achieve the transformation of the
Diels-Alder adduct toward calothrixin B, which was con-
structed in a total of 9 steps, in 17% overall yield from
commercially available 1,2,3,9-tetrahydro-4H-carbazol-4-one 7.
Benzyl 4-Hydroxy-9H-carbazole-9-carboxylate (10). To a
stirred solution of 9 (2 g, 4.43 mmol) in a 3:1 mixture of dioxane
and water (80 mL) at 40 °C was added sodium hydroxide (350
mg, 8.86 mmol). After it was stirred for 3 h, the reaction mixture
was quenched with water (50 mL) and neutralized with aqueous
saturated NH4Cl. The mixture was extracted several times with CH2-
Cl2 and the combined organic layers were dried over anhydrous
MgSO4 and concentrated in vacuo. Purification of the residue by
column chromatography on silica gel (4:1 CH2Cl2:hexanes) pro-
vided 1.26 g of 10 (3.97 mmol, 90%) as a pale brown solid: mp
Experimental Section
(E,E)-N,N-Dimethyl-N′-(3-oxocyclohex-1-en-1-yl)imidoforma-
mide (3). A solution of 3-aminocyclohex-2-ene-1-one14 (1.59 g,
14.32 mmol) and dimethylformamide dimethylacetal (3.8 mL, 28.42
mmol) was heated in THF (30 mL) at 80 °C for 4 h. After being
cooled to room temperature the solution was concentrated in vacuo.
Purification of the residue by column chromatography on silica gel
(9:1 EtOAc:MeOH containing 2% Et3N) afforded 2.19 g (13.19
mmol, 92%) of diene 3 as a 3:1 mixture of E,E and Z,E isomers.
The mixture of dienes was next heated in refluxing toluene for 1.5
1
159 °C (AcOEt:hexanes). H NMR (CDCl3, 300 MHz) δ 5.41 (s,
1H), 5.57 (s, 2H), 6.72 (d, J ) 7.8 Hz, 1H), 7.24-7.30 (m, 2H),
7.34-7.46 (m, 5H), 7.46-7.56 (m, 2H), 7.91 (d, J ) 8.4 Hz, 1H),
8.28 (d, J ) 7.8 Hz, 1H). 13C NMR (CDCl3, 75 MHz) δ 68.7,
109.1, 109.5, 114.2, 115.8, 122.9, 123.6, 125.1, 126.5, 127.7, 128.6
(2C), 128.7 (2C), 128.8, 135.2, 137.7, 140.0, 151.3, 152.4. FT-IR
(KBr, cm-1) 3315, 3036, 1694, 1629, 1593. MS (EI, 70 eV) m/z
(rel intensity) 317 (M+, 16), 273 (14), 272 (11), 182 (4), 91 (100).
HRMS (EI) calcd for C20H15NO3 317.1052, found 317.106 [M+].
Benzyl 3-Bromo-4-hydroxy-9H-carbazole-9-carboxylate (11).
To a solution of 10 (1.0 g, 3.15 mmol) in dry CH3CN (50 mL) at
room temperature was added freshly recrystallized NBS (560 mg,
3.15 mmol). After it was strirred for 30 min, the solution was
concentrated and the residue was purified by column chromatog-
raphy on silica gel (1:1 CH2Cl2:hexanes) to give 1.17 g of 11 (2.95
1
h then concentrated to give diene 3 as the single E,E-isomer. H
NMR (CDCl3, 300 MHz) δ 1.95-2.04 (m, 2H), 2.32 (t, J ) 6.5
Hz, 2H), 2.46 (t, J ) 6.3 Hz, 2H), 3.03 and 3.06 (2s, 6H), 5.43 (s,
1H), 7.62 (s, 1H). 13C NMR (CDCl3, 75 MHz) δ 22.1, 30.5, 34.4,
36.8, 40.4, 110.7, 152.0, 172.5, 199.9. FT-IR (liquid film, cm-1
)
2939, 2815, 1615, 1557. MS (EI, 70 eV) m/z (rel intensity) 166
(M+, 91), 138 (18), 123 (46), 109 (32), 71 (27), 44 (100), 28 (9).
HRMS (EI) calcd for C9H14N2O 166.1106, found 166.110 [M+].
9H-carbazol-4-ol (8).11,15 To a stirred solution of 1,2,3,9-
tetrahydro-4H-carbazol-4-one 7 (1 g, 5.41 mmol) in a 10:1 mixture
of Ph2O and 1,2,5-trimethylbenzene (55 mL) at room temperature
was added 10% Pd/C (810 mg, 0.75 mmol). The mixture was heated
for 18 h at 250 °C under an inert atmosphere of N2 then cooled to
room temperature. The catalyst was removed by filtration on Celite
and the filter cake was rinsed several times with EtOAc. Volatiles
were removed under reduce pressure and the resulting mixture was
loaded onto a silica gel column. Elution of the column with hexanes
(removal of Ph2O) then with CH2Cl2 provided 890 mg of 8 (4.86
mmol, 90%) as a pale brown solid: mp 173 °C (AcOEt:hexanes).
1H NMR (CDCl3, 300 MHz) δ 5.30 (s, 1H), 6.58 (d, J ) 7.8 Hz,
1H), 7.02 (d, J ) 7.8 Hz, 1H), 7.21-7.27 (m, 3H), 7.37-7.43 (m,
2H), 8.07 (br s, 1H), 8.27 (d, J ) 7.7 Hz, 1H). 13C NMR (CDCl3,
75 MHz) δ 103.3, 105.2, 110.0, 111.8, 119.7, 122.3, 122.8, 125.1,
126.6, 138.8, 141.4, 151.9. FT-IR (KBr, cm-1) 3399, 3260, 1638,
1609, 1586, 1450. MS (EI, 70 eV) m/z (rel intensity) 183 (M+,
100), 155 (31), 154 (55),127 (15).
Benzyl 4-{[(Benzyloxy)carbonyl]oxy}-9H-carbazole-9-car-
boxylate (9). To a solution of 9H-carbazol-4-ol 8 (1 g, 5.46 mmol)
in dry DMF (15 mL) at 0 °C was added NaH (60% dispersion in
mineral oil, 500 mg, 12.6 mmol). The mixture was stirred for 1 h
at 0 °C and allowed to warm to room temperature. Benzyl
chloroformate (1.8 mL, 12.6 mmol) was then added and the mixture
was stirred for an additional 1 h. After it was quenched with water
(100 mL), the mixture was extracted several times with CH2Cl2
and the combined organic layers were dried over anhydrous MgSO4
and concentrated in vacuo. The residue was purified by column
1
mmol, 94%) as a white solid: mp 130 °C (AcOEt:hexanes). H
NMR (CDCl3, 300 MHz) δ 5.55 (s, 2H), 6.06 (s, 1H), 7.33-7.48
(m, 6H), 7.53-7.56 (m, 2H), 7.60 (d, J ) 9.0 Hz, 1H), 8.22-8.25
(m, 2H). 13C NMR (CDCl3, 75 MHz) δ 68.9, 103.8, 109.9, 114.6,
115.8, 123.0, 123.7, 124.3, 127.0, 128.6 (2C), 128.8 (3C), 129.4,
135.0, 137.7, 139.0, 147.4, 152.0. FT-IR (KBr, cm-1) 3405, 1733,
1588, 1461. MS (EI, 70 eV) m/z (rel intensity) 397 (M+, 81Br, 3),
397 (M+, 79Br, 4), 353 (3), 153 (5), 91 (100). HRMS (EI) calcd
for C20H14NO379Br 395.0157, found 395.017 [M+].
Benzyl 3-Bromo-1,4-dioxo-1,4-dihydro-9H-carbazole-9-car-
boxylate (12). To a solution of 11 (1.0 g, 2.53 mmol) in a 2:3
mixture of AcOH and TFA (25 mL) at room temperature was
successively added a few drops of water and diacetoxy iodobenzene
(2.45 g, 7.6 mmol). After the mixture was stirred for 30 min at 40
°C, it was cooled to room temperature, MeOH (75 mL) was added,
and stirring was continued for an additional 30 min. The mixture
was extracted with CH2Cl2 several times and the combined organic
layers were dried over anhydrous MgSO4 and concentrated in vacuo.
Purification of the residue by column chromatography on silica gel
(1:1 CH2Cl2:hexanes) afforded 860 mg (2.10 mmol, 83%) of 12 as
1
a red solid: mp 175 °C (AcOEt:hexanes). H NMR (CDCl3, 300
MHz) δ 5.51 (s, 2H), 7.22 (s, 1H), 7.39-7.52 (m, 7H), 7.99 (d, J
) 8.4 Hz, 1H), 8.29 (d, J ) 8.1 Hz, 1H). 13C NMR (CDCl3, 75
MHz) δ 71.0, 114.6, 121.3, 123.2, 123.8, 126.1, 128.8 (3C), 129.2
(2C), 129.4, 133.7, 134.9, 137.3, 138.0, 138.3, 149.8, 174.7, 175.9.
FT-IR (KBr, cm-1) 3441, 1756, 1668, 1593, 1531. MS (EI, 70 eV)
m/z (rel intensity) 411 (M+, 81Br, 1), 409 (M+, 79Br, 1), 367 (5),
91 (100). HRMS (EI) calcd for C20H12NO479Br 408.9950, found
408.997 [M+].
(14) Huang, Y.; Hartmann, R. W. Synth. Commun. 1998, 28, 1197.
(15) (a) Scott, T. L.; So¨derberg, C. G. Tetrahedron 2003, 59, 6323. (b)
Dubois, E. A.; van den Bos, J. C.; Doornbos, T.; van Doremalen, P. A. P.
M.; Somsen, G. A.; Vekemans, J. A. J. M.; Janssen, A. G. M.; Batink, H.
D.; Boer, G. J.; Pfaffendorf, M.; van Royen, E. A.; van Zwieten, P. A. J.
Med. Chem. 1996, 39, 3256.
9-Benzyl-1,2,3,9-tetrahydro-4H-carbazol-4-one (13). To a
solution of 1,2,3,9-tetrahydro-4H-tetrahydrocarbazol-4-one 710,11
(4.0 g, 21.6 mmol) in dry DMF (50 mL) at 0 °C was added sodium
J. Org. Chem, Vol. 71, No. 22, 2006 8387

Sissouma et al.
hydride (60% dispersion in mineral oil, 990 mg, 24.7 mmol) in
small portions. After the mixture was stirred for 2 h at 0 °C it was
allowed to warm to room temperature and benzyl bromide (2.9 mL,
24.7 mmol) was added dropwise. The reaction mixture was then
stirred at room temperature for an additional 2 h and quenched
with water (50 mL). The precipitate that formed was filtered and
washed with pentane to provide 5.6 g (20.3 mmol, 95%) of 13 as
a pale brown solid that was used without further purification. An
analytically pure sample of 13 was obtained by crystallization from
7.51 (m, 3H), 8.31 (d, J ) 7.6 Hz, 1H). 13C NMR (CDCl3, 100
MHz) δ 48.4, 111.8, 116.4, 123.4, 124.4, 125.3, 126.9 (2C), 127.7,
128.2, 129.0 (2C), 133.3, 136.0, 137.4, 139.3, 140.4, 175.2, 178.2.
FT-IR (KBr, cm-1) 3380, 3036, 1663, 1646, 1583, 1518. MS (EI,
70 eV) m/z (rel intensity) 367 (M+, 81Br, 11), 365 (M+, 79Br, 11),
91 (100), 65 (11). HRMS (EI) calcd for C19H12NO279Br 365.0051,
found 365.006 [M+].
3,4-Dihydro-1H-indolo[3,2-j]phenanthridine-1,7,13(2H,12H)-
trione (18). To a stirred solution of diene 3 (520 mg, 3.13 mmol)
in dry CH3CN (15 mL) at room temperature was added a solution
of dienophile 12 (850 mg, 2.07 mmol) in dry CH3CN (15 mL).
After the mixture was heated for 18 h at 40 °C, it was cooled to
room temperature and concentrated in vacuo. The residue was
purified by column chromatography on silica gel (elution with 1:1
EtOAc:hexanes then with pure EtOAc) to give 520 mg of
cycloadduct 18 (1.64 mmol, 79%) as a red solid: mp >300 °C
dec (CHCl3:hexanes). 1H NMR (CDCl3, 400 MHz) δ 2.20 (qt, J )
6.6 Hz, 2H), 2.90 (t, J ) 6.6 Hz, 2H), 3.14 (t, J ) 6.6 Hz, 2H),
7.40 (t, J ) 7.2 Hz, 1H), 7.50 (t, J ) 7.2 Hz, 1H), 7.62 (m, 1H),
8.17 (d, J ) 8.1 Hz, 1H), 9.22 (s, 1H), 13.21 (s, 1H). 13C NMR
(CDCl3, 75 MHz) δ 21.0, 32.5, 38.7, 113.9, 116.6, 122.3, 123.4,
124.3, 126.9, 127.4, 128.2, 137.4, 138.5, 138.9, 149.3, 168.1, 176.0,
178.8, 197.6. FT-IR (KBr, cm-1) 3289, 1701, 1657, 1570, 1523.
MS (EI, 70 eV) m/z (rel intensity) 316 (M+, 56), 288 (100), 260
(24), 232 (10). HRMS (EI) calcd for C19H12N2O3 316.0848, found
316.084 [M+].
1
EtOAc: mp 148 °C (AcOEt). H NMR (CDCl3, 300 MHz) δ 2.22
(qt, J ) 6.0 Hz, 2H), 2.58 (t, J ) 6.0 Hz, 2H), 2.86 (t, J ) 6.0 Hz,
2H), 5.32 (s, 2H), 7.00-7.03 (m, 2H), 7.18-7.32 (m, 6H), 8.30
(d, J ) 6.9 Hz, 1H). 13C NMR (CDCl3, 75 MHz) δ 22.3, 23.4,
37.9, 47.0, 109.6, 113.1, 121.7, 122.7, 123.2, 124.9, 126.1 (2C),
127.9, 129.0 (2C), 136.0, 137.1, 151.8, 193.9. FT-IR (KBr, cm-1
)
3080, 2941, 1641, 1631, 1612, 1531. MS (EI, 70 eV) m/z (rel
intensity) 275 (M+, 52), 247 (22), 91 (100), 65 (20). Anal. Calcd
for C19H17NO: C, 82.88; H, 6.22; N, 5.09. Found: C, 82.72; H,
6.29; N, 4.97.
9-Benzyl-9H-carbazol-4-ol (14). To a stirred solution of 13 (2.0
g, 7.27 mmol) in dry THF (20 mL) at room temperature was added
sodium hydride (60% dispersion in mineral oil, 670 mg, 16.7 mmol)
in small portions. After the solution was stirred for 2 h, methyl-
benzene sulfinate (1.23 mL, 9.36 mmol) was added dropwise. The
mixture was then refluxed for 2 h and subsequently quenched with
water (20 mL) and aqueous saturated NH4Cl (10 mL) at room
temperature. The mixture was extracted several times with EtOAc
and the combined organic layers were dried over anhydrous MgSO4
and concentrated. The residue was taken up in dioxane (20 mL)
and the resulting solution was refluxed for 15 h. After it was cooled
to room temperature, the solution was concentrated and the residue
was purified by column chromatography on silica gel (8:2 CH2-
Cl2:hexanes) to afford 1.86 g (6.81 mmol, 94%) of 14 as a pale
brown solid: mp 172 °C (AcOEt:hexanes). 1H NMR (CDCl3, 300
MHz) δ 5.39 (s, 1H), 5.50 (s, 2H), 6.60 (d, J ) 7.8 Hz, 1H), 6.97
(d, J ) 8.1 Hz,1H), 7.12-7.15 (m, 2H), 7.24-7.29 (m, 5H), 7.33-
7.41 (m, 2H), 8.33 (d, J ) 7.8 Hz, 1H). 13C NMR (CDCl3, 75
MHz) δ 46.7, 101.8, 105.2, 108.5, 111.3, 119.6, 122.1, 122.9, 125.1,
126.5 (2C), 126.6, 127.5, 128.8 (2C), 137.2, 140.1, 142.7, 152.0.
FT-IR (KBr, cm-1) 3345, 3020, 1636, 1603, 1585. MS (EI, 70 eV)
m/z (rel intensity) 273 (M+,25), 91 (100), 65 (14). HRMS (EI) calcd
for C19H15NO 273.1154, found 273.116 [M+].
12-Benzyl-3,4-dihydro-1H-indolo[3,2-j]phenanthridine-1,7,-
13(2H,12H)-trione (19). To a stirred solution of diene 3 (100 mg,
0.60 mmol) in dry CH3CN (4 mL) at room temperature was added
a solution of dienophile 16 (200 mg, 0.55 mmol) in dry CH3CN (4
mL). The reaction mixture was heated for 48 h at 40 °C, cooled to
room temperature, and concentrated under reduced pressure. The
residue was purified by column chromatography on silica gel (95:5
CH2Cl2:EtOAc) to give 180 mg of cycloadduct 19 (0.44 mmol,
1
80%) as an orange solid: mp 118 °C (CHCl3:hexanes). H NMR
(CDCl3, 300 MHz) δ 2.27 (qt, J ) 6.6 Hz, 2H), 2.93 (t, J ) 6.6
Hz, 2H), 3.18 (t, J ) 6.6 Hz, 2H), 5.89 (s, 2H), 7.20-7.32 (m,
5H), 7.40-7.47 (m, 3H), 8.43 (d, J ) 7.8 Hz, 1H), 9.42 (s, 1H).
13C NMR (CDCl3, 75 MHz) δ 21.4, 33.1, 39.2, 48.6, 111.7, 118.4,
123.6, 123.9, 125.1, 126.8, 127.0 (2C), 128.0, 128.1, 128.2, 128.9
(2C), 135.6, 136.0, 140.0, 140.7, 150.3, 168.2, 177.5, 179.3, 198.3.
FT-IR (KBr, cm-1) 3409, 3041, 2958, 1702, 1666, 1569, 1509.
MS (EI, 70 eV) m/z (rel intensity) 406 (M+, 100), 378 (21), 315
(6), 91 (93), 65 (11). HRMS (EI) calcd for C26H18N2O3 406.1317,
found 406.133 [M+].
9-Benzyl-3-bromo-9H-carbazol-4-ol (15). To a stirred solution
of 14 (1.8 g, 6.59 mmol) in dry CH3CN (70 mL) at room
temperature was added freshly recrystallized NBS (880 mg, 4.95
mmol). After being stirred for 30 min, the solution was concentrated
in vacuo and the residue purified by column chromatography on
silica gel (1:1 CH2Cl2:hexanes) to provide 1.57 g (4.46 mmol, 90%)
7,13-Dioxo-12,13-dihydro-7H-indolo[3,2-j]phenanthridin-1-
yl Trifluoromethanesulfonate (21). To a solution of 18 (150 mg,
0.47 mmol) in THF (3 mL) at -78 °C were added HMPA (2 mL)
and LiHMDS (1 M solution in THF, 1.2 mL, 1.2 mmol). After the
solution was stirred for 1 h at -78 °C, a solution of PhNTf2 (254
mg, 0.71 mmol) in THF (1 mL) was added via cannula. The reaction
mixture was stirred for an additional 1 h at -78 °C, quenched with
water (5 mL), and neutralized with aqueous 1 M HCl. The mixture
was then extracted with EtOAc and the combined organic layers
were dried over anhydrous MgSO4 and concentrated in vacuo. The
residue was purified by column chromatography on silica gel (1:1
EtOAc/hexanes) to afford a mixture of 20 and 21. To this mixture,
dissolved in dioxane (2 mL), was added DDQ (160 mg, 0.70 mmol)
in one portion. After the solution was refluxed for 2 h, it was cooled
to room temperature and concentrated under reduced pressure. The
residue was taken up in EtOAc and the resulting solution was
filtered through a short plug of neutral alumina to give 107 mg of
21 (0.24 mmol, 51%) as a red solid: mp 276 °C (CHCl3). 1H NMR
(DMSO- d6, 300 MHz) δ 7.42 (dd, J ) 7.2 and 7.9 Hz, 1H), 7.50
(dd, J ) 6.5 and 7.9 Hz, 1H), 7.63 (d, J ) 8.1 Hz, 1H), 8.06-8.20
1
of 15 as a white solid: mp 175 °C (AcOEt:hexanes). H NMR
(CDCl3, 300 MHz) δ 5.29 (s, 2H), 6.12 (s, 1H), 6.84 (d, J ) 8.7
Hz, 1H), 7.08-7.11 (m, 2H), 7.22-7.35 (m, 5H), 7.40-7.46 (m,
2H), 8.36 (d, J ) 7.8 Hz, 1H). 13C NMR (CDCl3, 75 MHz) δ 46.7,
99.2, 102.7, 108.6, 111.6, 119.9, 121.7, 123.3, 125.7, 126.3 (2C),
127.6, 128.4, 128.8 (2C), 136.8, 140.2, 141.6, 148.1. FT-IR (KBr,
cm-1) 3394, 3050, 1597, 1484. MS (EI, 70 eV) m/z (rel intensity)
353 (M+, 81Br, 8), 351 (M+, 79Br, 8), 91 (100), 65 (12). HRMS
(EI) calcd for C19H14NO79Br 351.0259, found 351.025 [M+].
9-Benzyl-3-bromo-1H-carbazole-1,4(9H)-dione (16). To a stirred
solution of 15 (1.22 g, 3.46 mmol) in a 2:3 mixture of AcOH and
TFA (25 mL) containing a few drops of water was added diacetoxy
iodobenzene (3.34 g, 10.38 mmol). The solution was stirred for 30
min at 50 °C then MeOH (75 mL) was added and stirring was
continued for an additional 30 min. The solution was extracted
several times with CH2Cl2 and the combined organic layers were
dried over anhydrous MgSO4 and concentrated. Purification of the
residue by column chromatography on silica gel (1:1 CH2Cl2:
hexanes) afforded 1.21 g (3.30 mmol, 96%) of 16 as red needles:
mp 208 °C (AcOEt). 1H NMR (CDCl3, 400 MHz) δ 5.84 (s, 2H),
7.12 (s, 1H), 7.15 (d, J ) 8.0 Hz, 2H), 7.25-7.30 (m, 3H), 7.37-
(m, 3H), 8.35 (d, J ) 8.1 Hz, 1H), 9.66 (s, 1H), 13.43 (s, 1H). 13
C
NMR (DMSO-d6, 75 MHz) δ 114.0, 114.8, 115.9, 117.9 (CF3, J
) 319 Hz), 122.2, 123.2, 124.1, 124.6, 126.9, 127.4, 131.1, 131.8,
136.5, 137.5, 138.6, 143.6, 148.8, 151.4, 177.5, 178.7. FT-IR (KBr,
8388 J. Org. Chem., Vol. 71, No. 22, 2006

Concise and Efficient Synthesis of Calothrixin B
1
cm-1) 3391, 3281, 1673, 1653, 1616, 1555, 1529. MS (EI, 70 eV)
m/z (rel intensity) 446 (M+, 62), 314 (56), 297 (42), 285 (100),
269 (2), 257 (54). HRMS (EI) calcd for C20H9N2O5F3S 446.0184,
found 446.021 [M+].
hexanes). H NMR (CDCl3, 300 MHz) δ 5.92 (s, 2H), 7.23-7.34
(m, 4H), 7.42-7.46 (m, 2H), 7.65 (d, J ) 7.8 Hz, 1H), 7.90 (t, J
) 8.1 Hz, 1H), 8.29 (d, J ) 8.2 Hz, 1H), 8.42-8.46 (m, 1H), 9.83
(s, 1H). 13C NMR (CDCl3, 75 MHz) δ 48.9, 112.1, 116.3, 117.5,
118.5 (CF3, J ) 322 Hz), 123.0, 123.7, 125.2, 126.4, 126.7 (2C),
127.8, 128.8 (2C), 130.9, 131.3, 135.9, 136.4, 138.2, 140.1, 144.4,
149.2, 152.3, 178.9, 179.2 (NB: 25 out of 27 resonance peaks were
observed). FT-IR (KBr, cm-1) 3430, 2933, 1657, 1655, 1615, 1517.
MS (EI, 70 eV) m/z (rel intensity) 536 (M+, 5), 404 (33), 91 (100),
65 (17). HRMS (EI) calcd for C26H15N2O3 403.1083, found 403.109
[M - SO2CF3]+.
Removal of the Trifluoromethanesulfonyloxy Group of 21:
Obtention of 1b. To a stirred solution of 21 (100 mg, 0.22 mmol)
in dioxane (5 mL) at room temperature were added NEt3 (0.125
mL, 0.88 mmol), Pd(PPh3)4 (12 mg, 4 mol %), and formic acid
(0.030 mL, 0.59 mmol). The mixture was refluxed for 4 h under
an inert atmosphere of N2 then cooled to room temperature. The
precipitate was collected by filtration, washed with EtOAc, then
crystallized from acetone to give 50 mg (0.17 mmol, 75%) of 1b
12-Benzyl-7H-indolo[3,2-j]phenanthridine-7,13(12H)-dione (N-
Benzyl Calothrixin B)(24). To a stirred solution of 23 (70 mg,
0.13 mmol) in dioxane (4 mL) at room temperature were added
NEt3 (0.073 mL, 0.52 mmol), Pd(PPh3)4 (6 mg, 4 mol %), and
formic acid (0.014 mL, 0.35 mmol). The mixture was refluxed for
20 min under an inert atmosphere of N2. After it was cooled to
room temperature the mixture was quenched with brine and
extracted with CH2Cl2. The combined organic layers were dried
over anhydrous MgSO4 and concentrated in vacuo. The residue
was purified by column chromatography on silica gel (2:98 EtOAc:
CH2Cl2) to provide 48 mg (0.12 mmol, 95%) of 24 as a red solid:
mp 264 °C (CHCl3:hexanes). 1H NMR (CDCl3, 300 MHz) δ 6.03
(s, 2H), 7.21-7.34 (m, 5H), 7.44-7.49 (m, 3H), 7.74 (t, J ) 7.2
Hz, 1H), 7.84 (t, J ) 6.9 Hz, 1H), 8.20 (d, J ) 8.4 Hz, 1H), 8.47-
8.49 (m, 1H), 9.56 (d, J ) 8.1 Hz, 1H), 9.81 (s, 1H). 13C NMR
(CDCl3, 75 MHz) δ 48.5, 111.6, 117.7, 123.1, 123.3, 123.9, 124.5,
125.1, 126.6 (2C), 127.7, 127.9, 128.0, 128.9 (2C), 130.1, 130.3,
131.4, 133.3, 135.1, 136.2, 140.1, 147.9, 152.2, 181.0, 182.0. FT-
IR (KBr, cm-1) 3058, 1651, 1612, 1568, 1523. MS (EI, 70 eV)
m/z (rel intensity) 388 (M+, 64), 149 (18), 91 (100), 65 (17). HRMS
(EI) calcd for C26H16N2O2 388.1212, found 388.119 [M+].
7H-Indolo[3,2-j]phenanthridine-7,13(12H)-dione (Calothrixin
B) (1b). To a stirred solution of 24 (70 mg, 0.18 mmol) in benzene
(7 mL) was added AlCl3 (120 mg, 0.9 mmol) in one portion. The
mixture was refluxed for 2 h and subsequently quenched at room
temperature by the addition of water (10 mL). It was then extracted
with CH2Cl2 and the combined organic layers were dried over
anhydrous MgSO4. Volatiles were removed in vacuo. Purification
of the residue by column chromatography on silica gel (1:3 EtOAc:
hexanes) afforded 31 mg (0.10 mmol, 57%) of 1b as a red solid.
1
as a red solid: mp >300 °C dec (acetone). H NMR (DMSO-d6,
300 MHz) δ 7.39-7.52 (m, 2H), 7.63-7.66 (m, 1H), 7.89-8.01
(m, 2H), 8.18-8.21 (m, 2H), 9.60 (d, J ) 8.4 Hz, 1H), 9.64 (s,
1H), 13.20 (br s, 1H). 13C NMR (DMSO-d6, 100 MHz) δ 113.9,
115.5, 122.2, 122.5, 123.3, 124.3, 124.8, 127.1, 129.8, 130.2, 131.4,
131.5, 132.5, 137.9, 138.4, 147.5, 151.2, 180.3, 180.8. FT-IR (KBr,
cm-1) 3292, 2921, 1653,1558, 1532. MS (EI, 70 eV) m/z (rel
intensity) 298 (M+, 100), 270 (52), 242 (14), 121 (67). HRMS (EI)
calcd for C19H10N2O2 298.0742, found 298.074 [M+].
12-Benzyl-1-hydroxy-7H-indolo[3,2-j]phenanthridine-7,13-
(12H)-dione (22). To a stirred solution of 19 (150 mg, 0.37 mmol)
in Ph2O (4 mL) was added 10% Pd/C (97 mg, 0.09 mmol). The
mixture was heated for 1 h at 250 °C under an inert atmosphere of
N2. After the mixture was cooled to room temperature, the catalyst
was removed by filtration on Celite and the filter cake was rinsed
several times with EtOAc. Volatiles were removed under reduced
pressure and the resulting mixture was loaded onto a silica gel
column. Elution of the column with hexanes (removal of Ph2O)
and then with CH2Cl2 provided 110 mg of 22 (0.27 mmol, 73%)
as a brown solid: mp 245 °C (CHCl3:hexanes). 1H NMR (DMSO-
d6, 300 MHz) δ 5.93 (s, 2H), 7.11 (d, J ) 7.5 Hz, 1H), 7.24-7.50
(m, 7H), 7.59 (d, J ) 7.8 Hz, 1H), 7.72-7.79 (m, 2H), 8.21 (d, J
) 7.8 Hz, 1H), 9.43 (s, 1H), 10.92 (s, 1H). 13C NMR (CDCl3, 75
MHz) δ 49.1, 111.7, 114.2, 117.3, 118.3, 123.0, 123.1, 124.2, 124.5,
125.8, 126.6 (2C), 128.1, 129.0 (2C), 129.2, 133.2, 135.8, 136.9,
141.5, 147.6, 153.6, 154.4, 180.1, 185.0 (NB: 25 out of 26
resonance peaks were observed). FT-IR (KBr, cm-1) 3420, 3080,
1651, 1646, 1608, 1552. MS (EI, 70 eV) m/z (rel intensity) 404
(M+, 53), 313 (3), 91 (100), 65 (11). HRMS (EI) calcd for
C26H16N2O3 404.1161, found 404.118 [M+].
12-Benzyl-7,13-dioxo-12,13-dihydro-7H-indolo[3,2-j]phenan-
thridin-1-yl Trifluoromethanesulfonate (23). To a solution of 22
(100 mg, 0.25 mmol) in CH2Cl2 (5 mL) maintained at -78 °C
under a nitrogen atmosphere were added Et3N (0.174 mL, 1.25
mmol) and Tf2O (0.084 mL, 0.5 mmol). The mixture was stirred
for 2 h at -78 °C, allowed to warm to room temperature, and
quenched by the addition of water. It was then extracted with CH2-
Cl2 and the combined organic layers were dried over anhydrous
MgSO4 and concentrated in vacuo. Purification of the residue by
column chromatography on silica gel (eluent: CH2Cl2) afforded
126 mg (0.23 mmol, 94%) of 23 as a red solid: mp 220 °C (CHCl3:
Acknowledgment. This work was supported by the “Re´gion
des Pays de la Loire” (CER 2000-2006). D.S. is thankful to the
AUF (Agence Universitaire pour la Francophonie) for a post-
doctoral fellowship.
Supporting Information Available: General experimental
details and copies of 1H and 13C NMR spectra for compounds 1b,
3, 8, 9-16, 18, 19, and 21-24. This material is available free of
charge via the Internet at http://pubs.acs.org.
JO061270O
J. Org. Chem, Vol. 71, No. 22, 2006 8389