An efficient synthesis of pyrrolo[2,1-c][1,4]benzodiazepine. Synthesis of the antibiotic DC-81

Full text of DOI:10.1021/jo010043d

J . Org. Chem. 2001, 66, 2881-2883
2881
An Efficien t Syn th esis of
P yr r olo[2,1-c][1,4]ben zod ia zep in e.
Syn th esis of th e An tibiotic DC-81
Wan-Ping Hu, J eh-J eng Wang,* Fu-Lung Lin,
Yu-Chen Lin, Shinne-Ren Lin, and Mei-Hui Hsu
School of Chemistry, Kaohsiung Medical University,
Kaohsiung City 807, Taiwan, R.O.C.
jjwang@cc.kmu.edu.tw
Received J anuary 16, 2001
F igu r e 1. PBD analogues.
Pyrrolo[2,1-c][1,4]benzodiazepines (1) (PBDs) are a
group of potent, naturally occurring antitumor antibiotics
produced by Streptomyces species.1 The cytotoxic and
antitumor effects of these compounds are believed to arise
from modification of DNA, which leads to inhibition of
nucleic acid synthesis and production of excision-depend-
ent single- and double-strand breaks in cellular DNA.2
These antibiotics have been proposed to covalently bond
to N2 of guanine to form a neutral minor groove adduct
(2) (Scheme 1).3 Tomaymycin (3), cross-linker DSB-120
(4),3d,4 and DC-81 (5) are the best known examples of the
PBDs (Figure 1). Synthetic approaches to these com-
pounds have been reported;4-6 however, most of them are
tedious. For instance, one of the widely used methods
involves the cyclization of amino dithioacetals using
mercuric chloride to give the imine products (Scheme 2).
Nevertheless, it takes six steps to synthesize the starting
material, (2S)-pyrrolidine-2-carboxaldehyde diethyl thio-
acetal (7), from L-proline (6). The overall yield of this 10-
step synthesis of DC-81 is about 15-20%.4,6 More re-
cently, Wang et al. reported the total synthesis of DC-81
over 13 steps in 4% yield.5b
Herein we would like to report a very short route and
efficient synthesis of PBD analogue, DC-81. The synthe-
sis started with reduction of substituted 2-nitrobenzoic
acid (8)6 with stannous chloride to give amine 10 in 92%
yield (Scheme 3). Reaction of the resulting amine 10 with
triphosgene in THF under reflux generated isatoic an-
hydride (11) in excellent yield (98%).7 Compound 11 was
coupled to L-proline in DMSO at 120 °C to produce
dilactam 12, followed by conversion of the amide to N-(10-
methoxymethyl)-8-benzyloxy-7-methoxypyrrolo[2,1-c][1,4]-
benzodiazepine-5,11-dione (13) with MOMCl in high
yield.
* To whom correspondence should be addressed. Tel: (886)-7-312-
1101 ext 2275. Fax: (886)-7-312-5339.
(1) (a) Tendler, M. D.; Korman, S. Nature 1963, 199, 501. (b) Hurley,
L. H. J . Antibiot. 1977, 30, 349.
The key step of this synthesis is hydride reduction of
MOM-protected dilactam 13. Mori reported that the
imine form of PBD analogues could be prepared via
reduction of MOM-protected dilactam with 10 molar
equiv of NaBH4 in MeOH at 0 °C, followed by silica gel
chromatography.8 Unfortunately, after the MOM-pro-
tected dilactam 13 was prepared, as Thurston reported,9
it also failed in our hands to afford the corresponding
imine 14, using either the reported8 condition or a
number of variations. Instead, ring-opening products
were obtained via 3-aza-Grob fragmentation.10 In light
of straightforward reaction sequence consideration, we
explored this step with different reagents for conversion
of compound 13 to its imine form. After careful studies,
we found that MOM-protected dilactam 13 was success-
fully converted to benzyl DC-81 (14) in 50% yield (95%
on the basis of the recovered starting material), by
treating with LiBH4 (1 molar equiv) in THF at -10 °C
for 9 h. Further attempts to complete the reaction with
longer reaction time, more reagents, or higher temper-
ature would produce overreduction amine product. Fi-
nally, benzyl DC-81 (14) was converted to DC-81 (5) as
reported.6 The overall yield of the six-step synthesis of
(2) (a) Kohn, K. W. Anthramycin. In Antibiotics III Mechanism of
Action of Antimicrobial and Antitumor Agents; Corcoran, J . W., Hahn,
F. E., Eds.; Springer-Verlag: New York,1975; pp 3-11. (b) Petrusek,
R. L.; Uhlenhopp, E. L.; Duteau, N.; Hurley, L. H. J . Biol. Chem. 1982,
257, 6207.
(3) (a) Hurley, L. H.; Petrusek, R. L. Nature 1979, 282, 529. (b)
Cheatham, S.; Kook, A.; Hurley, L. H.; Barkley, M. D.; Remers, W. J .
Med. Chem. 1988, 31, 583. (c) Wang, J . J .; Hill, G. C.; Hurley, L. H. J .
Med. Chem. 1992, 35, 2995. (d) Mountzouris, J . A.; Wang, J . J .;
Thurston, D. E.; Hurley, L. H. J . Med. Chem. 1994, 37, 3132.
(4) Thurston, D. E.; Bose, D. S.; Thompson, A. S.; Howard, P. W.;
Leoni, A.; Croker, S. J .; J enkins, T. C.; Neidle, S.; Hartley, J . A.; Hurley,
L. H. J . Org. Chem. 1996, 61, 8141.
(5) (a) Kamal, A.; Laxman, E.; Reddy, P. S. M. M. Tetrahedron Lett.
2000, 41, 8631. (b) Wang, T.; Lui, A. S.; Cloudadale, I. S. Org. Lett.
1999, 1, 1835. (c) Thurston, D. E.; Bose, D. S.; Howard, P. W.; J enkins,
T. C.; Leoni, A.; Baraldi, P. G.; Guiotto, A.; Cacciari, B.; Kelland, L.
R.; Foloppe, M.; Rault, S. J . Med. Chem. 1999, 42, 1951. (d) Kraus, G.
A.; Selvakumar, N. Tetrahedron Lett 1999, 40, 2039. (e) Kraus, G. A.;
Melekhov, A. Tetrahedron 1998, 54, 11749. (f) O′Neil, I. A.; Thompson,
S.; Murray, C. L.; Kalindjian, S. B. Tetrahedron Lett. 1998, 39, 7787.
(g) Bose, D. S.; Srinivas, P.; Gurjar, M. K.; Tetrahedron Lett. 1997,
38, 5839. (h) Kamal, A.; Damayanthi, Y.; Reddy, B. S. N.; Lakmi-
narayana, B.; Reddy, B. S. P. Chem. Commun. 1997, 1015. (i) Kamal,
A.; Howard, P. W.; Reddy, B. S. N.; Reddy, B. S. P.; Thurston, D. E.
Tetrahedron 1997, 53, 3223. (j) Kamal, A.; Reddy, B. S. P.; Reddy, B.
S. N. Tetrahedron Lett. 1996, 37, 6803. (k) Kamal, A.; Rao, N. V. Chem.
Commun. 1996, 385. (l) Kamal, A.; Reddy, B. S. P.; Reddy, B. S. N.
Tetrahedron Lett. 1996, 37, 2281. (m) Eguchi, S.; Yamashita, K.;
Matsushita, Y.; Kakehi, A. J . Org. Chem. 1995, 60, 4006. (n) Molina,
P.; Diaz, I.; Tarraga, A. Tetrahedron 1995, 51, 5617. (o) Thurston, D.
E.; Bose, D. S. Chem. Rev. 1994, 94, 433 and references therein. (p)
Thurston, D. E. Advances in the Study of Pyrrolo[2,1-c][1,4]benzo-
diazepine (PBD) Antitumor Antibiotics. In Molecular Aspects of
Anticancer Drug-DNA Interactions; Neidle, S., Waring, M. J ., Eds.;
Macmillan Press: New York, 1993; Vol. 1, pp 54-88.
(7) Reed, J . N.; Snieckus, V. Tetrahedron Lett. 1984, 25, 5505.
(8) Mori, M.; Kimura, M.; Uozumi, Y.; Ban, Y. Tetrahedron Lett.
1985, 26, 5947.
(9) Langley, D. R.; Thurston, D. E J . Org. Chem. 1987, 52, 91.
(10) Wang, J . J .; Hu, W. P.; Chung, H. W.; Wang, L. F.; Hsu, M. H.
Tetrahedron 1998, 54, 13149.
(6) Thurston, D. E.; Murty, V. S.; Langley, D. R.; J ones, G. B.
Synthesis 1990, 81.
10.1021/jo010043d CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/20/2001

2882 J . Org. Chem., Vol. 66, No. 8, 2001
Sch em e 1. P ossible Mech a n ism of th e F or m a tion of P BD-DNA Ad d u ct 2
Notes
Sch em e 2. Syn th esis of DC-81 (5) via Cycliza tion of Am in o Dith ioa ceta l 9
Sch em e 3. Tota l Syn th esis of DC-81 (5)a
a
Yield in parentheses is based upon the recovered starting material.
2-Am in o-4-ben zyloxy-5-m eth oxyben zoic a cid (10). A so-
lution of substituted nitrobenzoic acid 8 (18.18 g, 60 mmol) and
SnCl2‚2 H2O (137 g, 0.6 mol) in MeOH (800 mL) was stirred at
70 °C for 5 h. The mixture was concentrated under vacuum to
a thick syrup; then ethyl acetate (300 mL) was added. The
organic phase was washed with water until it turned to a clear
solution, and then it washed with brine and dried over MgSO4.
After removal of solvent, the crude material was recrystallized
from ethyl acetate to afford a yellow solid. 10: yield 15.1 g (92%);
mp 159-161 °C; 1H NMR (CDCl3 + DMSO-d6, 400 MHz) δ 7.43-
7.29 (m, 6H), 6.22 (s, 1H), 5.12 (s, 2H), 3.80 (s, 3H); 13C NMR
(CDCl3 + DMSO-d6, 100 MHz) δ 169.6 (s), 153.5 (s), 146.9 (s),
140.3 (s), 136.1 (s), 128.2 (d), 127.6 (d), 126.8 (d), 114.0 (d), 102.4
(s), 100.8 (d), 70.0 (t), 56.3 (q); LRMS (EI, m/z) 273 (M+); HRMS
(EI, m/z) for C15H15NO4 calcd 273.1002, found 273.0995. Anal.
Calcd for C15H15NO4: C, 65.93; H, 5.53; N, 5.13. Found: C, 65.67;
H, 5.53; N, 4.89.
DC-81 is about 35% (67% yield based upon the recovered
starting material 13, at the fifth step). The other advan-
tage of this methodology is that the reaction can be
carried out at much larger scale (10 g) than previously
reported syntheses. Furthermore, in the first three steps,
the products were easily recrystallized and pure enough
for next subsequent reactions. The intermediate 11 can
serve as a versatile leaping point for further analogue
synthesis to establish the SAR of substituted prolidine
C ring.
In conclusion, we have described a practical and large-
scale total synthesis of antibiotic DC-81. The efficiency
and adaptability of the synthetic procedure detailed
above make possible the application of this methodology
to the preparation of other PBD analogues, such as
compound 3 and 4.
7-Ben zyloxy-6-m eth oxyisa toic a n h yd r id e (11). To a solu-
tion of compound 10 (20.1 g, 73.6 mmol) in THF (300 mL) was
added triphosgene (16.2 g, 51.5 mmol) in one portion. The
reaction mixture was refluxed for 3 h. After being cooled to room
temperature, the solution was poured into ice/water. The result-
ing precipitate was filtered and recrystallized from MeOH to give
Exp er im en ta l Section
Melting points are uncorrected. 1H NMR and 13C NMR spectra
were recorded at 400 and 100 MHz, respectively, using CDCl3
1
1
a white solid 11: yield 21.5 g (98%); mp 232-235 °C; H NMR
as a solvent. H NMR chemical shifts are referenced to TMS or
CDCl3 (7.26 ppm). 13C NMR was referenced to CDCl3 (77.0 ppm).
Multiplicities were determined by the DEPT sequence as s, d,
t, q. Mass spectra and high-resolution mass spectra (HRMS)
were measured using the electron-impact (EI, 70 eV) technique
by Taichung Regional Instrument Center of NSC at NCHU.
Elemental analyses were performed by Tainan Regional Instru-
ment Center of NSC at NCKU. Flash chromatography was
carried out on silica gel 60 (E. Merck, 230-400 mesh).
(CDCl3 + DMSO-d6, 400 MHz) δ 11.44 (s, NH), 7.47-7.33 (m,
6H), 6.74 (s, 1H), 5.21 (s, 2H), 3.89 (s, 3H); 13C NMR (CDCl3
+
DMSO-d6, 100 MHz) δ 159.5 (s), 156.0 (s), 147.9 (s), 146.4 (s),
137.6 (s), 135.3 (s), 128.6 (d), 128.3 (d), 127.6 (d), 109.2 (d), 101.7
(s), 99.4 (d), 70.9 (t), 56.2 (q); LRMS (EI, m/z) 299 (M+); HRMS
(EI, m/z) for C16H13NO5 calcd 299.0794, found 299.0792. Anal.
Calcd for C16H13NO5: C, 64.21; H, 4.38; N, 4.60. Found: C, 63.99;
H, 4.43; N, 4.50.

Notes
8-Ben zyloxy-7-m eth oxypyr r olo[2,1-c][1,4]ben zodiazepin e-
J . Org. Chem., Vol. 66, No. 8, 2001 2883
°C, and the reaction mixture was stirred at same temperature
for 8 h. The reaction mixture was poured into ice/water (100
mL) and extracted four times with ethyl acetate. The combined
organic phases were washed with saturated NaHCO3, H2O, and
brine and dried over MgSO4. After removal of solvent, the
residue was purified by flash chromatography (CH2Cl2/MeOH
) 70:1) to give a light yellow solid product 14 (yield 3.89 g, 50%)
and compound 13 (4.77 g, 95% yield based upon the recovered
starting material): mp 58-61 °C; 1H NMR (CDCl3, 400 MHz) δ
7.47-7.29 (m, 6H), 7.18 (s, 1H), 5.38 (d, J ) 9.8 Hz, 1H), 5.21
(s, 2H), 4.45 (d, J ) 9.8 Hz, 1H), 4.12-4.07 (m, 1H), 3.95 (s,
3H), 3.75-3.54 (m, 2H), 3.40 (m, 3H), 2.70 (t, J ) 2.0 Hz, 1H),
2.06-1.95 (m, 4H); 13C NMR (CDCl3, 100 MHz) δ 164.6 (s), 162.4
(d), 150.4 (s), 148.0 (s), 140.5 (s), 136.2 (s), 128.6 (d), 128.0 (d),
127.3 (d), 120.5 (s), 111.7 (d), 111.3 (d), 70.8 (t), 56.1 (q), 53.6-
(d), 46.6 (t), 29.6 (t) 24.1 (t); LRMS (EI, m/z) 336 (M+); HRMS
(EI, m/z) for C20H20N2O3 calcd 336.1475, found 336.1473.
5,11-d ion e (12). The mixture of anhydride 11 (15.9 g, 53.2
mmol) and L-proline (8.2 g, 68.4 mmol) in DMSO (300 mL) was
heated at 120 °C for 4 h. After being cooled to room temperature,
the solution was poured into water (600 mL) and kept in freezer
for 6 h. The resulting precipitate was filtered and recrystallized
from MeOH to give a white solid. 12: yield 18.0 g (96%); mp
190-193 °C; 1H NMR (CDCl3, 400 MHz) δ 8.07 (br s, NH), 7.43-
7.33 (m, 5H), 7.27 (s, 1H), 6.45 (s, 1H), 5.17 (s, 2H), 4.30 (d, 3.2
Hz, 1H), 3.93 (s, 3H), 3.78-3.75 (m, 1H), 3.63-3.56 (m, 1H),
2.74-2.70 (m, 1H), 2.05-1.97 (m, 3H); 13C NMR (CDCl3, 100
MHz) δ 170.8 (s), 165.2 (s), 151.4 (s), 147.0 (s), 135.9(s), 129.2
(s), 128.8 (d), 128.3 (d), 127.2 (d), 119.8 (s), 112.6 (d), 106.5 (d),
71.1 (t), 56.8 (d),. 56.2 (q), 47.3 (t), 26.2 (t) 23.5 (t); LRMS (EI,
m/z) 352 (M+); HRMS (EI, m/z) for C20H20N2O4 calcd 352.1424,
found 352.1429. Anal. Calcd for C20H20N2O4: C, 68.17; H, 5.72;
N, 7.95. Found: C, 68.01; H, 5.84; N, 7.80.
N-(10-Meth oxym eth yl)-8-ben zyloxy-7-m eth oxyp yr r olo-
[2,1-c][1,4]ben zod ia zep in e-5,11-d ion e (13). To a stirred solu-
tion of aromatic lactam 12 (12 g, 34.1 mmol) in THF (150 mL)
was added NaH (3.4 g, 85.3 mmol) under nitrogen at 0 °C, and
the reaction mixture was stirred at the same temperature for
30 min. MOMCl (6.4 mL, 77.6 mmol) was added dropwise into
the reaction mixture. The resulting solution was stirred at room
temperature for 24 h. The reaction mixture was poured into ice/
water (150 mL) and extracted four times with ethyl acetate. The
combined organic phases were washed with saturated NaHCO3,
H2O, and brine, and dried over MgSO4. After removal of solvent,
the residue was purified by flash chromatography (CH2Cl2/
MeOH ) 40:1) to give a yellow oil product. 13: yield 12.2 g (90%);
1H NMR (CDCl3, 400 MHz) δ 7.45-7.29 (m, 6H), 7.18 (s, 1H),
5.37 (d, J ) 10 Hz, 1H), 5.21 (s, 2H), 4.44 (d, J ) 10 Hz, 1H),
4.09 (dd, J ) 8, 2.4 Hz, 1H), 3.95 (s, 3H), 3.77-3.72 (m, 1H),
3.59∼3.52 (m, 1H), 3.40 (s, 3H), 2.72-2.68 (m, 1H), 2.11-1.94
(m, 3H); 13C NMR (CDCl3, 100 MHz) δ 170.3 (s), 165.1 (s), 150.7
(s), 147.5 (s), 136.0 (s), 133.7 (s), 128.6 (d), 128.1 (d), 127.4 (d),
122.3 (s), 111.5 (d), 107.1 (d), 79.7 (t), 71.0 (t), 57.5 (d), 56.8 (q),
56.2 (q), 46.7 (t), 26.5 (t) 23.7 (t); LRMS (EI, m/z) 396 (M+);
HRMS (EI, m/z) for C22H24N2O5 calcd 396.1686, found 396.1688.
Anal. Calcd for C22H24N2O5: C, 66.65; H, 6.10; N, 7.07. Found:
C, 66.42; H, 6.20; N, 6.85.
8-Hyd r oxy-7-m eth oxyp yr r olo[2,1-c][1,4]ben zod ia zep in -
5-on e (5, DC-81). To a solution of compound 14 (5.93 g, 17.64
mmol) in absolute EtOH (130 mL) was added 10% Pd/C (8.8 g)
under nitrogen.6 1,4-Cyclohexadiene (17 mL, 176 mmol) was
added to the solution dropwise. The resulting solution was
stirred at room temperature for 2.5 h until TLC (reversed-phase
C18: H2O/MeOH ) 1:3) indicated that the reaction was complete.
The reaction mixture was filtered through Celite. Purification
of the residue by flash chromatography (CH2Cl2/MeOH ) 40:1)
gave a colorless solid product. 5: yield 3.91 g (90%); mp 135-
1
138 °C; H NMR (CDCl3, 400 MHz) δ 7.67 (d, J ) 4.4 Hz, 1H),
7.52 (s, 1H), 6.89 (s, 1H), 6.41 (br s, -OH), 3.96 (s, 3H), 3.85-
3.79 (m, 1H), 3.74-3.70 (m, 1H), 3.61-3.54 (m, 1H), 2.35-2.29
(m, 2H), 2.09-2.01 (m, 2H); 13C NMR (CDCl3, 100 MHz) δ164.8
(s), 162.6 (d), 148.5 (s), 145.5 (s), 141.2 (s), 120.0 (s), 112.7 (d),
111.2 (d), 56.3 (q), 53.7 (d), 26.6 (t) 24.2 (t); LRMS (EI, m/z) 246
(M+); HRMS (EI, m/z) for C13H14N2O3 calcd 246.1005, found
246.0996.
Ack n ow led gm en t. We thank the National Science
Council of the Republic of China for financial support.
Su p p or tin g In for m a tion Ava ila ble: Proton and carbon
spectra for compounds 10-14 and 5. This material is available
free of charge via the Internet at http://pubs.acs.org.
8-Ben zyloxy-7-m eth oxypyr r olo[2,1-c][1,4]ben zodiazepin -
5-on e (14, 8-Ben zyl DC-81). To a solution of MOM-protected
aromatic lactam 13 (9.6 g, 23.2 mmol) in THF (50 mL) was added
lithium borohydride (516 mg, 23.2 mmol) in one portion at -10
J O010043D