One-Step Preparation of Pyrrolo[1,4]benzodiazepine Dilactams: Total Synthesis of Oxoprothracarcin, Boseongazepines B and C

Article abstract of DOI:10.1055/s-0034-1378877

A one-step synthesis of pyrrolo[1,4]benzodiazepine dilactams has been developed. The high yielding method involves direct coupling of unprotected anthranilic acids with proline esters. This transformation was successfully applied in the first total syntheses of boseongazepines B and C as well as oxoprothracarcin and limazepine E.

Full text of DOI:10.1055/s-0034-1378877

SYNLETT
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
© Georg Thieme Verlag Stuttgart · New York
2015, 26, 2272–2276
letter
2272
G. Smits, R. Zemribo
Letter
Synlett
One-Step Preparation of Pyrrolo[1,4]benzodiazepine Dilactams:
Total Synthesis of Oxoprothracarcin, Boseongazepines B and C
Gints Smits
O
O
H
MeO
+
N
Ronalds Zemribo*
H
H
R1
NH2
OH
BOP reagent
R2
R2
Latvian Institute of Organic Synthesis,
Aizkraukles 21, Riga 1006, Latvia
ronalds@osi.lv
R1
HN
N
HOBt, Et3N, DMF
O
O
10 examples
62–96% yield
R = Br, NO2, CF3, OMe, OH
Received: 07.05.2015
Accepted after revision: 03.07.2015
Published online: 01.09.2015
OH
Me
10
N
OMe
H
H
11
9
8
N
9a
5a
H
1
11a
DOI: 10.1055/s-0034-1378877; Art ID: st-2015-d0345-l
B
5
A
7
C
N
N
2
NH2
6
4
Abstract A one-step synthesis of pyrrolo[1,4]benzodiazepine dilact-
ams has been developed. The high yielding method involves direct cou-
pling of unprotected anthranilic acids with proline esters. This transfor-
mation was successfully applied in the first total syntheses of
boseongazepines B and C as well as oxoprothracarcin and limazepine E.
3
O
O
O
1
anthramycin (2)
N
N
O
(CH2)3
O
H
H
Key words pyrrolo[1,4]benzodiazepine, dilactams, total synthesis,
oxoprothracarcin, boseongazepine
N
N
OMe
MeO
O
O
SJG-136
Figure 1 Representative PBD structures
Pyrrolo[1,4]benzodiazepines (PBD’s) 1 are naturally oc-
curring tricyclic antitumor antibiotics, possessing an S-con-
figuration chiral center at C11a, resulting in right-handed
twist of the molecule (Figure 1). The first PBD class mem-
ber, anthramycin (2)1 was isolated in 1968 by Leimgruber et
al. and immediately attracted the attention of scientific
community due to its interesting biological properties. It
was found that PBD’s covalently bind to C-2 amino group of
guanine residues within the minor groove of DNA.2
Nowdays a significant amount of information3 (over 200
publications and 40 patents) on PBD’s can be found in the
scientific literature. The reports include the isolation of
new natural products, novel synthetic pathways, as well as
new applications of PBD’s. Furthermore, one PBD class
member, SJG-136 is in Phase II clinical trials for treating
ovarian carcinoma.4
growing interest in PBD dilactams themselves, since they
have shown interesting biological activity towards several
different targets.5
The published protocols for the synthesis of the PBD di-
lactam skeleton usually consist of several synthetic steps.3a
Typically, the last step of PBD dilactam 3 synthesis is the B-
ring formation of prefunctionalized substrates (Scheme 1).
Classical methods include a reductive cyclization of azi-
doesters 4 or nitroesters 5, or a condensation of proline es-
ters 7 with isatoic anhydrides 6. Furthermore, PBD dilact-
ams can also be obtained via transition-metal-catalyzed
processes (8 and 9).
The number of synthetic steps as well as the harsh con-
ditions and reagents used in published protocols encour-
aged us to develop an alternative protocol for the synthesis
of PBD dilactams.
PBD dilactams, containing the oxidized N10-C11 amidic
moiety have become popular due to their robustness to-
wards a number of synthetic transformations and nowa-
days several protocols for conversion of the N10-C11 amide
functionality to the DNA-alkylating imine function are
known.3d Furthermore, in the past decade there has been a
In our previous studies6 we found that PBD dilactam 12
could be readily synthesized in a two-step sequence by first
cleaving the nitrogen protecting group of proline 10, fol-
lowed by coupling the intermediate proline ester hydro-
chloride to the corresponding unprotected anthranilic acid
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2272–2276

2273
G. Smits, R. Zemribo
Letter
Synlett
N3
O
1) ACE-Cl, CH2Cl2
NO2
CO2R3
then MeOH reflux
CO2R3
R1
2)
R1
OH
R2
R2
N
N
MeO
11
NH2
5
4
O
OH
CO2H
O
H
N
MeO
H
O
H
N
H
N
H
CO2R3
BOP, HOBt
Et3N, DMF
O
H
MeO2C
R1
N
R1
N
+
R2
NH
R2
64% yield in 2 steps
HN
N
PMB
10
12
O
O
O
3
Scheme 2 Synthesis of dilactam 12; ACE-Cl = 1-chloroethyl chlorofor-
mate; BOP = (benzotriazol-1-yloxy)tris(dimethylamino)phosphonium
hexafluorophosphate; HOBt = 1-hydroxybenzotriazole hydrate
6
7
OMe
OTf
HN
O
CO2R3
R1
R1
11 (Scheme 2). These noteworthy results directed us to
study the scope and limitations of this novel method for the
synthesis of PBD dilactams. Herein we report an efficient
method for the synthesis of PBD dilactams in a single step
from unprotected anthranilic acids and proline esters.
A number of commercially available anthranilic acids
and proline esters were examined for the synthesis of the
corresponding PBD dilactams, and the results are shown in
Table 1.
R2
R2
N
N
O
O
8
9
Scheme 1 Retrosynthetic analysis of PBD dilactams3a
Table 1 Synthesis of PBD Dilactams
Entry
Anthranilic acid
Proline derivative
Product
Yield (%)
71
O
H
N
H
NH2
1
HO2C
N
N
N
N
H
CO2H
O
13
13
14
O
H
N
H
NH2
2
3
4
92a
61a
68
MeO2C
N
+
Cl–
H2
CO2H
O
O
H
OH
N
H
NH2
MeO2C
MeO2C
N
CO2H
+
OH
Cl–
H2
O
O
H
N
H
NH2
N
H2
N
HO
+
Cl–
HO
CO2H
O
15
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2272–2276

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G. Smits, R. Zemribo
Letter
Synlett
Table 1 (continued)
Entry
Anthranilic acid
Proline derivative
Product
Yield (%)
Br
O
H
Br
N
H
NH2
5
96a
MeO2C
N
+
N
Cl–
H2
CO2H
O
16
O
H
N
F3C
H
F3C
NH2
6
7
8
96a
MeO2C
N
N
+
Cl–
H2
CO2H
O
17
O
H
N
H
NH2
92a
MeO2C
N
H2
N
O2N
+
Cl–
O2N
CO2H
O
18
O
H
N
H
NH2
73a,b
N
MeO2C
CO2H
N
H2
+
Cl–
O
19
OMe
O
H
OMe
N
H
NH2
9
62a,b
N
MeO2C
N
H2
+
CO2H
Cl–
O
20
OH
O
H
OH
N
MeO
H
MeO
NH2
10
64b
N
MeO2C
N
H2
+
CO2H
Cl–
O
12
a Cat. HCl in THF–H2O was necessary to effect the cyclization.
b Yield in three steps, including the PMB protecting group cleavage of the proline nitrogen using ACE-Cl.7
After optimization of the reaction conditions initially
found, the best results were obtained using BOP as the am-
ide coupling reagent in combination with HOBt·H2O and
Et3N in DMF.8 The transformation generally proceeds with
good yields of up to 96% (Table 1, entries 5 and 6) and a
number of functional groups are tolerated; even unprotect-
ed hydroxyl groups (Table 1, entry 3) and phenol groups
(Table 1, entries 4 and 10). Unprotected proline also could
serve as a coupling partner (Table 1, entry 1). However
higher yields were obtained by using a proline ester (Table
1, entry 2). In contrast, the reported methods for the syn-
thesis of these PBD dilactams often contain multistep syn-
thesis (six steps in case of 159) or harsh reaction conditions
(several hours at 120 °C–150 °C in DMSO in case of 13,10
1411 and 1812). The transformation was also applicable to
the total synthesis of two natural products, namely,
boseongazepine B13 (20) and oxoprothracarcin14 (19) as
well as the key intermediate 12 used in the total synthesis
of limazepine E.6 Oxoprothracarcin (19) was selectively
transformed into boseongazepine C (23) by reduction of the
N10-C11 amide group (Scheme 3).
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2272–2276

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G. Smits, R. Zemribo
Letter
Synlett
References and Notes
O
H
H
N
N
H
H
(1) Leimgruber, W.; Batcho, A. D.; Czajkowski, R. C. J. Am. Chem. Soc.
1968, 90, 5641.
(2) Thurston, D. E. In Molecular Aspects of Anticancer Drug–DNA
Interactions; Vol. 1; Neidle, S.; Waring, M. J., Eds.; The Macmil-
lan Press Ltd: London, 1993, 54.
NaBH4, TFA
N
N
THF, 14%
O
O
oxoprothracarcin (19)
boseongazepine C (23)
(3) For reviews, see: (a) Antonow, D.; Thurston, D. E. Chem. Rev.
2011, 111, 2815. (b) Cipolla, L.; Araujo, A. C.; Airoldi, C.; Bini, D.
Anti-Cancer Agents Med. Chem. 2009, 9, 1. (c) Kamal, A.; Rao, M.
V.; Laxman, N.; Ramesh, G.; Reddy, G. S. K. Curr. Med. Chem.:
Anti-Cancer Agents 2002, 2, 215. (d) Thurston, D. E.; Bose, D. S.
Chem. Rev. 1994, 94, 433.
NaH, SEMCl
DMF, 79%
NaBH4, MeOH
27% in 2 steps
SEM
O
NaBH4, MeOH, then
RP prep. LC/MS
N
N
H
H
(4) See: https://clinicaltrials.gov/show/NCT01200797.
N
N
(5) (a) Jiao, R. H.; Xu, H.; Cui, J. T.; Ge, H. M.; Tan, R. X. J. Appl. Micro-
biol. 2013, 114, 1046. (b) Antonow, D.; Jenkins, T. C.; Howard, P.
W.; Thurston, D. E. Bioorg. Med. Chem. 2007, 15, 3041.
(c) Kamal, A.; Reddy, K. L.; Devaiah, V.; Shankaraiah, N.; Reddy,
G. S. K.; Raghavan, S. J. Comb. Chem. 2007, 9, 29. (d) Biel, M.;
Deck, P.; Giannis, A.; Waldmann, H. Chem. Eur. J. 2006, 12, 4121.
(6) Smits, G.; Zemribo, R. Org. Lett. 2013, 15, 4406.
O
O
21
prothracarcin (22)
Scheme 3 Total synthesis of boseongazepine C
The reported method15 for selective reduction of the
N10-C11 amide group gave very low yields of the desired
boseongazepine C (23) and the reaction was also not repro-
ducible. A reduction of the double bond was also observed
using these conditions. To increase the yield of the reduc-
tion we applied the described3d concept by introducing an
N10 nitrogen protecting group to lower the electron densi-
ty on nitrogen N10, thereby increasing the electrophilicity
of the C11-carbonyl. Although the N10 protecting group
was successfully installed, the subsequent reduction se-
quence was again low yielding. Nevertheless, the spectro-
scopic data of boseongazepine C (23) obtained by either re-
duction method were in a good agreement with the litera-
ture data.
In summary, we have developed a convenient and high
yielding method for the synthesis of pyrrolo[1,4]benzodiaz-
epine dilactams in one step from unprotected anthranilic
acids and proline ester derivatives. Notably, a number of
functional groups are tolerated, even unprotected hydroxyl
and phenol groups. The broad number of commercially
available proline esters and anthranilic acids makes this
method a valuable tool for the synthesis of large PBD dilact-
am libraries in short time.
(7) Yang, B. V.; O’Rourke, D.; Li, J. Synlett 1993, 195.
(8) General Procedure for Synthesis of PBD Dilactams: To a
stirred solution of anthranilic acid (2.2 mmol, 2 equiv), BOP
reagent (2.2 mmol, 2 equiv) and HOBt hydrate (2.2 mmol, 2
equiv) in anhyd DMF (10 mL) was added Et3N (10.9 mmol, 10
equiv). After stirring for 15 min, proline ester hydrochloride
(1.1 mmol, 1 equiv) was added and the resultant mixture was
stirred for 16 h. The volatiles were removed in vacuo and the
residue was partitioned between CH2Cl2 and brine. The organic
phase was dried over anhyd Na2SO4, filtered and concentrated
in vacuo. The residue was purified by flash column chromatog-
raphy (EtOAc–MeOH) or reversed-phase flash column chroma-
tography (MeOH + 0.5% HCOOH–H2O + 0.5% HCOOH).
(S)-9-Bromo-2,3-dihydro-1H-benzo[e]pyrrolo[1,2-a][1,4]-
diazepine-5,11(10H,11aH)-dione (23): colorless solid; [α]D
+423 (c = 0.1, MeOH). 1H NMR (300 MHz, CDCl3): δ = 7.95 (d, J =
7.9 Hz, 1 H), 7.69–7.81 (m, 2 H), 7.14 (t, J = 7.9 Hz, 1 H), 4.05 (d, J
= 6.2 Hz, 1 H), 3.76–3.91 (m, 1 H), 3.50–3.70 (m, 1 H), 2.67–2.86
(m, 1 H), 1.92–2.18 (m, 3 H). 13C NMR (100 MHz, CDCl3): δ =
170.0, 164.4, 136.0, 133.1, 130.9, 129.2, 126.0, 115.5, 56.8, 47.5,
26.4, 23.5. HRMS–ESI: m/z [M + H] calcd for C12H12N2O2Br:
295.0082; found: 295.0090.
(S)-8-(Trifluoromethyl)-2,3-dihydro-1H-benzo[e]pyrrolo-
[1,2-a][1,4]diazepine-5,11(10H,11aH)-dione (24): yellowish
solid; Lit. 9: [α]D +41 (c = 0.1, MeOH). 1H NMR (400 MHz,
DMSO): δ = 7.93 (dd, J = 7.9, 1.5 Hz, 1 H), 7.76 (br s, 1 H), 7.73
(dd, J = 7.9, 1.5 Hz, 1 H), 7.12 (t, J = 7.9 Hz, 1 H), 4.03 (d, J = 6.0
Hz, 1 H), 3.72–3.88 (m, 1 H), 3.52–3.69 (m, 1 H), 2.66–2.85 (m, 1
H), 1.95–2.14 (m, 3 H). 13C NMR (100 MHz, DMSO): δ = 170.7,
163.4, 137.1, 131.9, 131.9 (q, J = 32.2 Hz), 129. 8, 123.5 (q, J =
272.8 Hz), 119.9, 118.1, 56.1, 47.0, 25.7, 23.0. HRMS–ESI: m/z
[M + H] calcd for C13H12N2O2F3: 285.0851; found: 285.0855.
(S,E)-2-Ethylidene-10-{[2-(trimethylsilyl)ethoxy]methyl}-
2,3-dihydro-1H-benzo[e]pyrrolo[1,2-a][1,4]diazepine-
5,11(10H,11aH)-dione (21): To a stirred solution of oxoprothra-
carcin (140 mg, 0.6 mmol, 1 equiv) in anhyd DMF (1.5 mL)
under an argon atmosphere was added NaH (80% in mineral oil,
21 mg, 0.7 mmol, 1.2 equiv) at 0 °C. After stirring for 30 min
SEM-Cl (118 mg, 0.7, 1.2 equiv) was added and the mixture was
allowed to warm to r.t. overnight and then partitioned between
CH2Cl2 and brine. The organic phase was dried over anhyd
Na2SO4, filtered and concentrated in vacuo. The residue was
Acknowledgment
We thank the European Social Fund (ESF) for financial support (proj-
ect No. 1DP/1.1.1.2.0/13/APIA/VIAA/003).
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0034-1378877.
S
u
p
p
ortioInfgrmoaitn
S
u
p
p
ortiInfogrmoaitn
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2272–2276

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G. Smits, R. Zemribo
Letter
Synlett
purified by flash column chromatography (EtOAc–PE,
9:1 → 1:1). The title compound was isolated as a yellowish oil
(170 mg, 79%); [α]D +333.00 (c = 0.1, CHCl3). 1H NMR (300 MHz,
CDCl3): δ = 7.90 (dd, J = 7.8, 1.6 Hz, 1 H), 7.69 (dd, J = 7.8, 1.6 Hz,
1 H), 7.48–7.57 (td, J = 7.8, 1.6 Hz, 1 H), 7.34 (td, J = 7.8, 1.6 Hz, 1
H), 5.46–5.60 (m, 2 H), 4.72 (d, J = 9.8 Hz, 1 H), 4.24–4.40 (m, 2
H), 4.17 (d, J = 15.7 Hz, 1 H), 3.59–3.82 (m, 2 H), 3.48 (d, J = 16.3
Hz, 1 H), 2.55–2.70 (m, 1 H), 1.74 (d, J = 6.6 Hz, 3 H), 0.99 (t, J =
8.6 Hz, 2 H), 0.02 (s, 9 H). 13C NMR (100 MHz, CDCl3): δ = 170.0,
165.3, 139.9, 133.1, 132.4, 130.0, 129.5, 126.4, 122.7, 118.5,
78.1, 67.1, 57.6, 51.1, 28.2, 18.4, 14.6, –1.3. HRMS–ESI: m/z [M +
Na] calcd for C20H28N2O3SiNa: 395.1767; found: 395.1758.
Boseongazepine C: To a stirred solution of 21 (150 mg, 0.4
mmol, 1 equiv) in MeOH (3 mL) was added NaBH4 (46 mg, 1.2
mmol, 3 equiv) at 0 °C. The mixture was then stirred at r.t. and
then an additional 2 equiv of NaBH4 were added. The process of
addition (2 equiv of NaBH4) was repeated three times to achieve
full consumption of 21. The reaction mixture was then parti-
tioned between CH2Cl2 and brine. The organic phase was sepa-
rated and the aq phase was extracted with CH2Cl2 (2 ×). The
combined organic extracts were dried over anhyd Na2SO4, fil-
tered and concentrated in vacuo. The residue was purified by
preparative LC–MS (MeOH + 0.5% HCOOH–H2O + 0.5% HCOOH).
The collected prothracarcin was used in the next step without
further purification. The obtained material was dissolved in a
mixture of MeOH (1 mL) and CH2Cl2 (0.3 mL) and treated with
NaBH4 (46 mg, 1.208 mmol, 3 equiv). The process was repeated
several times to achieve full consumption of the prothracarcin.
The reaction mixture was then partitioned between CH2Cl2 and
brine. The organic phase was separated and the aq. Phase was
extracted with CH2Cl2 (2 ×). The combined organic extracts
were dried over anhyd Na2SO4, filtered and concentrated in
vacuo. The residue was purified by flash column chromatogra-
phy (EtOAc–PE, 1:1 → 1:0). The title compound was isolated as a
yellowish solid (25 mg, 27%). See the Supporting Information
for compound characterization.
(9) Madani, H.; Thompson, A. S.; Threadgill, M. D. Tetrahedron 2002,
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2272–2276