Synthesis of C-8 alkylamino substituted pyrrolo[2,1-c][1,4]benzodiazepines as potential anti-cancer agents

Article abstract of DOI:10.1016/S0960-894X(02)00324-4

The design and facile synthesis of C-8 alkylamino substituted pyrrolo[2,1-c][1,4]benzodiazepines is described. These have been prepared by linking the amines at C-8 position with propane spacer to improve solubility in water, and their in vitro cytotoxici

Full text of DOI:10.1016/S0960-894X(02)00324-4

Bioorganic & Medicinal Chemistry Letters 12 (2002) 1917–1919
Synthesis of C-8 Alkylamino Substituted Pyrrolo[2,1-c][1,4]-
benzodiazepines as Potential Anti-Cancer Agents
Ahmed Kamal,* N. Laxman, G. Ramesh, O. Srinivas and P. Ramulu
Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 27 March 2002; accepted 6 May 2002
Abstract—The design and facile synthesis of C-8 alkylamino substituted pyrrolo[2,1-c][1,4]benzodiazepines is described. These have
been prepared by linking the amines at C-8 position with propane spacer to improve solubility in water, and their in vitro cyto-
toxicity studies have been carried out. # 2002 Elsevier Science Ltd. All rights reserved.
Natural and synthetic pyrrolo[2,1-c][1,4]benzodiaze-
pines (PBDs) are well known for their ability to bind
covalently in the minor groove of DNA. Recently, there
is much impetus for the PBD ring systems as they can
recognize and bind to specific sequence of DNA.1À5
Synthetic derivatives possessing PBD ring system are
generating interest as potential anti-cancer and gene
targeting agents due to their sequence specificity when
binding to duplex DNA,6À8 examples of naturally
occurring PBDs include anthramycin, tomaymycin,
sibiromycin and DC-81. The cytotoxicity and anti-
tumour activity of these compounds are attributed to
their property of covalent binding to the N-2 of guanine
in the minor groove of duplex DNA via acid-labile
aminal bond to the electrophilic imine at the N-10–C-11
position in a sequence selective manner. It has been well
established that the imine functionality or its equivalent
carbinolamine form is a primary requirement for the
covalent binding.9,10
also towards the development of new synthetic
strategies.13À19 In continuation of these efforts, we
report here the synthesis of PBD ring system linked to
alkyl amines by a propane spacer at C-8 position with a
view to enhance their lipophilicity.
Recently a large number of structurally modified PBDs
have been prepared and evaluated for their biological
activity, particularly their anti-tumour potential.11,12
A
The compounds (3a–c) were prepared from vanillic acid
methyl ester 4 by its etherification with dibromo pro-
pane to afford 5. The mono alkylation of 5 has been
achieved by using 3 molar equivalents of the dibromo
propane. Nitration of 5 followed by ester hydrolysis and
coupling 2S-pyrrolidine methyl ester hydrochloride gives
8, which is reduced with DIBAL-H to give correspond-
ing aldehyde 9. The aldehyde is protected with diethyl-
thioacetal by using TMSCl–EtSH to afford 10. The
compound 10 is coupled with morpholine, N-methyl-
piperzine and N,N-diethyl amine to give respectively the
desired intermediates 11a–c. The nitro thioacetal has
number of these compounds have been selected for pre-
clinical studies but unfortunately most of them did not
proceed beyond, due to problems related to water solu-
bility. Therefore, it has been envisaged in the present
work that incorporation of an amino functionality
linked to the PBD ring system could improve its water
solubility profile. We have been interested in the
structural modifications of the PBD ring system and
*Corresponding author. Tel.: +91-40-719-3157; fax: +91-40-716-
0512; e-mail: ahmedkamal@iict.ap.nic.in
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00324-4

1918
A. Kamal et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1917–1919
Scheme 1. (a) Br(CH2)3Br, K2CO3, CH3COCH3, reflux, 48 h, 82%; (b) SnCl4–HNO3, CH2Cl2, À25 ꢀC, 5 min, 88%; (c) 1 M LiOH, THF–MeOH–
H2O (3:1:1), 12 h, rt, 90%; (d) SOCl2, l-proline methyl ester hydrochloride, Et3N, H2O, 0 ꢀC, 3 h, 80%; (e) DIBAL-H, CH2Cl2, À78 ꢀC, 45 min,
65%; (f) EtSH–TMSCl, CHCl3, rt 18 h, 82%; (g) morpholine or N-methyl piperazine or N,N-diethyl amine, K2CO3, CH3CN, reflux, 48 h, 70–72%;
.
(h) SnCl2 2H2O, MeOH, reflux, 40 min, 70–75%; (i) HgCl2, CaCO3, CH3CN–H2O (4:1), 10 h, 50–52%.
.
been reduced with SnCl2 2H2O in methanol to give 12a–c.
Table 1. Log LC50 (concentration in mol/L causing 50% lethality)
values for compounds 3a–ca in different cancer cell lines
Deprotection of the thioacetal group by using the
method of Thurston and co-workers afforded the target
molecules (3a–c) (Scheme 1).20
Cancer
LC50 values
3a
3b
3c
A series of PBD analogues (3a–c) were tested at the
National Cancer Institute (Bethesda, MD, USA) using
Leukaemia
Non-small-cell lung
Colon
À4.00
À4.02
À4.18
À4.02
À4.06
À4.00
À4.01
À4.00
À4.01
À4.28
À4.17
À4.26
À4.19
À4.51
À4.21
À4.25
À4.00
À4.19
À4.00
À4.00
À4.06
À4.00
À4.03
À4.00
À4.00
À4.00
À4.00
a
Developmental Therapeutics Program involving
CNS
tumour cell line cytotoxicity. From these screenings
GI50 and LC50 values were obtained. The GI50 value is a
response parameter and represents the concentration of
the compound that induces 50% cell line growth inhi-
bition. The LC50 value represents the compound con-
centration in mol/L causing 50% lethality (Table 1).
Each cancer type represents the arithmetic average of
six to eight different cancer cell lines. These compounds
show moderate cytotoxic activity against some cancer
cell lines. It is observed from the cytotoxicity data that
the N-methylpiperizine linked to the PBD moiety (3b)
has promising activity in comparison to the other alkyl
amines like morpholine and N,N-diethyl amine and
interestingly this compound is more potent for the mel-
anoma cell lines.
Melanoma
Ovarian
Renal
Prostate
Breast
aEach cancer type represents the average of six to eight different cancer
cell lines. For each histolasic cancer type, the average Àlog LC50 value
was determined from an NCI panel consisting of six to eight human
cancer cell lines. The lower log LC50 values show the increase of cyto-
toxicity.
In conclusion, new C-8 alkylamino linked PBDs have
been synthesized which exhibit cytotoxic activity in
some cancer cell lines. Moreover, these compounds with
improved lyophilicity are promising for the develop-

A. Kamal et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1917–1919
1919
ment of new cytotoxic agents and for which detailed
investigations are in progress.
9. Thurston, D. E.; Bose, D. S. Chem. Rev. 1994, 94, 433.
10. Kamal, A.; Rao, M. V.; Laxman, N.; Ramesh, G.; Reddy,
G. S. K. Curr. Med. Chem. Anti-cancer Agents 2002, 2, 215.
11. Gregson, S. J.; Howard, P. W.; Corcoran, K. E.; Jenkins,
T. C.; Kelland, L. R.; Thurston, D. E. Bioorg. Med. Chem.
Lett. 2001, 11, 2859.
Acknowledgements
12. Langlois, N.; Rousseau, A. R.; Gaspard, C.; Werner,
G. H.; Darro, F.; Kiss, R. J. Med. Chem. 2001, 44, 3754.
13. Kamal, A.; Laxman, E.; Laxman, N.; Rao, N. V. Bioorg.
Med. Chem. Lett. 2000, 10, 2311.
14. Kamal, A.; Laxman, E.; Arifuddin, M. Tetrahedron Lett.
2000, 41, 7743.
We thank the National Cancer Institute, Maryland for
the anticancer assay in human cell lines. The authors
(N.L., G.R., O.S., P.R.) are grateful to CSIR (New
Delhi) for the award of the Research Fellowships.
15. Kamal, A.; Laxman, E.; Reddy, P. S. M. M. Tetrahedron
Lett. 2000, 41, 8631.
16. Kamal, A.; Laxman, E.; Reddy, P. S. M. M. Synlett 2000,
10, 1476.
References and Notes
1. Thurston, D. E. In Molecular Aspects of Anticancer Drug-
DNAInteractions ; Macmillan: London, UK, 1993; Vol. 1,
p54.
2. Hurley, L. H.; Needham-Van Devanter, D. R. Acc. Chem.
Res. 1986, 19, 230.
3. Hurley, L. H. J. Med. Chem. 1989, 32, 2027.
4. Hartley, J. A.; Souhami, R. L. In Cancer Chemotherapy.
Frontiers in Pharmacology and Therapeutics; Hickman, J. A.,
Tritton, T., Eds.; Blackwell Scientific: Oxford, 1993; p251.
5. Mountzouris, J. A.; Hurley, L. H. In Advances in DNA
Sequence Selective Agents; Padwa, A. Ed.; JAI: London, 1992;
Vol. 1, p263.
6. Thurston, D. E. Br. J. Cancer 1999, 80, 65.
7. Kamal, A.; Laxman, N.; Ramesh, G.; Neelima, K.; Kon-
dapi, A. K. Chem. Commun. 2001, 437.
17. Kamal, A.; Reddy, G. S. K.; Raghavan, S. Bioorg. Med.
Chem. Lett. 2001, 11, 387.
18. Kamal, A.; Reddy, G. S. K.; Reddy, K. L. Tetrahedron
Lett. 2001, 42, 6969.
19. Kamal, A.; Reddy, G. S. K.; Reddy, K. L.; Raghavan, S.
Tetrahedron. Lett. 2002, 43, 2103.
20. Selected data for compound 3a: 1H NMR (200 MHz,
CDCl3) d 1.95–2.40 (m, 6H), 2.45–2.60 (m, 6H), 3.50–3.90 (m,
7H), 3.95 (s, 3H), 4.05–4.20 (m, 2H), 6.80 (s, 1H), 7.48 (s, 1H),
7.65 (d, 1H); MS (EI) m/z 373. Compound 3b: 1H NMR
(200 MHz, CDCl3) d 1.75–2.15 (m, 6H), 2.35 (s, 3H), 2.45–2.75
(m, 10H) 3.50–3.90 (m, 3H), 3.98 (s, 3H), 4.05–4.20 (m, 2H),
6.85 (s, 1H), 7.55 (s, 1H), 7.68 (d, 1H); MS (EI) m/z 386.
1
Compound 3c: H NMR (200 MHz, CDCl3) d 1.50–2.40 (m,
8H), 2.90–3.20 (m, 10H) 3.50–3.90 (m, 3H), 3.98 (s, 3H), 4.05–
4.20 (m, 2H), 6.80 (s, 1H), 7.48 (s, 1H), 7.65 (d, 1H); MS (EI)
m/z 359.
8. Reddy, B. S. P.; Damayanthi, Y.; Reddy, B. S. N.; Lown,
J. W. Anti-Cancer Drug Des. 2000, 15, 225.