A novel approach for the synthesis of lophocladines A, B and C1 analogues

Article abstract of DOI:10.1016/j.tetlet.2011.09.032

A novel approach for the syntheses of lophocladines A and B has been developed. These compounds were prepared in 4-6 steps with moderate to excellent overall yields. The key step involved the nucleophilic substitution of 4-chloronicotinic acid with the ca

Full text of DOI:10.1016/j.tetlet.2011.09.032

Tetrahedron Letters 52 (2011) 6142–6144
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A novel approach for the synthesis of lophocladines A, B and C1 analogues
Wannaporn Disadee a,b, , Poonsakdi Ploypradith a,b, Thammarat Aree c, Narongsak Chaichit d,
⇑
Somsak Ruchirawat a,b,e
a Laboratory of Medicinal Chemistry, Chulabhorn Research Institute, Lak Si, Bangkok 10210, Thailand
b Program on Chemical Biology, Chulabhorn Graduate Institute, Lak Si, Bangkok 10210, Thailand
c Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
d Department of Physics, Faculty of Science and Technology, Thammasat University, Pathum Thani 12121, Thailand
e The Center of Excellence on Environmental Health, Toxicology and Management of Chemicals, Lak Si, Bangkok 10210, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 July 2011
Revised 28 August 2011
Accepted 7 September 2011
Available online 10 September 2011
A novel approach for the syntheses of lophocladines A and B has been developed. These compounds were
prepared in 4–6 steps with moderate to excellent overall yields. The key step involved the nucleophilic
substitution of 4-chloronicotinic acid with the carbanion generated from phenylacetonitrile. Subsequent
reduction of the cyano group, lactamization and oxidation furnished lophocladine A in 50% yield over 4
steps. Further amination with various amines led to lophocladine B and its C1 analogues in good yields. In
addition, the synthesized compounds were evaluated for their cytotoxicity against leukaemia cells.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Nitrogen heterocycles
Nucleophilic substitution
2,7-Naphthyridine
Lophocladine
Naphthyridine derivatives have prompted considerable scien-
tific interest because of their broad spectrum of application in
the pharmaceutical sciences, agriculture and chemical industry.1
In a number of studies, the 2,7-naphthyridine structural fragment
was found to be a component of various polycyclic alkaloids such
as the marine alkaloid ascididemine (1) (Fig. 1) which exhibits a
wide range of pharmacological activity.2a Recently, Gross et al. iso-
lated the marine algal alkaloids, lophocladines A (2) and B (3a),
from a red alga, Lophocladia sp.3 These 2,7-naphthyridines exhib-
ited different modes of biological activity as observed from their
preliminary screening. Lophocladine A showed affinity for the
a number of syntheses.8 An intermolecular version of this reaction
could provide a valuable route to further transformations such as
second ring fusion to the napthyridine ring system. Herein, we re-
port the application of this methodology for the synthesis of lop-
hocladines A and B.
At the outset, unprotected 4-chloronicotinic acid (4)9 was trea-
ted with a solution of 2-phenylacetonitrile under various condi-
tions. After some experimentation, we found that this reaction
required 1.5 equiv of 2-phenylacetonitrile and 2 equiv of potas-
sium tert-butoxide (KOtBu) in N-methylpyrrolidone (NMP) at
130 °C for 30 min and gave cleanly the desired product 5 in good
yield (65%). We also examined the use of microwave irradiation
(MW) for this reaction and found that the best result (85% yield)
was obtained using conditions at 110 °C and 30 W for 4 min. As a
consequence of this successful procedure, we turned our attention
to the preparation of nicotinate 6, as shown in Scheme 1. The most
effective conditions for the methylation of 5 were found to involve
NMDA (N-methyl-D-aspartic acid) receptor while also exhibiting
d-opioid receptor antagonist activities. On the other hand, lophocl-
adine B exhibited cytotoxicity towards human lung tumour cells
and breast cancer cells.3 Although these compounds were found
to be of biological interest,2b,2c synthetic accessibility has been lim-
ited.1,4 To date, only two groups have reported the total synthesis
of lophocladines A and B via a similar key step reaction, namely
cyclisation of enamines generated from 4-benzyl- or 4-methyl-3-
cyanopyridine5 and Bredereck’s reagent.6 Thus, despite the earlier
approaches, a new and more efficient synthetic route is desirable.
N
O
Ph
Ph
NH
N
N
Addition of nucleophiles to the a- and c-position of p-electron
O
NH2
deficient N-heterocyclic compounds7 is an important method for
N
N
N
the synthesis of functionalized compounds and has been used in
ascididemine (1)
lophocladine A (2)
lophocladine B (3a)
⇑
Corresponding author. Tel.: +66 2 574 0622; fax: +66 2 574 0622x1513
E-mail address: wannapor@cri.or.th (W. Disadee).
Figure 1. Examples of naturally occurring bioactive compounds containing the 2,7-
naphthyridine ring system.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.09.032

W. Disadee et al. / Tetrahedron Letters 52 (2011) 6142–6144
6143
Ph
CN
CO2H
Ph
CN
CO2Me
Cl
Ph
CN
Me2SO4
CO2H
KO tBu, NMP, MW
110 °C, 30 W, 4 min
(85%)
K2CO3, acetone
rt, 40 min
N
4
N
5
N
6
(88%)
Ph
Ph
NH
NH
.
CoCl2 6H2O, NaBH4
O2, DBU
O
O
benzene-MeOH (1:3)
0 °C, 5 h
ClCH2CH2Cl
80 °C, 2 d
(92%)
N
7
N
lophocladine A (2)
(72%)
Scheme 1. Synthesis of lophocladine A (2).
N
Ph
OMe
OH
Ph
CN
CO2H
Ph
NaH, MeI
DMF
rt, 2.5 h
NH
O
O
N
Me
N
5
N
8
9
Me2SO4, base
Scheme 2. Base-mediated dearomatization of naphthyridone 8.
the reaction in acetone with a stoichiometric amount of K2CO3 and
dimethyl sulfate (Me2SO4)10 at room temperature for 40 min. The
yields decreased slightly with the weaker bases NaHCO3 and
Na2CO3, due to incomplete methylation. For the next step, we
investigated the lactamization and subsequent oxidation of 7 to
lophocladine A. We first explored the reduction of 6 using catalytic
hydrogenation (Pd/C or PtO2), however, these methods failed to re-
duce the cyanide group, even under high pressures of H2 (75 psi),
and products of pyridine ring hydrogenation were observed. After
some experimentation, we found that 6 could be converted into 7
by using NaBH4 and CoCl2 in a solvent mixture of benzene and
MeOH.11 The cyclised product 7 was obtained in 72% yield. With
the naphthyridine framework in hand, we further oxidized com-
pound 7 into laphocladine A (2) using various reagents: 2,3-di-
chloro-5,6-dicyanobenzoquinone (DDQ), KOtBu/O2, and 1,8-
diazabicycloundec-7-ene (DBU)/O2. In general, the best results
were obtained using strong bases in the presence of O2 (KOtBu in
DMF at rt for 1 h, 87%; or DBU in dichloroethane at 80 °C for 2 days,
92%).
It should be noted that compound 5 was unstable and slowly
cyclised into 8 in a polar solvent such as dimethylsulfoxide (DMSO)
(Scheme 2).12 In general, nitrile 5 could be prepared in good yields,
despite being contaminated with the cyclised product 8 in <5%
yield. Transformation of 8 into lophocladine A (2) was possible
via palladium-catalyzed deoxygenation, however, preparation of
the O-triflate proved to be problematic. The reaction of 8 in the
presence of NaH in DMF at room temperature gave complete con-
version into an intermediate that was trapped with iodomethane
Figure 2. ORTEP structure of compound 9.
Table 1
Synthesis of lophocladine B (3a) and its C1 analogues
Ph
N
Ph
N
POCl3
MeCN
N
-Nucleophiles
2
Cl
R
DMSO
120 oC
100 oC, 2 h
(72%)
N
N
3a-g
10
Entry
R
Product
3a
Yielda (%)
48b
1
2
NH2
HN
3b
95
Ph
3
4
3c
96
98
HN
HN
3d
5
3e
98
N
O
6
7
N3
3f
97
89
(MeI) to give N-methyl-1,4-dihydropyridine
9 in good yield
MeONMe
3g
(91%).13 The structure of this dearomatized product was confirmed
by X-ray crystallographic analysis (Figure 2).14 Thus, the prepara-
tion of 6 was carefully carried out under controlled conditions to
avoid the formation of 9.
a
Isolated yield.
b
Two-step yield (see Supplementary data).
Having successfully developed a route for the synthesis of
lophocladine A (2), we subsequently synthesized lophocladine B
(3a) and its derivatives using the known chlorination/amination
method shown in Table 1. Chlorination of 2 was performed using
phosphorus oxychloride (POCl3) and the desired product 10 was
obtained in good yield (72%).15 In general, lophocladine derivatives
3b-g were prepared in excellent yields.

6144
W. Disadee et al. / Tetrahedron Letters 52 (2011) 6142–6144
5. 4-Benzyl-3-cyanopyridine and 4-methyl-3-cyanopyridine were prepared in
Table 2
low to good overall yields, see: (a) Pfleger, K.; Fuchs, W.; Pailer, M. Monatsh.
Chem. 1978, 109, 597–602; (b) Cameron, D. W.; Deutscher, K. R.; Feutrill, G. I.;
Hunt, D. E. Aust. J. Chem. 1982, 35, 1451–1498; (c) Shing, T.-L.; Chia, W.-L.;
Shiao, M.-J.; Chau, T.-Y. Synthesis 1991, 849–850; (d) Reimann, E.; Wichmann,
P.; Höfner, G. Sci. Pharm. 1996, 64, 637–646.
Cytotoxic activities
Entry
Compound
IC50 (l
M)a
MOLT-3
HL-60
6. (a) Lotter, M.; Schilling, J.; Reimann, E.; Bracher, F. Arch. Pharm. Chem. Life Sci.
2006, 339, 677–679; (b) Zhang, A.; Ding, C.; Cheng, C.; Yao, Q. J. Comb. Chem.
2007, 9, 916–919.
1
2
2
3a
3b
3c
3d
3e
3f
3g
10
> 150
32
24
59
27
> 150
> 150
80
31
0.04
> 150
45
51
> 150
43
> 150
> 150
> 150
17
3b
4b
5
7. For previous studies on nucleophilic substitution of N-heteroaromatic
compounds, see: (a) Yin, Z.; Zhang, Z.; Kadow, J. F.; Meanwell, N. A.; Wang, T.
J. Org. Chem. 2004, 69, 1364–1367; (b) Cherng, Y.-J. Tetrahedron 2002, 58, 4931–
4935; (c) Sommer, M. B.; Begtrup, M.; Boegesoe, K. P. J. Org. Chem. 1990, 55,
4817–4821; (d) Yamanaka, H.; Ohba, S. Heterocycles 1990, 31, 895–909.
8. For selected publications, see: (a) Seto, M.; Aramaki, Y.; Imoto, H.; Aikawa, K.;
Oda, T.; Kanzaki, N.; Iizawa, Y.; Baba, M.; Shiraishi, M. Chem. Pharm. Bull. 2004,
52, 818–829; (b) Adam, S. Tetrahedron 1991, 47, 7609–7614; (c) Sakamoto, T.;
Kondo, Y.; Yamanaka, H. Chem. Pharm. Bull. 1985, 33, 626–633.
9. Due to its stability and ease of preparation, 4-chloronicotinic acid (4) was used
instead of other halo compounds. 4-Iodonicotinic acid and 4-bromopyridine
were found to be unstable and easily underwent polymerization, see: (a)
Lazaar, J.; Rebstock, A.-S.; Mongin, F.; Godard, A.; Trécourt, F.; Marsais, F.;
Quéguiner, G. Tetrahedron 2002, 58, 6723–6728; (b) Feast, W. J.; Tsibouklis, J.
Polym. Int. 1994, 35, 67–74.
10. Dimethyl sulfate was used instead of iodomethane due to ease of handling.
11. For selected publications on NaBH4/CoCl2 reduction of nitriles, see: (a) Bhat, A.
S.; Gervay-Hague, J. Org. Lett. 2001, 3, 2081–2084; (b) Gaucher, A.; Zuliani, Y.;
Cabaret, D.; Wakselman, M.; Mazaleyrat, J.-P. Tetrahedron: Asymmetry 2001, 12,
2571–2580; (c) Zhang, F.-Y.; Corey, E. J. Org. Lett. 2000, 2, 1097–1100; (d) Osby,
J. O.; Heinzman, S. W.; Ganem, B. J. Am. Chem. Soc. 1986, 108, 67–72.
12. This type of reaction is not well known and only one example was found in the
literature, see: Huffman, J. J. Heterocycl. Chem. 1987, 24, 549–553.
13. It is interesting to note that compound 9 was found in CDCl3 solution as an
equilibrating mixture of two diastereomers (ratio = 1:4 at 23 °C, and 1:3 at
50 °C). The assignment of the major isomer was strongly supported by the
shielded OMe resonance (d 2.99), due to the anisotropic effect, with respect to
6b
7
8
9b
10
VP-16
1.3
a
XTT assay. The IC50 values are given as the mean of three duplicate experiments.
Compounds did not completely dissolve in the test medium.
b
Lophocladines A, B and C1 analogues were evaluated for in vitro
cytotoxic activity against human acute promyelocytic leukaemia
(HL-60)16 and mouse acute lymphoblastic leukaemia (MOLT-3) cell
lines using an XTT [2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-
tetrazolium-5-carboxanilide] assay. As shown in Table 2, although
all the compounds were less potent than the reference compound
(etoposide, VP-16), the order of potency as a function of the naph-
thyridine C1 position was halogen, primary- and secondary-ami-
ne > tert-amine, suggesting that this position is also important for
enhanced cytotoxic activity.
In conclusion, we have developed a novel method for the prep-
aration of lophocladine A and B, as well as its C1 analogues and
have evaluated their cytotoxicity. Our strategy can be applied for
the synthesis of other C4 analogues using commercially available
heteroaromatic aldehydes to generate various cyano derivatives.
that of the minor isomer (d 3.92).
mediated dearomatization of naphthyridone 8 to dihydropyridine 9 is shown
below. The reaction is probably based on the formation of ketene
A proposed mechanism for the base-
a
intermediate B, which undergoes cyclisation via an intramolecular reaction
into C. Subsequent lactone ring-opening and trapping of the intermediate E
with the electrophile leads to 9.
Acknowledgments
O
NH
Ph
Ph
NH
O
B
The authors gratefully acknowledge all researchers from the
Central Facilities, the Chulabhorn Research Institute for the evalu-
ation of cytotoxic activity. The authors would like to thank Dr. M.
Paul Gleeson, the Kasetsart University, for careful reading of the fi-
nal manuscript. This work was partially supported by the Thailand
Research Fund (Grant No. RTA 4880008 to N.C. and T.A.; TRG
5280001 to W.D.), Chulabhorn Research Institute and the Center
of Excellence on Environmental Health, Toxicology and Manage-
ment of Chemicals, Thailand.
8
•
O
O
N
N
A
B
NH
N
N
O
Ph
Ph
Ph
O
O
O
O
O
Supplementary data
N
N
H
D
N
H
Supplementary data (General methods, experimental proce-
dures, spectroscopic data, copies of 1H and 13C NMR of all new
compounds and crystallographic information (CIF) for compound
9.) associated with this paper can be found, in the online version,
at doi:10.1016/j.tetlet.2011.09.032.
E
C
2 MeI
B
9
14. Complete crystallographic data for compound 9 have been deposited with the
Cambridge Crystallographic Data Centre, CCDC No. 728653.
References and notes
15. For selected publications on POCl3 chlorination of N-heteroaromatic
compounds, see: (a) England, D. B.; Padwa, A. Org. Lett. 2008, 10, 3631–3634;
(b) Cappelli, A.; Giuliani, G.; Anzini, M.; Riitano, D.; Giorgi, G.; Vomero, S.
Bioorg. Med. Chem. 2008, 16, 6850–6859; (c) Vanejevs, M.; Jatzke, C.; Renner, S.;
Mueller, S.; Hechenberger, M.; Bauer, T.; Klochkova, A.; Pyatkin, I.; Kazyulkin,
D.; Aksenova, E.; Shulepin, S.; Timonina, O.; Haasis, A.; Gutcaits, A.; Parsons, C.
G.; Kauss, V.; Weil, T. J. Med. Chem. 2008, 51, 634–647.
1. For a review on naphthyridines, see: Litvinov, V. P.; Roman, S. V.; Dyachenko, V.
D. Russ. Chem. Rev. 2001, 70, 299–320.
2. For reviews on marine alkaloids, see: (a) Molinski, T. F. Chem. Rev. 1993, 93,
1825–1838; (b) Sithranga Boopathy, N.; Kathiresan, K. J. Oncol. 2010, 2010,
214186; (c) Güven, K. C.; Percot, A.; Sezik, E. Mar. Drugs 2010, 8, 269–284.
3. Gross, H.; Goeger, D. E.; Hills, P.; Mooberry, S. L.; Ballantine, D. L.; Murray, T. F.;
Valeriote, F. A.; Gerwick, W. H. J. Nat. Prod. 2006, 69, 640–644.
16. Lotter previously reported that lophocladine
B showed cytotoxic activity
against HL-60 in an MTT assay with IC50 = 1 M, see: Ref. 6a.
l
4. For a recent synthesis of 2,7-naphthyridine-containing analogues, see: Dai, X.;
Cheng, C.; Ding, C.; Yao, Q.; Zhang, A. Synlett 2008, 2989–2992.