One-pot synthesis of dibenzo[b,h][1,6]naphthyridines from 2-acetylaminobenzaldehyde: Application to a fluorescent DNA-binding compound

Article abstract of DOI:10.1039/c4cc07807a

Dibenzo[b,h][1,6]naphthyridines were synthesized in one pot by reacting 2-acetylaminobenzaldehyde with methyl ketones under basic conditions via four sequential condensation reactions. This method was also applied to the synthesis of 1,2-dihydroquinolines. 6-Methyl-1,6-dibenzonaphthyridinium triflates showed strong fluorescence, and the fluorescence intensities were changed upon intercalation into double-stranded DNA. This journal is

Full text of DOI:10.1039/c4cc07807a

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One-pot synthesis of dibenzo[b,h][1,6]naphthyridines
from 2-acetylaminobenzaldehyde: application to a
fluorescent DNA-binding compound†
Cite this:DOI: 10.1039/c4cc07807a
Received 3rd October 2014,
Accepted 23rd October 2014
Kentaro Okuma,* Tomohiro Koga, Saori Ozaki, Yutaro Suzuki, Kenta Horigami,
Noriyoshi Nagahora, Kosei Shioji, Masatora Fukuda and Masanobu Deshimaru
DOI: 10.1039/c4cc07807a
www.rsc.org/chemcomm
Dibenzo[b,h][1,6]naphthyridines were synthesized in one pot by Recently, three-component reactions that yielded naphthyri-
reacting 2-acetylaminobenzaldehyde with methyl ketones under dine derivatives were reported.9 Although 2,2,4-trimethyl-
basic conditions via four sequential condensation reactions. This [1,2]dihydroquinoline was obtained by reacting aniline with
method was also applied to the synthesis of 1,2-dihydroquinolines. acetone and iodine,10 there is no report on the direct synthesis
6-Methyl-1,6-dibenzonaphthyridinium triflates showed strong of dibenzo[b,h][1,6]naphthyridines 1 from easily available
fluorescence, and the fluorescence intensities were changed upon starting materials, such as 2-aminobenzaldehydes. Those
intercalation into double-stranded DNA.
results prompted us to look into ways for the synthesis of
dibenzo[b,h][1,6]naphthyridines 1 and their alkylation. Our
Cyclic polyaza compounds, such as quinolines, dihydroquino- synthetic plan consists of four sequential condensation reactions
lines, naphthyridines, benzonaphthyridines, and phenanthridines, (aldol, imination, aza-Morita–Baylis–Hillman, and intramolecular
are important structural components of naturally occurring imination) of 2-acetylaminobenzaldehyde (2) with methyl ketones
alkaloids and synthetic analogues possessing interesting bio- (3), as shown in Fig. 1. In this communication, we report the
logical activities.1,2 Ethidium bromide (EtBr) and the analogues synthesis of 1, their fluorescence properties, and their DNA inter-
of phenanthridine and benzonaphthyridine are used for the calation properties.
visualization of nucleic acids in agarose gels. Those dyes
2-Acetylaminobenzaldehyde 2 was synthesized by reducing
produce red-orange fluorescence under ultraviolet light and 2-nitrobenzaldehyde with Sn/HCl or Fe/HCl and subsequently
show enhanced fluorescence when bound to double-stranded adding acetic anhydride.11 Treatment of 2 with 2 eq. of
DNA. Because of their fluorescence properties, those dyes are 4-methylacetophenone 3a in the presence of aq NaOH (5 M
used as DNA intercalating agents.3 In this regard, the develop- solution, 2 eq.) in refluxing EtOH gave 2-(2-acetylaminophenyl)-
ment of efficient routes for the preparation of phenanthridines 3-(4-methylbenzoyl)-1,2-dihydroquinoline (4a) in 80% yield.12
1
and benzonaphthyridines is of great importance in synthetic The structure of 4a was confirmed by H NMR, 13C NMR, and MS
organic, pharmaceutical, and material chemistry.4 It is well analyses. The 1H NMR spectrum of 4a showed a singlet at 6.06 ppm,
known that benzaldehyde reacts with acetophenone under basic which unambiguously indicated the existence of a methine proton,
conditions to afford chalcone.5 We have reported a one-pot synth- along with peaks of 13 aromatic protons. Other reactions were carried
esis of quinolines and 2-arylquinoline N-oxides that involved out in a similar manner (55–99% yields, Scheme 1, Table S1, ESI†).
reacting 2-nitrochalcones with Sn/HCl or Zn/HCl, the intermedi-
6
¨
ates of which were 2-aminochalcones, or the Friedlander reaction.
Methods for the synthesis of dibenzo[b,h][1,6]naphthyridines (1)
include the reaction of quinolines with 2-aminobenzoic acid or
2-aminoacetophenone7 and the reaction of quinolinones with
8
¨
2-aminoacetophenones, all of which are based on the Friedlander
reaction that proceeds at a high reaction temperature (160–180 1C).
Department of Chemistry, Faculty of Science, Fukuoka University, Jonan-ku,
Fukuoka 814-0180, Japan. E-mail: kokuma@fukuoka-u.ac.jp
† Electronic supplementary information (ESI) available. CCDC 1003324 and
1003843. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c4cc07807a
Fig. 1 Retrosynthetic analysis of dibenzonaphthyridine 1.
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Scheme 2 One-pot synthesis of dibenzo[b,h][1,6]naphthyridines 1.
Scheme 3 Thermal transformation of 4f–4j.
Scheme 1 Reaction of 2-acetylaminobenzaldehyde 2 with 3.
1
The structure of 1a was confirmed by H NMR, 13C NMR, MS,
and elemental analyses. As single crystals of 1a were obtained
by reaction with 4-methylacetophenone, X-ray crystallographic
analysis was performed (Fig. S6, ESI†).18
Compound 4e was finally determined by X-ray crystallographic
analysis (Fig. S5, ESI†). Compound 4c was gradually oxidized in
a CH2Cl2 solution to give the corresponding quinoline 4c0 after
24 h at rt.
The reaction is speculated to proceed as follows: the aldol
reaction of benzaldehyde 2 with acetophenone 3 furnished
chalcone, which underwent hydrolysis and imination with 2 to
give imine a. Intramolecular aza-Morita–Baylis–Hillman cycliza-
tion of imine a produced dihydroquinoline derivative 4. Under
specific conditions (20 eq. NaOH in refluxing ethanol for 10 h),
dihydroquinoline 4 was hydrolyzed, oxidized, and intramolecu-
larly condensed to give naphthyridine 1 (Scheme 4).
Previously, dibenzo[b,h][1,6]naphthyridines 1 were synthe-
sized in two steps: the reaction of formylquinolin-2-one with
aniline and the reaction of this imine with polyphosphoric
acid.19 1,6-Dibenzonaphthyridin-6-ones were synthesized by
It is well known that the reaction of 2-aminobenzaldehyde
¨
with acetophenones gives 2-arylquinolines via the Friedlander
reaction.13 However, there is no report on the synthesis of
1,2-dihydroquinolines by reacting 2-aminobenzaldehyde deriva-
tives with acetophenones. There are several methods for the
synthesis of dihydroquinolines.1,9 Recent examples include
the AuCl3/AgSbF6-catalyzed intramolecular allylic amination of
2-tosylaminophenylprop-1-en-3-ols,14 the electrocyclization of
N-methyl-2-hydroxyalkylanilines,15 the intramolecular cycliza-
tion of o-(1-hydroxy-2-alkenyl)phenyl isocyanides in the presence
of BF3ꢀEt2O as a catalyst,16 and the tandem Michael-aldol reac-
tion of N-tosyl-2-aminobenzaldehyde with an a,b-unsaturated
carbonyl compound.17 A synthesis conducted under mild basic
conditions is expected to offer more advantages than the above-
mentioned methods. In the method developed in this study,
readily available 2 and 3 were used as substrates.
If the N-acetylamino group of 4 could be hydrolyzed and
condensed with a carbonyl group, it would be possible to
synthesize dibenzo[b,h][1,6]naphthyridines 1. Because only aq
NaOH was used as the base for the synthesis of dihydroquino-
lines 4, we reacted 2 with 3 in the presence of NaOH hoping
that 1 would be obtained in a one-pot operation (Scheme 2).
Treatment of 2a with 3a in the presence of 20 M NaOH (1 eq.) in
refluxing ethanol for 6 h furnished polymeric products. When
2 eq. of 5 M NaOH was added to refluxing ethanol followed by
20 eq. of 20 M NaOH, and then reflux was conducted for 10 h,
naphthyridine 1a was formed in 70% yield. Under similar condi-
tions, naphthyridines 1b–j were synthesized as well (Scheme 2,
Table S2, ESI†). However, as the yields of 1f–1j were low (21–23%),
aerobic oxidation, deacetylation, and imination of 4 at an elevated
temperature (B230 1C) were performed, and the yields of
1f–1j were improved to 75–96% (Scheme 3, Table S3, ESI†).
Scheme 4 Plausible reaction mechanism.
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Fig. 2 (a) UV/vis and fluorescence spectra of 1a (5 mM, EtOH, excitation at
360 nm) and 5a (5 mM, EtOH, excitation at 400 nm). (b) Photograph of EtOH
solution of 1a (left, 0.1 mM) and 5a (right, 0.1 mM) under 365 nm light irradiation.
Scheme 5 Synthesis of dibenzonaphthyridinium triflates 5a–c.
reaction with diethyl 2-(3-methylbut-2-enyl)malonate with
anilines in refluxing diphenyl ether, but the yields were very
low at 2–4%.20 The present reaction requires only one step
using commercially available substituted methyl ketones 3 and
N-acetyl o-aminobenzaldehyde 2, which was synthesized from
o-nitrobenzaldehyde. Thus, this method provides a versatile
alternative for the synthesis of substituted naphthyridines in a
one-pot operation.
If dibenzonaphthyridines 1 were selectively methylated at
6-position, their structures would be very similar to that of EtBr,
a well-known DNA intercalating agent. Then, the alkylation
of those compounds with methyl triflate was carried out
(Scheme 5). As expected, the methylation proceeded regioselec-
tively at 6-position to afford N-methylation products (5a–c)
almost quantitatively.
EtBr is characterized by its intrinsic fluorescence. In
contrast, the fluorescence properties of methylated naphthyridines
5a–c remain unexplored. Therefore, we examined the differences in
spectroscopic properties between 1a and 5a. Although dibenzo-
naphthyridine 1a did not show significant fluorescence, methylated
dibenzonaphthyridine 5a exhibited strong fluorescence (Fig. 2a and b).
The fluorescence quantum yield of 5a is 0.15 (0.17 for 5b and 0.18
for 5c; see Fig. S1, ESI†).
We then examined the intercalation property of 5b with the
expectation that it would behave as an EtBr-like DNA staining
reagent. As shown in Fig. 3a, a titration experiment revealed a
gradual decrease in the fluorescence intensity of 5b in response
to the increasing concentration of plasmid DNA. This result
can be explained by considering the ability of the nitrogen-
containing planar ring to coordinate to DNA, because the
structure of the ring system is very similar to that of EtBr.
Interestingly, such a negative effect of the intercalation on the
fluorescence is in contrast to the characteristics of EtBr, whose
fluorescence is strongly enhanced by the intercalation into
DNA. The difference in fluorescence properties between 5b
and EtBr was visually confirmed when DNA fragments electro-
Fig. 3 Change in fluorescence of compound 5b upon intercalation into
DNA. (a) Fluorescence spectra of 0.10 mM serial solutions of 5b containing
the indicated concentrations of 31mer oligonucleotide, spanning from
440 nm to 600 nm (excitation at 370 nm). (b) Difference in DNA-
intercalation-induced fluorescence between 5b and EtBr. Upper, EtBr
(0.5 mM in water); lower, compound 5b (0.5 mM in 0.1% DMSO in water);
left, without DNA; right, containing 7.5 mg mLꢁ1 pUC-18 plasmid DNA.
Fluorescence was observed by excitation with 254 nm UV light. (c) DNA
staining of 5b. 100 bp DNA ladder fragments electrophoresed on 8%
acrylamide gels were stained with 5b (left) and EtBr (right). The excitation
conditions are the same as those for (b).
phoresed on acrylamide gels were stained with those two reagents. fragments were visible as dark bands against the bright back-
When 5b-stained gel was observed under UV irradiation, DNA ground of the gel (Fig. 3b and c). As typical DNA intercalators have
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We have synthesized dibenzo[b,h][1,6]naphthyridines in one pot
by reacting 2-acetylaminobenzaldehyde with acetophenone under
basic conditions. This method was also applied to the synthesis
of 1,2-dihydroquinolines. 6-Methyl-1,6-dibenzonaphthyridinium
triflates showed significant fluorescence and intercalated into
double-stranded DNA. Further studies on the novel features of
those reagents are in progress.
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(0.50 mmol) in EtOH was added 5 M aq. NaOH (0.24 mL, 1.2 mmol).
After refluxing for 8 h, the reaction mixture was concentrated and
left to stand for 2 h to yield orange crystals. Recrystallization from
EtOH gave orange crystals of 4a (0.41 mmol). Mp 234–236 1C.
¨
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