Synthesis of pyrimido[1,6-a]quinoxalines via intermolecular trapping of thermally generated acyl(quinoxalin-2-yl)ketenes by Schiff bases

Article abstract of DOI:10.3762/bjoc.14.147

Acyl(quinoxalin-2-yl)ketenes generated by thermal decarbonylation of 3-acylpyrrolo[1,2-a]quinoxaline-1,2,4(5H)-triones react regioselectively with Schiff bases under solvent-free conditions to form pyrimido[1,6-a]quinoxaline derivatives in good yields.

Full text of DOI:10.3762/bjoc.14.147

Synthesis of pyrimido[1,6-a]quinoxalines via intermolecular
trapping of thermally generated acyl(quinoxalin-2-yl)ketenes
by Schiff bases
Svetlana O. Kasatkina, Ekaterina E. Stepanova*, Maksim V. Dmitriev, Ivan G. Mokrushin
and Andrey N. Maslivets*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 1734–1742.
Department of Chemistry, Perm State University, ul. Bukireva 15,
Perm 614990, Russian Federation
doi:10.3762/bjoc.14.147
Received: 06 April 2018
Accepted: 26 June 2018
Published: 11 July 2018
Email:
Ekaterina E. Stepanova* - caterina.stepanova@psu.ru;
Andrey N. Maslivets* - koh2@psu.ru
Associate Editor: T. J. J. Müller
* Corresponding author
© 2018 Kasatkina et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
acyl(quinoxalin-2-yl)ketenes; cycloaddition;
pyrimido[1,6-a]quinoxalines; Schiff bases; thermolysis
Abstract
Acyl(quinoxalin-2-yl)ketenes generated by thermal decarbonylation of 3-acylpyrrolo[1,2-a]quinoxaline-1,2,4(5H)-triones react
regioselectively with Schiff bases under solvent-free conditions to form pyrimido[1,6-a]quinoxaline derivatives in good yields.
Introduction
Quinoxaline is a 4-aza isostere of quinoline, which rarely occurs Pyrimido[1,6-a]quinoxalines are one of the most intriguing and
in structures of natural products. Its derivatives are gaining unexplored structures representing isosteres of this scaffold.
popularity in medicinal chemistry and pharmacology because Only few synthetic procedures towards these compounds are
many of them exhibit various biological activities [1,2].
described in the literature: heterocyclizations of α-chloroiso-
cyanates with quinoxalin-2-ylideneacetates [9], multicompo-
Quinoxaline-based 6/6/6-angularly fused scaffolds (quinoxa- nent Mannich–Ritter transformations of quinoxalin-2(1H)-ones
line fused by a six-membered heterocycle at the [a]-side) are under the action of nitriles and 3,4-dihydro-2H-pyran [10] and a
promising biologically active compounds. Recent research microwave-assisted cascade strategy via in situ-generated
studies revealed that they can act as inhibitors of poly(ADP- N-acyliminium ion precursors and amines [11] (Figure 2).
ribose) polymerase (PARP) [3], inhibitors of hepatitis C virus
[4], 5-HT2C agonists [5-7], substances for controlling intraoc- To develop a new synthetic approach towards pyrimido[1,6-a]-
ular pressure (IOP) [8] etc. (Figure 1).
quinoxalines we looked through the procedures to their closest
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Figure 1: Quinoxaline-based 6/6/6-angularly fused scaffolds and respective examples of biologically active compounds.
Figure 2: Synthetic routes towards the pyrimido[1,6-a]quinoxaline scaffold.
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Beilstein J. Org. Chem. 2018, 14, 1734–1742.
analogues – pyrido[1,2-a]quinoxalines, the synthesis of which
has been explored more frequently [3,4,12-46]. The analysis
helped us to disclose a tempting but challenging methodology,
which has the potential to be extended for the synthesis of the
desired heterocyclic system, via intermolecular trapping
of thermally generated acyl(quinoxalin-2-yl)ketenes
[20,21,23,24,28,29,38] (Figure 2).
Syntheses utilizing acylketenes are of practical and theoretical
interest due to the high reactivity of acylketenes and the struc-
tural diversity of the reaction products [47-54]. The introduc-
tion of the quinoxalin-2-yl substituent into acylketenes results in
the formation of a peculiar system of conjugated double bonds,
which can potentially act as either oxo-diene or aza-diene
(Figure 3).
Scheme 1: Thermolysis of five-membered 2,3-dioxoheterocycles re-
sulting in acyl(quinoxalin-2-yl)ketenes.
Figure 3: Acyl(quinoxalin-2-yl)ketene.
First, we studied the decarbonylation of precursors III –
3-acylpyrrolo[1,2-a]quinoxaline-1,2,4(5H)-triones (PQTs,
To the best of our knowledge, there is no example of the 1a–h) by simultaneous thermal analysis (STA, Table 1). Ac-
involvement of the aza-diene fragment of acyl(quinoxalin-2- cording to the data obtained, PQTs 1a–h underwent thermal de-
yl)ketenes into intermolecular trapping by hetero-dienophiles composition with a mass loss accompanied by an endothermic
published so far. In this article we report a synthetic protocol effect and CO evolution (Figure 4). The values of the mass loss
towards pyrimido[1,6-a]quinoxalines via the intermolecular corresponded to the elimination of a CO molecule from a PQT.
trapping of acyl(quinoxalin-2-yl)ketenes by Schiff bases.
Table 1: Thermal characteristics of decarbonylation of PQTs 1a–h.
Results and Discussion
The most convenient method for the generation of acyl(quinox-
PQT
temp. of decarbonylation (°C)
extrapolated onset
alin-2-yl)ketenes is the thermal decarbonylation (thermolysis)
of five-membered 2,3-dioxoheterocycles having a quinoxaline
fragment. Currently, three types of such precursors are known:
5-aryl-4-quinoxalin-2-ylfuran-2,3-diones I [21], 3-aroyl-4-
arylpyrrolo[1,2-a]quinoxaline-1,2-diones II [55], and
3-acylpyrrolo[1,2-a]quinoxaline-1,2,4(5H)-triones III [23,56]
(Scheme 1).
onset
peak
1a
1b
1c
1d
1e
1f
187
174
173
148
172
179
184
171
207
209
183
174
204
198
203
197
216
220
206
183
217
212
213
206
1g
1h
According to the literature data, precursors I and II are unsuit-
able for achieving the proposed goal as the generated ketene IV
reacts only at its oxo-diene fragment in intermolecular trapping
reactions with various dienophiles [57-62]. Under these circum-
stances precursors III generating ketenes V seemed to be the
only suitable candidates for the development of a strategy
towards pyrimido[1,6-a]quinoxalines.
Having taken into account the results of the thermal analysis,
we examined the feasibility and conditions of the intermolecu-
lar reaction of the ketene generated from PQT 1a with benzal-
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Figure 4: STA plot of thermolysis of PQT 1a. Blue solid curve: DSC; green solid curve: TG; greed dashed curve: DTG; violet solid curve:
MID (m/z = 28); brown solid curve: MID (m/z = 44); heating rate: 5 °C/min.
aniline (2a). The reaction mixtures obtained were investigated expectations and gave the best yields of the target compound 3a
by UPLC–MS and the results are summarized in Table 2.
(Table 2, entry 6).
The reaction mixtures contained only three types of products, Being inspired by the optimization results obtained, we exam-
and we succeeded to identify each of them. The structures of the ined the scope of the reaction applying the developed methodol-
reaction products were elucidated as the desired pyrimido[1,6- ogy with PQTs 1a–h and Schiff bases 2a–d. The results are
a]quinoxaline 3a, quinoxalinone 4a [29] and pyrido[1,2- shown in Figure 5.
a]quinoxaline 5a [29] (Scheme 2). Product IV of an alternative
intermolecular trapping reaction (Table 1) was not detected.
Unfortunately, our attempts to involve Schiff bases synthesized
from aliphatic aldehydes and ketones did not give any satisfac-
The most likely way of the formation of quinoxalinone 4a is tory results because of various nucleophilic side-reactions.
hydration of the ketene with subsequent decarboxylation
(Scheme 2); more careful drying the reaction vials and solvents We found that the intermolecular trapping worked perfectly in
easily reduced the amount of compound 4a.
case of N5-substituted PQTs 1a–f and did not work at all with
N5-unsubstituted PQT 1g and 1h. The failure to obtain prod-
The formation of pyrido[1,2-a]quinoxaline 5a can be explained ucts 3o and 3p from PQTs 1g and 1h can be explained by the
by a concurrent process of ketene dimerization (Scheme 2) [29] occurrence of intramolecular cyclization in these ketenes result-
in comparison to the intermolecular trapping of it by benzal- ing in the formation of furoquinoxalines 6a,b [56,63,64] which
aniline (2a). Since the yields of the target product 3a decreased were confirmed by UPLC–MS data as the sole products of the
and the yields of compound 5a increased at prolonged time of reaction (Scheme 3).
reaction, the formation of the target compounds deemed to be
reversible.
The formation of pyrimido[1,6-a]quinoxalines 3a–n was unam-
biguously confirmed by the crystal structure of compounds
Performing the reaction under solvent-free conditions at the 3g and 3j (CCDC 1834011, Figure 6; CCDC 1834012,
onset decarbonylation temperature (Table 1) exceeded our Figure 7).
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Table 2: Intermolecular trapping of ketene generated from PQT 1a by benzalaniline (2a)a.
entry
yield of 3a (%)b
time (min)
temp. (°C)
1
2
3
4
5
6
65
50
57
30
80
85c
5
190
190
200
200
175
187
15
5
60
3
2
aConditions: suspension of 1a (1 mmol) and 2a (1.1 mmol) in Dowtherm A (5 mL). bYields were determined by UPLC. cSolvent-free reaction.
Scheme 2: Side-reactions concurring with intermolecular trapping of ketene generated from PQT 1a by benzalaniline (2a).
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Figure 5: Scope of the intermolecular trapping of ketenes generated from PQTs 1a–h by Schiff bases 2a–d under solvent-free conditions.
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Scheme 3: Formation of furoquinoxalines 6a,b via intramolecular cyclization in ketenes generated from PQTs 1g,h.
Conclusion
We have developed a facile synthesis of pyrimido[1,6-
a]quinoxaline derivatives via the intermolecular trapping
of thermally generated acyl(quinoxalin-2-yl)ketenes by Schiff
bases. The reaction proceeds under solvent-free conditions
without any additives and catalysts. The elaborated method
might be applicable to the syntheses of pharmaceutically impor-
tant substances.
Supporting Information
Supporting Information File 1
Experimental details, copies of 1H and 13C NMR spectra of
pyrimido[1,6-a]quinoxalines 3a–n, STA plots of PQT 1a–h
and X-ray crystal structure details of compounds 3g,j.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-14-147-S1.pdf]
Acknowledgements
Figure 6: ORTEP drawing of compound 3g (CCDC 1834011) showing
thermal ellipsoids at the 30% probability level.
This work was supported by the Russian Science Foundation,
project # 17-73-10210.
ORCID® iDs
Svetlana O. Kasatkina - https://orcid.org/0000-0002-7943-7525
Ekaterina E. Stepanova - https://orcid.org/0000-0002-5851-3082
Andrey N. Maslivets - https://orcid.org/0000-0001-7148-4450
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