Revisiting the three component synthesis of isoxazolo[5,4-b]pyridines, 4-aryl-3,7,7-trimethyl-isoxazolo[5,4-b]quinolin-5(6H)-ones and related heterocycles

Article abstract of DOI:10.1016/j.poly.2016.09.011

The one-step three-component microwave assisted synthesis between aromatic aldehydes with tetronic acid or indan-1,3-dione readily formed the Knoevenagel adducts that underwent addition of 3-methylisoxazol-5-amine 1 to form the isoxazolo[5,4-b]pyridine pr

Full text of DOI:10.1016/j.poly.2016.09.011

Accepted Manuscript
Revisiting the Three Component Synthesis of Isoxazolo[5,4-b]pyridines, 4-Ar-
yl-3,7,7-trimethyl-isoxazolo[5,4-b]quinolin-5(6H)-ones and Related Heterocy-
cles
Anthony R. Lingham, John A. Hawley, Tamar Greaves, Neale Jackson, Frank
Antolasic, Helmut M Hügel
PII:
S0277-5387(16)30431-4
DOI:
http://dx.doi.org/10.1016/j.poly.2016.09.011
POLY 12199
Reference:
Polyhedron
To appear in:
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Revised Date:
Accepted Date:
31 May 2016
6 September 2016
7 September 2016
Please cite this article as: A.R. Lingham, J.A. Hawley, T. Greaves, N. Jackson, F. Antolasic, H.M. Hügel, Revisiting
the Three Component Synthesis of Isoxazolo[5,4-b]pyridines, 4-Aryl-3,7,7-trimethyl-isoxazolo[5,4-
b]quinolin-5(6H)-ones and Related Heterocycles, Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.
2016.09.011
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Revisiting the Three Component Synthesis of Isoxazolo[5,4-b]pyridines, 4-Aryl-3,7,7-
trimethyl-isoxazolo[5,4-b]quinolin-5(6H)-ones and Related Heterocycles.
Anthony R. Lingham1*, John A. Hawley2, Tamar Greaves1, Neale Jackson1,
Frank Antolasic1, Helmut M Hügel1*.
1RMIT University, School of Science, GPO Box 2476, Melbourne VIC 3001 Australia.
2 Australian Catholic University, Locked Bag 4115, Fitzroy, VIC 3065 Australia
*Corresponding authors.
Abstract
The one-step three-component microwave assisted synthesis between aromatic aldehydes with
tetronic acid or indan-1,3-dione readily formed the Knoevenagel adducts that underwent addition
of 3-methylisoxazol-5-amine 1 to form the isoxazolo[5,4-b]pyridine products 8[a,b,d,e,g] in 67-
90% yield. The multicomponent reaction using dimedone formed the respective addition products
4-aryl-3,7,7-trimethyl-isoxazolo[5,4-b]quinolin-5(6H)-ones 8[c,f,h] (36-79%). In contrast, the
sonication of an aryl aldehyde and dimedone with an equivalent amount of 2-hydroxyammonium
formate exclusively generated the Knoevenagel adduct 4c via hydro-4c (Scheme 6). When
microwaved either with 3-amino-5-methylisoxazole 1 or 3,4,5-trimethoxyaniline 14 in ethanoic
acid-ethylacetate (1:1), tetrahydroacridones 15[a,b] formed in high yields (88-92%). Importantly,
this two-step reaction sequence generates highly reproducible and pure products.
Keywords: Multicomponent Microwave Synthesis, Isoxazolopyridines, Isoxazoloquinolinones,
Tetrahydroacridones
Introduction
The atom efficient and step-economic accessibility of heterocyclic scaffolds particularly
pyridine-one containing rings is of medicinal importance and has been actively pursued1-3. The
three component synthesis of substituted bi- and tri- cyclic pyridine derivatives provides a rapid
route for the preparation of a family of potentially bioactive compounds.4,5 However, Kappe and
coworkers6 demonstrated that the control of chemo- and regioselectivity in multicomponent
experimental protocols also influences the product pathway when aryl-aldehydes, dimedone, 5-
aminopyrazoles are condensed.
The reactions of aryl-aldehydes and 1,3-dicarbonyls with 2-aminoazoles have been reviewed by
Chebanov and coworkers7,8 to produce polycyclic substituted 4-aryl-3,7,7-trimethyl-6,7,8,9-
tetrahydroisoxazolo[5,4-b]quinolin-5(6H)-one
and
4-aryl-3,7,7-trimethyl-isoxazolo[5,4-
b]quinolin-5(6H)-ones. More recently Chen and Micalizio9 and also Chung10 et al. developed an
expedient three-component coupling reaction.
Our aim was to utilize 3-methylisoxazol-5-amine 1, aromatic aldehydes 2a-c and 1,3-dicarbonyls
3a-c compounds to prepare isoxazolo-tricylic products by MCRs in water for the rapid synthesis
of a range of targets for investigation as nuclear hormone receptor agonists (NR4A). Tu11, 12a et al.
suggested the reaction proceeds from 2a and 3a by the formation of the Knoevenagel addition
product 4 that acts as an electrophile for the addition of the 3-methylisoxazol-5-amine 1 to 4 to
give 5A. Then follows the cyclization of 5A, followed by dehydration/oxidation leading to both
the furo[3’,4’:5,6]pyrido[2,3-d]pyrimidine11 7 and 4-(4-fluoro-phenyl)-3-methyl-7H-1,6-dioxa-
2,8-diaza-s-indanen-5-one 12a 8 as outlined in Scheme 1. The evidence to support the proposed
mechanism is based on the reaction between the Knoevenagel adduct 4 and the amine 1 to give the

same yields as achieved in the one pot reaction.12a The obvious reaction byproducts include imine
formation by condensation of the aryl aldehyde with 1.
Scheme 1. Tu12a et al. MW 3-component synthesis of isoxazolo[5,4-b]pyridine 8.
Results and discussion
The MCR shown in Scheme 1 between 1, 4-fluorobenzaldehyde (2a) and tetronic acid (3a)
gave 4-(4-fluoro-phenyl)-3-methyl-7H-1,6-dioxa-2,8-diaza-s-indacen-5-one (8a), a target of
particular interest due to the activity of the fused lactone and fluorinated aromatic substituents6 on
the isoxazolo[5,4-b]pyridine system. The reported conditions involved the MCR in water at
120ºC (200 W) for 6 min to give 8 in yields above 90%. The authors mentioned that reactions
performed in EtOH and AcOH at 90ºC for 10 min gave modest yields of 8a (54 and 57%
respectively), although yields in aqueous media reportedly superseded hydrocarbon solvents under
all conditions investigated.12a
In our hands, using Tu’s reported conditions12a on a 1 mmolar scale in a CEM Discover
microwave12b and 1, 2a and 3a in H2O (2mL) with microwave heating at 120ºC (200 W) for 6
minutes, upon workup the residue contained the expected product 8a in < 10% yield (GC-MS)
along with 4-fluorobenzoic acid [formed via the oxidation of 2a]. With GC-MS monitoring, the
yields of 8a on a 2 mmol scale at 140ºC in 2 mL EtOH were 45% and in 2 mL AcOH 65% that
were similar to published results12a along with some observable decomposition products.
The absence of the proposed Knoevenagel intermediate 4 (Scheme 1) during GC-MS reaction
monitoring caused concern in that the one-pot reaction may be limited by this condensation step.
Reaction of 2a with 3a in H2O (2 mL) at 120ºC (200 W) for 10 min followed by extraction with
CHCl3 provided the β-enone 4 in only 60% yield, indicating that both the formation of 4
accompanied by oxidation of the 2a were responsible for low yields of 8a. Studies by Zimmer13 et
al. discuss the necessity of acidic conditions to catalyze the formation of 4 and to avoid the 2:1
reaction of 3a:2a known to produce the consequent bis-Michael addition adduct. To accelerate the
formation of the Knoevenagel condensation process and simultaneously avoid aryl aldehyde
oxidation, we avoided aqueous conditions and optimized a general reaction medium consisting of

EtOAc:AcOH (1:1, 50 wt.%) as mild acidic reaction conditions. Using our experimental
conditions with MW heating at 140ºC for 8 min, we successfully produced 8a in 90% yield by
GC-MS monitoring, (Scheme 2) accompanied by 4-fluorobenzoic acid as the side product.
Aqueous work-up involving extraction into hot CHCl3 isolated 8a as a crystalline solid (53%) and
accompanied by an isomer (3%) with identical m/z that had a tendency to co-crystallize during
recrystallization from EtOH. After successive recrystallizations only a low yield of 8a (26%) of
sufficient purity suitable for bioassay was obtained and the structure was confirmed by X-ray
crystallography. The ORTEP diagram in Figure 1 clearly shows the planar isoxazolo[5,4-
b]pyridine system in 8a, as expected for the aromatized product having undergone oxidation after
cyclization.
Figure 1. X-ray crystal structure of 4-(4-fluorophenyl)-3-methyl-7H-1,6-dioxa-2,8-diaza-s-
indacen-5-one (8a) (ORTEP view). Hydrogen atoms have been removed for clarity and
displacements ellipsoids are scaled to the 50% probability level.
Scheme 2. MW-assisted MCR synthesis of isoxazolo[5,4-b]pyridine and isoxazolo[5,4-
b]quinolin-5(6H)-ones 8a-h.
Our optimized conditions were then applied to reactions involving 2a, 4-nitrobenzaldehyde
(2b) and 4-methylbenzaldehyde (2c) to monitor the influence of aryl substituents on the yield of 8.
The MCR reactions that gave the highest yields of 8 were the aryl aldehydes with electron

withdrawing substituents (Scheme 2), consistent with the increased aldehyde electrophilic effect,
enhancing reactivity and product yields. We found that the nature of the aromatic substituent can
influence the reaction between intermediates and the amine nucleophile 1, resulting in a
substantial decreased yield of 8h (Scheme 2).
Besides 3a, other 1,3-dicarbonyls were investigated including 1,3-indandione (3b) that forms
stable Knoevenagel condensation products (Scheme 3), however dimedone (3c) has been reported
to react with aromatic aldehydes to preferentially form the more stable bis-Michael adduct 1114
followed by dehydration to produce xanthenedione products 12 under acidic conditions (Scheme
4)15. Aldehydes bearing electron withdrawing substituents gave high yields of 8 for reactions
involving 3b and 3c, despite differences in the reactivity of intermediates formed.
In the one-pot process, side products appeared from undesired reactions between intermediates.
We therefore considered that investigations into the reaction mechanisms involved with this
family of reactions was warranted in order to determine the merits of MCR compared to a step-
wise synthesis.
Mechanisms of condensation reactions
To further investigate the molecular pathway to isoxazolo[5,4-b]quinolin-5(6H)-ones 8, the
Knoevenagel adducts 4 were prepared by MW irradiation at 140ºC for 8 min (Scheme 3). The
condensation of 2a with 3a in EtOAc:AcOH [1:1] gave comparable yields to neat AcOH and
provided 94% conversion to 4a by GC-MS analysis (Scheme 3). Reacting 2a with 3b gave 97%
conversion to 4b and showed no sign of byproduct formation or decomposition. The same reaction
at 273ºC over a 3 min reaction time gave quantitative conversion to 4b and demonstrated that this
intermediate is robust at higher temperatures. Similar results were obtained for the preparation of
4d.
Scheme 3. Stepwise pathway to product 8.

Remarkably, attempts to prepare 4c under the general conditions gave >90% conversion to the
bis-Michael product 11a after 30 seconds of MW heating as shown by NMR analysis.
Dehydration to the xanthenedione product 12a was complete upon MW heating for 1.5 min
(Scheme 4) and the structure was confirmed by X-ray crystallography. The ORTEP diagram in
Figure 2 shows the bent tricyclic ring in 12a formed upon dehydration of the bis-Michael adduct.
Both 2a and 2b with 3c formed the xanthenediones 12a and 12b and these derivatives were
isolated for use in further studies.
Figure 2. X-ray crystal structure of 9-(4-fluorophenyl)-3,4,5,6,7,9-hexahydro-3,3,6,6-tetramethyl-
1H-xanthene-1,8(2H)-dione (12a) (ORTEP view). Hydrogen atoms have been removed for clarity
and displacement ellipsoids are scaled to the 50% probability level.
The reaction between 1 and 4a occurred spontaneously in CDCl3 and was monitored by
1HNMR analysis whereby a water layer was visible suggesting this was the result of the
1
condensation reaction. The water of condensation was confirmed during HNMR studies,
appearing as a broad singlet at 2.05 ppm. However, neither Tu’s 5A (Scheme 1) or the bis-Michael
1
1
adduct were detected by H NMR or GC-MS analysis. Further HNMR monitoring revealed the
H2O condensation byproduct had hydrated 10A forming a white precipitate in CDCl3 and from
analysis in D6-DMSO gave the structure 10B (Scheme 3) that was corroborated by HRMS data.
This was also confirmed by using trifluoroacetic acid to revert 10B to 10A in D6-DMSO with
reaction monitoring by 1HNMR.
Previous MCR studies have shown that bis-Michael adducts 11 participate as intermediates in
the preparation of 12-aryl-8,9,10,12-tetrahydro-7H-benzo[b][4,7]phenanthrolin-11-ones14 and
tetrahydroacridones.10 The microwave reaction of 1 and 11b in EtOAc:AcOH (1:1) returned only
a mixture of 1 and 12b, whereas studies conducted in neutral media with catalytic amounts of
piperidine as described by Chung et al.10 returned complex mixtures. The base catalyzed reaction
i
between 11a and 3,4,5-trimethoxyaniline (14) in PrOH:H2O (9:1) with MW irradiation at 125 ºC
(100 W) for 7.5 min produced the tetrahydroacridone 15 isolated in 22% yield (Scheme 4). This
indicates that the nucleophilicity of the amine 1 is too weak to react with the bis-Michael adduct
11. Furthermore, the comparative product yields derived by the acid or base MW assisted routes to
15 found that our EtOAc:AcOH (1:1) conditions provided 88% and the base catalyzed medium
10% yield (Scheme 4).

Scheme 4. Acidic/Basic microwave-assisted reaction pathways to 8e and 15b.
We also investigated the obvious alternative condensation pathway between 1 and 2b that
resulted in formation of the imine (16) as shown in Scheme 5. Studies by Ma et al.16 reported that
imines act as an intermediate in the MCR preparation of spiro{isoxazolo[1,3]dioxanopyridine}-
4,6-diones. When 16 was microwaved for four minutes with 3c in EtOAc:AcOH [1:1] two
condensation products formed in a 1:1 ratio that were identified as 13e and 8e from which 13e
could be recrystallized in 31% yield (Scheme 5). Noteworthy is that longer MW reaction times
yielded 8e as the sole product. The structure of 13e was confirmed by X-ray crystallography. The
ORTEP diagram in Figure 3 shows a tetrahedral geometry around the benzylic carbon, as expected
for the unoxidised heterocyclic product 13e.
NO2
NO2
N
O
AcOH:EtOAc [1:1]
MW 140 oC, 4 min
cat. AcOH
EtOH, rt
O
O
N
+
1
2b
+
N
N
3c
20%
H
O
O
N
8e
N
H
NO2
13e
isolated 31%
16
[1:1]
Scheme 5. The imine reaction conditions and pathway that produced a 1:1 mixture of 13e and 8e.

Figure
tetrahydroisoxazolo[5,4-b]quinolin-5(6H)-one (13e) (ORTEP view). Hydrogen atoms have been
3.
X-ray
crystal
structure
of
3,7,7-trimethyl-4-(4-nitrophenyl)-4,7,8,9-
removed for clarity and displacement ellipsoids are scaled to the 50% probability level.
The failure to isolate dimedone-Knoevenagel adducts turned our attention to the utilization of
the ionic liquid buffer 2-hydroxyethylammonium formate (17) that had previously been used in
o
Knoevenagel condensations.17 The sonication at 2 C of a mixture of 2a and 3c in methanol with
17 for 45 minutes followed by solvent evaporation generated the required Knoevenagel adduct
hydro-4c in 94% yield. Attempts to isolate hydro-4c by column chromatography led to
decomposition and the product proved to be unstable during analysis in CDCl3 and D6-DMSO, but
was characterized by NMR in CD3OD. Freshly prepared hydro-4c underwent Michael addition
with 1 or 14 when microwaved for eight minutes in EtOAc:AcOH [1:1] to provide products 8c in
74% [quantitative GC] or 15a [> 94% GC] (Scheme 6).
Scheme 6. Two-step three-component microwave synthesis of 4-fluorophenyl-3,7,7-trimethyl-
isoxazolo[5,4-b]quinolin-5(6H)-one 8c and 3,3-dimethyl-9-[4-fluorophenyl]-1,2,3,4,9,10-
hexahydro-11,12,13-trimethoxy-]-acridine-1-one 15a.
Conclusion
We have found that the three-component MW assisted reaction pathway between aromatic
aldehydes, 1,3-cyclocarbonyls with 3-methylisoxazol-5-amine to furnish isoxazolo[5,4-b]pyridine,
4-aryl-3,7,7-trimethyl-isoxazolo[5,4-b]quinolin-5(6H)-one or
4-aryl-3,7,7-trimethyl-6,7,8,9-
tetrahydroisoxazolo[5,4-b]quinolin-5(6H)-one products, 8a-h always requires the formation of the
Knoevenagel adduct before undergoing Michael addition with the amine; whereas the bis-Michael

adducts 11a,b (Scheme 4) were unreactive with this amine. This was further confirmed whereby
only the two-step synthesis sequence via the ionic liquid assisted generation of the dimedone-
Knoevenagel adduct (hydro-4c, Scheme 6) provided the respective tricyclic products upon
microwave irradiation with the amines 1 or 14.
Appendix A. Supplementary Data
CCDC 1452875 (8a), CCDC 1452873 (12a) and CCDC 1452874 (13e) contains the
supplementary crystallographic data. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB12 1EZ, UK; fax; (+44) 1223-336-033; or email:
deposit@ccdc.cam.ac.uk.
Acknowledgements
We are grateful to Steven Privér and Sam Jackson for obtaining X-ray diffraction
crystallographic structures for our compounds and to Paul Morrison for assistance with high
resolution GC-MS analysis. We acknowledge Lisa Stevens for in kind research support.
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Abstract: The one-step three-component microwave assisted synthesis between aromatic aldehydes with
tetronic acid or 1,3-indandione readily produced Knoevenagel adducts that underwent addition of 3-
methylisoxazol-5-amine (1) to form the isoxazolopyridine products (8). With dimedone, the Knoevenagel
adduct was generated using 2-hydroxyammonium formate and then reacted with 1 to give pure products.