Aldonitrones as aldehyde equivalents: An efficient, green, and novel protocol for the synthesis of 1,8-dioxo-octahydroxanthenes

Article abstract of DOI:10.1080/00397911.2012.736584

A novel, self-catalyzed, solvent-free, microwave-enhanced, green, and efficient protocol for the synthesis of 1,8-dioxo-octahydroxanthenes and bis-5,5-dimethyl-1,3-cyclohexanediones by condensing aldo-nitrones (imine oxide) and dimedone (5,5-dimethyl-1,3-

Full text of DOI:10.1080/00397911.2012.736584

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Aldonitrones as Aldehyde Equivalents:
An Efficient, Green, and Novel Protocol
for the Synthesis of 1,8-Dioxo-
octahydroxanthenes
Dhruva Kumar a , Suresh a & Jagir S. Sandhu a
a Department of Chemistry , Punjabi University , Patiala , Punjab ,
India
Published online: 08 Jul 2013.
To cite this article: Dhruva Kumar , Suresh & Jagir S. Sandhu (2013) Aldonitrones as
Aldehyde Equivalents: An Efficient, Green, and Novel Protocol for the Synthesis of 1,8-Dioxo-
octahydroxanthenes, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 43:20, 2739-2747, DOI: 10.1080/00397911.2012.736584
To link to this article: http://dx.doi.org/10.1080/00397911.2012.736584
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Synthetic Communications1, 43: 2739–2747, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.736584
ALDONITRONES AS ALDEHYDE EQUIVALENTS: AN
EFFICIENT, GREEN, AND NOVEL PROTOCOL FOR THE
SYNTHESIS OF 1,8-DIOXO-OCTAHYDROXANTHENES
Dhruva Kumar, Suresh, and Jagir S. Sandhu
Department of Chemistry, Punjabi University, Patiala, Punjab, India
GRAPHICAL ABSTRACT
Abstract A novel, self-catalyzed, solvent-free, microwave-enhanced, green, and efficient
protocol for the synthesis of 1,8-dioxo-octahydroxanthenes and bis-5,5-dimethyl-1,3-
cyclohexanediones by condensing aldo-nitrones (imine oxide) and dimedone (5,5-dimethyl-
1,3-cyclohexanedione) under microwave irradiation is disclosed. Product formation (viz.
xanthenes and bis-5,5-dimethyl-1,3-cyclohexanediones) is dependent on reaction con-
ditions. The entire process is green and sustainable in terms of solvents, chemicals, reaction
procedure, and yields. A plausible reaction mechanism of this reaction is also proposed.
Supplementary materials are available for this article. Go to the publisher’s online
edition of Synthetic Communications1 for full experimental and spectral details.
Keywords Bis-5,5-dimethyl-1,3-cyclohexanediones;
dimedone
(5,5-dimethyl-1,3-
cyclohexanedione); 1,8-dioxo-octahydroxanthenes; microwave irradiation (MWI);
nitrones; solvent-free
INTRODUCTION
Nitrones (imine oxides) are reputed 1,3-dipoles and have been extensively
explored for the synthesis of five-membered heterocycles by combining them with
several types of multiple bonds.[1] Apart from this major utility, their general
Received August 22, 2012.
Address correspondence to Jagir S. Sandhu, Department of Chemistry, Punjabi University, Patiala
147002, Punjab, India. E-mail: j_sandhu2002@yahoo.com
2739

2740
D. KUMAR, SURESH, AND J. S. SANDHU
chemistry has been little studied.[1c] There are few reports of successful 1,3-additions
of nitrones.[2] In contrast, their counterpart imines have been extensively explored to
expose their utility as aldehyde equivalent.[3]
Within oxygen heterocycles, xanthenes gained much attention because of their
numerous applications in medicinal chemistry and material science such as anti-
viral,[4] antibacterial,[5] and anti-inflammatory activities.[6] They also act as sensiti-
zers in photodynamic therapy (PDT) for destroying the tumor cells.[7] Xanthenes
are further used in the dye industry,[8] as antagonists for the paralyzing action of
zoxazolamine,[9] as pH-sensitive fluorescent materials for visualization of biomole-
cules,[10] in biodegradable agrochemicals,[11] in cosmetics and pigments,[12] and in
laser technology because of their spectroscopic properties.[13] Furthermore,
xanthenes-based dyes are extracted naturally from soil and from plants such as
Indigofera longeracemosa.[14]
Because of these extensive utilities of these molecules, the search for efficient,
facile synthesis of these molecules continues to attract much attention, and to
achieve this goal several catalysts have been used, such as Yb(PFO)3,[15] SmCl3,[16]
p-dodecylbenzenesulfonic acid,[17] para-toluencesulfonic acid (pTSA),[18] methane-
sulfonic acid,[19] sulfamic acid,[20] Amberlyst-15,[21] NaHSO4SiO2 or WO3=ZrO2,[22]
diammoniumhydrogen
phosphate,[23]
HClO4-SiO2,[24]
HClO4-SiO2
and
PPA-SiO2,[25] Yb(OTf)3,[26] InCl3 Á 4H2O,[27] silica-bonded N-propyl sulfamic acid
(SBNPSA),[28] b-cyclodextrin,[29] Dowex 50w,[30] cyanuric chloride,[31] polyaniline-p-
proline
toluenesulfonate,[32]
LiBr,[36]TiO2=SO42À
ZrOCl2 Á 8H2O,[33]
H3PW12O40,[34]
triflate,[35]
,
[37] ionic liquids,[38] and ultrasound irradiation[39]. Some of these
already reported procedures are not very efficient, require expensive catalyst and
harsh reaction conditions, and leave harmful waste on workup. There is still scope
for development of a green, sustainable process.
The present protocol is the first ever use of aldo-nitrones for the preparation of
1,8-dioxo-octahydroxanthenes and bis-5,5-dimethyl-1,3-cyclohexanediones using
various environmentally benign reaction conditions. This reaction plausibly pro-
ceeds via addition–elimination pathway followed by cyclization to afford the desired
products in very good to excellent yields.
RESULTS AND DISCUSSION
Initially, a series of nitrones was prepared using a variety of aldehydes and
hydroxyl amine as per already reported methods.[40] A mixture of freshly prepared
N-phenyl-N-phenylmethylidenamine oxide (10 mmol) 1a and 5,5-dimethyl-1,3-
cyclohexanedione (20 mmol) 2 was stirred under different reaction conditions
(Table 1). In the absence of solvents, the reaction did not proceed up to 15 h
(Table 1, entry 1). Among solvents, distilled water and ethyl alcohol only accelerated
the reaction in a minimum reaction time at ambient temperature (Table 1, entry 4,5)
and afforded 3a in 86 and 90% yields respectively. Under these reaction conditions,
only bis-5,5-dimethyl-1,3-cyclohexanedione products were obtained (Table 2)
instead of cyclized product 4 (Scheme 1).
Following the same reaction procedure, a variety of nitrones 1a–i were reacted
with 5,5-dimethyl-1,3-cyclohexanedione in water and EtOH at ambient temperature
with similar success. Nitrones bearing electron-withdrawing 1d–f, 1h–i and donating

1,8-DIOXO-OCTAHYDROXANTHENES
2741
Table 1. Reaction of 1a and 2 under different solventsa
Entry
Solvent
Time (h)
Yield (%)
1
2
3
4
5
6
7
No solvent
EtOAc
PEG
15
9
–
56
45
86
90
39
12
5
H2O
EtOH
3
1
CHCl3
Toluene
14
13
aReaction conditions: 1 (10 mmol), 2 (20 mmol), and solvent (10 mL)
stirred at rt. PEG stands for polyethylene glycol.
Table 2. Synthesis of bis-5,5-dimethyl-1,3-cyclohexanedione 3a–i at ambient temperature
Time (h)
Yield (%)
Entry
Aldo-nitrone
Producta
A
B
A
B
Melting point (ꢀC)
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
3a
3b
3c
3d
3e
3f
3.0
4.0
4.5
3.5
3.5
3.0
4.5
3.0
4.0
1.0
1.5
2.0
1.5
1.5
1.0
2.0
2.0
2.0
86
88
85
85
87
91
86
89
88
90
91
88
90
92
93
91
90
90
192–194[25]
139–141[16]
146–148[25]
196–197[36d]
185–187[25]
143–144[36d]
188–190[16]
203–204[37]
188–190[25]
1g
1h
1i
3g
3h
3i
aThe products were characterized by comparison of their melting points and spectral (IR, 1H NMR)
data with those of authentic samples. A and B stand for reaction conditions H2O, rt and EtOH, rt,
respectively.
Scheme 1. Synthesis of xanthenes 4a–h and bis-5,5-dimethyl-1,3-cyclohexanediones 3a–i.

2742
D. KUMAR, SURESH, AND J. S. SANDHU
Table 3. Effects of solvent variations under MWIa
Entry
Solvent
Time (min)
Yield 4a (%)
1
2
3
4
5
Solvent free
H2O
2.0
2.0
2.0
0.5
2.0
84
82
80
89
84
H2O–EtOH (50:50)
PEG
PEG–H2O (50:50)
aReaction conditions: A mixture of 1 (10 mmol), 2 (20 mmol), and solvent
(5 mL) was irradiated under MWI.
group 1b–c on the aldehyde ring did not have significant effect either on rate of
reaction or on yields of bis-products 3a–i (Table 2).
Further, the same reaction was investigated under microwave irradiations
(Prolabo Microwave Module Synthwave S-402) using different reaction conditions
(Table 3). Interestingly, in all cases reaction of N-phenyl-N-phenylmethylidenamine
oxide 1 and 5,5-dimethyl-1,3-cyclohexanedione 2 under microwave irradiations
(Scheme 1) afforded only 1,8-dioxo-octahydroxanthenes 4a–h, in almost comparable
yields and reaction time (Table 3).
As per current awareness of green and sustainable chemistry, reaction con-
ditions were optimized as condensation of N-phenyl-N-phenylmethylidenamine
oxide 1a and 5,5-dimethyl-1,3-cyclohexanedione 2 was carried out under microwave
irradiations without any solvent or catalyst to afford product 4a in a fraction of a
minute (Table 4). This procedure is fairly large in scope and is extendable to several
nitrones 1b–i to obtain xanthenes 4a–h (for other details, see Table 4).
In further study of this investigation, when bis-5,5-dimethyl-1,3-cyclo
hexanediones 3a–h were subjected to microwave irradiations for 1–2 min in the
presence of polyethylene glycol and solvent-free conditions, cyclized products were
obtained 4a–h in good to excellent yields.
Table 4. Synthesis of 1,8-dioxo-octahydroxanthenes under MWI
Time (min)
Yield %
Entry
Aldo-nitrone
Producta
A
B
A
B
Melting point (ꢀC)
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
4a
4b
4c
4d
4e
4f
2.0
0.5
1.0
1.5
0.5
0.5
1.0
1.0
1.5
84
86
84
88
86
87
87
85
89
90
88
92
91
92
91
90
205–206[37a]
216–217[37a]
241–243[37a]
226–228[37a]
190–192[25]
230–233[33]
246–248[37a]
221–223[37a]
2.5
3.0
2.0
2.0
2.0
2.5
3.0
1g
1i
4g
4h
aThe products were characterized by comparison of their melting points and spectral (IR, 1H NMR)
data with those of authentic samples. A and B stand for reaction conditions solvent-free, MWI and
PEG, MWI, respectively.

1,8-DIOXO-OCTAHYDROXANTHENES
2743
Scheme 2. Plausible reaction mechanism for the syntheses of 3a–i and 4a–h.
A plausible mechanism for this reaction could be via nucleophilic attack of 2
onto electron-deficient imino carbon of nitrone 1, yielding intermediate 5, which
after elimination can follow two routes (as shown in Scheme 2). This unstable
intermediate can adopt two routes, A and B. In route A the Knoevenagel product
of 5,5-dimethyl-1,3-cyclohexanedione (viz., 2-benzylidene-5,5-dimethylcyclo-
hexane-1,3-dione 6) is formed via a simple addition–elimination reaction, in which
one molecule of 2 adds to aldehydic carbon of nitrone 1 with elimination of phenlhy-
droxylamine followed by nucleophilic addition of a second molecule of 2 in a
Michael way to afford 3a (Scheme 2). In route B, without the formation of any
Knoevenagel product 6, nucleophilic addition of the second molecule of 2 occurs
on intermediate 5, followed by elimination of phenylhydroxylamine to afford 3a,
Which undergoes dehydration under microwave heating to afford 4a (Scheme 2).
Overall, this reaction seems to be self-catalyzed as nitrones are highly polar.
Facile addition of dimedone could take place on these with concommittant elimin-
ation of phenlhydroxylamine. In this sequence, one can presume that either basicity
of the eliminated base or acidity of the dimidione is responsible for the autocatalysis.
CONCLUSION
In summary, the present protocol is an efficient, green, sustainable, convenient,
and mild strategy for the synthesis of 1,8-dioxo-octahydroxanthenes and bis-5,5--
dimethly-1,3-cyclohexanedione derivatives. The method reported herein, in contrast
to earlier ones, does not employ any toxic or expensive catalysts or stringent reaction
conditions and is free of any corrosive or unwanted by-products. Desired products
are obtained in very good to excellent yields.
EXPERIMENTAL
Melting points were determined in open capillaries and are uncorrected.
Reagent-grade chemicals were purchased from a commercial source and used

2744
D. KUMAR, SURESH, AND J. S. SANDHU
without further purification. The reactions were carried out in Prolabo Microwave
Module Synthwave S-402 (preliminary experiments were carried out in a domestic
microwave oven). Infrared (IR) spectra were recorded in KBr discs on a Perkin-Elmer
1
240C analyzer. H NMR spectra were recorded on a Bruker Avance II 400 NMR
spectrometer using tetramethylsilane (TMS) as internal standard. Elemental analysis
were recorded on a Vario Micro V1.9.6 analyzer. The progress of the reaction was
monitored by thin-layer chromatography (TLC) using silica gel G (Merck).
Typical General Methods
Synthesis of bis-5,5-dimethyl-1,3-cyclohexanedione (3a–i). A mixture of
nitrone (10 mmol) 1, 5,5-dimethyl-1,3-cyclo-hexandione (20 mmol) 2, and distilled
water (10 ml) or ethyl alcohol (10 ml) was stirred at ambient temperature for an
appropriate time (see Table 2). Reaction progress was monitored via TLC. After
reaction completion, the crude product was precipitated out in aqueous media and
in ethyl alcohol precipitates formed on addition of cold water. The obtained product
was filtered, dried, and (for further purification) recrystallized from ethanol.
Synthesis of 1,8-dioxo-octahydroxanthenes (4a–h). A mixed reaction
mixture of nitrone (10 mmol) and 5,5-dimethyl-1,3-cyclo-hexandione (20 mmol)
using solvent-free or polyethylene glycol (5 mL) was irradiated in an Erlenmeyer
flask under microwave irradiations for an appropriate time (see Table 4). Reaction
progress was monitored via TLC. After reaction completion, the reaction mass
was cooled and stirred with cold water (10 mL) for 15 min. The obtained product
was filtered, dried, and (for further purification) recrystallized from ethanol.
Representative Spectral Data
Compound 3a. IR (KBr): 3063, 2962, 1593, 1448, 1372 cmÀ1 1H NMR
.
(400 MHz, CDCl3): d 11.90 (s, 1H, OH), 11.59 (s, 1H, OH), 7.28–7.10 (m, 5H,
ArH), 5.54 (s, 1H, CH), 2.48–2.17 (m, 8H, 4 CH2), 1.26 (s, 6H, 2 CH3), 1.10 (s,
6H, 2 CH3). Anal. calcd. for C23H28O4: C,74.97; H, 7.66. Found: C, 75.01 H, 7.60.
Compound 4a. IR (KBr): 3312, 2957, 1662, 1626, 1466, 1362 cmÀ1. 1H NMR
(400 MHz, CDCl3): d 7.29–7.26 (m, 2H, ArH), 7.23–7.19 (m, 2H, ArH), 7.11–7.07
(m, 1H, ArH), 4.74 (s, 1H, CH), 2.46 (s, 4H, 2 CH2), 2.25–2.21 (d, 2H, CH2),
2.18–2.14 (d, 2H, CH2) 1.10 (s, 6H, 2 CH3), 0.98 (s, 6H, 2 CH3). Anal. calcd. for
C23H26O3: C, 78.83; H, 7.48. Found: C, 78.99 H, 7.31.
Supporting Information
Full experimental details, IR, 1H NMR, and elemental analyses with literature
references for all compounds are provided online.
ACKNOWLEDGMENTS
The authors are thankful to the Council of Scientific and Industrial Research,
New Delhi, India, for financial assistance and to the Indian National Science

1,8-DIOXO-OCTAHYDROXANTHENES
2745
Academy for additional financial support for this research project. The authors are
also thankful to the Sophisticated Analytical Instrument Facility Central Instrument
Laboratory (SAIF-CIL), Panjab University, Chandigarh, for spectral analysis.
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