An efficient synthesis of pyrano [4,5-c] pyrrole derivatives through microwave-accelerated intramolecular Knoevenagal hetero Diels-Alder reaction

Article abstract of DOI:10.1016/j.tetlet.2009.09.136

Synthesis of novel pyranopyrrole derivatives has been achieved by a one-pot IMHDA reaction of N-substituted aldehydes with various diones in the presence of ethylene diaminediacetate (EDDA) in excellent yields under mild conditions.

Full text of DOI:10.1016/j.tetlet.2009.09.136

Tetrahedron Letters 50 (2009) 6886–6890
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An efficient synthesis of pyrano [4,5-c] pyrrole derivatives through
microwave—accelerated intramolecular Knoevenagal hetero
Diels–Alder reaction
*
Mathesan Jayagobi, Raghavachary Raghunathan
Department of Organic Chemistry, University of Madras, Guindy Campus, Chennai 600 025, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of novel pyranopyrrole derivatives has been achieved by a one-pot IMHDA reaction of N-substi-
tuted aldehydes with various diones in the presence of ethylene diaminediacetate (EDDA) in excellent
yields under mild conditions.
Received 7 August 2009
Revised 18 September 2009
Accepted 23 September 2009
Available online 27 September 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Hetero Diels–Alder reaction
N-Substituted aldehydes
1,3-Diones
Microwave
Pyrano[4,5-c]pyrroles
The intramolecular Diels–Alder reaction is a widely used meth-
od for the synthesis of six-membered heterocyclic compounds.1a–e
Many heterocycles such as quinolines, pyrroles, and pyrans have
received enormous attention from synthetic chemists from the
standpoint of both their preparation and their reactivity. The syn-
thesis of pyran and its hydrogenated form has gained importance,
since many natural products such as carbohydrates, talaromycines,
milbemycines, avermectins, pheromones, and iridoids contain pyr-
an skeleton2a–e and possess interesting biological activities with
potential medical applications.3a,b The domino hetero Diels–Alder
reaction is one of the most important methods for the construction
of heterocyclic compounds.4a–h In the present work, we propose to
utilize hetero Diels–Alder reaction as a tool for the construction of
pyranopyrroles which are structurally related to natural products.
Symmetrical 1,3-diones as heterodienes have been subjected to
intramolecular cycloaddition reaction for the synthesis of complex
polycyclic heterocycles.5 A number of reports are available for the
synthesis of pyrans and their benzopyran analogues but there is no
literature available on pyranopyrrole derivatives.6a–f
our presentstudy, we haveused N-alkenylaldehyde for thefirst time
in an intramolecular hetero Diels–Alder reaction for the synthesis of
pyranopyrrole derivatives in a stereoselective manner.
The strategy we have developed begins with the reaction of cyclic
1,3-dicarbonyl compounds and N-alkenyl aldehydes in the presence
of EDDA in toluene. The required substrates for hetero Diels–Alder
reaction were prepared in three steps from commercially available
starting materials. In the first step, sulfonylation of ethanolamines
1a–b with tosyl chloride under standard PTC reaction conditions
proceeded well to provide the sulfonamide derivatives 2a–b. N-
Alkylation of sulfonamides 2a–b with prenyl bromide/cinnamyl
bromide in the presence of potassium carbonate yielded N-tosyl-
N-prenyl/cinnamyl-aminoethanols 3a–d which on oxidation with
IBX in DMSO afforded the required N-alkenyl aldehydes 4a–d in
good yield (98%) (Scheme 1).
R1
R
R2
O
R
Ts
HO
H
N
Ts
(i)
(iii)
N
(ii)
H2N
Ts
N
R
R
In the course of our investigation directed toward the synthesis of
pyrrole derivatives, herein we report a precise and new route for the
synthesis of pyranopyrrole derivatives based on tandem Knoevena-
gel HDA reaction. Domino hetero Diels–Alder reaction has been well
exploited by Tietze for the synthesis of polycyclic compounds.7a,b In
HO
1a-b
OH
R2
2a-b
R1
3a-d
4a-d
R
= H, CH3CH2
R1 = H, CH3
R2 = CH3, C6H5
Scheme 1. Synthesis of N-alkenyl aldehydes. Reagents and conditions: (i) TBAB,
10% NaOH/benzene, tosyl chloride, 0 °C–rt; 8 h, 90%; (ii) prenyl/cinnamyl bromide
K2CO3/acetone, 12 h, 90%; (iii) iodoxybenzoic acid, DMSO, 2–2.5 h, 98%.
* Corresponding author. Tel.: +91 44 22202811; fax: +91 44 22300488.
E-mail address: ragharaghunathan@yahoo.com (R. Raghunathan).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.136

M. Jayagobi, R. Raghunathan / Tetrahedron Letters 50 (2009) 6886–6890
6887
R
O
N
O
O
N
O
O
Ts
R
N
N
Ts
N
N
O
O
R1
R2
4a-d
4a = R = H, R1 = R2 = Me
5
R2
R1
4b = R = Et, R1 = R2 = Me
4c = R = H, R1 =H, R2 = Ph
4d = R = Et, R1= H, R2 = Ph
R1
Ha
R1 R2
Ha
R2
O
O
Ts
N
Ts
N
R
N
N
Hb
O
Hb
O
R
N
O
N
O
6a = R = H, R1= R2 = Me
6b = R = Et, R1= R2 = Me
6c = R = H, R1 = H, R2 = Ph
6d = R = Et, R1 = H, R2 = Ph
7a = R = H, R1= R2 = Me
7b = R = Et, R1= R2 = Me
7c = R = H, R1 = H, R2 = Ph
7d = R = Et, R1 = H, R2 = Ph
Scheme 2. Synthesis of pyranopyrroles through IMKHDA reaction. Reagents and
conditions: Method A: EDDA, toluene, reflux; Method B: toluene, MW.
The alkenyl aldehydes so obtained were reacted with 1,3-
diones under suitable reaction conditions to achieve the synthesis
of pyranopyrrole ring systems through intramolecular Knoevena-
gel hetero Diels–Alder reaction. Thus, the reaction of alkenyl alde-
hyde 4a with barbituric acid 5 in the presence of EDDA in reflux
in toluene proceeded through intramolecular Knoevenagel HDA
pathway to afford a mixture of pyranopyrrole 6a and 7a (35:65)
with an overall yield of 62%. The products were separated by col-
umn chromatography and the structures were assigned on the
basis of spectroscopic studies. The 1H NMR spectrum of com-
pound 6a exhibited two singlets at d 1.22 and 1.49 for geminal
Figure 1. ORTEP diagram of 6b.
dimethyl protons, a singlet at d 2.42 for the aromatic methyl,
and two singlets at d 3.27and 3.28 for N-CH3 protons of the
pyrimidine ring. The N-CH2 protons of the pyrrolidine ring were
observed as multiplets in 2.96–3.04 (2H) and two doublet of dou-
blets at d 3.58 (J = 9.0, 9.0, 1H) and d 4.36 (J = 6.0, 9.0, 1H). Partic-
ularly diagnostic were the protons Ha and Hb situated at the ring
Table 1
Reaction times and yields of the domino reactions of 4a–d with 5 under various conditions
Entry
Aldehydes
1,3-Diones
Conditions
Ratio of the products
Time
Cis
Trans
Overall yield (%)
O
N
O
Ts
N
1
Method A
Method B
6 h
2 min
35
30
65
70
62
74
O
O
O
N
O
O
O
4a
5
O
O
O
Ts
N
N
2
3
Method A
Method B
6 h
2 min
20
15
80
85
67
80
N
5
4b
Ts
N
O
N
Method A
Method B
6 h
2 min
35
30
65
70
60
72
N
H
5
4c
O
O
Ts
N
N
4
Method A
Method B
6 h
2 min
33
25
67
75
63
75
O
N
O
H
5
4d
Reaction conditions: Method A: toluene, reflux; Method B: toluene, MW.

6888
M. Jayagobi, R. Raghunathan / Tetrahedron Letters 50 (2009) 6886–6890
junctions showing doublets of triplets at d 1.89 (J = 6.0, 12.0) and
d 2.48 (J = 6.0, 12.0), respectively. On the other hand, the 1H NMR
spectrum of 7a exhibited one doublet of triplet at d 2.59 (J = 9.0,
9.0) and one multiplet at d 2.96–3.04 for Ha and Hb protons,
respectively. The low value of the coupling constant (6 Hz) be-
tween Ha and Hb in 6a indicated the cis fusion at the ring junc-
tions. Similar type of d values were observed for trans product 7a,
but the trans fusion at the ring junction was discerned by the
high coupling constant value (9 Hz).
To improve the reaction yields and chemoselectivity, we carried
out the reaction under microwave conditions.8a–f We were pleased
to find that the reaction in toluene under microwave irradiation
without base worked well to provide adducts 6a and 7a (30:70)
with an overall yield of 74%. Thus, there was an increase in the
Figure 2. ORTEP diagram of 7b.
Table 2
Reaction times and yields of the domino reactions of 4a–d with various symmetrical 1,3-diones under various conditions
Entry
Aldehydes
1,3-Diones
Conditions
Ratio of the products
Trans
Time
Cis
Overall yield (%)
O
Ts
N
1
Method A
Method B
6 h
2 min
31
30
69
70
63
75
O
O
8
4a
O
Ts
N
2
Method A
Method B
6 h
2 min
25
20
75
80
66
80
O
O
8
4b
O
Ts
N
3
Method A
Method B
6 h
2 min
23
15
77
80
60
77
O
O
H
8
4c
O
Ts
N
4
5
6
Method A
Method B
6 h
2 min
19
15
81
85
64
81
O
O
H
8
4d
O
O
O
Ts
N
Method A
Method B
8 h
2.5 min
18
15
80
85
57
68
4a
11
O
O
O
Ts
N
Method A
Method B
8 h
2 min
17
15
83
85
60
75
11
4b

M. Jayagobi, R. Raghunathan / Tetrahedron Letters 50 (2009) 6886–6890
6889
Table 2 (continued)
Entry
Aldehydes
1,3-Diones
Conditions
Ratio of the products
Trans
Time
Cis
Overall yield (%)
O
Ts
N
O
7
Method A
Method B
8 h
2.5 min
12
10
88
90
58
68
H
O
11
4c
O
Ts
N
O
8
Method A
Method B
8 h
2.5 min
15
11
85
89
63
72
H
O
11
4d
Reaction conditions: Method A: toluene, reflux; Method B: toluene, MW.
chemical yield with a slight improvement in the chemoselectivity.
Encouraged by these findings, we expanded the scope of the hetero
Diels–Alder reaction by changing the alkenyl substituent of the
internal dipolarophile. Thus, the reaction of 4b with barbituric acid
5 (Scheme 2) was tested under optimized reaction conditions and
the results are summarized in Table 1. The substrate 4b also re-
acted in the same way as 4a, but gave better yield of the product
in shorter duration of time. These results show the efficiency of
the prenyl-substituted dienophile.
Having optimized the reaction conditions for the stereoselective
synthesis of pyrano[4,5-c]pyrrole derivatives, we conducted the
intramolecular Knoevenagel hetero Diels–Alder reaction by
employing various symmetrical diones, namely dimedone and in-
dane-1,3-dione with aldehydes 4a–d (Schemes 3 and 4). The re-
sults are summarized in Table 2. The intramolecular Knoevenagel
hetero Diels–Alder reaction of 4a–d with indane-1,3-dione in
refluxing toluene completed in 6 h (Scheme 4). As observed in
the former case, we readily isolated the expected cis and trans pyr-
ano[4,5-c]pyrrole derivatives simply by changing from toluene/re-
flux conditions to microwave irradiation.9
O
R
R
O
O
Ts
N
Ts
N
O
R1
O
R2
R2
R1
11
4a-d
4a = R = H, R1 = R2 = Me
4b = R = Et, R1 = R2 = Me
4c = R = H, R1 =H, R2 = Ph
4d = R = Et, R1= H, R2 = Ph
R1 R2
O
R1 R2
Ha
Ha
O
Ts
N
Ts
N
Hb
R
R
Hb
O
O
13a = R = H, R1= R2 = Me
13b = R = Et, R1= R2 = Me
13c = R = H, R1 = H, R2 = Ph
13d = R = Et, R1 = H, R2 = Ph
12a = R = H, R1= R2 = Me
12b = R = Et, R1= R2 = Me
12c = R = H, R1 = H, R2 = Ph
12d = R = Et, R1 = H, R2 = Ph
Scheme 4. Synthesis of pyranopyrroles through IMKHDA reaction. Reagents and
conditions: Method A: toluene, reflux; Method B: toluene, MW.
The structures of the cis and trans products were determined on
the basis of detailed 2D NMR studies. The structures of the cycload-
ducts were further unambiguously supported by the single crystal
X-ray diffraction analysis of some of the products (see Figs. 1 and
2).10a,b Similarly, the protocol was extended further to other 1,3-
diones and alkenyl aldehydes 4a–d to form a series of pyranopyr-
roles in moderate to good yields.
In conclusion, we have synthesized cis and trans isomers of the
pyranopyrrole derivatives through intramolecular domino Knoe-
venagel hetero Diels–Alder reaction. The preceding studies dem-
onstrate the scope of the intramolecular Knoevenagel hetero
Diels–Alder reaction for the synthesis of cis and trans pyrrolo
pyrrole products. We have shown that use of microwave irradia-
tion improves the overall yield of the products as well as
stereoselectivity.
R
O
R
O
Ts
EDDA
N
Ts
N
reflux, toluene
12h
O
O
O
R2
R1
R2
R1
8
4a-d
4a = R = H, R1 = R2 = Me
4b = R = Et, R1 = R2 = Me
4c = R = H, R1 =H, R2 = Ph
4d = R = Et, R1= H, R2 = Ph
1 R2
O
R1
Ha
R
R2
O
Ha
Acknowledgment
Ts
N
Ts
N
R
Hb
O
Hb
O
M.J. and R.R. thank UGC New Delhi for financial support. We are
grateful to Dr. K.K. Balasubramanian, Shasun Chemicals Ltd. for
Microwave Synthesiser facility.
R
10a = R = H, R1= R2 = Me
10b = R = Et, R1= R2 = Me
10c = R = H, R1 = H, R2 = Ph
10d = R = Et, R1 = H, R2 = Ph
9a = R = H, R1= R2 = Me
9b = R = Et, R1= R2 = Me
9c = R = H, R1 = H, R2 = Ph
9d = R = Et, R1 = H, R2 = Ph
References and notes
Scheme 3. Synthesis of pyranopyrroles through IMKHDA reaction. Reagents and
1. For reviews of the intramolecular Diels–Alder reaction, see: (a) Roush, W. R.. In
conditions: Method A: toluene, reflux; Method B: toluene, MW.
Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon:

6890
M. Jayagobi, R. Raghunathan / Tetrahedron Letters 50 (2009) 6886–6890
Oxford, UK, 1991; pp 513–550. Chapter 4.4; (b) Borger, D. L.; Weinreb, S. M.
removal of the solvent, the crude reaction mixture was subjected to column
chromatography using hexane–ethyl acetate (7.5:2.5) mixture.
Hetero-Diels–Alder Methodology in Organic Synthesis; Academic Press: San
Diego, CA, 1987; (c) Oppolzer, W. Angew. Chem., Int. Ed. Engl. 1984, 23,
876–889; (d) Tietze, L. F.; Geissler, H.; Fennen, J.; Brumby, T.; Brand, S.; Schulz,
G. J. Org. Chem. 1994, 59, 182–191; (e) Taber, D. F. Intramolecular Diels–Alder and
Alder-Ene Reactions; Springer: Berlin, 1984.
Representative spectral data of the products; Compound 6a: white solid; mp:
180–182 °C; IR (KBr): 1695.3, 1637.4, 1340.4, 1159.1 cmÀ1 1H NMR (300 MHz,
;
CDCl3) d 1.22 (s, 3H),1.49 (s, 3H), 1.89 (dt, Ha, J = 6.0, 12.0 Hz), 2.42 (s, 3H), 2.48
(dt, Ha, J = 6.0, 12.0 Hz), 2.96–3.04 (m, 2H), 3.27 (s, 3H), 3.28 (s, 3H), 3.58 (dd,
1H, J = 9.0, 9.0 Hz), 4.36 (dd, 1H, J = 6.0, 9.0 Hz), 7.33 (d, 2H, J = 9.0 Hz), 7.40 (d,
2H J = 6.0 Hz); 13C NMR: 20.67, 21.49, 27.70, 28.16, 28.96, 33.81, 47.63, 49.65,
52.20, 83.87, 86.54, 127.26, 129.82, 134.54, 143.57, 150.97, 155.79, 161.92. MS
m/z: 419.79 (M+); Anal. Calcd for C20H25N3O5S: C, 57.19; H, 5.97; N, 9.98.
Found: C, 57.31; H, 6.12; N, 10.10.
2. (a) Tietze, L. F.; Geissler, H.; Fennen, J.; Brumby, T.; Brand, S.; Shulz, G. J. Org.
Chem. 1994, 59, 182–191. and references cited therein; (b) House, H. O.; Cronin,
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Pelletier, S. W., Ed.; Wiley: New York, 1986; Vol. 4, pp 1–274; (b) Foder, G. B.;
Colasanti, B.. In Alkaloids: Chemical and Biological Perspectives; Pelletier, S. W.,
Ed.; Wiley: New York, 1985; Vol. 3, pp 1–90; (c) Buomora, P.; Olsen, J. C.; Oh, T.
Tetrahedron 2001, 57, 6099–6138; (d) Carruthers, W. In Cycloaddition Reactions
in Organic Synthesis; Pergamon: New York, 1990; (e) Boger, D. L. Combining C–C
Compound 7a: white solid; mp: 160–162 °C; IR (KBr): 1695.3, 1647.1,
1342.4,1159.1 cmÀ1 1H NMR (300 MHz, CDCl3) d 1.24 (s, 3H), 1.41 (s, 3H),
;
2.40 (s, 3H), 2.59 (dt, Ha, J = 9.0, 9.0 Hz), 2.96–3.04 (m, 2H), 3.14 (s, 3H), 3.20 (s,
3H), 3.58–3.65 (m, 2H), 3.98 (d, 1H, J = 12.0 Hz), 7.17 (d, 2H, J = 9.0 Hz), 7.57 (d,
2H J = 9.0 Hz); 13C NMR: 21.57, 25.02, 26.25, 27.61, 28.13, 30.87, 32.68, 45.21,
47.34, 51.74, 80.45, 85.07, 127.26, 128.91, 133.57, 143.73, 150.42, 154.18,
162.21. MS m/z: 419.86 (M+); Anal. Calcd for C20H25N3O5S: C, 57.28; H, 6.00; N,
10.02. Found: C, 57.12; H, 6.11; N, 10.18.
Compound 9a: white solid; mp: 146–148 °C; IR (KBr): 1619.4, 1334.7,
1155.1 cmÀ1 1H NMR (300 MHz, CDCl3) d 1.01 (s, 6H), 1.09 (s, 3H), 1.36 (s,
;
p
-Bonds. In Comprehensive Organic Chemistry; Paquette, L. A., Ed.; Pergamon
Press: Oxford, 1991; Vol. 5, pp 451–512; (f) Ho, T.-L. Tandem Organic Reactions;
Wiley: New York, 1992; (g) Ziegler, F. E. Combining C–C -Bonds. In
3H), 1.78 (dt, Ha, J = 6.0, 12.0 Hz), 2.18 (m, 5H), 2.42 (s, 3H), 2.89 (m, H, Hb), 3.51
(dd, 1H, J = 6.0, 9.0 Hz),4.33 (dd, 1H, J = 6.0, 12.0 Hz), 7.31 (m, 2H), 7.73 (d, 2H
J = 6.0 Hz); 13C NMR: 20.54, 21.51, 27.90, 28.28, 28.59, 32.04, 33.67, 42.44,
47.56, 49.75, 50.45, 52.09, 80.07, 109.75, 127.26, 129.74, 134.82, 143.34,
169.66, 196.66. MS m/z: 403.32 (M+);Anal. Calcd for C22H29NO4S: C, 65.32; H,
7.21; N, 3.42. Found: C, 65.45; H, 7.10; N, 3.50.
p
Comprehensive Organic Chemistry; Paquette, L. A., Ed.; Pergamon Press:
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Ed. Engl. 1993, 32, 131–163.
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Compound 10a: white solid; mp: 199–201 °C; IR (KBr): 1612.2, 1334.6,
6. (a) Ciganek, E.. In Organic Reactions; Dauben, W. G., Ed.; John Wiley and Sons:
New York, 1984; Vol. 32, p 79; (b) Ingal, A. H.. In Comprehensive Heterocyclic
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820; (b) Tietze, L. F. Chem. Rev. 1996, 96, 115–136.
1159.1 cmÀ1 1H NMR (300 MHz, CDCl3) d 0.736 (s, 3H), 0.94 (s, 3H), 1.17
;
(s,3H), 1.29 (s, 3H), 1.87–2.11 (m, 4H), 2.39–245 (m, 1H), 240 (s, 3H), 2.87 (t,
1H, J = 9.0 Hz), 2.98 (t,1H, J = 6.0), 3.37–3.41 (distorted dt, 1H, J = 3.0, 12.0 Hz),
3.50–3.64 (m, 2H), 7.28 (d, 2H, J = 6.0 Hz), 7.65 (d, 2H J = 6.0 Hz); 13C NMR:
21.47, 25.17, 26.36, 27.56, 28.30, 31.80, 31.86, 42.67, 45.30, 47.46, 50.70, 52.30,
80.05, 109.27, 127.65, 129.61, 134.65, 143.15, 168.53, 197.48. MS m/z: 403.92
(M+); Anal. Calcd for C22H29NO4S: C, 65.41; H, 7.16; N, 3.37. Found: C, 65.52; H,
7.26; N, 3.52.
Compound 12a: white solid; mp: 149–151 °C; IR (KBr): 1721.1, 1335.7,
1158.1 cmÀ1 1H NMR (300 MHz, CDCl3) d 1.24 (s, 3H), 1.52 (s, 3H), 1.80 (dt,
;
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Lett. 2006, 47, 2265–2270; (e) Shanmugasundram, M.; Manikandan, S.;
Raghunathan, R. Tetrahedron 2002, 58, 997–1000; (f) Manian, D. R. S.;
Jayashankaran, J.; Raghunathan, R. Tetrahedron Lett. 2007, 48, 1385–1389.
9. General procedure for the intramolecular domino Knoevenagel hetero Diels–Alder
reaction: Method A: To a solution of 4-hydroxyquinolinone (1 mmol) in toluene
the corresponding 2(N-prenyl-N-tosylamino)acetaldehyde (1 mmol) and the
base EDDA (1 mmol) were added and the reaction mixture was refluxed, till the
completion of the reaction. The solvent was then evaporated under reduced
pressure and the residue was subjected to column chromatography using
hexane–ethyl acetate (7.5:2.5) mixture.
Ha, J = 9.0, 12.0 Hz), 2.28–2.38 (m, 1Hb), 2.95–3.05 (m, 2H), 3.57 (dd, Ha, J = 6.0,
12.0 Hz), 4.17 (dd, 1H, J = 6.0, 12.0 Hz), 7.07–7.76 (m, 8ArH); 13C NMR: 20.64,
21.50, 27.98, 33.05, 47.64, 50.63, 50.70, 83.57, 106.47, 118.13, 121.14, 127.30,
129.84, 130.31, 131.88, 133.10, 134.66, 137.44, 143.61, 174.06. MS m/z: 409.43
(M+); Anal. Calcd for C23H23NO4S: C, 67.31; H, 5.68; N, 3.28. Found: C, 67.43; H,
5.51; N, 3.39.
Compound 13a: white solid; mp: 171–173 °C; IR (KBr): 169.3, 1334.7,
1159.1 cmÀ1 1H NMR (300 MHz, CDCl3) d 1.30 (s, 3H), 1.46 (s, 3H), 2.00 (s,
;
3H), 2.52 (dt, Ha, J = 6.0, 12.0 Hz), 2.90 (t, 1H, J = 12.0), 2.97 (t, 2H, J = 6.0 Hz),
3.55–3.64 (m, 2H), 3.57 (d, 1H, J = 12.0 Hz), 6.91–7.53(m, 8ArH); 13C NMR:
21.42, 25.83, 26.44, 31.13, 45.95, 46.79, 50.74, 80.63, 106.54, 117.72, 120.74,
127.16, 129.32, 129.99, 131.78, 133.28, 137.41, 143.46, 172.61. MS m/z: 409.36
(M+); Anal. Calcd for C23H23NO4S: C, 67.49; H, 5.51; N, 3.45. Found: C, 67.60; H,
5.62; N, 3.59.
Method B: A solution of 1,3-diones (1 mmol) and the corresponding aldehyde
(1 mmol) in toluene (2 ml) without base was irradiated with microwave
(MODEL = Chem Discover benchmate microwave 300 W, P = 100, T = 110 °C,
20 MHz) until the TLC showed the disappearance of the starting material. After
10. (a) Chinnakali, K.; Sudha, D.; Jayagobi, M.; Raghunathan, R.; Fun, H.-K. Acta
Crystallogr., Sect. E 2007, 63, o4434–o4435; (b) Chinnakali, K.; Sudha, D.;
Jayagobi, M.; Raghunathan, R.; Fun, H.-K. Acta Crystallogr., Sect. E 2009, 65,
o1862–o1863.