Phosphomolybdic acid (PMA) catalyzed highly efficient and rapid synthesis of ss-enaminones

Article abstract of DOI:10.1080/00397911.2010.524339

ss-Enamino derivatives have been synthesized by the condensation of amines and ss-dicarbonyl compounds in the presence of 1 mol% of phosphomolybdic acid (PMA) at room temperature in good to excellent yields. Taylor & Francis Group, LLC.

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Phosphomolybdic Acid (PMA) Catalyzed
Highly Efficient and Rapid Synthesis of β-
Enaminones
K. Nagaiah a , K. V. Purnima a , D. Sreenu a , S. Jhansi a , R. Srinivasa
Rao a & J. S. Yadav a
a Division of Organic Chemistry, Indian Institute of Chemical
Technology, Hyderabad, India
Accepted author version posted online: 20 Jul 2011.Published
online: 17 Oct 2011.
To cite this article: K. Nagaiah , K. V. Purnima , D. Sreenu , S. Jhansi , R. Srinivasa Rao & J. S. Yadav
(2012) Phosphomolybdic Acid (PMA) Catalyzed Highly Efficient and Rapid Synthesis of β-Enaminones,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 42:4, 461-468, DOI: 10.1080/00397911.2010.524339
To link to this article: http://dx.doi.org/10.1080/00397911.2010.524339
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Synthetic Communications1, 42: 461–468, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.524339
PHOSPHOMOLYBDIC ACID (PMA) CATALYZED
HIGHLY EFFICIENT AND RAPID SYNTHESIS
OF b-ENAMINONES
K. Nagaiah, K. V. Purnima, D. Sreenu, S. Jhansi,
R. Srinivasa Rao, and J. S. Yadav
Division of Organic Chemistry, Indian Institute of Chemical Technology,
Hyderabad, India
GRAPHICAL ABSTRACT
Abstract b-Enamino derivatives have been synthesized by the condensation of amines and
b-dicarbonyl compounds in the presence of 1 mol% of phosphomolybdic acid (PMA) at
room temperature in good to excellent yields.
Keywords Amines; di carbonyl compounds; b-enaminones; phosphomolybdic acid
INTRODUCTION
b-Enamino compounds are important functionalized building blocks for the
synthesis of a variety of valuable biologically active heterocyclic compounds,[1] which
are important intermediates in the synthesis of pharmaceuticals,[2] amino acids,[3a–e]
peptides,[3f] and alkaloids.[3g,h] Because of the importance of these compounds as
intermediates in organic synthesis, a number of synthetic methods have been
developed for their synthesis, including the reaction of amines and 1,3-dicarbonyl
compounds supported on silica with microwave irradiation,[4] silica-supported per-
chloric acid,[5] NaAuCl4 ꢀ 2H2O,[6] Zn(ClO4)2 ꢀ 6H2O,[7] CeCl3 ꢀ 7H2O,[8] bismuth(III)
triflouroacetate,[9] scandium(III) triflate,[10] erbium(III) triflate[11] or zirconium tetra
chloride,[12] cyclization of amino acids,[13] yetterbium(III) triflate,[14a] and ionic
liquids.[15] Recently, tungstophosphoric acid (PW) supported on different metal oxi-
des (TiO2, g-Al2O3, K-10, and KSF) and activated carbon were used in the synthesis
of enaminones.[16] In addition, these compounds have been prepared by direct
condensation of b-dicarbonyl compounds and primary amines under solvent-free
Received May 21, 2010.
Address correspondence to K. Nagaiah, Division of Organic Chemistry, Indian Institute of
Chemical Technology, Hyderabad-500007, India. E-mail: nagaiah@iict.res.in
461

462
K. NAGAIAH ET AL.
Scheme 1. Synthesis of b-enamine from b-carbonyl compound with amine.
conditions.[14b] However, these methods may suffer from one or more disadvantages
such as the use of expensive or less readily available reagents, vigorous reaction
conditions, longer reaction times, unsatisfactory yields, or the use of toxic solvents
that limit these methods to small-scale synthesis. Because of the importance of these
compounds in organic synthesis, the development of facile and ‘‘green’’ synthetic
methods to the b-enaminones under mild reaction conditions is still worthwhile.
Heteropoly acids (HPAs) are environmentally, commercially, and economi-
cally feasible solid acids because of their catalytic reactivies, ease of handling, cleaner
reactions in comparison to conventional catalysts, nontoxicity, and experimental
simplicity and hence regarded as green catalysts. Among the HPAs, phosphomolyb-
dic acid (PMA) is an inexpensive, commercially available catalyst. It is a new type of
acid that is different from the traditional Lewis acids such as AlCl3, BF3 ꢀ OEt2,
metal halides, and metal triflates. Therefore, in recent years, PMA has been exploited
as a promising and selective reagent for a variety of functional group transforma-
tions, especially as a HPA in dehydration of alcohols,[17] tetrahedropyranylation,[18]
Friedel–Crafts reactions,[19] aza-Diels–Alder reactions,[20] aziridinization of
olefines,[21] ring opening of aziridines,[22] and synthesis of homoallylic amines.[23] How-
ever, there is no report of the application of PMA as catalyst for the direct one-pot
condensation of b-dicarbonyl compounds with amines to produce b-enaminones.
As part of our program[20] in developing new selective, environmental friendly,
commercial, and economical methodologies for the synthesis of fine chemicals, we
were interested to find another enamination reaction. Herein we describe an efficient
method for the condensation of b-dicarbonyls with amines (aliphatic and aromatic)
in acetonitrile to afford enaminones in good to excellent yields using PMA as
Table 1. Optimization of the catalytic and solvent conditions for the reaction of
dimidone 1 with aniline 2aa
No.
Catalyst (mol%)
Solvent
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
1
1
Tolune
DCM
1.5
3.0
3.0
1.0
00
85
68
76
87
92
88
87
1
THF
CH3CN
0.5
1
00
00
00
2
00
00
5
aReaction conditions: dimidone (1 mmol), aniline (1 mmol), and solvent (5 ml).
bIsolated and unoptimized yields.

SYNTHESIS OF b-ENAMINONES
463
catalyst. Accordingly, b-dicarbonyl compound 1 with amine 2 in the presence of
1 mol% of PMA in acetonitrile to get the corresponding b-enamino compound 3
in 92% yield (Scheme 1) is described.
Initially we made a systematic study of this reaction for catalytic evolution of
PMA for the reaction of b-dicarbonyl compound 1 (dimedone) and aniline 2a with
various solvents with different catalyst loads (Table 1). The best result was obtained
with 1 mol% of the catalyst in acetonitrile as the solvent in terms of reaction time and
yields.
RESULTS AND DISCUSSIONS
To investigate the scope of the PMA-catalyzed synthesis of b-enaminones, the
reaction between dimedone and various amines was examined, and the results are
summarized in Table 2. To show the efficacy of this method, several aromatic amines
containing electron-withdrawing groups (such as chloro, fluoro, trifluoromethyl,
nitro groups) and electron-donating groups (such as alkyl, aryl, methoxy) were
treated with dimedone to afford the corresponding enaminones in good to excellent
yields. This clearly demonstrates that the electronic nature of substituents of the aro-
matic ring did not show any obvious effects in terms of yields under these reaction
conditions. Interestingly, aliphatic primary amines reacted more efficiently than aro-
matic amines with b-dicarbonyl compounds to afford the corresponding enaminones
in excellent yields. Also this method offers several advantages such as better yields,
shorter reaction times, cleaner reaction profiles, and simple experimental and
workup procedures. The additional advantage of this method is the survival of
various functional groups such as halo, alkyl, alkoxy, and nitro, thus allowing a wide
range of substitution patterns in the substrates. All reactions were clean and efficient,
and the products were obtained in good to excellent yields. The products were
purified by column chromatography on silica gel and determined by 1H NMR, mass,
and infrared (IR) spectroscopic data.
In conclusion, this procedure offers a highly efficient new procedure for the
PMA-catalyzed synthesis of b-enaminones in good to excellent yields by the reaction
of dimedone with aliphatic and aromatic amines. This method offers several advan-
tages such as good yields, short reaction times, clean reaction profiles, and simple
experimental and workup procedures.
EXPERIMENTAL
General Procedure
A mixture of dimedone (1 mmol), amines (1 mmol), and phosphomolybdic acid
(1 mol%) in CH3CN (5 ml) was stirred at room temperature for an appropriate time.
After completion of the reaction as indicated by thin-layer chromatography (TLC),
the reaction mixture was extracted with EtOAc (3 ꢁ 5 ml), and the organic layer was
washed with water and brine and dried over Na2SO4. The reaction mixture was
concentrated in vacuum, and the residue was purified by column chromatography
to afford the corresponding pure b-enamino compounds.

464
K. NAGAIAH ET AL.
Table 2. PMA-catalyzed synthesis of enamino compounds
Dicarbonyl
compound 1
Amine
(R-NH2) 2
Time
(h)
Yield
(%)b
Entry
Producta
3
=
a
R
Phꢂ
1.5
2.0
3.0
1.5
1.0
3.0
2.5
2.0
1.0
92
90
87
89
92
87
85
83
97
00
00
00
00
00
00
00
00
b
c
d
e
f
3-CI-4-CH3-C6H3ꢂ
2,4-Difluoro-C6H3-
4-CH3-C6H4ꢂ
3-CH3O-C6H4ꢂ
4-CF3-C6H4ꢂ
2-NO2-C6H4ꢂ
3-NO2-C6H4ꢂ
Ph-CH2ꢂ
g
h
i
(Continued )

SYNTHESIS OF b-ENAMINONES
465
Table 2. Continued
Dicarbonyl
compound 1
Amine
(R-NH2) 2
Time
(h)
Yield
(%)b
Entry
Producta
3
00
00
j
CH3-CH(CH3)ꢂ
1.0
1.0
97
95
k
aAll products were characterized by 1H NMR, IR, and mass spectroscopy.
bIsolated and unoptimized yields.
Spectral Data for Selected Compounds
5,5-Dimethyl-3-phenylamino-cyclohex-2-enone (3a). Yellow solid; mp:
115–117 ꢃC; IR (KBr): nmax 3236, 3060, 2953, 2888, 1629, 1599, 1571, 1492, 1446,
1
1241, 1150, 763, 706, 605, 519 cmꢂ1; H NMR (200 MHz, CDCl3): d 1.10 (s, 6H),
2.10 (s, 2H), 2.35 (s, 2H), 5.50 (s, 1H), 7.00–7.20 (m, 3H), 7.25–7.40 (m, 2H),
8.30–8.40 (brs, 1H, NH); EIMS: m=z (%): 215 (Mþ), 159, 144, 130, 118, 93, 92,
77, 65, 39.
3-(3-Chloro-4-methyl-phenylamino)-5,5-dimethyl-cyclohex-2-enone (3b).
Yellow solid; mp: 210–212 ꢃC; IR (KBr): nmax 3219, 2957, 2927, 1726, 1571, 1516,
1
1462, 1274, 1149, 786, 727 cmꢂ1; H NMR (200 MHz, CDCl3): d 1.10 (s, 6H), 2.10
(s, 2H), 2.22 (s, 3H), 2.40 (s, 2H), 4.80 (s, 1H), 7.00–7.20 (m, 2H,), 7.30 (d, J ¼ 7.8 Hz,
1H,), 8.50 (brs, 1H, NH); EIMS: m=z (%): 263 (Mþ), 221, 167, 149, 95, 69, 57.
3-(2,4-Difluoro-phenylamino)-5,5-dimethyl-cyclohex-2-enone (3c).
Yellow solid; mp: 192–194 ꢃC; IR (KBr): nmax 3240, 3051, 2959, 2891, 1607, 1578,
1
1505, 1424, 1244, 1142, 961, 861, 815, 723, 656 cmꢂ1; H NMR (400 MHz, CDCl3):
d 1.10 (s, 6H), 2.10 (s, 2H), 2.38 (s, 2H), 5.00 (s, 1H), 6.90 (m, 2H), 7.25 (m, 1H), 8.20
(brs, 1H, NH); EIMS: m=z(%): 251 (Mþ), 195, 155, 141, 121, 113, 93, 66, 39.
5,5-Dimethyl-3-p-tolylamino-cyclohex-2-enone (3d). Yellow solid; mp:
202–204 ꢃC; IR (KBr): nmax 3243, 3183, 3057, 2954, 1609,1576, 1516, 1409, 1275,
1
1239, 1145, 810, 715 cmꢂ1; H NMR (200 MHz, CDCl3): d 1.10 (s, 6H), 2.10 (s,
2H), 2.34 (s, 5H), 5.35 (s, 1H), 7.00–7.10 (m, 4H), 8.20 (brs, 1H, NH); EIMS: m=z
(%): 229 (Mþ), 199, 172, 143, 130, 106, 91, 77, 55, 38.
3-(3-Methoxy-phenylamino)-5,5-dimethyl-cyclohex-2-enone (3e).
Yellow solid; mp: 188–190 ꢃC; IR (KBr): nmax 3257, 3034, 2957, 1580, 1539, 1490,
1
1430, 1368, 1248, 1151, 1044, 778, 700 cmꢂ1; H NMR (200 MHz, CDCl3): d 1.05
(s, 6H), 2.10 (s, 2H), 2.32 (s, 2H), 3.75 (s, 3H), 5.50 (s, 1H), 6.55–6.75 (m, 3H), 7.15
(t, J ¼ 8.2 Hz, 1H), 8.30 (brs, 1H, NH); EIMS: m=z (%): 245 (Mþ), 230, 95, 69, 55.

466
K. NAGAIAH ET AL.
5,5-Dimethyl-3-(4-trifluoromethyl-phenylamino)-cyclohex-2-enone (3f).
Brown solid; mp: 201–203 ꢃC; IR (KBr): nmax 3244, 3183, 3105, 3047, 2959, 1614,
1
1581, 1525, 1413, 1327, 1109, 855, 811, 697 cmꢂ1; H NMR (200 MHz, CDCl3): d
1.10 (s, 6H), 2.12 (s, 2H), 2.38 (s, 2H), 5.60 (s, 1H), 7.30 (d, J ¼ 8.0 Hz, 2H), 7.55
(d, J ¼ 8.0 Hz, 2H), 8.75 (brs, 1H, NH); EIMS: m=z (%): 283 (Mþ), 240, 227, 198,
141, 68, 57.
5,5-Dimethyl-3-(2-nitro-phenylamino)-cyclohex-2-enone
(3g). Yellow
solid; mp: 163–165 ꢃC; IR (KBr): nmax 3236, 3179, 3022, 2955, 1611, 1577, 1521,
1
1353, 1240, 1144, 860, 781, 655 cmꢂ1; H NMR (200 MHz, CDCl3): d 1.10 (s, 6H),
2.15 (s, 2H), 2.42 (s, 2H), 5.60 (s, 1H), 7.20 (m, 1H), 7.60 (d, J ¼ 7.2 Hz, 2H), 8.10
(d, J ¼ 8.2 Hz, 1H), 8.70 (brs, 1H, NH); EIMS: m=z (%): 260 (Mþ), 234, 206, 177,
162, 133, 95, 84, 56.
5,5-Dimethyl-3-(3-nitro-phenylamino)-cyclohex-2-enone
(3h). Yellow
solid; mp: 172–174 ꢃC; IR (KBr): nmax: 3442, 3259, 3195, 3118, 3064, 2958, 1614,
1
1578, 1535, 1352, 1263, 1241, 1146, 795, 677 cmꢂ1; H NMR (200 MHz, CDCl3): d
1.10 (s, 6H), 2.18 (s, 2H), 2.40 (s, 2H), 5.60 (s, 1H), 7.30–7.40 (m, 2H), 7.90
(m, 1H), 8.00 (m, 1H), 8.80 (brs, 1H, NH); EIMS: m=z (%): 260 (Mþ), 231, 203,
195, 141, 97, 83, 55.
3-Benzylamino-5,5-dimethyl-cyclohex-2-enone (3i). Yellow solid; mp:
126–128 ꢃC; IR (KBr): nmax: 3224, 3030, 2954, 2866, 1594, 1498, 1362, 1277, 1230,
1
1148, 999, 873, 772, 728, 690 cmꢂ1; H NMR (200 MHz, CDCl3): d 1.05 (s, 6H),
2.00 (s, 2H), 2.25 (s, 2H), 4.25 (m, 2H), 4.85 (s, 1H), 7.10–7.15 (brs, 1H, NH),
7.20–7.40 (m, 5H); EIMS: m=z (%): 229 (Mþ), 169, 155, 141, 128, 115, 91, 69, 57, 41.
3-Isopropylamino-5,5-dimethyl-cyclohex-2-enone (3j). White solid; mp:
136–138 ꢃC; IR (KBr): nmax 3266, 3073, 2954, 2924, 2862, 1543, 1430, 1378, 1248,
1177, 1153, 797, 695 cmꢂ1; 1H NMR (200 MHz, CDCl3): d 1.05 (s, 6H), 1.19 (s, 3H),
1.21 (s, 3H), 2.05 (s, 2H), 2.15 (s, 2H), 3.55–3.56 (m, 1H), 4.95 (s, 1H), 6.35–6.45 (brs,
1H, NH); EIMS: m=z (%): 181 (Mþ), 180, 165, 152, 138, 125, 110, 96, 82, 77, 55, 41.
5,5-Dimethyl-3-(1-phenyl-ethylamino)-cyclohex-2-enone
(3k). Yellow
solid; mp: 106–108 ꢃC; IR (KBr): nmax 3270, 3055, 2958, 2924, 2862, 1600, 1536,
1
1447, 1376, 1249, 1150, 1124, 760, 698 cmꢂ1; H NMR (200 MHz, CDCl3): d 1.05
(s, 6H), 1.44 (d, J ¼ 7.17 Hz, 3H), 2.00 (s, 2H), 2.25 (s, 2H), 4.35–4.45 (m, 1H),
4.71 (s, 1H), 6.95–7.10 (brs, 1H, NH), 7.15–7.35 (m, 5H); EIMS: m=z (%): 243
(Mþ), 157, 105, 83, 77, 41.
ACKNOWLEDGMENTS
K. V. P., D. S., and R. S. R. thank CSR and S. J. thanks UCG for awarding
the fellowships.
REFERENCES
1. (a) Boyd, G. V. In The Chemistry of Amidines and Imidates, vol. 2; S. Patai, Z. Rappoport,
(Eds.); John Wiley & Sons: Chichester, New York, 1991, 339; (b) Spivey, A. C.; Srikaran, R.;

SYNTHESIS OF b-ENAMINONES
467
Diaper, C. M.; Turner, D. J. Traceless solid-phase synthesis of 2-substituted pyrimidines
using an ‘‘off-the-shelf’’ chlorogermane-functionalised resin. Org. Biomol. Chem. 2003,
1638; (c) Hassneen, H. M.; Abdallah, T. A. New routes to pyridino[2,3-d]pyrimidin-4-one
and pyridino-[2,3-d]triazolino[4,5-a]pyrimidin-5-one derivatives. Molecules 2003, 333.
2. (a) Sweeney, T. R.; Strube, R. E. In Burger’s Medicinal Chemistry, Part II, 4th ed.; M. E. Wolff
(Ed.); Wiley: New York, 1979; p. 333; (b) Foster, J. E. : Nicholson, J. M.; Butcher, R.; Stables,
J. P.; Edafiogho, I. O.; Goodwin, A. M.; Henson, M. C.; Smith, C. A.; Scott, K. R. Synthesis,
characterization and anticonvulsant activity of enaminones, part 6: Synthesis of substituted
vinylic benzamides as potential anticonvulsants. Bioorg. Med. Chem. 1999, 2415.
3. (a) Palmieri, G.; Cimarelli, C. Stereoselective reduction of Enantiopure b-Enamino esters
by hydride: A convenient synthesis of both Enantiopure b-amino esters. J. Org. Chem.
1996, 5557; (b) Bartoli, G.; Cimarelli, C.; Marcantoni, E.; Palmieri, G.; Petrini, M.
Chemo- and diastereoselective reduction of b-Enamino esters: A convenient synthesis of
both cis- and trans-c-Amino alcohols and b-amino esters. J. Org. Chem. 1994, 5328; (c)
Lubell, W. D.; Kitamura, M.; Noyori, R. Enantioselective synthesis of p-Amino acids
based on BINAP-Ruthenium(II) catalyzed hydrogenation. Tetrahedron: Asymmetry
1999, 10915; (d) Potin, D.; Dumas, F.; D’Angelo, J. New chiral auxiliaries: Their use in
the asymmetric hydrogenation of beta-acetamidocrotonates. J. Am. Chem. Soc. 1990,
3483; (e) Cimarelli, C.; Palmiri, G.; Volpini, E. An improved synthesis Of b–aminoacids.
Synth. Com. 2001, 2943; (f) Beholz, L. G.; Benovsky, R.; Ward, D. L.; Barta, N. S.; Stille,
J. R. Formation of dihydropyridone- and pyridone-based peptide analogs through
Aza-Annulation of b-Enamino ester and amide substrates with a-Amido acrylate deriva-
tives. J. Org. Chem. 1997, 1033; (g) David, O.; Blot, J.; Bellec, C.; Fargeau-bellassoue, M.
C.; Haviari, G.; Celerier, J. P.; Lhommet, G.; Gramain, J. C.; Gardette, D. Enamino ester
reduction: A short Enantioselective route to pyrrolizidine and indolizidine alkaloids.
Synthesis of (þ)-laburnine, (þ)-tashiromine, and (ꢂ)-isoretronecanol. J. Org. Chem.
1999, 3122; (h) Ferraz, H. M. C.; Pereira, F. L. C.; Leite, F. S.; Nunes, M. R. S.; Payret-
Arru´a, M. E. Synthesis of N-substituted pyrrole and tetrahydroindole derivatives from
alkenyl b-dicarbonyl compounds. Tetrahedron 1999, 10915.
4. Rechsteiner, B.; Texier-Bollet, F.; Hamelin, J. Synthesis in dry media coupled with micro-
wave irradiation : Application to the preparation of enaminoketones. Tetrahedron Lett.
1993, 5071.
5. Das, B.; Venkateswarlu, K.; Majhi, A.; Reddy, M. R.; Reddy, K. N.; Rao, Y. K.;
Ravikumar, K.; Sridhar, B. Highly efficient, mild, and chemo- and stereoselective
synthesis of enaminones and enamino esters using silica-supported perchloric acid under
solvent-free conditions. J. Mol. Catal. A: Chem. 2006, 246, 276.
6. Arcadi, A.; Bianchi, G.; Giuseppe, S. D.; Marinelli, F. Gold catalysis in the reactions of
1,3-dicarbonyls with nucleophiles. Green Chem. 2003, 5, 64.
7. Bartoli, G.; Bosco, M.; Locatelli, M.; Marcantoni, E.; Malchiorre, P.; Sambri, L.
Zn(ClO4)2 ꢀ 6H2O as a powerful catalyst for the conversion of b-Ketoesters into
b-enamino esters. Synlett 2004, 239.
8. Khodaei, M. M.; Khosropour, A. R.; Kookhazadeh, M. Enamination of b-dicarbonyl
compounds catalyzed by CeCl3 ꢀ 7H2O at ambient conditions: Ionic liquid and solvent-free
media. Synlett 2004, 1980.
9. Khosropour, A. R.; Khodaei, M. M.; Kookhazadeh, M. A mild, efficient, and environ-
mentally friendly method for the regio- and chemoselective synthesis of enaminones using
Bi(TFA)3 as a reusable catalyst in aqueous media. Tetrahedron Lett. 2004, 1725.
10. Yadav, J. S.; Naveen Kumar, V.; Srinivasa Rao, R.; Diana Priyadarshini, A.;
Purushothama Rao, P.; Reddy, B. V. S.; Nagaiah, K. Sc(OTf)3-catalyzed highly rapid
and efficient synthesis of b-enamino compounds under solvent-free conditions. J. Mol.
Catal. A. Chem. 2006, 256, 234.

468
K. NAGAIAH ET AL.
11. Dalpozzo, R.; De Nino, A.; Nardi, M.; Russo, B.; Procopio, A. Erbium(III) triflate: A
valuable catalyst for the synthesis of aldimines, ketimines, and enaminones. Synthesis
2006, 1127.
12. Lin, J.; Zhang, L. F. ZrCl4-catalyzed efficient synthesis of enaminones and Enamino esters
under solvent-free conditions. Monatsh. Chem. 2007, 77, and references cited therein.
13. Turunen, B. J.; George, G. I. Amino acid–derived enaminones: A study in ring formation
providing valuable asymmetric synthons. J. Am. Chem. Soc. 2006, 8702.
14. (a) Epifano, F.; Genovese, S.; Curini, M. Ytterbium triflate–catalyzed synthesis of
b-enaminones. Tetrahedron Lett. 2007, 2717; (b) Vohra, R. K.; Renaud, J. L.; Bruneau,
C. Tertiary 3-Aminopropenones and 3-Aminopropenoates: Their preparation, with and
without Lewis acids, from secondary amines and 1,3-Diketo compounds. Synthesis.
2007, 731.
15. Karthikeyan, G.; Perumal, P. T. Ionic liquid–promoted simple and efficient synthesis
of _enamino esters and _enaminones from 1,3-dicarbonyl compounds: One-pot, three-
component reaction for the synthesis of substituted pyridines. Can. J. Chem. 2005, 83,
1746.
16. Rafiee, E.; Mahdavi, H.; Eavani, S.; Joshaghani, M.; Shiri, F. Catalytic activity of tung-
stophosphoric acid supported on carriers of diverse acidity in the synthesis of enaminones.
Appl. Catal. A: Gen. 2009, 352, 202.
´
´
17. Vazquez, P.; Pizzio, L.; Caceres, C.; Blanco, M.; Thomas, H.; Alesso, E.; Finkielsztein, L.;
Lantan˜o, B.; Moltrasio, G.; Aguirre, J. Silica-supported heteropolyacids as catalysts in
alcohol dehydration reactions. J. Mol. Catal. A: Chem. 2000, 161, 223.
´
´
18. Romanelli, G.; Vazquez, P.; Pizzio, L.; Quaranta, N.; Autino, J.; Blanco, M. Caceres. C.
Phenol tetrahydropyranylation catalyzed by silica-alumina supported heteropolyacids
with Keggin structure. Appl. Catal. A: Gen. 2004, 163.
´
´
19. Pizzio, L. R.; Vazquez, P. G.; Caceres, C. V.; Blanco, M. N.; Alesso, E. N.; Torviso, M.
R.; Lantan˜o, B.; Moltrasio, G. Y.; Aguirre, J. M. C-Alkylation reactions catalyzed by
silica-supported Keggin heteropolyacids. Appl. Catal. A: Gen. 2005, 287, 1.
20. Nagaiah, K.; Sreenu, D.; Srinivasa Rao, R.; Vashishta, G.; Yadav, J. S. Phosphomolybdic
acid–catalyzed efficient one-pot three-component aza-Diels–Alder reactions under solvent-
free conditions: A facile synthesis of trans-fused pyrano- and furanotetrahydroquinolines
Tetrahedron Lett. 2006, 4409.
21. Kishore Kumar, G. D.; Baskaran, S. Heteropoly acid as a novel nitrene transfer agent:
A facile and practical aziridination of olefins with chloramine-T. Chem. Commun. 2004,
1026.
22. Kishore Kumar, G. D.; Baskaran, S. Opening of N-tosyl aziridines with nucleophiles
catalyzed by phosphomolybdic acid supported on silica gel. Synlett. 2004, 1719.
23. Smitha, G.; Bruhaspathy, M.; Williamson, J. S. Phosphomolybdic acid-catalyzed efficient
three-component reaction: A facile synthesis of protected homoallylic amines. Synlett.
2005, 839.