Diastereoselective synthesis of cyclopentanediols by InCl3/Al mediated intramolecular pinacol coupling reaction in aqueous media

Article abstract of DOI:10.3390/molecules13102652

A "green" and practical intramolecular pinacol coupling reaction promoted by InCl3/Al catalysts in aqueous media has been developed. Under mild conditions, a novel class of polysubstituted cyclopentane-1,2-diols have been obtained with excellent diastereo

Full text of DOI:10.3390/molecules13102652

Molecules 2008, 13, 2652-2658; DOI: 10.3390/molecules13102652
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Communication
Diastereoselective Synthesis of Cyclopentanediols by InCl3/Al
Mediated Intramolecular Pinacol Coupling Reaction in Aqueous
Media
Yunhua Chen 2, Jieping Wan 1, Chunyan Wang 1 and Cuirong Sun 1,*
1
Department of Chemistry, Zhejiang University, Hangzhou, 310027, P.R. China
2
Zhejiang Hisun Pharm. Co. Ltd. Taizhou, 318000, P.R. China
* Author to whom correspondence should be addressed; Email: suncuirong@zju.edu.cn.
Received: 2 August 2008; in revised form: 14 October 2008 / Accepted: 15 October 2008 / Published:
27 October 2008
Abstract: A “green” and practical intramolecular pinacol coupling reaction promoted by
InCl3/Al catalysts in aqueous media has been developed. Under mild conditions, a novel
class of polysubstituted cyclopentane-1,2-diols have been obtained with excellent
diastereoselectivity.
Keywords: Intramolecular; Pinacol coupling; Indium, Aluminium; Aqueous media
Introduction
In organic synthesis, the formation of carbon-carbon bonds is one of the most fundamental and
important operations in the construction of functional molecules. Pinacol coupling, which involves the
homo- or cross reductive dimerization of two carbonyls represents one of the most classical ways of
generating carbon-carbon bonds [1]. Since its discovery in 1859 [2], the pinacol coupling reaction has
been a focal point in synthetic chemistry not only for its carbon-carbon bond generating power, but
also for the wide utility of the 1,2-diols obtained from these reactions. These 1,2-diols are valuable
synthons in the synthesis of biologically active natural products or molecular fragments [3-4].
After the first report over a century ago, numerous modified pinacol coupling methods have been
developed. Instead of the early use of sodium as catalyst, a variety of other metals or metal/Lewis
acids such as Zn [5], Mg [6], Ti [7], Sm [8-9], Cr [10], In [11], and VCl3/Al [12], etc. have been

Molecules 2008, 13
2653
proven to be efficient metal sources for pinacol coupling reactions. Hirao et al. [12] reported the main
pinacol coupling protocol that is successful in water. Recently, another water-mediated pinacol
coupling process has been reported by Hosono and co-workers [13]. However, most of the reported
catalyst systems have unfavorable properties due to the toxicity of the metal catalyst(s) and/or the use
of toxic organic solvents as reaction media. Therefore, environmentally benign methodologies to
produce this valuable reaction are highly desirable. Unlike the corresponding intermolecular reactions,
intramolecular pinacol coupling reactions furnish cyclized diols, which usually limits their applications
[14]. Much fewer systematic intramolecular pinacol coupling methods are available than their
intermolecular counterparts [15-16].
Indium as a nontoxic metal has been extensively studied due to its versatile catalytic activity and
tolerance to water and air. A number of organic reactions have been successfully achieved in water or
in aqueous media employing indium or indium salts as catalysts [17-18]. It is also our interest to
explore broader application of indium as a practical metal catalyst [19-21]. In our previous work
studying indium catalysis, we developed InCl3/Al catalyzed pinacol coupling reactions in aqueous
media, in which a class of vicinal diols derived from aromatic aldehydes, ketones and aldehyde-ketone
cross coupling have been synthesized in good yields [22]. In order to further explore the scope of this
catalyst system, we have also investigated the intramolecular coupling reactions of 1,5-dicarbonyl
substrates, and we present here our results from this study.
Results and Discussion
Screening of reaction temperature
Based on the results of our intermolecular pinacol coupling method, we applied the InCl3/Al co-
catalyst system to the intramolecular coupling reaction of 1,3,5-triphenylpentane-1,5-dione, which
contains a 1,5-dicarbonyl skeleton. At the presence of 1 equivalent mol InCl3 (0.5 mol scale) and 3.5
equivalent mol of aluminum powder, the substrate was heated at 80 oC for 8 h in 4 mL EtOH/H2O (1:1)
to gave the target diol 2a in 51 % isolated yield. We then optimized the temperature on this model
reaction, and found that at 70 oC, the pinacol coupling product was formed as the main product after 8
h, and it could be isolated in 65 % yield with column chromatography on silica gel. It is noteworthy
that at room temperature, the reaction also proceeded well, but much longer time is required for
satisfactory conversion.
Investigation of the scope of the InCl3/Al catalyzed intramolecular pinacol coupling
With the proper conditions in hand, we than turned to examine the scope of application of this
protocol for the intramolecular pinacol coupling reaction. A group of 1,5-dicarbonyl substrates
analogous to 1,3,5-triphenylpentane-1,5-diketone were subjected to the reaction. The representative
results are summarized in Table 1. Symmetrical aromatic 1,5-diketones with diversified functional
groups furnished the corresponding five-membered vicinal 1,2-diols in moderate to good yields. While
alkyl, alkoxy as well as halogen substitutions were tolerated by this catalyst system, the amino group
didn’t give the expected product. Unsymmetrical 1,5-diketones and 1,4-ketones (entries 13 and 14)

Molecules 2008, 13
2654
were also subjected to the reaction conditions, but unfortunately none of the corresponding pinacols
were observed under the standard conditions. What should be noted is that most of the substrates gave
the corresponding diols with excellent diastereoselectivity, as only one diastereoisomer was observed
after chromatography purification. However, the highly symmetrical structure of the products and the
1
lack of vicinal hydrogen atom make it impossible to identify their relative configuration by H-NMR
[23]. Further work on this issue is presently in progress in our group.
Figure 1. Intramolecular pinacol coupling of aromatic 1,5-diketones.
R1
R1
InCl3/Al
O
O
HO
OH
NH4Cl, EtOH/H2O
R2
R2
R2
1
R2
2
Table 1. InCl3/Al mediated intramolecular pinacol coupling or various aromatic 1,5-diketones.a
Entry
Diketone
Product
Yield (%)b
d. e. (%)c
R1
H
R2
H
1
2
2a
2b
2c
2d
2e
2f
2g
2h
2i
65
78
> 99
> 99
58
H
4-CH3
4-CH3O
4-Cl
H
3
H
75
4
H
62
> 99
> 99
> 99
> 99
44
5
4-CH3
4-Cl
4-CH3O
2-Cl
4-CH3
4-F
80
6
H
83
7
H
88
8
H
61
9
4-CH3
H
90
> 99
> 99
-
10
11
12
2j
-
79
4-N(CH3)2
H
trace
Cl
O
O
-
-
-
13d
H3CO
O
14e
nr
O
a
Reaction conditions: 0.5 mmol 1, 0.5 mmol InCl3, 1.8 mmol aluminum powder and 0.5 g NH4Cl
o
b
mixed in 4 mL solvent (VEtOH:VH2O=1:1) and stirred at 70 C for 8 h. Isolated yield.
c
Diastereomeric excess was calculated from the 1H-NMR of products based on clearly
distinguishable signals. d Complex mixture of reduced products were formed in the reaction, but no
target diol was observed in the reaction mixture by ESI-MS. e No conversion was observed.

Molecules 2008, 13
Experimental
General
2655
All 1,5-dicarbonyl compounds were synthesized following literature procedures [24-25].
InCl3·4H2O was dehydrated by reflux in SOCl2 prior to use, and all other chemicals used in the
experiments were obtained from commercial sources and used without further purification. The
reactions were carried out under an open atmosphere. Melting points were determined using an XT-4
1
apparatus and were not corrected. H- and 13C-NMR spectra were recorded in CDCl3 on a Bruker
AVANCE DMX-500 spectrometer at 500 and 125 MHz, respectively. Chemical shifts are reported in
ppm (δ), relative to an internal tetramethylsilane (TMS) standard. Mass spectra were recorded on a
Bruker Esquire 3000plus mass spectrometer (Bruker-Franzen Analytik GmbH Breman, Germany)
equipped with an ESI interface and ion trap analyzer.
General procedure of the intramolecular pinacol coupling reaction
1,5-Dicarbonyl compound (0.5 mmol), InCl3 (0.5 mmol), Al powder (1.8 mmol) and NH4Cl (0.5 g)
were placed in a vessel, mixed solvent (H2O: EtOH=1:1, 4 mL) was added and the mixture was stirred
at 70 oC for 8 h. The completion of the reaction was monitored with TLC. After cooling down to rt, the
mixture was extracted with ethyl ether (3×10 mL). The combined organic layer was dried overnight
with anhydrous sodium sulphate. After removal of solvent, the residue was subjected to silica gel
chromatography to give the corresponding product.
1,2,4-Triphenylcyclopentane-1,2-diol (2a). White solid; m.p. 133-135 oC; 1H-NMR: δ = 7.34 (d, 4H, J
= 5.5 Hz), 7.25 (t, 1H, J = 6.0 Hz), 7.00 (t, 10H, J = 8.0), 4.06-3.98 (m, 1H), 3.41 (s, 2H), 2.75 (dd,
13
2H, J1 = 10.4 Hz, J2 = 3.4 Hz), 2.52 (dd, 2H, J1 = 8.9 Hz, J2 = 5.0 Hz); C-NMR: δ = 144.4, 143.4,
128.8, 127.6, 127.5, 127.4, 127.3, 127.1, 85.8, 46.5, 38.7; IR (KBr, cm-1): 3420, 3310, 3026, 2924,
1496, 1278, 1191, 940. ESI-MS [M+Na]+: m/z 353. HRMS: Calcd. for C23H22O2Na [M+Na]+:
353.1512; Found, 353.1513.
4-Phenyl-1,2-dip-tolylcyclopentane-1,2-diol (2b). White solid; m.p. 155-157 oC; 1H-NMR: δ = 7.32 (d,
4H, J = 5.7 Hz) 7.20 (t, 1H, J = 5.9 Hz), 6.87 (d, 4H, J = 8.0 Hz), 6.77 (d, 4H, J = 7.9 Hz), 4.02-3.97
(m, 1H), 3.28 (s, 2H), 2.72 (dd, 2H, J1 = 10.2 Hz, J2 = 3.5 Hz), 2.51 (dd, 2H, J1 = 9.0 Hz, J2 = 4.9 Hz),
13
2.09 (s, 6H); C-NMR: δ = 144.6, 140.5, 136.5, 128.8, 128.3, 127.3, 126.5, 126.4, 85.7, 45.8, 38.5,
21.2; IR (KBr, cm-1): 3467, 3301, 1922, 1514, 1281, 1190, 941; ESI-MS [M+Na]+: m/z 381; HRMS:
Calcd. for C25H26O2Na [M+Na]+: 381.1825; Found, 381.1828.
o
1
1,2-bis(4-Methoxyphenyl)-4-phenylcyclopentane-1,2-diol (2c). White solid; m.p. 128-129 C; H-
NMR: δ = 7.28 (d, 4H, J = 6.5 Hz), 6.83 (t, 2H, J = 8.0 Hz), 6.52-6.47 (m, 7H); 3.93-3.80 (m, 1H),
3.69 (s, 2H), 3.38 (s, 6H), 2.64 (dd, 2H, J1 = 10.0 Hz, J2 = 3.8 Hz), 2.43 (dd, 2H, J1 = 9.0 Hz, J2 = 4.9
Hz); 13C-NMR: δ =144.9, 128.9, 127.9, 127.2, 126.2, 119.0, 113.1, 112.8, 111.5, 85.6, 55.2, 45.2, 38.3.

Molecules 2008, 13
2656
IR (KBr, cm-1): 3466, 3303, 2926, 1601, 1489, 1290, 1169, 1047, 962; ESI-MS [M+Na]+: m/z 413;
HRMS: Calcd. for C25H26O4Na [M+Na]+: 413.1723; Found, 413.1714.
1,2-bis(4-Chlorophenyl)-4-phenylcyclopentane-1,2-diol (2d). White solid; m.p. 147-150 oC; 1H-NMR:
δ= 7.43-7.38 (m, 4H), 7.32 (t, 1H, J = 6.8 Hz), 7.05 (d, 4H, J = 8.6 Hz), 7.00 (d, 4H, J = 8.6 Hz), 4.14-
4.06 (m, 1H), 3.38 (s, 2H), 2.76 (dd, 2H, J1 = 10.0 Hz, J2 = 3.9 Hz), 2.60 (dd, 2H, J1 = 9.0 Hz, J2 = 5.0
13
Hz); C-NMR: δ = 143.9, 141.7, 133.3, 129.0, 127.9, 127.8, 127.2, 126.7, 85.4, 45.5, 38.4; IR (KBr,
cm-1): 3462, 3292, 1949, 1600, 1493, 1278, 1095, 939; ESI-MS [M+Na]+: m/z 421; HRMS: Calcd. for
C23H20Cl2O2Na [M+Na]+: 421.0733; Found, 421.0717.
1,2-Diphenyl-4-p-tolylcyclopentane-1,2-diol (2e). White solid; m.p. 145-147 oC; 1H-NMR: δ = 7.20 (d,
2H, J = 8.0 Hz), 7.11 (d, 2H, J = 10.0 Hz), 6.96-6.90 (m, 10H), 4.00-3.93 (m, 1H), 3.27 (s, 2H), 2. 71
(dd, 2H, J1 = 10.2 Hz, J2 = 4.0 Hz), 2.48 (dd, 2H, J1 = 8.8 Hz, J2 = 5.5 Hz), 2.27 (s, 3H); 13C-NMR: δ
= 143.7, 141.6, 136.2, 129.8, 127.8, 127.4, 127.3, 126.7, 86.2, 46.1, 38.6, 21.5; IR (KBr, cm-1): 3470,
3300, 2923, 1516, 1369, 1058, 943; ESI-MS [M+Na]+: m/z 367. HRMS Calcd. for C24H24O2Na
[M+Na]+: 367.1669; Found, 367.1661.
o
1
4-(4-Chlorophenyl)-1,2-diphenylcyclopentane-1,2-diol (2f). White solid; m.p. 142-145 C; H-NMR :
δ = 7.29 (d, 2H, J = 8.1 Hz), 7.24 (d, 2H, J = 8.1 Hz), 6.94 (s, 10H), 3.97-3.93 (m, 1H), 3.46 (s, 2H),
2.66 (dd, 2H, J1 = 10.7 Hz, J2 = 1.6 Hz), 2.48 (dd, 2H, J1 = 9.1 Hz, J2 = 4.4 Hz); 13C-NMR: δ = 143.2,
142.9, 132.1, 128.9, 128.6, 127.6, 127.2, 126.4, 85.8, 45.4, 38.1; IR (KBr, cm-1): 3467, 3294, 2922,
1493, 1092, 1014, 945; ESI-MS [M+Na]+: m/z 387; HRMS: Calcd. for C23H21ClO2Na [M+Na]+:
387.1122; Found, 387.1122.
4-(4-Methoxyphenyl)-1,2-diphenylcyclopentane-1,2-diol (2g). White solid; m.p. 122-125 oC; 1H-NMR:
δ = 7.36 (d, 2H, J = 8.5 Hz), 7.25 (d, 2H, J = 9.5 Hz), 7.09-7.02 (m, 8H), 6.97 (d, 2H, J = 8.6 Hz),
4.10-4.03 (m, 1H), 3.86 (s, 3H), 3.39 (s, 2H), 2.80 (dd, 2H, J1 = 10.2 Hz, J2 = 3.9 Hz), 2.60 (dd, 2H, J1
=8.8 Hz, J2 = 5.4 Hz); 13C-NMR: δ = 143.2, 142.3, 131.9, 128.2, 127.6, 127.1, 126.5, 114.2, 86.1, 55.6,
46.0, 38.0; IR (KBr, cm-1): 3464, 3300, 2952, 1610, 1514, 1180, 1059, 938; ESI-MS [M+Na]+: m/z 383;
HRMS: Calcd. for C24H24O3Na [M+Na]+: 383.1618; Found, 383.1608.
o
1
4-(2-Chlorophenyl)-1,2-diphenylcyclopentane-1,2-diol (2h). White solid; m.p. 132-135 C; H-NMR:
δ = 7.48 (d, 1H, J = 8.0 Hz), 7.40-7.38 (m, 1H), 7.30-7.27 (m, 1H), 7.10-7.08 (m, 1H), 7.01-6.98 (m,
10H), 4.36-4.27 (m, 1H), 3.34 (s, 2H), 2.70 (dd, 2H, J1 = 10.2 Hz, J2 = 3.8 Hz), 2.56 (dd, 2H, J1 = 8.7
13
Hz, J2 = 5.4 Hz); C-NMR: δ = 143.2, 130.3, 129.4, 128.6, 127.8, 127.5, 127.4, 127.1, 126.7, 126.5,
85.6, 44.1, 36.7; IR (KBr, cm-1): 3549, 3462, 3061, 2981, 1601, 1489, 1083, 941; ESI-MS [M+Na]+:
m/z 387; HRMS: Calcd. for C23H21ClO2Na [M+Na]+: 387.1122; Found, 387.1113.
1,2,4-tri-p-Tolylcyclopentane-1,2-diol (2i). White solid; m.p. 146-149 oC; 1H-NMR: δ = 7.21 (d, 2H, J
= 7.8 Hz), 7.12 (d, 2H, J = 7.8 Hz), 6.86 (d, 4H, J = 8.2 Hz), 6.76 (d, 4H, J =8.0 Hz), 3.95-3.91 (m,
1H), 3.31 (s, 2H), 2.66 (dd, 2H, J1 = 10.1 Hz, J2 = 3.7 Hz), 2.45 (dd, 2H, J1 = 9.0 Hz, J2 = 4.9 Hz),
2.30 (s, 3H), 2.12 (s, 6H); 13C-NMR: δ = 141.5, 140.6, 136.5, 135.8, 129.4, 128.2, 127.2, 126.5, 85.7,

Molecules 2008, 13
2657
45.8, 38.1, 21.3, 21.2; IR (KBr, cm-1): 3469, 3283, 3026, 2920, 1514, 1190, 1022, 943; ESI-MS
[M+Na]+: m/z 395. HRMS: Calcd. for C26H28O2Na [M+Na]+: 395.1982; Found, 395.1974.
4-(4-Fluorophenyl)-1,2-diphenylcyclopentane-1,2-diol (2j). White solid; m.p. 163-165 oC; 1H-NMR: δ
= 7.68 (d, 2H, J = 8.0 Hz), 7.54 (d, 2H, J = 8.0 Hz), 7.05 (d, 10H, J = 5.1 Hz), 4.18-4.14 (m, 1H), 3.48
(s, 2H), 2.83 (dd, 2H, J1 = 10.3 Hz, J2 = 3.5 Hz), 2.65 (dd, 2H, J1 = 8.9 Hz, J2 = 5.0 Hz); 13C-NMR: δ
= 147.3, 143.1, 127.7, 127.6, 127.3, 126.4, 125.8, 125.7, 85.7, 45.4, 38.6; IR (KBr, cm-1): 3450, 3185,
2930, 1510, 1089, 943; ESI-MS [M+Na]+: m/z 371; HRMS: Calcd. for C23H21FO2Na [M+Na]+:
371.1418; Found, 371.1423.
Acknowledgements
This work was financially supported by the Natural Science Foundation of China (grant no.
20775069).
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
Kahn, B. E.; Rieke, R. D. Carbonyl coupling reactions using transition metals, lanthanides, and
actinides. Chem. Rev. 1988, 88, 733-745.
Fittig, R. Ueber einige metamorphosen des acetones de essigisäure. Justus Leibigs Ann. Chem.
1859, 110, 13-45.
McMurry, J. E.; Rico, J. G.; Shih, Y. N. Synthesis and stereochemistry of sarcophytol B: An
anticancer cembranoid. Tetrahedron Lett. 1989, 30, 1173-1176.
McMurry, J. E.; Dushin, R. G.; Total synthesis of (+-)-isolobophytolide and (+-)-crassin by
tatanium-induced carbonyl coupling. J. Am. Chem. Soc. 1990, 112, 6942-6949.
Li, T.-Y.; Cui, W.; Liu, J.-G.; Zhao, J.-Z.; Wang, Z.-M. A highly dl-stereoselective
pinacolization of aromatic aldehydes mediated by TiCl4-Zn. Chem. Commun. 2000, 139-140.
Handy, S. T.; Omune, D. A.; Chelation effect on the pathway between intramolecular
hydrodimerization and pinacol coupling. Org. Lett. 2005, 7, 1553-1555.
Yamamoto, Y.; Hattori, R.; Itoh, K. Highly trans-selective intramolecular pinacol coupling of
diols catalyzed bulky Cp2TiPh. Chem. Commun. 1999, 825-826.
Ueda, T.; Kanomata, N.; Machida, H. Synthesis of planar-chiral paracyclophanes via samarium
(II)-catalyzed intramolecular pinacol coupling. Org. Lett. 2005, 7, 2365-2368.
Molander, G. A.; Kenny, C. Intramolecular reductive coupling reactions promoted by samarium
diiodide. J. Am. Chem. Soc. 1989, 111, 8236-8246.
10. Svatos, A. Boland, W. Reductive pinacol coupling reactions of aromatic carbonyl compounds
catalytic in chromium(II). Synlett. 1998, 549.
11. Nair, V.; Ros, S.; Jayan, C. N. ; Rath, N, P. Indium/indium trichloride mediated pinacol cross-
coupling reaction of aldehydes and chalcones in aqueous media: a facile stereoselective synthesis
of substituted but-3-ene-1,2-diols. Tetrahedron Lett. 2002, 43, 8967-8969.
12. Xu, X.; Hirao, T. Vanadium-catalyzed pinacol coupling reaction in water. J. Org. Chem. 2005,
70, 8594-8596.

Molecules 2008, 13
2658
13. Buchammagari, H.; Toda, Y.; Hirano, M.; Hososno, H.; Takeuchi, D.; Osakada, K. Room
temperature-stable electride as a synthetic organic reagent: application to pinacol coupling
reaction in aqueous media. Org. Lett. 2007, 9, 4287-4289.
14. Corey, E. J.; Danheiser, R. L.; Chandrasekaran, S. New reagent for the intermolecular and
intramolecular pinacolic coupling of ketones and aldehydes. J. Org. Chem. 1976, 41, 260-265.
15. Hays, D. S.; Fu. G. C. Metal hydride mediated intramolecular pinacol couplings of dialdehydes
and ketoaldehydes. J. Am. Chem. Soc. 1995, 117, 7283-7284.
16. Yamamoto, Y.; Hattori, R.; Itoh, K. Highly trans-selective intramolecular pinacol coupling of
dials catalyzed by bulky Cp2TiPh. Chem. Commun. 1999, 825-826.
17. Li, C.-J.; Chan, T.-H. Organic syntheses using indium-mediated and catalyzed reactions in
aqueous media. Tetrahedron 1999, 55, 11149-11176.
18. Auge, J.; Lubin-Germain, N.; Uziel, J. Recent advances in indium-promoted organic reactions.
Synthesis 2007, 1739-1764.
19. Wang, C.-Y.; Su, H.; Yang, D.-Y. Regio- and stereoselective dimerization of terminal alkynes to
enynes in InCl3-NaBH4 system. Synlett. 2004, 561-563.
20. Wang, C.-Y.; Yan, L.; Zheng, Z.-G.; Yang, D.-Y.; Pan, Y.-J. Synthesis of (E)-alkenes via
hydroindation of C=C in InCl3-NaBH4 system. Tetrahedron 2006, 62, 7712-7717.
21. Wang, C.-Y.; Wan, J.-P.; Zheng, Z.-G.; Pan, Y.-J. A new InCl3-catalyzed reduction of anthrones
and anthraquinones by using aluminum powder in aqueous media. Tetrahedron 2007, 63, 5071-
5075.
22. Wang, C.-Y.; Pan, Y.-J.; Wu, A.-X. InCl3/Al mediated pinacol coupling reactions of aldehydes
and ketones in aqueous media. Tetrahedron 2007, 63, 429-434.
23. Hirsch, S. S.; Bailey, W. J. Base-catalyzed alkylation of cyclopentadiene rings with alcohols and
amines. J. Org. Chem. 1978, 43, 4090.
24. Yanagisawa, A.; Takahashi, H.; Arai, T. One-pot synthesis of 1,5-diketones catalyzed by barium
isopropoxide. Tetrahedron 2007, 63, 8581-8585.
25. Rong, L.-C.; Li, X.-Y.; Wang, H.-Y.; Shi, D.-Q.; Tu, S.-J. Solvent-free Michael addition reaction
of cyclohexanone with chalcone. Chin. J. Org. Chem. 2007, 27, 1292-1295.
Sample Availability: Samples of compounds 2a-2j are available from the authors.
© 2008 by the authors; licensee Molecular Diversity Preservation International, Basel, Switzerland.
This article is an open-access article distributed under the terms and conditions of the Creative
Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).