1370
B.M.Choudary et al./ Tetrahedron Letters 46 (2005) 1369–1371
Table 1. Synthesis of flavanones using different crystallites of MgOa
reactions, to some extent, and that it is largely driven
by Lewis basic O2ꢀ ions. MgO is polyhedrally shaped,
while NA-MgO has the shape of hexagonal platelets
with the same average concentration of OH groups to
that of NAP-MgO. A possible rationale for the display
of the higher yields of flavanones using NAP-MgO is
that the OH groups present on the corner sites of the
NAP-MgO crystals are more accessible for the reac-
tants. NAP-MgO has a single crystalline polyhedral
structure, which a high surface concentration of edges/
corners and various exposed crystal planes (such as
002, 001, 111), which leads to inherently high surface
reactivity per unit area. Thus, NAP-MgO displayed
the highest activity compared to NA-MgO and CM-
MgO.11
Entry Catalyst
1NAP-MgO 2
Time (h)
Conversion (%)b
Flavanone Chalcone
d
1
90, 60
c, 1 00
10, 40c
40
50
2
3
4
5
NA-MgO
CM-MgO
18
36
60
50
85
50
Sil–NAP-MgO 40
Sil–NA-MgO 48
15
50
a Conditions: 2-hydroxyacetophenone (3.0 mmol), benzaldehyde
(2.5 mmol), catalyst (0.100 g), solvent (ethanol, 5 mL).
b Conversions based on 1H and G.C. according to Ref. 7b.
c Using toluene as a solvent.
d Using DMSO as a solvent.
Table 2. Synthesis of flavanones by nanocrystalline MgOa
To conclude, we have shown that the NAP-MgO is a
highly active, reusable12 catalyst for the synthesis of
flavanones. Thus nanocrystalline MgO with its definite
shape, size, and accessible OH groups, and higher
density of Mg+ at the edges/corner shows higher activity
in the synthesis of flavanones.
Entry R1
R2
R3
Time (h)
Conversion (%)b
Flavanone Chalcone
c
1H
2
H
H2, 20 1
90, 90c
95
93
10, 10c
H
NO2
Br
H
H
H
H
1
1
1
1
8
8
5
1
3
H
5
5
8
7
6
4
H
OH
Cl
H
94
86
5
4
Acknowledgements
6
H
OMe
CH3
H
H
H
H
H
H
H
H
18
1
60
40
7
H
74
26
K.V.S.R and J.Y thank CSIR for the award of research
fellowships. Nanocrystalline MgO samples were pur-
chased from NanoScale Materials Inc., Manhattan,
KS 66502, USA.
8
NO2
Br
20
20
24
18
15
60
70
50
70
80
40
30
50
30
20
9
H
10
11
12
OMe H
NO2 OH
H
OH
1
3
4
H
H
H
H
N3O0
Cl
95
86
5
2
1
30
32
96
80
4
References and notes
15
1
H
H
OMe 36
OH
14
6
H
H
20
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Chulia, A-J. Bioorg.Med.Chem.Lett.
a Reaction conditions: 2-hydroxyacetophenone (3.0 mmol), benzalde-
hyde (2.5 mmol), catalyst (0.100 g), solvent (ethanol, 5 mL).
b Conversions calculated based on 1H NMR and GC according to
Ref. 7b.
c After 4th cycle. (The catalyst can be regenerated at 250 °C under N2
flow for 1h).
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than for substrates bearing electron-donating groups
(Table 2, entries 2–4 and 5–7). When the benzaldehyde
was substituted with either an electron-withdrawing
group or donating group at the 2-position, the forma-
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8–11). The formation of flavanone was 100%, when the
reaction was carried out in DMSO with total conversion
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