R. Mondal et al. / Tetrahedron Letters 52 (2011) 5020–5024
5023
Table 3
Anhyd Cs2CO3 catalyzed synthesis of flavanones by (i) cyclization of 20-hydroxychalcones and (ii) condensation between 20-hydroxyacetophenones and benzaldehydes
Entry
1
Reaction
Reaction condition
Time
Product(s) (Yield, %)
Cyclization of 1a
(i) Acetone, reflux
(ii) MW irradiation
(i) Acetone, reflux
(ii) MW irradiation
(i) Acetone, reflux
(ii) MW irradiation
(i) Acetone, reflux
(ii) MW irradiation
(i) Acetone, reflux
(ii) MW irradiation
(i) Acetone, reflux
(ii) MW irradiation
3 h
3 min
3 h
3 min
5 h
3 min
3 h
3 min
3 h
3 min
3 h
2a (70)
2a (65)
2d (66)
2d (62)
1e (74)
1e (68)
2a (58) + 1a (28)
2a (54) + 1a (21)
2d (62) + 1d (24)
2d (55) + 2d (18)
2e (24) + 1e (45)
2e (28) + 1e (39)
2
3
4
5
6
Cyclization of 1d
Cyclization of 1e
Condensation of 20-hydroxyacetophenone and benzaldehyde
Condensation of 20-hydroxyacetophenone and p-chlorobenzaldehyde
Condensation of 20-hydroxyacetophenone and p-N,N-dimethylamino-benzaldehyde
3 min
mixture containing 1a and the isomeric flavanone (2a), the latter
being the predominant component. In order to investigate the sol-
vent effects, the reaction was performed with two 20-hydroxychal-
cones (1a and 1e) in three different solvents, viz., acetone, ethanol,
and DMF. It was observed that the optimum yield of the flavanone
was almost the same in these three solvents within comparable
time spans, which led us to choose the lowest boiling solvent ace-
tone for all the cases studied.10 In all the cases good yields of flav-
anones except 1g were obtained. We then studied this cyclization
on K2CO3 surface by use of microwave irradiation. Here also similar
yields of flavanones were obtained.10 All the results are presented
in Table 1.
It was then our interest to develop a synthetic route to
flavanones starting directly from 20-hydroxyacetophenones
(Scheme 2). Thus, when an equimolar mixture of 20-hydroxyace-
tophenone and benzaldehyde was refluxed in dry ethanol with
added anhyd K2CO3 or subjected to microwave irradiation on
K2CO3 surface,12 the resulting flavanone was obtained in moder-
ate yield (Table 2).
Supplementary data
Supplementary data associated with this article can be found, in
References and notes
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Financial assistance from the UGC-CAS and DST-PURSE Pro-
grams, Department of Chemistry is gratefully acknowledged. The
authors also acknowledge the DST-FIST Program to the Depart-
ment of Chemistry, Jadavpur University for providing the NMR
spectral data. R.M. and A.D. are thankful to UGC and CSIR, New
Delhi for their research fellowships.