Tetrahedron Letters 47 (2006) 8827–8829
Heterogeneous catalyst: Amberlyst-15 catalyzes the synthesis
of 14-substituted-14H-dibenzo[a, j]xanthenes under
solvent-free conditions
Shengkai Ko and Ching-Fa Yao*
Department of Chemistry, National Taiwan Normal Universtry, 88, Tingchow Road, Taipei 116, Taiwan, ROC
Received 6 July 2006; revised 12 October 2006; accepted 13 October 2006
Abstract—A one-pot condensation of b-naphthol with aldehydes in the presence of Amberlyst-15 to give 14-substituted-14H-di-
benzo[a,j]xanthenes under solvent-free condition is described.
Ó 2006 Elsevier Ltd. All rights reserved.
The synthesis of xanthene derivatives has been of con-
siderable interest to chemists because of their wide range
of biological and pharmaceutical properties such as anti-
viral,1 antibacterial,2 and anti-inflammatory activities.3
Furthermore, these compounds can be used as dyes,4
in laser technology,5 pH-sensitive fluorescent materials
for the visualization of biomolecular assemblies.6 It is
also noteworthy that dibenzoxanthenes derivatives are
candidates as sensitizers in photodynamic therapy.7
Thus, the synthesis of benzoxanthene derivatives cur-
rently is of great interest. Various methods have been
reported for the synthesis of benzoxanthenes, including
the reaction of b-naphthol with formamide,8 2-naph-
thol-1-methanol,9 and carbon monoxide.10 However,
these methods have many disadvantages such as low
yields, the need for a prolonged reaction time, the use
of toxic organic solvents, excess reagents, and harsh
reaction conditions. Because of these drawbacks, the
reaction has been improved by mixing b-naphthol with
aldehydes in the presence of a catalyst, such as pTSA,11
sulfamic acid,12 I2,13 AcOH–H2SO4.14 These methods
also suffer from some disadvantages such as a long reac-
tion time, the use of toxic solvent, special apparatus, and
the use of toxic catalysts. Thus, the development of a
new catalyst for the synthesis of dibenzoxanthene deriv-
atives would be highly desirable.
example, operational simplicity, environmental compa-
tibility, nontoxic, reusability, low cost, and ease of isola-
tion. A tremendous upsurge of interest in various
chemical transformations processes by catalysts under
heterogeneous conditions has occurred. One of those
heterogeneous catalysts is Amberlyst-15. It makes reac-
tion processes convenient, more economic, and environ-
mentally benign. Owing to the numerous advantages
associated with this cheap and nonhazardous catalyst,
Amberlyst-15 has been explored as a powerful catalyst
for various organic transformations under mild condi-
tions,15 such as the synthesis of bisindoylmethanes,
dioxo-octahydroxanthenes, dioxo-decahydroacridines,
and 1,2-diaryl-2-bromo-alcohols.
We had the opportunity to further explore the catalytic
activity of Amberlyst-15 for the synthesis of 14-sub-
stituted-14H-dibenzo[a,j]xanthenes. Herein, we wish to
report on a novel synthesis of 14-substituted-14H-di-
benzo[a,j]xanthenes in a high yield, promoted by the
catalytic amounts of Amberlyst-15 under solvent-free
conditions. In an initial endeavor, 1 equiv of benzalde-
hyde 1a and 2 equiv of b-naphthol 2 were stirred at
125 °C. After 24 h, only 36% of the expected product 3
was obtained after recrystallizing the crude product
from ethanol. To improve the yield and optimize the
reaction conditions, the same reaction was carried out
in the presence of Amberlyst-15 under similar condi-
tions. Surprisingly, a significant improvement was
observed and the yield of 3 was increased to 94%.
In recent years, the use of solid acidic catalysts has
offered important advantages in organic synthesis, for
*
Based on the above results, we also conducted the same
Corresponding author. Tel./fax: +886 2 29309092; e-mail: cheyaocf@
reactions using aldehydes 1b–k and 2 in the presence of
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.10.072
Ko, Shengkai
