JOURNAL OF CHEMICAL RESEARCH 2018 339
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Scheme 2 Possible mechanism for xanthene synthesis.
Synthesis of xanthenes; general procedure
As reported in the literature, benzyl alcohols react with
ChPS in the absence of solvent to form aldehydes and ChBS,
which is highly acidic in nature.23 The reaction of 2-naphthol
with aldehydes in the presence of an acid catalyst is known to
give o-QMs,13 which can condense with another molecule of
2-naphthol to produce the corresponding dibenzo[a,j]xanthenes
(Scheme 2).
Similarly, in the case of octahydroxanthene synthesis, after
oxidation of the alcohol to the corresponding aldehyde, ChBS
activates the carbonyl group. Subsequent nucleophilic attack by
dimedone forms the o-QM intermediate, which then condenses
with another molecule of dimedone to produce the desired
product.
In summary, an efficient and novel protocol for the direct
synthesis of xanthene derivatives from the reactions of benzyl
alcohols and 2-naphthol (or dimedone) by a tandem catalytic
process using ChPS is described. The procedure offers several
advantages including metal-free oxidation, high yields,
operational simplicity and clean reactions, which makes it a useful
and attractive process for the synthesis of these compounds.
A mixture of alcohol (1 mmol) and ChPS (2 mmol) was stirred at
80 °C for 35 min. Then, 2-naphthol (or dimedone) (2 mmol) was
added in one portion and the mixture was stirred at 105 °C for the
appropriate time (Table 3). After completion, the mixture was cooled
to room temperature and then water (10 mL) was added. The mixture
was stirred for 5 min and the solid product obtained was separated by
filtration. The product was purified by recrystallisation from aqueous
EtOH or using silica gel column chromatography with hexane/ethyl
acetate (95:5) as the eluent to obtain the pure compound (Table 3). All
products are known and selected spectroscopic data are given below.
14-Phenyl-14H-dibenzo[a,j]xanthene (5a): Pale yellow solid; m.p.
189–190 °C (lit.17 191–192 °C); IR (KBr) νmax: 3074, 3020, 2887, 1620,
1591, 1512, 1489, 1457, 1402, 1253, 1079, 1026, 964, 827, 744, 700 cm–1;
1H NMR (CDCl3, 500 MHz): δ 8.40 (d, J = 8.5 Hz, 2H), 7.84 (d, J = 7.9
Hz, 2H), 7.80 (d, J = 8.8 Hz, 2H), 7.59 (app. t, J = 7.7 Hz, 2H), 7.53 (d, J
= 7.5 Hz, 2H), 7.49 (d, J = 8.8 Hz, 2H), 7.41 (app. t, J = 7.5 Hz, 2H), 7.15
(app. t, J = 7.5 Hz, 2H), 7.00 (t, J = 7.5 Hz, 1H), 6.50 (s, 1H) ppm. Anal.
calcd for C27H18O: C, 90.47; H, 5.06; found: C, 90.43; H, 5.04%.
3,3,6,6-Tetramethyl-9-phenyl-3,4,5,6,7,9-hexahydro-1H-xanthene-
1,8(2H)-dione (6a): White solid; m.p. 202–203 °C (lit.18 200–202 °C);
IR (KBr) νmax: 3030, 2950, 1669, 1470, 1363, 1200, 1170, 740, 700 cm–1;
1H NMR (DMSO-d6, 500 MHz): δ 0.90 (s, 6H), 1.03 (s, 6H), 2.09 (d,
J = 16.1 Hz, 2H), 2.28 (d, J = 16.2 Hz, 2H), 2.54 (d, J = 17.1 Hz, 2H),
2.57 (d, J = 17.7 Hz, 2H), 4.53 (s, 1H), 7.09 (t, J = 7.0 Hz, 1H), 7.18 (d,
J = 7.0 Hz, 2H), 7.22 (t, J = 7.20 Hz, 2H) ppm. Anal. calcd for C23H26O3:
C, 78.83; H, 7.48; found: C, 78.80; H, 7.46%.
Experimental
All reagents were purchased from Merck, Aldrich or Dae Jung and
used without further purification. All yields refer to isolated products
after purification. ChPS was prepared according to the reported
procedure.23,24 Products were characterised by comparison with
authentic samples and by spectroscopic data (IR and 1H NMR spectra)
and melting point range. NMR spectra were recorded on Bruker
Avance DPX 500 or 300 instruments. The spectra were measured in
CDCl3 or DMSO-d6 relative to TMS (0.00 ppm). IR spectra were
recorded on a JASCO FTIR 460 plus spectrophotometer. All of the
compounds were solids and solid-state IR spectra were recorded using
the KBr disk technique. Melting point ranges were determined in open
capillaries with a BUCHI 510 melting point apparatus. Elemental
analyses were performed with a Heraeus CHN-O-Rapid analyser.
Thin-layer chromatography (TLC) was performed on silica-gel
polygram SIL G/UV 254 plates.
Acknowledgement
We are grateful to the University of Hormozgan Research
Council for the partial support of this research.
Received 22 February 2018; accepted 26 May 2018
Paper: 1805268
Published online: 26 June 2018
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