J IRAN CHEM SOC (2012) 9:483–488
485
Compound 2a: White solid; mp: 106–108 8C; yield:
0.43 g (10%)
DMSO-d6): d = 19.37, 37.3, 38.6, 123.8, 196.4. MS: m/z
(%) = 234 [M?] (69), 216 (85), 187 (76), 150 (100), 136
(60), 77 (90), 55 (54), 41(80).
IR (KBr): 2,961, 1,737 (C=O), 1,664 (a, b unsaturated
C=O), 1,607, 1,466, 1,422, 1,379, 1,202, 1,160, 1,041, 870,
1
742, 572 cm-1; H NMR (500 MHz, CDCl3): d = 1.11
Results and discussion
(12H, s, C(CH3)2), 1.27 (6H, t, J = 7.1 Hz, OCH2CH3),
2.26–2.35 (4H, AB q, J = 16.3 Hz, 2CH2), 2.41–2.50 (4H,
AB q, J = 16 Hz, 2CH2), 3.92 (1H, d, J = 3.6 Hz,–CH–
CH(CO2Et)2), 4.13 (2H, q, J = 7.1 Hz, OCH2CH3),
4.38 (1H, d, J = 3.1 Hz,–CH–CH(CO2Et)2); 13C NMR
(125.7 MHz, CDCl3): d = 14.4, 26.9, 27.0, 30.1, 32.2,
41.4, 51.1, 53.3, 61.6, 112.0, 166.3, 168.3, 197.01; MS: m/z
(%) = 432 [M?] (5), 340 (34), 286 (17), 273 (100), 257
(38), 217 (13), 201 (12), 161 (8), 77 (3).
In our initial studies, we isolated 9-substituted 1,8-di-
oxooctahydroxanthene 2a (Table 1, entry1) as sole
product, from Si (CH3)3Cl catalyzed reaction of DEEM
with dimedone in C2H5OH in quantitative yield (based
on consumed starting materials) after 3 days at 35 °C.
Other experiments were conducted under different con-
ditions. Results of all experiments are summarized in
Table 1.
Refluxing C2H5OH shortened the reaction time, but led
to partial hydrolysis of product 2a to 3a, Scheme 1
(Table 1, entry 2). When p-TsOH is used as catalyst, both
2a and 3a are produced in varying temperature dependent
proportions (Table 1, entries 3, 4). The reaction did not
occur when heterogeneous catalysts such as PPA/SiO2 or
HPA were used (Table 1, entries 6, 7). Unexpectedly, in
the case of cyclohexane-1,3-dione, the reactions did not
occur under acidic conditions even after prolonged reaction
times. When alkoxides were used in catalytic amounts at
ambient temperature, only starting materials were recov-
ered. By raising the temperature to reflux in ethanol or
toluene, 3a was produced in very low yields as sole
product, and mostly starting materials were again recov-
ered (Table 1, entries 8, 9). Increasing the alkoxides type
catalysts to equimolar ratios, give rise to the production of
4b in excellent yield.
Compound 3a: White solid; mp: 125.2–127 °C; yield:
3 g (83%)
IR (KBr): 2,963, 2,877, 1,720 (C=O), 1,678, 1,655 (a, b
unsaturated C=O), 1,623 (C=C), 1,466, 1,422, 1,380, 1,201,
1
1,164, 1,137, 1,033, 742, 573 cm-1; H NMR (500 MHz,
CDCl3): d = 1.11 (12H, s, C(CH3)2), 1.16 (3H, t, J =
7.1 Hz, OCH2CH3), 2.29 (4H, s, –CH2C–O), 2.39 (4H, s,
–CH2C(O)), 2.72 (2H, d, J = 4.1 Hz, CH2 a-ester), 3.8
(1H, t, J = 7.1 Hz, CH–CH2), 3.98 (2H, q, J = 7.1 Hz,
OCH2CH3); 13C NMR (125.7 MHz, CDCl3): d = 14.6,
24.1, 27.4, 29.8, 32.4, 36.8, 41.2, 51.2, 60.3, 113.5, 164.9,
172.5, 197.4. MS: m/z (%) = 360 [M?] (3), 286 (26), 273
(100), 257 (5), 217 (12), 161 (7), 55 (5).
Compound 3b: White solid; mp: 102–104 °C; yield:
2.43 g (80%)
After many trials, we finally discovered that 1,4-diaza-
bicyclo [2.2.2] octane ‘‘DABCO’’ is the most efficient
catalyst for this reaction with molar ratios of: dimedone/
DEEM/DABCO = 1:1.5:0.01, in a solvent-free reaction at
60 °C (Table 1, entries 13, 15).
IR (KBr): 2,949, 2,899, 1,737 (C=O), 1,654 (a, b unsatu-
rated C=O), 1,615 (C=C), 1,382, 1,278, 1,176, 1,132,
1
1,043, 1,011, 959, 847, 566 cm-1; H NMR (500 MHz,
DMSO-d6): d = 1.11 (3H, t, J = 7.1 Hz, OCH2CH3), 1.83
(2H, m, CH2), 1.94 (2H, m, CH2), 2.32 (6H, t, J = 7.7 Hz,
3CH2), 2.47 (4H, m, 2CH2), 3.69 (1H, t, J = 4.5 Hz,CH),
3.89 (2H, q, J = 7.1, OCH2CH3); 13C NMR (125.7 MHz,
DMSO-d6): d = 14.8, 20.8, 24.1, 27.3, 37.2, 38.6, 60.4,
114.1, 166.8, 171.3, 197.5; MS: m/z (%) = 304 [M?] (3),
231 (21), 217 (100), 175(7), 55(5).
A plausible explanation of ‘‘DABCO’’ being a better
catalyst than alkoxides may be due to the solubility of the
transient intermediate enolate in comparison with alkali
metal enolate salts produced when using alkoxides. In
addition, protonated ‘‘DABCO’’ may act as an activator of
DEEM, being a proton donor (Scheme 2).
The reaction seems to occur via a reaction sequence of
addition–elimination, Michael addition, cyclization via con-
densation the two adjacent carbonyl groups (Schemes 2, 3).
Compound 4b: White solid; mp: 155–157 °C; yield:
0.23 g (10%)
1
The HNMR spectra of the xanthenediones in general
show the characteristic signals. The methyl groups present
in the 3 and 6 positions due to 12 protons appear as a
singlet around a 1.0–1.1 ppm. The protons at the 2 and 7
positions and at the 4 and 5 positions appear around
2.3–2.5 ppm as singlets in general. Compound 2a shows
IR (Nujol): 3,200, 1,670, 1,630, 1,610, 1,530, 1,460, 1,390,
1,200 cm-1 1H NMR (500 MHz, DMSO-d6): d = 1.89
.
(4H, s, 2CH2), 2.36 (6H, s, 3CH2), 2.49 (2H, s, CH2), 7.25
(1H, s, CH), 7.44 (1H, s, OH).13C NMR (125.7 MHz,
123