Oxidation of some alkoxy-cycloheptatriene derivatives: unusual formation of furan and furanoids from cycloheptatrienes

Article abstract of DOI:10.1016/j.tet.2007.03.145

The oxidation of some alkoxy tropone and tropone ketal derivatives with singlet oxygen and m-chloroperbenzoic acid was investigated. In most cases furan and furanoid derivatives were isolated. The structures of the formed products were determined by means of spectral data and the formation mechanism of these unusual products is discussed.

Full text of DOI:10.1016/j.tet.2007.03.145

Tetrahedron 63 (2007) 4944–4950
Oxidation of some alkoxy-cycloheptatriene derivatives: unusual
formation of furan and furanoids from cycloheptatrienes
b
b,
c,
Ahmet Coxskun,a,b Murat Guney, Arif Daxstan and Metin Balci
*
*
€
aSelc¸uk University, Faculty of Education, Department of Chemistry, 42099 Konya, Turkey
bAtatu€rk University, Department of Chemistry, 25240 Erzurum, Turkey
cMiddle East Technical University, Department of Chemistry, 06531 Ankara, Turkey
Received 8 January 2007; revised 6 March 2007; accepted 22 March 2007
Available online 27 March 2007
Abstract—The oxidation of some alkoxy tropone and tropone ketal derivatives with singlet oxygen and m-chloroperbenzoic acid was
investigated. In most cases furan and furanoid derivatives were isolated. The structures of the formed products were determined by means
of spectral data and the formation mechanism of these unusual products is discussed.
Ó 2007 Elsevier Ltd. All rights reserved.
O
O
O
1. Introduction
HO
HO
HO
OH
Since their discovery, natural and synthetic tropone (1) and
tropolone (2) derivatives have attracted considerable interest
due to the unique structure and properties of the tropolone
ring.1
O
CO2Et
HO
CO2Et
OH
OH
3
4
5
O
O
O
OH
CO2Et
OH
OH
O
OH
O
O
8
2
1
6
7
OH
Consequently, over the last 50 years, a large number of inge-
nious and elaborate approaches for synthesizing this ring
system have been developed.2 The photooxygenation of
cyclic 1,3-dienes is one of the most valuable methods for the
synthesis of oxygen functionalized target molecules.3 In
connection with the development of new synthetic strategies
to tropolones, we recently studied the suitability of bicyclic
endoperoxides derived by the cycloaddition of singlet oxy-
gen to the appropriate cyclic dienes and subsequently syn-
thesized isomeric stipitatic acids 3 and 4,4 polyoxygenated
tropolones 5 and 6,5 and some benzotroponoid systems 7
and 8.6
In addition to the synthetic aspects, the chemistry of endo-
peroxides has fascinated organic chemists for long years.7
As a direct consequence of this, we have been interested in
the oxidation reactions of different tropone and tropolone
derivatives in order to develop new synthetic strategies and
to furthermore study the chemistry of the formed endoper-
oxides. We herein describe an expedient route to the forma-
tion of some furanoid structures derived from bicyclic
endoperoxides.
2. Results and discussion
Keywords: Cycloheptatriene; Tropone; Tropolone; Endoperoxide; Singlet
oxygen; Rearrangement; Furan derivatives.
* Corresponding authors. Tel.: +90 312 2105140; fax: +90 312 2101280
(M.B.); tel.: +90 442 2314405; fax: +90 442 2360948 (A.D.); e-mail
addresses: adastan@atauni.edu.tr; mbalci@metu.edu.tr
In our first attempted synthesis of tropolone derivatives, we
proceeded to subject tropone ketal 98 (that was synthesized
from tropone as described in the literature), to an epoxida-
tion reaction.
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.03.145

A. Coxskun et al. / Tetrahedron 63 (2007) 4944–4950
4945
Treatment of ketal 9 with m-chloroperbenzoic acid (m-
CPBA) afforded a mixture, with ethylene glycol monoben-
zoate 10a9 as the major product (Scheme 1). The expected
epoxidation product 11 was formed as the minor product.
The products were separated by column chromatography
on silica gel. For characterization of 10 it was converted to
the corresponding acetate 10b.10 We assume that the benzoic
acid ester 10a is a secondary product and formed from the
rearrangement of the primarily formed epoxide 11. Opening
of the ketal ring in 11 under the reaction conditions may
form 12, which can undergo a pinacol–pinacolone type rear-
rangement to give 10a (Scheme 2).
O
O
O
O
O
CoTPP
CH2Cl2, 0 °C
O
O
O
+
O
O
OHC
O
16/17 21%
14
15 61%
SiO2
OR
OH
rt, CHCl3
O
O
O
O
HO
O
13 days, 98%
O
18/19
20
a R = H
OR
Ac2O/Py
100%
b R = COCH3
O
O
NH3, MeOH
rt, quant.
O
O
O
O
O
HO
OH
m-CPBA
O
+
.
BF3 OEt2
Na2CO3
CH2Cl2
OH
OH
NH2
+
O
1
9
10 64%
a R = H
11 6%
O
21
Ac2O/Py
100%
b R = COCH3
Scheme 3.
the electron transfer reaction between the Co2+ species and
endoperoxide 14 serves as a key intermediate, as shown in
Scheme 4. This intermediate can undergo two different reac-
tions; (i) forming the bisepoxide 15 by the attack of oxygen
radicals to the double bond and (ii) cleavage of the carbon–
carbon bond in 22 can form the allyl radical 23, which can
then be trapped by oxygen atom to give 16/17. The formed
product has an orthoester structure, which will not be stable
and is easily converted to 18/19 via intermediates 24–26,
respectively (Scheme 4).
Scheme 1.
OH
OH
O
O
O
H
O
O
O
O
H+/H2O
OH
-H2O
O
10a
11
Scheme 2.
12
13
Next, the tetraphenylporphyrin-sensitized photooxygenation
of 27b,11 obtained from the NEt3-catalyzed rearrangement of
14 followed by acetylation, was investigated. The expected
singlet oxygen cycloaddition product 28 was converted into
31 under the reaction conditions. Column chromatography
of the reaction mixture on silica gel isolated a furanoid 31
in 83% yield. The COSY, HMQC, and HMBC experimental
data are in agreement with the proposed structure.
Unfortunately, the epoxidation route did not lead to any tro-
polone derivatives. We therefore turned our attention to the
bicyclic endoperoxide 1411 synthesized by the addition of
singlet oxygen to tropone ketal 9.
Foote et al.12 and our group13 have reported that cobalt-
meso-tetraphenylporphyrin (CoTPP) promotes the rearrange-
ment of the bicyclic endoperoxides to the corresponding
bisepoxide with a syn-configuration. In addition to this
previously observed reaction of CoTPP, we further demon-
strated that some bicyclic endoperoxides derived from
cycloheptatriene can be converted into tropolone derivatives
upon treatment with CoTPP.4 For this reason CoTPP-cata-
lyzed decomposition of endoperoxide 14 was investigated.
The 1H NMR spectrum of crude mixture showed the forma-
tion of bisepoxide 15 and an isomeric mixture of aldehydes
16/17. After column chromatography, bisepoxide 15 and
a mixture of isomeric diols 18/19 were isolated. The initially
formed aldehydes 16/17 were rearranged into the rather la-
bile diols 18/19 upon treatment with column material (SiO2).
The diol mixture 18/19 underwent H2O elimination upon
standing at room temperature in CHCl3 and produced the fu-
ran derivative 20a, which was then converted to acetate 20b,
which was confirmed by analysis of the spectral data. Am-
monolysis of 20b gave the known amide 21 (Scheme 3).14
CH3
O
168.1
O1'
4.26-4.30 A2 part of A2B2 system
O
5'
O
169.3
21.0*
/ 2.05*
O
103.6
2'
CH3
63.1
1
2
164.4
4'
O
3
3'
170.1
O
139.4 123.3
6.15 d J =12.4 Hz
62.0
123.85
O
6.2 d, J = 5.5 Hz
151.55
6.38 d J =12.4 Hz
4.30-4.36 B2 part of A2B2 system
7.8 d J=5.5 Hz
*exchangeable
31
The formation mechanism of the formed furanoid 31 can be
rationalized as depicted in Scheme 5. The peroxide linkage
in 28 first undergoes a homolytic ring cleavage under the re-
action conditions to form the diradical 29. This cleavage is
followed by a C–C cleavage and the formation of the ester
carbonyl group. Ring closure takes place between the alkoxy
radical and acyl radical in 30 (Scheme 5). We assume that the
oxygen functionalities attached to bridgehead carbon atoms
in 28 makes the molecule labile.
In the light of these observations, we suggest the following
mechanism. The initially formed radical 22 resulting from

4946
A. Coxskun et al. / Tetrahedron 63 (2007) 4944–4950
O
O
O
O
15
-Co2+
O
Co3+
O
Co3+
O
O
O
O
O
O
O
O
O
CoTPP
O
O
O
Co3+
O
O
23
22
14
-Co2+
cyclization
OH
O
O
O
O
O
HO
O
O
18/19
16/17
double-bond
isomerization
ring-closure
HO
O
O
O
SiO2/H2O
ring-opening
O
O
O
O
O
O
OH
25
OH
O
OH
26
24
Scheme 4.
OH
In order to test the generality of this interesting transforma-
tion of a cycloheptatriene unit into a furanoid structure, we
tested the reactivity of the tropolone derivative 34b with
singlet oxygen. The diacetate 34b15 was synthesized by
the base-catalyzed rearrangement of the tropone endoper-
oxide 322g,16 followed by acetylation with acetic anhydride
in pyridine to give 34a17 and 34b. The diacetate 34b was
then subjected to photooxygenation reaction. NMR spectra
of reaction mixture showed the formation of endoperoxide
35 and a rearranged product 36 in 41 and 44%, yields, re-
spectively (Scheme 6). The endoperoxide 35 was isolated by
column chromatography in a 4% yield though the rearranged
product 36 decomposed. NMR spectral data of 36 was
obtained from the NMR spectrum of the crude reaction
mixture.
OR
NEt3
CH2Cl2, rt
Ac2O, pyridine
rt, 16h,
O
O
O
O
O
HO
O
AcO
32
33
34
a R = H , 12%
b R = COCH3, 72%
1O2, TPP
CCl4, 4 d
CH3
OAc
O
O
O
O
O
+
O
O
O
O
CH3
AcO
36, 44%
35, 41%
Scheme 6.
OR
OAc
O
O
O
O
NEt3
CH2Cl2
O
O
1. O2, TPP, hν, 2h
O
O
O
O
CCl4, 10 °C,
2. hν, 4d, 83%
RO
AcO
14
28
27
a R = H
Ac2O/Py
90%
hν
O-O cleavage
b R = COCH3
OAc
O
O
OAc
CH3
O
O
O
O
O
O
C-C cleavage
AcO
ring-closure
O
O
CH3
O
O
O
O
O
AcO
O
31
30
29
Scheme 5.

A. Coxskun et al. / Tetrahedron 63 (2007) 4944–4950
4947
Finally, we investigated the m-chloroperbenzoic acid (m-
CPBA) oxidation reaction of the cycloheptatriene derivative
14. The isolated product was identified as the hemiketal 38.
The configuration of the epoxide ring was determined by
measuring the coupling constant between the H5 and H6 pro-
tons. AM1 calculations show that the dihedral angle between
those protons is w65ꢀ in the case of exo-configuration and
w11ꢀ for endo-configuration. The dihedral relationship is
sufficiently distinctive to be revealed by the magnitude of the
spin–spin interaction. Thus, no measurable coupling con-
stant between H5 and H6 proton is uniquely accommodated
by the exo-orientation of the epoxy ring. The formation of
38 can be rationalized by the epoxidation of the sterically
less-hindered double bond in 14, followed rearrangement
of the peroxide linkage and intramolecular ring closure
(Scheme 7).
Spiro[1,3-dioxolane-2,20-[8]oxabicyclo[5.1.0]octa-3,5-di-
ene] (11): 1H NMR (200 MHz, CDCl3): 6.03 (m, 3H, H4, H5
and H6), 5.67 (br d, J3,4¼12.2 Hz, 1H, H3), 4.13–3.91 (m,
4H, 2ꢁOCH2), 3.66 (dd, A part of AB system, J1,7¼4.4,
J1,3¼1.6, 1H, H1), 3.38 (br d, B part of AB system,
J1,7¼4.4, 1H, H7). 13C NMR (50 MHz, CDCl3): 132.80,
131.57, 129.81, 129.40, 109.52, 76.61, 67.27, 67.02,
53.18. IR (NaCl film): 3055, 3030, 3004, 2979, 2953,
2902, 1268, 1166, 1140, 1114, 1089, 1013. Anal. Calcd
for C9H10O3: C, 65.05; H, 6.07. Found: C, 64.81; H, 6.14.
The elution of the column was continued with n-hexane/
ethyl acetate (4:1) and as the third fraction, the benzoic acid
ester 10a9 was isolated (1.44 g, 64%, as a pale yellow wax).
2-Hydroxyethyl benzoate (10a): 1H NMR (200 MHz,
CDCl3): 8.05 (m, 2H, aryl), 7.60–7.39 (m, 3H, aryl), 4.45
(m, A2 part of A2B2 system, 2H, OCH2), 3.95 (m, B2 part
of A2B2 system, 2H, OCH2), 2.37 (m, 1H, OH). 13C NMR
(50 MHz, CDCl3): 168.92, 135.09, 131.95, 131.65, 130.36,
68.62, 63.35. IR (NaCl film): 3438, 3030, 2979, 2953,
2928, 2902, 1702, 1472, 1370, 1319, 1294, 1268, 1114, 706.
O
O
O
O
O
rt
5
m-CPBA
O
O
O
O
O
6
CH2Cl2, rt, 4h
O
OH
O
O
37
14
38 40%
3.3. Acetylation of 2-hydroxyethyl benzoate (10a)
HO
O
O
ring closure
To the benzoic acid ester 10a (100 mg, 0.6 mmol) a solution
of acetic anhydride (224 mg, 2.20 mmol) and pyridine
(3 mL) was added. The reaction mixture was kept under
nitrogen at room temperature for 16 h. The excess acetic an-
hydride and the solvent were removed under vacuum and the
1H NMR analysis of residue indicated the formation of the
acetate 10b10 (pale yellow liquid, 125 mg, 100%).
O
O
39
Scheme 7.
3. Experimental
3.1. General
2-(Acetyloxy)ethyl benzoate (10b): 1H NMR (200 MHz,
CDCl3): 8.05 (m, 2H, aryl), 7.60–7.39 (m, 3H, aryl), 4.51
(m, A2 part of A2B2 system, 2H, OCH2), 4.40 (m, B2 part
of A2B2 system, 2H, OCH2), 2.08 (s, 3H, CH3). 13C NMR
(50 MHz, CDCl3): 172.74, 168.28, 135.09, 131.85, 131.69,
130.38, 64.69, 64.16, 22.75. IR (NaCl film): 3081, 3055,
3030, 2979, 2953, 2927, 1753, 1727, 1447, 1370, 1268,
1243, 1217, 1115, 1064, 1038. MS (EI, 70 eV) m/z 208
(M+, trace), 166 (M+ꢂacetyl, 1), 136 (2), 106 (100), 77 (26).
Melting points are uncorrected. Infrared spectra were ob-
tained from solution in 0.1 mm cells or KBr pellets on a reg-
ular instrument. The 1H and 13C NMR spectra were recorded
on 400 (100), 200 (50) MHz spectrometers. Apparent split-
ting is given in all cases. Column chromatography was per-
formed on silica gel (60 mesh, Merck). TLC was carried out
on Merck 0.2 mm silica gel 60 F254 analytical aluminum
plates. Methylene chloride was distilled over CaH2. Pyridine
and n-hexane were distilled from dry NaOH tablets. Ethyl
acetate and carbon tetrachloride was distilled over P2O5.
Methanol was distilled from magnesium metal activated with
iodine. Acetic anhydride was distilled before use at reduced
pressure. All substances reported in this paper are in their
racemic form.
3.4. CoTPP-catalyzed reaction of endoperoxide 14
To a magnetically stirred solution of endoperoxide 14 (1.6 g,
8.8 mmol) in CH2Cl2 (100 mL) at 0 ꢀC was added cobalt-
meso-tetraphenylporphyrin (CoTPP) (15 mg, 0.022 mmol)
in portions. The mixture was stirred at 0 ꢀC for 20 min. After
evaporation of the solvent the residue was chromatographed
on silica gel (20 g) eluting with n-hexane/ethyl acetate (4:1).
The first fraction was identified as bisepoxide 1511 (0.98 g,
61%, pale yellow crystals from n-hexane/ethyl acetate 1:1
mp 75–76 ꢀC, given as a liquid in the literature11).
3.2. Oxidation of 1,4-dioxaspiro[4.6]undeca-6,8,10-tri-
ene (9) with m-chloroperbenzoic acid (m-CPBA)
To a solution of 9 (2.0 g, 13.3 mmol) in methylene chloride
(100 mL) were added Na2CO3 (5.65 g, 53.3 mmol) and
m-CPBA (2.30 g, 13.37 mmol). The resulting mixture was
stirred for 4 h at room temperature in an ultrasound bath.
The precipitate was filtered and dried by evaporating the sol-
vent. The residue was chromatographed on flurosil column
(60 g) eluting with n-hexane/ethyl acetate (9:1). The first
fraction was identified as m-chloroperbenzoic acid. The sec-
ond fraction was monoepoxide 11 isolated as a yellow wax
(133 mg, 6%).
Spiro[6,9-dioxatricyclo[6.1.0.05,7]nona-2-ene-4,20-[1,3]di-
oxolane] (15): 1H NMR (200 MHz, CDCl3): 5.99 (br dd, A
part of AB system, J2,3¼11.3, J1,2¼2.4, 1H, H2), 5.63 (br d,
B part of AB system, J2,3¼11.3, 1H, H3), 4.10–3.78 (m, 4H,
2ꢁOCH2), 3.51–3.40 (m, 3H, H1, H7, H8), 3.27 (dd,
J5,7¼4.0, J3,5¼1.4, 1H, H5). 13C NMR (50 MHz, CDCl3):
136.29, 128.66, 108.99, 67.43, 66.79, 59.77, 55.84, 54.75,
50.79. IR (KBr film): 3055, 3029, 3004, 2902, 1472, 1446,

4948
A. Coxskun et al. / Tetrahedron 63 (2007) 4944–4950
1165, 1140, 1114, 1089, 1063, 1012, 987, 936. MS (EI,
70 eV) m/z 182 (M+, 1), 153 (14), 125 (5), 112 (64), 99
(14), 95 (9), 81 (60), 73 (16), 68 (100).
3.7. Ammonolysis of 2-(acetyloxy)ethyl (2E)-3-(2-
furyl)prop-2-enoate (20b)
The furan derivative (20b) (300 mg, 1.34 mmol) was dis-
solved in 15 mL of absolute methanol. While dry NH3 was
passed through the solution, the mixture was stirred for 2 h
at room temperature. After evaporation of methanol and
formed acetamide, the 1H NMR spectrum of the crude prod-
uct showed formation of 21 and ethylene glycol in quantita-
tive yield as a yellow wax (183 mg).
Then the column was eluted with ethylacetate/methanol
(95:5) and gave as the second fraction a mixture of isomeric
18 and 19 was isolated (369 mg, 21%, in 3:2 ratio).
cis and trans 2-Hydroxyethyl (2E)-3-(5-hydroxy-2,5-di-
hydrofuran-2-yl)prop-2-enoate (18 and 19): 1H NMR
(400 MHz, CDCl3): 6.88 (dd, J¼15.6, J¼4.8, 1H), 6.75
(dd, J¼15.6, J¼5.4, 1H), 6.11–5.96 (m, 4H), 5.81 (m, 2H),
5.47 (m, 1H), 5.18 (m, 1H), 5.10 (m, 1H), 5.05 (m, 1H),
4.19–4.11 (m, A2 parts of A2B2 systems, 4H), 3.76–3.68
(m, B2 parts of A2B2 systems, 4H). 13C NMR (100 MHz,
CDCl3): 166.91, 166.78, 147.41, 146.53, 132.28, 132.19,
128.83, 128.70, 120.93, 120.70, 103.51, 103.47, 83.99,
83.77, 66.33 (2C), 60.71, 60.33.
(2E)-3-(2-Furyl)prop-2-enamide
0
(21):14
1H
0
NMR
0
(400 MHz, CDCl3): 7.48 (d, J4 ,5 ¼2.0, 1H, H5 ), 7.42 (d,
A part of AX system, J2,3¼15.6, 1H, H3), 6.60 (d, A
0
0
0
part of AX system, J3 ,4 ¼3.4, 1H, H3 ), 6.46 (dd, X part
0
0
0
0
0
of AX system, J3 ,4 ¼3.4, J4 ,5 ¼2.0, 1H, H4 ), 6.30 (d, X
part of AX system, J2,3¼15.6, 1H, H2). 13C NMR
(CDCl3): 167.75, 151.07, 144.97, 131.44, 115.62, 115.07,
112.50.
3.5. The formation of 20a from 18/19
3.8. Acetylation of 5-hydroxy-2-(2-hydroxyethoxy)-
cyclohepta-2,4,6-trien-1-one (27a)
Compounds 18/19 (40 mg, 0.20 mmol) were dissolved in
0.5 mL of CDCl3 in the NMR tube and its rearrangement
was followed by 1H NMR spectroscopy. After 13 days, the
compounds 18/19 were completely rearranged to the furan
derivative 20a at room temperature as a brown wax.
The reaction was carried out according to the above-men-
tioned procedure (Section 3.3) by using 1.2 g (6.6 mmol)
of 27a,11 pyridine (30 ml), and acetic anhydride (2.69 g,
26.36 mmol). After 16 h, the excess acetic anhydride and
the solvent were removed under vacuum. The product was
purified by a short column on silica gel using a mixture of
hexane:ethyl acetate (85:15 ratio), yielding 1.58 g (90%)
of the diacetate (pale yellow crystals from methylene chlo-
ride/n-hexane 1:4, mp 85–86 ꢀC).
2-Hydroxyethyl (2E)-3-(2-furyl)prop-2-enoate (20a): 1H
0
0
0
NMR (400 MHz, CDCl3): 7.45 (d, J4 ,5 ¼1.7, 1H, H5 ),
7.43 (d, A part of AX system, J2,3¼15.7, 1H, H3), 6.60
0
0
0
(d, A part of AB system, J3 ,4 ¼3.2, 1H, H3 ), 6.45 (dd, B
0
0
0
0
0
part of AB system, J3 ,4 ¼3.2, J4 ,5 ¼1.7, H4 ), 6.30 (d,
X part of AX system, J2,3¼15.7, 1H, H2), 4.28 (m, A2 part
of A2B2 system, 2H, OCH2), 3.85 (m, B2 part of A2B2
system, 2H, OCH2). 13C NMR (100 MHz, CDCl3): 167.66,
150.94, 145.19, 131.94, 115.48, 115.29, 112.59,
66.41, 61.39. IR (NaCl film): 3362, 3183, 3080, 3055,
2979, 2953, 2928, 1676, 1625, 1395, 1243, 1217, 783.
Anal. Calcd for C9H10O4: C, 59.34; H, 5.53. Found: C,
59.74; H, 5.38.
4-[2-(Acetyloxy)ethoxy]-5-oxocyclohepta-1,3,6-trien-1-yl
acetate (27b): 1H NMR (400 MHz, CDCl3): 7.11 (d, A part
of AB system, J6,7¼12.8, 1H, H6), 6.96 (dd, B part of AB
system, J6,7¼12.8, J2,7¼2.2, 1H, H7), 6.72 (dd, A part of
AB system, J2,3¼11.0, J2,7¼2.2, 1H, H2), 6.67 (br d, B
part of AB system, J2,3¼11.0, 1H, H6), 4.41 (m, A2 part of
A2B2, 2H, OCH2), 4.20 (m, B2 part of A2B2, OCH2), 2.22
(s, 3H, CH3), 2.01 (s, 3H, CH3). 13C NMR (100 MHz,
CDCl3): 179.59, 171.11, 169.54, 163.41, 149.80, 137.15,
133.79, 123.38, 113.05, 67.47, 62.32, 21.13, 21.04. IR
(NaCl film): 3055, 3030, 2979, 2953, 2928, 1753, 1574,
1498, 1447, 1370, 1294, 1243, 1217, 1191, 1140, 1114,
936.
3.6. Acetylation of 2-hydroxyethyl (2E)-3-(2-furyl)prop-
2-enoate (20a)
The reaction was carried out according to the above-
mentioned procedure (3.3) by using 100 mg (0.55 mmol)
of 20a, pyridine (3 ml), and acetic anhydride (224 mg,
2.20 mmol). After 16 h, the solvent was evaporated and
3.9. Photooxygenation of 4-[2-(acetyloxy)ethoxy]-5-oxo-
cyclohepta-1,3,6-trien-1-yl acetate (27b)
1
the H NMR analysis of a residue indicated the formation
of the acetate 20b (pale yellow liquid, 111 mg, 90%).
The diester 27b (552 mg, 2.07 mmol) and tetraphenylpor-
phyrin (5 mg, 0.008 mmol) were dissolved in 40 mL of
CCl4. The solution was then irradiated with a projection
lamp (500 W) while a slow stream of dry oxygen was passed
through the solution at 10 ꢀC. After 2 h, the 1H NMR
spectral studies indicated the formation of a mixture consist-
ing of 31 and 28 in a ratio of 2:1. The residue was chromato-
graphed on silica gel (20 g) eluting with n-hexane/ethyl
acetate (7:3). The initially formed endoperoxide 28 was
decomposed on the column material. The furanoid 31
(205 mg) was isolated in a yield of 33%. However, when
the irradiation was continued with the same projection
lamp (500 W) in the absence of oxygen, after 4 days the
1
2-(Acetyloxy)ethyl (2E)-3-(2-furyl)prop-2-enoate (20b): H
0
NMR (200 MHz, CDCl3): 7.49 (m, 1H, H5 ), 7.44 (d, A
part of AX system, J2,3¼15.8, 1H, H3), 6.62 (d, A part of
0
0
0
AB system, J3 ,4 ¼3.4, 1H, H3 ), 6.47 (dd, B part of AB sys-
0
0
0
0
0
tem, J3 ,4 ¼3.4, J4 ,5 ¼1.8, 1H, H4 ), 6.33 (d, X part of AX
system, J2,3¼15.8, 1H, H2), 4.42–4.29 (m, A2B2 system,
4H, 2ꢁOCH2), 2.09 (s, 3H, CH3). 13C NMR (CDCl3):
172.70, 168.63, 152.87, 146.83, 133.59, 117.21, 116.91,
114.26, 64.25, 64.17, 22.73. IR (NaCl film): 3055, 3030,
2979, 2953, 1728, 1446, 1395, 1243, 1217, 1185, 1063,
757. Anal. Calcd for C11H12O5: C, 58.93; H, 5.39. Found:
C, 59.05; H, 5.24.

A. Coxskun et al. / Tetrahedron 63 (2007) 4944–4950
4949
endoperoxide 28 was completely converted into the furanoid
3.11. Photooxygenation of 4-(acetyloxy)-5-oxocyclo-
hepta-1,3,6-trien-1-yl acetate (34b)
31. A flash chromatography on silica gel allowed to
isolate the furanoid 31 in 83% yield as a pale yellow wax,
513 mg.
The reaction was carried out according to the above-men-
tioned procedure (Section 3.9) by using 2.0 g (9.0 mmol)
of 34b and TPP (10 mg, 0.016) in 200 mL of CCl4. After
2-(Acetyloxy)ethyl(2Z)-3-[2-(acetyloxy)-5-oxo-2,5-dihydro-
furan-2-yl]prop-2-enoate (31): 1H NMR (400 MHz,
1
4-day irradiation, the solvent was evaporated and the H
0
0
CDCI3): 7.80 (d, A part of AB system, J3 ,4 ¼5.5 Hz,
NMR analysis of residue indicated the formation of the
endoperoxide 35 (41%) and furan derivative 36 (44%). The
crude mixture was chromatographed on silica gel (20 g)
eluting with n-hexane/ethyl acetate (5:1). The first fraction
was identified as endoperoxide 35 (isolated yield 88 mg,
4%, pale green wax).
1H, H3), 6.38 (d, A part of AB system, J2,3¼12.4 Hz,
0
0
1H, H3), 6.21 (d, B part of AB system, J3 ,4 ¼5.5 Hz, 1H,
0
H4 ), 6.15 (d, B part of AB system J2,3¼12.4 Hz, 1H, H2),
4.34–4.30 (m, A2 part of A2B2 system OCH2), 4.29–4.25
(m, B2 part of A2B2 system OCH2), 2.06 (s, 3H, CH3),
2.05 (s, 3H, CH3). 13C NMR (100 MHz, CDCI3): 170.90,
169.32, 168.08, 164.38, 151.55, 139.38, 123.85, 123.28,
103.58, 63.08, 62.01, 21.04, 20.98. IR (NaCl film): 3113,
3061, 2962, 1790, 1733, 1440, 1392, 1373, 1228, 1195,
1079, 1016, 915, 820. MS (EI, 70 eV) m/z 195
(M+ꢂAcOH–CO2, 2), 168 (4), 153 (4), 137 (6), 125 (5),
109 (5), 88 (100), 83 (20), 71 (10), 54 (24). Anal.
Calcd for C13H14O8: C, 52.35; H, 4.73. Found: C, 52.60;
H, 5.09.
5-(Acetyloxy)-2-oxo-6,7-dioxabicyclo-[3.2.2]nona-3,8-dien-
1-yl acetate (35): H NMR (400 MHz, CDCI3): 6.84 (d,
1
J3,4¼8.4, 1H, H4), 6.12 (dd, J8,9¼8.8, J3,9¼1.8, 1H, H9),
5.25 (dd, J8,9¼8.8, J3,8¼0.7, 1H, H8), 5.20 (ddd, J3,4¼8.4,
J3,9¼1.8, J3,8¼0.7, 1H, H3), 2.23 (s, 3H, CH3), 2.22 (s, 3H,
CH3). 13C NMR (100 MHz, CDCI3): 188.32, 168.42, 168.26,
157.68, 146.45, 129.70, 103.25, 83.13, 76.65, 21.00, 20.49.
IR (NaCl film): 3093, 2926, 1766, 1706, 1372, 1199, 1171,
1117, 1017, 923, 830. Anal. Calcd for C11H10O7: C, 51.98;
H, 3.97. Found: C, 51.24; H, 4.09.
3.10. Acetylation of 2,5-dihydroxycyclohepta-2,4,6-
trien-1-one (33)
The furan derivative could not be isolated from column, but
spectral analyses were performed on the crude products.
The reaction was carried out according to the above-
mentioned procedure (Section 3.3) by using 2.0 g
(14.5 mmol) of 33, pyridine (12 ml), and acetic anhydride
(5.5 mL). After 16 h, the solvent and excess acetic anhydride
Acetic (2Z)-3-[2-(acetyloxy)-5-oxo-2,5-dihydrofuran-2-yl]-
prop-2-enoic anhydride (36): 1H NMR (400 MHz,
1
0
0
0
were evaporated and H NMR analysis of the residue indi-
CDCI3): 7.70 (d, J3 ,4 ¼5.5, 1H, H3 ), 6.31 (d, A part of
cated the formation of the diacetate 34b15 and monoacetate
34a.17 The residue was chromatographed on a silica gel
(30 g) column eluting with n-hexane/ethyl acetate (95:5).
The first fraction was identified as 34a17 (321 mg, 12%,
brown crystals from methylene chloride/n-hexane 1:4, mp
109–110 ꢀC, lit.17 mp 107–108 ꢀC).
AB system, J2,3¼12.5, 1H, H3), 6.17 (d, J3 ,4 ¼5.5, 1H,
0 0
0
H4 ), 5.99 (d, B part of AB system, J2,3¼12.5, 1H, H2),
2.02 (s, 3H, CH3), 2.00 (s, 3H, CH3). 13C NMR (50 MHz,
CDCI3): 169.95, 168.80, 168.73, 151.83, 146.19, 138.98,
124.26, 123.23, 103.71, 20.92, 20.40.
3.12. Oxidation of 14 with m-chloroperbenzoic acid
4-Hydroxy-5-oxocyclohepta-1,3,6-trien-1-yl acetate (34a):
1H NMR (400 MHz, CDCl3): 8.57 (m, 1H, OH), 7.22 (AA0
part of AA0BB0 system, 2H, H3, H6 or H2, H7), 7.10 (BB0
part of AA0BB0 system, 2H, H3, H6 or H2, H7), 2.26 (s,
3H, CH3). 13C NMR (100 MHz, CDCl3): 172.76, 171.37,
151.29, 133.64, 124.57, 22.84. IR (NaCl film): 3218, 1750,
1563, 1463, 1420, 1268, 1200, 1149, 853, 758. MS (EI,
70 eV) m/z 180 (M+, 3), 168 (6), 139 (100), 131 (9), 111
(63), 91 (38), 75 (30), 65 (17), 55 (22).
The reaction was carried out according to the above-men-
tioned procedure (Section 3.2) by using 0.85 g (4.67 mmol)
of 14 and m-CPBA (0.97 g, 5.60) in 25 mL of CH2Cl2. The
solvent was removed. The crude was extracted with Et2O
(3ꢁ30 mL) and the combined ethereal extracts were dried
over MgSO4 and concentrated in vacuo. The residue was pu-
rified from silica gel (20 g) using n-hexane/ethyl acetate 6:4
(white crystals from CH2Cl2/n-hexane (1:3), 370 mg 40%,
mp 138–139 ꢀC).
The second fraction was identified as 34b15 (2.32 g, 72%,
brown crystals from methylene chloride/n-hexane 1:4, mp
97–98 ꢀC, lit.15 mp 93–95 ꢀC).
Spiro[1-hydroxy-7,9-dioxatricyclo[3.3.1.06,8]nona-3-ene-
1
2,20-[1,3]dioxolane] (38): H NMR (400 MHz, CDCI3):
6.29 (dd, A part of AB system, J3,4¼9.5, J4,5¼4.0, 1H, H4),
5.67 (d, B part of AB system, J3,4¼9.5, 1H, H3), 4.50 (d,
J4,5¼4.0, 1H, H5), 4.21–4.04 (m, 4H, OCH2), 3.96 (m,
1H, OH), 3.73 (d, A part of AB system, J6,8¼3.1, 1H, H8),
3.66 (d, B part of AB system, J6,8¼3.1, 1H, H6). 13C NMR
(50 MHz, CDCI3): 135.11, 132.35, 104.64, 102.53, 74.34,
68.57, 68.15, 59.09, 53.49. IR (NaCl film): 3399, 2983,
2900, 1474, 1373, 1291, 1195, 1167, 1027, 954, 875,
793. MS (EI, 70 eV) m/z 198 (M+, 0.2), 169 (8), 153
(28), 141 (4), 125 (11), 109 (24), 97 (17), 86 (31), 81
(100), 68 (55). Anal. Calcd for C9H10O5: C, 54.55; H,
5.09. Found: C, 53.65; H, 4.84.
4-(Acetyloxy)-5-oxocyclohepta-1,3,6-trien-1-yl acetate (34b):
1H NMR (200 MHz, CDCl3): 7.15 (d, A part of AB system,
J2,3¼J6,7¼11.7, 2H, H2, H7 or H3, H6), 6.88 (br d, B part of
AB system, J2,3¼J6,7¼11.7, 2H, H2, H7 or H3, H6), 2.32 (s,
3H, CH3), 2.28 (s, 3H, CH3). 13C NMR (50 MHz, CDCl3):
171.00, 168.95, 168.23, 153.66, 131.92, 122.75, 21.12,
20.81. IR (NaCl film): 3048, 1763, 1591, 1370, 1196,
1171, 1143, 1071, 916, 853. MS (EI, 70 eV) m/z 222 (M+,
0.1), 180 (14), 138 (100), 110 (46), 82 (4), 54 (5). Anal.
Calcd for C11H10O5: C, 59.46; H, 4.54. Found: C, 59.16;
H, 4.64.

4950
A. Coxskun et al. / Tetrahedron 63 (2007) 4944–4950
1987, 60, 1429–1432; (m) G€uney, M.; C¸ elik, Z. C.; Daxstan,
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_
The authors are indebted to TUBITAK (project number
¨
104T401 and TUBITAK-BAYG National Postdoctoral
_
€
Fellowship Program for A.C.), Ataturk University, TUBA
5. Dastan, A.; Balci, M. Tetrahedron 2006, 62, 4003–4010.
€
(Turkish Academy of Sciences) and Middle East Technical
University for financial supports. Thanks also to Dr. Cavit
Kazaz, Dr. Hamdullah Kılıc¸, Res. Assistant Barıxs Anıl, Dr.
Ebru Mete for NMR experiments, mass spectra and elemen-
tal analyses.
6. Guney, M.; Daxstan, A.; Balci, M. Helv. Chim. Acta 2005, 88,
830–838.
7. Singlet Oxygen; Wassermann, H. H., Murray, R. W., Eds.;
Organic Chemistry, A Series of Monograph; Academic: New
York, NY, 1979; Vol. 40.
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