Synthesis of functionalized p-dihydrobenzoquinones and p-benzoquinones based on [3+3] cyclizations of 1,3-bis-silyl enol ethers

Article abstract of DOI:10.1016/j.tet.2006.03.019

Functionalized mono-protected p-dihydrobenzoquinones were prepared by [3+3] cyclization of 1,3-bis-silyl enol ethers with 2-acyloxy-3-(silyloxy)alk-2-en-1-ones. Deprotection and oxidation of the products afforded the corresponding p-benzoquinones.

Full text of DOI:10.1016/j.tet.2006.03.019

Tetrahedron 62 (2006) 4800–4806
Synthesis of functionalized p-dihydrobenzoquinones
and p-benzoquinones based on [3+3] cyclizations of
1,3-bis-silyl enol ethers
Zafar Ahmed,a,b Christine Fischer,b Anke Spannenbergb and Peter Langera,b,
*
aInstitut fur Chemie, Universitat Rostock, Albert-Einstein-Str. 3a, 18059 Rostock, Germany
¨
¨
bLeibniz-Institut fur Katalyse e. V. an der Universitat Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany
¨
¨
Received 14 February 2006; accepted 6 March 2006
Available online 29 March 2006
Abstract—Functionalized mono-protected p-dihydrobenzoquinones were prepared by [3+3] cyclization of 1,3-bis-silyl enol ethers with
2-acyloxy-3-(silyloxy)alk-2-en-1-ones. Deprotection and oxidation of the products afforded the corresponding p-benzoquinones.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
The reaction of 1a–c with sodium acetate and sodium benz-
oate afforded, according to a known procedure,10 the 2-(acy-
loxy)alk-2-ene-1,3-diones 2a–d. The reaction of 2a–d
with Me3SiCl/NEt3 gave the 2-acyloxy-3-(silyloxy)alk-2-
en-1-ones 3a–d (Scheme 2, Table 1). The TiCl4 mediated
[3+3] cyclization of 3a–d with 1,3-bis-silyl enol ethers
4a–d afforded the mono-protected p-dihydrobenzoquinones
5a–j. Product 5f contains one free and two orthogonally
p-Dihydrobenzoquinones1,2 and p-benzoquinones1,3,4 occur
in a number of natural products, such as sorrentanone and
a-tocopherolquinone (Scheme 1). They have found many
technical and medicinal applications and represent impor-
tant synthetic building blocks. Some years ago, Chan and
co-workers reported an elegant approach to salicylates based
on [3+3] cyclizations5,6 of 1,3-bis-silyl enol ethers.7 Re-
cently, we reported the [3+3] cyclization of 1,3-bis-silyl
enol ethers with 2-acyloxy-3-(silyloxy)alk-2-en-1-ones to
give p-dihydrobenzoquinones.8 Herein, we report full details
of these studies. With regard to our preliminary communica-
tion,8 we extended the methodology to the synthesis of
p-benzoquinones. The transformations reported offer a
convenient approach to functionalized mono-protected 1,4-
dihydrobenzoquinones and to p-benzoquinones.
O
O
O
O
i
R3
R4
R3
R4
OR5
Cl
1a-c
2a-d
Me3SiO OSiMe3
ii
R1
R2
OH
O
4a-d
R1
R3
Me3SiO
R3
O
R2
iii
R4
R4
O
O
O
OH
OR5
OR5
3a-d
5a-j
HO
O
O
iv
α-Tocopherolquinone
Sorrentanone
OH
O
Scheme 1. p-Benzoquinones in natural products.
O
O
R1
R3
R1
R3
R2
R2
v
2-Chloro-1,3-diketones 1a–c are readily available by reac-
tion of 1,3-diketones with N-chlorosuccinimide (NCS).9
R4
R4
OH
O
6a-g
7a-g
Keywords: Cyclization; Dihydroquinones; Oxidation; Quinones; Silyl enol
ethers.
* Corresponding author. Tel.: +381 4986410; fax: +381 4986412; e-mail:
peter.langer@uni-rostock.de
Scheme 2. Synthesis of p-dihydrobenzoquinones 5a–j and of p-benzoqui-
nones 7a–g; (i) NaOAc or NaOBz, DMSO, 3 h, 20 ꢀC; (ii) Me3SiCl,
NEt3, C6H6, 20 ꢀC, 3 d; (iii) TiCl4, CH2Cl2, ꢁ78/20 ꢀC, 20 h; (iv)
H2SO4 (5.0 M), THF, reflux, 36 h; (v) DDQ, C6H6, 3 h, 20 ꢀC.
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.03.019

Z. Ahmed et al. / Tetrahedron 62 (2006) 4800–4806
4801
Table 1. Products and yields
5,6,7 R1 R2 R3
OMe Me Me Ac 52
OH
O
OTf
O
R4
R5 % (5)a % (6)a % (7)a
OMe
OMe
i
a
b
c
d
e
f
H
99b
97b
79
90
84
86
82
82
86
73
H
OMe Me Ph Ac 43
OBz
OBz
Me
Et
Et
OEt
OEt
OEt
Me Me Ac 55
Me Me Ac 54
91b
97b
83
5i
8 (98%)
Me Ph
Ac 37
Ph
O
ii
OMe OMe Me Me Ac 50
g
h
i
H
Me
H
OMe Et
OEt Et
OMe Me Me Bz 55
OEt Me Me Bz 46
Et
Et
Ac 51
Ac 39
98b
OMe
c
—
—
c
c
OBz
9 (70%)
j
Et
—
a
b
c
Yields of isolated products.
Used in crude form for the following step.
Experiment not carried out.
Scheme 3. (i) Tf2O, pyridine, ꢁ78/–10 ꢀC; (ii) phenylboronic acid,
Pd(PPh3)4 (3 mol %), K3PO4 (1.5 equiv), dioxane, reflux.
protected hydroxyl groups. Treatment of a THF solution of
5a–g with H2SO4 (5.0 M) resulted in clean deprotection
to give 6a–g; oxidation of the latter by DDQ afforded the
p-benzoquinones 7a–g.
2. Experimental
2.1. General comments
All solvents were dried by standard methods and all reac-
tions were carried out under an inert atmosphere. For H
The structure of all products was proved by spectroscopic
methods. The structure of 5j was independently confirmed
by X-ray crystal structure analyses (Fig. 1).11
1
and 13C NMR spectra the indicated deuterated solvents
were used. Mass spectrometric data (MS) were obtained
by electron ionization (EI, 70 eV), chemical ionization
(CI, H2O) or electrospray ionization (ESI). For preparative
scale chromatography, silica gel (60–200 mesh) was used.
Melting points are uncorrected.
A great advantage of the methodology reported lies in the
fact that mono-protected 1,4-dihydrobenzoquinones could
be prepared without the need of regioselective protective
group manipulations. The free hydroxyl group was success-
fully functionalized. For example, benzoate protected dihy-
droquinone 5i was transformed into the corresponding enol
triflate 8. The Suzuki reaction of 8 with phenylboronic
acid afforded the biaryl 9 (Scheme 3).
2.2. General procedure for the synthesis of 2-acyloxy-
1,3-diones (2b,g)
The 2-chloro-1,3-dione (1.0 mmol) was treated with sodium
acetate (2.0 mmol) in anhydrous DMSO (2.60 mL/mmol)
for 3 h at room temperature. After addition of water
(50 mL), the organic layer was extracted with diethyl ether
(3ꢂ). The combined organic layers were filtered, dried
(Na2SO4) and concentrated. The residue was purified by
chromatography (silica gel, 20% ethylacetate/heptane) to
give the product.
In conclusion, we have reported the synthesis of func-
tionalized mono-protected p-dihydrobenzoquinones pre-
pared by [3+3] cyclization of 1,3-bis-silyl enol ethers with
2-acyloxy-3-(silyloxy)alk-2-en-1-ones. Deprotection and
oxidation of the products afforded the corresponding
p-benzoquinones.
2.2.1. Compound 2b. Starting with 2-chlorobenzoylacetone
1b (3.00 g, 15.3 mmol), sodium acetate (2.50 g, 30.5 mmol)
and DMSO (40 mL), 2b was isolated as a colourless oil
1
(2.40 g, 72%); H NMR (300 MHz, CDCl3): d¼2.22 (s,
3H, CH3), 2.28 (s, 3H, CH3), 6.25 (s, 1H, CH), 7.44–8.00
(m, 5H, ArH); 13C NMR (75 MHz, CDCl3): d¼20.4, 26.8
(CH3), 82.1 (CH), 128.0 (C), 128.7 (2C), 129.4 (2C),
134.2 (CH), 169.2, 190.8, 199.4 (C); IR (KBr): n¼3064
(w), 2934 (w), 1737 (s), 1693 (s), 1597 (m), 1449 (m),
1373 (m), 1228 (s), 1091 (m), 693 (m) cmꢁ1; MS (EI,
70 eV): m/z (%): 220.1 (M+, 1), 178.1 (38), 136.0 (60),
105.0 (100), 77.0 (92); HRMS (EI): calcd for C12H12O4
[M]+: 220.0730; found: 220.0726.
2.2.2. Compound 2g. Starting with 4-chloro-3,5-heptane-
dione 1g (3.0 g, 18.4 mmol), sodium acetate (3.00 g,
36.8 mmol) and DMSO (48 mL), 2g was isolated as a colour-
less oil (2.50 g, 73%); 1H NMR (300 MHz, CDCl3): d¼1.01
(t, 6H, J¼7.2 Hz, CH3), 2.18 (s, 3H, CH3), 2.50–2.74 (m,
4H, CH2), 5.46 (s, 1H, CH); 13C NMR (75 MHz, CDCl3):
d¼6.8 (2C), 20.4 (CH3), 84.3 (CH), 169.3, 202.0 (2C, C);
Figure 1. ORTEP plot of 5j (non-hydrogen atoms are represented as thermal
ellipsoids at the 30% probability level).

4802
Z. Ahmed et al. / Tetrahedron 62 (2006) 4800–4806
IR (KBr): n¼2982 (w), 2943 (w), 1754 (s), 1737 (s), 1717
(s), 1374 (w), 1233 (s), 1091 (w) cmꢁ1; MS (EI, 70 eV):
m/z (%): 186.0 (M+, 1), 144.0 (20), 130.0 (20), 88.0 (94),
57.0 (100), 43.0 (95); elemental analysis: calcd (%) for
C9H14O4 (186.20): C 58.05, H 7.57; found: C 58.45, H 7.50.
2970 (w), 2904 (w), 1753 (s), 1653 (s), 1404 (m), 1369
(m), 1312 (m), 1262 (m),1215 (s), 1197 (s), 1035 (w), 801
(m) cmꢁ1; GC–MS (EI, 70 eV): m/z (%): 266.1 (M+, 4),
224.1 (18), 178.0 (100), 150.1 (15), 43.1 (72); HRMS (EI):
calcd for C14H18O5 [M]+: 266.1149; found: 266.1147.
2.3. General procedure for the synthesis of alkyl
3-acyloxy-6-hydroxy salicylates (5a–j)
2.3.4. Compound 5d. Starting with 1,3-bis-silyl enol ether
4d (615 mg, 2.0 mmol), silyl enol ether 3a (472 mg,
2.0 mmol) and TiCl4 (0.22 ml, 2.0 mmol), 5d was isolated
1
ꢀ
To a stirred CH2Cl2 solution (2 mL/mmol) of 1,3-bis-silyl
enol ether (1.0 mmol) and silylꢀenol ether (1.0 mmol) was
added TiCl4 (1.0 mmol) at ꢁ78 C under argon atmosphere.
The temperature of the reaction mixture was allowed to rise
as a colourless solid (304 mg, 54%), mp 62 C; H NMR
(250 MHz, CDCl3): d¼1.10 (t, 3H, J¼7.3 Hz, CH3), 1.40
(t, 3H, J¼7.0 Hz, CH3), 2.11 (s, 3H, CH3), 2.29 (s, 3H,
CH3), 2.34 (s, 3H, CH3), 2.65–2.75 (m, 2H, CH2), 4.41 (q,
2H, J¼7.0 Hz, OCH2), 11.46 (s, 1H, OH); 13C NMR
(75 MHz, CDCl3): d¼13.0, 13.1, 14.1, 15.3 (CH3), 19.6
(CH2), 20.3 (CH3), 61.5 (CH2), 110.2, 128.8, 129.6, 135.6,
140.7, 158.2, 169.3, 171.7 (C); IR (KBr): n¼3431 (w),
2979 (s), 2936 (m), 1757 (s), 1651 (s), 1465 (m), 1445
(m), 1395 (s), 1377 (s), 1367 (s), 1320 (s), 1278 (s), 1219
(s), 1042 (m), 806 (m) cmꢁ1; GC–MS (EI, 70 eV): m/z
(%): 280.1 (M+, 7), 238.1 (16), 192.1 (100), 164.0 (26),
43.0 (90); HRMS (EI): calcd for C15H20O5 [M]+:
280.1305; found: 280.1308.
ꢀ
to 20 C during 20 h and saturated aqueous solution of
NaHCO3 (10 mL) was added. The organic layer was sepa-
rated and extracted with diethyl ether (3ꢂ30 mL). The com-
bined organic layers were dried (Na2SO4), filtered and the
filtrate was concentrated in vacuo. The residue was purified
by column chromatography using 20% ethylacetate/heptane
as eluent.
2.3.1. Compound 5a. Starting with 1,3-bis-silyl enol ether
4b (1.00 g, 3.8 mmol), silyl enol ether 3a (885 mg,
3.8 mmol) and TiCl4 (0.42 ml, 3.8 mmol), 5a was isolated
1
ꢀ
as a colourless solid (470 mg, 52%), mp 79 C; H NMR
(250 MHz, CDCl3): d¼2.12 (s, 3H, CH3), 2.30 (s, 3H,
CH3), 2.33 (s, 3H, CH3), 3.93 (s, 3H, OCH3), 6.73 (s, 1H,
ArH), 11.07 (s, 1H, OH); 13C NMR (75 MHz, CDCl3):
d¼15.3, 17.1, 20.3, 52.1 (CH3), 110.7 (C), 117.1 (CH),
132.1, 138.2, 141.0, 160.1, 169.1, 171.6 (C); IR (KBr):
n¼3490 (w), 2990 (w), 2961 (m), 1759 (s), 1664 (s), 1621
(m), 1476 (m), 1439 (m), 1362 (m),1319 (m), 1194 (s),
1072 (m), 802 (m) cmꢁ1; MS (EI, 70 eV): m/z (%): 238
(M+, 12), 196 (44), 164 (100); HRMS (EI): calcd for
C12H14O5 [M]+: 238.0836; found: 238.0837.
2.3.5. Compound 5e. Starting with 1,3-bis-silyl enol ether
4d (700 mg, 2.3 mmol), silyl enol ether 3b (676 mg,
2.3 mmol) and TiCl4 (0.25 ml, 2.3 mmol), 5e was isolated
1
ꢀ
as a colourless solid (290 mg, 37%), mp 79 C; H NMR
(300 MHz, CDCl3): d¼0.65 (t, 3H, J¼7.1 Hz, CH3), 1.17
(t, 3H, J¼7.5 Hz, CH3), 1.79 (s, 3H, CH3), 2.15 (s, 3H,
CH3), 2.78 (b, 2H, CH2), 3.89 (q, 2H, J¼7.1 Hz, OCH2),
7.08–7.30 (m, 5H, ArH), 11.34 (s, 1H, OH); 13C NMR
(75 MHz, CDCl3): d¼12.8, 13.0, 13.1 (CH3), 19.9 (CH2),
20.0 (CH3), 60.8 (CH2), 109.8 (C), 126.5, 127.3 (2C),
128.5 (2C, CH), 131.6, 133.3, 136.1, 138.3, 140.0, 157.7,
169.6, 170.9 (C); IR (KBr): n¼3444 (w), 2987 (m), 2937
(w), 1758 (s), 1655 (s), 1397 (m), 1376 (m), 1328 (m),
2.3.2. Compound 5b. Starting with 1,3-bis-silyl enol ether
4b (755 mg, 2.8 mmol), silyl enol ether 3b (847 mg,
2.8 mmol) and TiCl4 (0.31 ml, 2.8 mmol), 5b was isolated
1279 (m), 1211 (s), 1117 (m), 1009 (w), 811 (w) cmꢁ1
;
GC–MS (EI, 70 eV): m/z (%): 342 (M+, 4), 300 (38), 254
(95), 236 (28), 129 (57), 43 (100); elemental analysis: calcd
(%) for C20H22O5 (342.39): C 70.16, H 6.48; found: C 69.99,
H 6.80.
1
ꢀ
as a colourless solid (366 mg, 43%), mp 97 C; H NMR
(300 MHz, CDCl3): d¼1.80 (s, 3H, CH3), 2.17 (s, 3H,
CH3), 3.39 (s, 3H, OCH3), 6.91 (s, 1H, ArH), 7.08–7.33
(m, 5H, ArH), 10.89 (s, 1H, OH); 13C NMR (75 MHz,
CDCl3): d¼17.0, 19.9, 51.7 (CH3), 110.4 (C), 118.9,
126.8, 127.3 (2C), 128.2 (2C, CH), 136.4, 137.6, 138.8,
140.2, 159.5, 169.3, 170.7 (C); IR (KBr): n¼3526 (w),
3422 (w), 3055 (w), 3032 (w), 2954 (w), 1754 (s), 1670
(s), 1438 (m), 1358 (m), 1328 (m), 1237 (m), 1217 (s),
1205 (s), 1193 (s), 1072 (m), 705 (m) cmꢁ1; MS (EI,
70 eV): m/z (%): 300.1 (M+, 3), 258.1 (35), 226.0 (100);
elemental analysis: calcd (%) for C17H16O5 (300.31): C
67.99, H 5.37; found: C 67.93, H 5.69.
2.3.6. Compound 5f. Starting with 1,3-bis-silyl enol ether 4f
(600 mg, 2.0 mmol), silyl enol ether 3a (476 mg, 2.0 mmol)
and TiCl4 (0.22 ml, 2.0 mmol), 5f was isolaꢀted as a colour-
less solid (263 mg, 50%), mp 68–72 C; 1H NMR
(250 MHz, CDCl3): d¼2.10 (s, 3H, CH3), 2.25 (s, 3H,
CH3), 2.33 (s, 3H, CH3), 3.83 (s, 3H, OCH3), 3.94 (s, 3H,
OCH3), 11.16 (s, 1H, OH); 13C NMR (75 MHz, CDCl3):
d¼10.8, 15.4, 20.7, 52.7, 60.6 (CH3), 111.6, 127.2, 131.5,
140.8, 145.3, 154.4, 169.5, 172.1 (C); IR (KBr): n¼3501
(w), 3009 (w), 2960 (w), 2937 (w), 1753 (s), 1660 (s),
1440 (s), 1352 (s), 1214 (s), 1071 (m), 1041 (m), 806
(m) cmꢁ1; GC–MS (EI, 70 eV): m/z (%): 268 (M+, 10),
226 (17), 194 (100), 165 (38), 67 (46); HRMS (EI): calcd
for C13H16O6 [M]+: 268.0941; found: 268.0942.
2.3.3. Compound 5c. Starting with 1,3-bis-silyl enol ether
4c (808 mg, 2.8 mmol), silyl enol ether 3a (645 mg,
2.8 mmol) and TiCl4 (0.30 ml, 2.8 mmol), 5c was isolated
1
ꢀ
as a colourless solid (410 mg, 55%), mp 96 C; H NMR
(250 MHz, CDCl3): d¼1.40 (t, 3H, J¼8.4 Hz, CH3), 2.09
(s, 3H, CH3), 2.18 (s, 3H, CH3), 2.30 (s, 3H, CH3), 2.34 (s,
3H, CH3), 4.41 (q, 2H, J¼7.0 Hz, OCH2), 11.51 (s, 1H,
OH); 13C NMR (75 MHz, CDCl3): d¼9.7, 11.9, 12.2, 13.4,
18.5 (CH3), 59.7 (CH2), 108.2, 121.8, 126.8, 134.3, 138.7,
156.4, 167.5, 169.8 (C); IR (KBr): n¼3413 (w), 2989 (w),
2.3.7. Compound 5g. Starting with 1,3-bis-silyl enol ether
4b (500 mg, 1.9 mmol), silyl enol ether 3g (496 mg,
1.9 mmol) and TiCl4 (0.21 ml, 1.9 mmol), 5g was isolated
as a colourless oil (258 mg, 51%); H NMR (300 MHz,
CDCl3): d¼1.14 (t, 3H, J¼7.4 Hz, CH3), 1.25 (t, 3H,
1

Z. Ahmed et al. / Tetrahedron 62 (2006) 4800–4806
4803
J¼7.5 Hz, CH3), 2.32 (s, 3H, CH3), 2.40–2.89 (m, 4H, CH2),
2.4. General procedure for the synthesis of p-dihydro-
benzoquinones (6a–g)
3.95 (s, 3H, OCH3), 6.78 (s, 1H, ArH), 11.16 (s, 1H, OH);
13C NMR (75 MHz, CDCl3): d¼12.9, 14.5, 20.5 (CH3),
22.3, 23.6 (CH2), 52.3 (CH3), 110.2 (C), 115.6 (CH),
138.0, 140.3, 143.9, 160.4, 171.4 (C); IR (KBr): n¼2975
(m), 2879 (w), 1762 (s), 1665 (s), 1436 (m), 1369 (m),
H2SO4 (5 M, 8 mL/mmol) was added to a solution of starting
material (1.0 mmol) in THF (50 mL/mmol) and refluxed for
36 h. The reaction mixture was concentrated on rotary, taken
up in dichloromethane (20 mL) and washed with water
(50 mL). The organic layer was dried (Na2SO4) and concen-
trated. The residue was purified by chromatography (silica
gel, 20% ethylacetate/heptane) to give the product
1322 (m), 1232 (s), 1195 (s), 1079 (m), 808 (w) cmꢁ1
;
GC–MS (EI, 70 eV): m/z (%): 266 (M+, 6), 235 (9), 224
(44), 192 (100), 149 (25); HRMS (EI): calcd for C14H18O5
[M]+: 266.1144; found: 266.1149.
2.3.8. Compound 5h. Starting with 1,3-bis-silyl enol ether
4c (400 mg, 1.3 mmol), silyl enol ether 3g (359 mg,
1.3 mmol) and TiCl4 (0.15 ml, 1.3 mmol), 5h was isolated
2.4.1. Compound 6a. Starting with 5a (265 mg, 1.1 mmol),
5 M H2SO4 (9 mL) and THF (50 mL), 6a was isolated as
1
ꢀ
a colourless solid (215 mg, 99%), mp 120 C; H NMR
(300 MHz, CDCl3): d¼2.25 (s, 3H, CH3), 2.43 (s, 3H,
CH3), 3.95 (s, 3H, OCH3), 6.66 (s, 1H, ArH), 10.60 (s, 1H,
OH); 13C NMR (75 MHz, CDCl3): d¼14.5, 16.9, 52.0
(CH3), 110.8 (C), 116.6 (CH), 124.5, 132.5, 145.1, 156.0,
171.8 (C); IR (KBr): n¼3505 (s), 3086 (m), 2944 (m),
2862 (w), 1654 (s), 1621 (m), 1481 (s), 1442 (s), 1333 (s),
1237 (b), 1076 (m), 1050 (s), 967 (m), 796 (s), 719
(m) cmꢁ1; MS (EI, 70 eV): m/z (%): 196.0 (M+, 63), 163.9
(100), 135.9 (40), 107.0 (18), 79.0 (16); HRMS (EI): calcd
for C10H12O4 [M]+: 196.0725; found: 196.0730.
1
ꢀ
as a colourless solid (155 mg, 39%), mp 40 C; H NMR
(250 MHz, CDCl3): d¼1.09 (t, 3H, J¼7.6 Hz, CH3), 1.13
(t, 3H, J¼7.6 Hz, CH3), 1.14 (t, 3H, J¼7.3 Hz, CH3), 2.21
(s, 3H, CH3), 2.35 (s, 3H, CH3), 2.40–2.98 (m, 4H, CH2),
4.42 (q, 2H, J¼7.0 Hz, OCH2), 11.43 (s, 1H, OH); 13C
NMR (75 MHz, CDCl3): d¼11.5, 13.1, 13.9, 14.7, 20.6
(CH3), 21.7, 22.3, 61.7 (CH2), 109.8, 123.6, 134.9, 140.1,
141.9, 158.7, 170.3, 171.5 (C); IR (KBr): n¼3430 (w),
2971 (m), 2937 (m), 1755 (s), 1651 (s), 1407 (m), 1370
(m), 1314 (m), 1274 (m), 1219 (s), 1197 (s), 1022 (m),
807 (m) cmꢁ1; MS (EI, 70 eV): m/z (%): 294.2 (M+, 27),
252.2 (75), 206.1 (100), 163.1 (19); HRMS (EI): calcd for
C16H22O5 [M]+: 294.1462; found: 294.1466.
2.4.2. Compound 6b. Starting with 5b (264 mg, 0.8 mmol),
5 M H2SO4 (8 mL) and THF (40 mL), 6b was isolated as
1
ꢀ
a colourless solid (220 mg, 97%), mp 138 C; H NMR
(300 MHz, CDCl3): d¼2.28 (s, 3H, CH3), 3.39 (s, 3H,
OCH3), 6.85 (s, 1H, ArH), 7.18–7.49 (m, 5H, ArH), 10.56
(s, 1H, OH); 13C NMR (75 MHz, CDCl3): d¼16.9, 51.5
(CH3), 109.0 (C), 119.1 (CH), 126.5 (C), 127.9, 128.9
(2C), 129.2 (2C, CH), 133.7, 136.4, 144.2, 155.5, 170.9
(C); IR (KBr): n¼3530 (s), 3421 (b), 2945 (m), 2867 (w),
1670 (s), 1455 (s), 1434 (s), 1331 (s), 1184 (b), 1076 (m),
760 (m), 705 (m) cmꢁ1; MS (EI, 70 eV): m/z (%): 258.0
(M+, 53), 226.0 (100), 197.9 (29), 141.0 (25), 115.0 (15);
HRMS (EI): calcd for C15H14O4 [M]+: 258.0887; found:
258.0881.
2.3.9. Compound 5i. Starting with 1,3-bis-silyl enol ether
4b (1.00 g, 3.8 mmol), silyl enol ether 3i (1.12 g,
3.8 mmol) and TiCl4 (0.42 ml, 3.8 mmol), 5i was isolated
1
ꢀ
as a colourless solid (625 mg, 55%), mp 80 C; H NMR
(300 MHz, CDCl3): d¼2.17 (s, 3H, CH3), 2.35 (s, 3H,
CH3), 3.95 (s, 3H, OCH3), 6.78 (s, 1H, ArH), 7.51–8.26
(m, 5H, ArH), 11.16 (s, 1H, OH); 13C NMR (75 MHz,
CDCl3): d¼15.5, 17.3, 52.2 (CH3), 110.9 (C), 117.2, 128.7
(2C, CH), 128.9 (C), 130.2 (2C, CH), 132.5 (C), 133.8
(CH), 138.6, 141.1, 160.2, 164.7, 171.7 (C); IR (KBr):
n¼3431 (w), 3034 (w), 2985 (w), 2958 (w), 1732 (s), 1661
(s), 1622 (m), 1443 (m), 1360 (m), 1323 (m), 1262 (s),
1236 (s), 1202 (s), 1153 (m), 1085 (s), 1062 (s), 1023 (m),
803 (m), 705 (s) cmꢁ1; MS (EI, 70 eV): m/z (%): 300.1
(M+, 39), 269.1 (9), 105.0 (100), 77.0 (71); HRMS (EI):
calcd for C17H16O5 [M]+: 300.0992; found: 300.0990.
2.4.3. Compound 6c. Starting with 5c (70 mg, 0.2 mmol),
5 M H2SO4 (2 mL) and THF (14 mL), 6c was isolated as
a colourless solid (46 mg, 79%), mp 105 C; 1H NMR
ꢀ
(400 MHz, CDCl3): d¼1.41 (t, 3H, J¼5.3 Hz, CH3), 2.17
(s, 3H, CH3), 2.21 (s, 3H, CH3), 2.41 (s, 3H, CH3), 4.30
(s, 1H, OH), 4.41 (q, 2H, J¼7.1 Hz, OCH2), 10.97 (s,
1H, OH); 13C NMR (100 MHz, CDCl3): d¼11.9, 13.2,
14.2, 14.6 (CH3), 61.5 (CH2), 110.1, 121.1, 123.0, 131.1,
144.6, 154.4, 171.9 (C); IR (KBr): n¼3382 (b), 2983
(m), 2940 (w), 1655 (s), 1616 (m), 1468 (m), 1439 (m),
1394 (m), 1376 (m), 1223 (m), 1268 (s), 1213 (s), 1049
(m), 1034 (m), 799 (m), 746 (m) cmꢁ1; MS (EI, 70 eV):
m/z (%): 224.0 (M+, 26), 177.9 (100), 15.0 (61);
HRMS (EI): calcd for C12H16O4 [M]+: 224.1042; found:
224.1043.
2.3.10. Compound 5j. Starting with 1,3-bis-silyl enol ether
4d (800 mg, 2.6 mmol), silyl enol ether 3i (773 mg,
2.6 mmol) and TiCl4 (0.29 ml, 2.6 mmol), 5j was isolated
1
ꢀ
as a colourless solid (410 mg, 46%), mp 89 C; H NMR
(300 MHz, CDCl3): d¼1.13 (t, 3H, J¼7.5 Hz, CH3), 1.40
(t, 3H, J¼7.2 Hz, CH3), 2.16 (s, 3H, CH3), 2.35 (s, 3H,
CH3), 2.74 (q, 2H, J¼3.5 Hz, CH2), 4.42 (q, 2H, J¼7.0 Hz,
OCH2), 7.51–8.27 (m, 5H, ArH), 11.50 (s, 1H, OH); 13C
NMR (100 MHz, CDCl3): d¼13.2, 13.3, 14.2, 15.6 (CH3),
19.8, 61.7 (CH2), 128.7 (2C, CH), 129.1, 129.2, 129.8 (C),
130.2 (2C), 133.7 (CH), 135.9, 140.9, 158.4, 164.9, 171.9
(C); IR (KBr): n¼3437 (m), 2975 (m), 2933 (m), 2874 (w),
1731 (s), 1650 (s), 1611 (m), 1451 (m), 1394 (m), 1373
(m), 1321 (m), 1252 (s), 1205 (s), 1104 (s), 1066 (m), 1039
(m), 1026 (m), 807 (m), 715 (s) cmꢁ1; MS (EI, 70 eV): m/z
(%): 342.2 (M+, 67), 296.1 (50), 191.1 (23), 105.0 (100);
elemental analysis: calcd (%) for C20H22O5 (342.39): C
70.16, H 6.48; found: C 70.14, H 6.78.
2.4.4. Compound 6d. Starting with 5d (84 mg, 0.3 mmol),
5 M H2SO4 (3 mL) and THF (15 mL), 6d was isolated as
a colourless solid (65 mg, 91%), mp 98 C; 1H NMR
ꢀ
(300 MHz, CDCl3): d¼1.11 (t, 3H, J¼7.5 Hz, CH3), 1.43
(t, 3H, J¼7.1 Hz, CH3), 2.26 (s, 3H, CH3), 2.43 (s, 3H,
CH3), 2.72 (q, 2H, J¼7.5 Hz, CH2), 4.43 (q, 2H,
J¼7.1 Hz, OCH2), 10.92 (s, 1H, OH); 13C NMR (75 MHz,

4804
Z. Ahmed et al. / Tetrahedron 62 (2006) 4800–4806
CDCl3): d¼12.5, 13.5, 14.2, 14.6 (CH3), 19.6, 61.4 (CH2),
110.2, 121.2, 129.1, 130.5, 144.8, 154.2, 171.9 (C); IR
(KBr): n¼3450 (s), 1979 (s), 2936 (m), 2873 (m), 1655
(s), 1614 (m), 1465 (m), 1393 (m), 1375 (s), 1323 (m),
1276 (s), 1212 (s), 1109 (m), 1096 (m), 1047 (m), 802
(m), 764 (m) cmꢁ1; MS (EI, 70 eV): m/z (%): 238.1 (M+,
48), 192.1 (100), 177.1 (20), 164.1 (95), 149.1 (20), 121.1
(12), 91.1 (17); HRMS (EI): calcd for C13H18O4 [M]+:
238.1200; found: 238.1202.
temperature for 2 h. The reaction mixture was filtered, dried
(Na2SO4) and concentrated. The residue was purified by
chromatography (silica gel, 30% ethylacetate/heptane) to
give the product.
2.5.1. Compound 7a. Starting with 6a (107 mg, 0.5 mmol),
DDQ (124 mg, 0.5 mmol) and benzene (11 mL), 7a was iso-
1
ꢀ
lated as a yellow solid (95 mg, 90%), mp 48 C; H NMR
(300 MHz, CDCl3): d¼2.02 (s, 3H, CH3), 2.05 (d, 3H,
J¼1.5 Hz, CH3), 3.88 (s, 3H, OCH3), 6.57 (d, 1H,
J¼1.5 Hz, ArH); 13C NMR (75 MHz, CDCl3): d¼13.5,
16.0, 52.7 (CH3), 132.6 (CH), 137.0, 142.0, 146.0, 164.4,
183.5, 187.3 (C); IR (KBr): n¼3455 (w), 2955 (w), 1737
(s), 1658 (s), 1622 (m), 1440 (m), 1331 (s), 1238 (s), 1179
(m), 1043 (m), 898 (w) cmꢁ1; MS (EI, 70 eV): m/z (%):
222.0 (M+, 82), 193.9 (83), 175.9 (100), 147.9 (93), 119.9
(36), 91.0 (61); HRMS (EI): calcd for C10H10O4 [M]+:
194.0573; found: 194.0574.
2.4.5. Compound 6e. Starting with 5e (166 mg, 0.4 mmol),
5 M H2SO4 (4 mL) and THF (25 mL), 6e was isolated as
a colourless solid (140 mg, 97%), mp 68 C; 1H NMR
ꢀ
(300 MHz, CDCl3): d¼0.65 (t, 3H, J¼7.1 Hz, CH3), 1.17
(t, 3H, J¼7.5 Hz, CH3), 2.34 (s, 3H, CH3), 2.78 (q, 2H,
J¼7.5 Hz, CH2), 3.89 (q, 2H, J¼7.1 Hz, OCH2), 7.18–7.47
(m, 5H, ArH), 11.01 (s, 1H, OH); 13C NMR (75 MHz,
CDCl3): d¼12.4, 12.9, 13.3 (CH3), 19.7, 60.6 (CH2), 108.4
(C), 127.7 (CH), 128.5 (C), 128.9 (2C), 129.6 (2C, CH),
131.2, 131.9, 137.1, 143.8, 153.9, 171.1 (C); IR (KBr):
n¼3531 (s), 2977 (m), 2934 (m), 2872 (w), 1653 (s), 1444
(m), 1398 (s), 1373 (s), 1330 (s), 1293 (m), 1249 (m),
1209 (s), 1080 (m), 1059 (m), 763 (m), 699 (m) cmꢁ1; MS
(EI, 70 eV): m/z (%): 300.1 (M+, 38), 254.1 (100), 139.
(61), 221.0 (46), 183.1 (14), 129.0 (65); HRMS (EI): calcd
for C18H20O4 [M]+: 300.1356; found: 300.1358.
2.5.2. Compound 7b. Starting with 6b (125 mg, 0.4 mmol),
DDQ (110 mg, 0.4 mmol) and benzene (10 mL), 7b was iso-
1
ꢀ
lated as a yellow solid (105 mg, 84%), mp 128 C; H NMR
(300 MHz, CDCl3): d¼2.12 (s, 3H, CH3), 3.64 (s, 3H,
OCH3), 6.71 (d, 1H, J¼1.5 Hz, ArH), 7.25–7.29 (m, 5H,
ArH); 13C NMR (75 MHz, CDCl3): d¼16.2, 52.5 (CH3),
128.1 (2C), 129.1 (2C), 129.8 (C, CH), 130.9 (C), 132.7
(CH), 136.8, 142.4, 146.1, 164.1, 183.8, 186.5 (C); IR
(KBr): n¼3447 (w), 2950 (w), 1736 (s), 1653 (s), 1634
(m), 1432 (m), 1324 (m), 1237 (s), 1113 (m), 1036 (m),
755 (m), 698 (m) cmꢁ1; GC–MS (EI, 70 eV): m/z (%):
256.0 (M+, 29), 224.0 (100), 196.0 (36), 168.0 (28), 139.0
(60); elemental analysis: calcd (%) for C15H12O4 (256.25):
C 70.31, H 4.72; found: C 70.08, H 4.85.
2.4.6. Compound 6f. Starting with 5f (106 mg, 0.3 mmol),
5 M H2SO4 (3 mL) and THF (20 mL), 6f was isolated as
a colourless solid (73 mg, 83%), mp 81 C; 1H NMR
ꢀ
(300 MHz, CDCl3): d¼2.22 (s, 3H, CH3), 2.38 (s, 3H,
CH3), 3.82 (s, 3H, OCH3), 3.95 (s, 3H, OCH3), 4.44 (s,
1H, OH), 10.52 (s, 1H, OH); 13C NMR (75 MHz, CDCl3):
d¼9.7, 14.1, 52.1, 60.3 (CH3), 111.3, 119.2, 125.3, 144.6,
144.7, 149.6, 171.8 (C); IR (KBr): n¼3523 (s), 3152 (w),
3004 (w), 2964 (m), 2854 (w), 1642 (s), 1480 (m), 1441
(s), 1421 (s), 1371 (m), 1329 (s), 1274 (s), 1229 (s), 1122
(m), 1070 (m), 1048 (s), 1032 (m), 967 (m), 800 (s), 759
(m) cmꢁ1; MS (EI, 70 eV): m/z (%): 226.0 (M+, 37), 194.0
(100), 165.0 (80), 151.0 (41), 95.1 (28), 83.0 (40);
HRMS (EI): calcd for C11H14O4 [M]+: 226.0836; found:
226.0837.
2.5.3. Compound 7c. Starting with 6c (50 mg, 0.2 mmol),
DDQ (51 mg, 0.2 mmol) and benzene (5 mL), 7c was iso-
1
ꢀ
lated as a yellow solid (42 mg, 86%), mp 46 C; H NMR
(300 MHz, CDCl3): d¼1.35 (t, 3H, J¼7.1 Hz, CH3), 2.02
(s, 9H, CH3), 4.36 (q, 2H, J¼7.1 Hz, OCH2); 13C NMR
(75 MHz, CDCl3): d¼12.1, 12.4, 13.3, 14.1 (CH3), 61.9
(CH2), 137.0, 140.3, 141.11, 141.16, 164.4, 183.8, 186.9
(C); IR (KBr): n¼3452 (w), 2993 (w), 2930 (w), 1739 (s),
1648 (s), 1625 (m), 1374 (m), 1292 (s), 1216 (s), 1061
(m), 1023 (m), 713 (w) cmꢁ1; MS (EI, 70 eV): m/z (%):
222.0 (M+, 82), 193.9 (83), 175.9 (100), 147.9 (93), 119.9
(36), 91.0 (61); HRMS (EI): calcd for C12H14O4 [M]+:
222.0882; found: 222.0887.
2.4.7. Compound 6g. Starting with 5g (170 mg, 0.4 mmol),
5 M H2SO4 (5 mL) and THF (30 mL), 6g was isolated as
a colourless oil (140 mg, 98%); 1H NMR (400 MHz,
CDCl3): d¼1.19 (t, 3H, J¼7.5 Hz, CH3), 1.23 (t, 3H,
J¼7.5 Hz, CH3), 2.61 (q, 2H, J¼7.5 Hz, CH2), 2.92 (q,
2H, J¼7.5 Hz, CH2), 3.95 (s, 3H, OCH3), 6.69 (s, 1H,
ArH), 10.58 (s, 1H, OH); 13C NMR (100 MHz, CDCl3):
d¼12.9, 14.5 (CH3), 20.9, 22.3 (CH2), 52.2 (CH3), 110.1
(C), 115.6 (CH), 130.5, 138.4, 143.9, 156.1, 171.6 (C); IR
(KBr): n¼3442 (m), 2966 (m), 2875 (m), 1662 (s), 1437
(s), 1325 (m), 1252 (m), 1213 (m), 1081 (m), 806
(w) cmꢁ1; GC–MS (EI, 70 eV): m/z (%): 224.1 (M+, 18),
192.1 (100), 149.1 (38), 91.1 (10); HRMS (EI): calcd for
C12H16O4 [M]+: 224.1043; found: 224.1041.
2.5.4. Compound 7d. Starting with 6d (64 mg, 0.2 mmol),
DDQ (61 mg, 0.2 mmol) and benzene (6 mL), 7d was iso-
1
ꢀ
lated as a yellow solid (50 mg, 82%), mp 43 C; H NMR
(300 MHz, CDCl3): d¼1.02 (t, 3H, J¼7.5 Hz, CH3), 1.34
(t, 3H, J¼7.1 Hz, CH3), 2.00 (s, 3H, CH3), 2.03 (s, 3H,
CH3), 2.47 (q, 2H, J¼7.5 Hz, CH2), 4.36 (q, 2H,
J¼7.1 Hz, OCH2); 13C NMR (75 MHz, CDCl3): d¼11.8,
12.7, 13.2, 14.1 (CH3), 19.6, 61.9 (CH2), 137.1, 140.5,
140.9, 145.4, 164.4, 183.4, 187.3 (C); IR (KBr): n¼3531
(s), 2977 (m), 2934 (m), 2872 (w), 1653 (s), 1444 (m),
1398 (s), 1373 (s), 1330 (s), 1293 (m), 1249 (m), 1209 (s),
1080 (m), 1059 (m), 763 (m), 699 (m) cmꢁ1; MS (EI,
70 eV): m/z (%): 236.1 (M+, 68), 190.1 (99), 162.1 (100),
134.1 (18), 105.1 (11), 91.1 (43), 67.1 (70); HRMS (EI):
calcd for C13H16O4 [M]+: 236.1043; found: 236.1036.
2.5. General procedure for the synthesis of p-benzo-
quinones (7a–g)
A solution of dihydroquinones (6a–g) (1.0 mmol) and DDQ
(1.0 mmol) in benzene (20 mL/mmol) was stirred at room

Z. Ahmed et al. / Tetrahedron 62 (2006) 4800–4806
4805
2.5.5. Compound 7e. Starting with 6e (92 mg, 0.3 mmol),
DDQ (70 mg, 0.3 mmol) and benzene (7 mL), 7e was iso-
CF3), 121.3 (CH), 125.9, 128.2 (C), 128.8 (2C), 130.3 (2C,
CH), 132.5 (C), 134.2 (CH), 135.3, 143.8, 147.9, 163.7,
164.7 (C); IR (KBr): n¼3435 (w), 3069 (w), 3006 (w),
2956 (w), 1745 (s), 1725 (s), 1454 (m), 1417 (s), 1282
(m), 1259 (s), 1245 (s), 1217 (s), 1190 (m), 1137 (m),
1
ꢀ
lated as a yellow solid (73 mg, 82%), mp 43 C; H NMR
(300 MHz, CDCl3): d¼1.02 (t, 3H, J¼7.1 Hz, CH3), 1.06
(t, 3H, J¼7.5 Hz, CH3), 2.10 (s, 3H, CH3), 2.56 (q, 2H,
J¼7.5 Hz, CH2), 4.12 (q, 2H, J¼7.1 Hz, OCH2), 7.25–7.40
(m, 5H, ArH); 13C NMR (75 MHz, CDCl3): d¼12.1, 12.8,
13.7 (CH3), 19.8, 61.8 (CH2), 128.0 (2C), 129.2 (2C),
129.6 (C, CH), 131.2, 136.9, 140.6, 141.8, 145.7, 164.0,
183.8, 186.6 (C); IR (KBr): n¼3442 (m), 2974 (m), 2937
(w), 2875 (w), 1736 (s), 1651 (s), 1615 (m), 1445 (m),
1373 (m), 1286 (s), 1221 (s), 1152 (m), 1021 (m), 699
(m) cmꢁ1; GC–MS (EI, 70 eV): m/z (%): 298.1 (M+, 3),
252.0 (100), 237.0 (38), 224.1 (98), 129.0 (50); elemental
analysis: calcd (%) for C18H18O4 (298.33): C 72.47, H
6.08; found: C 72.28, H 5.83.
1079 (m), 1065 (m), 1015 (m), 827 (m), 707 (m) cmꢁ1
;
MS (EI, 70 eV): m/z (%): 432.1 (M+, 1), 401.0 (6), 105.0
(100); HRMS (EI): calcd for C18H15O7SF3 [M]+:
432.0485; found: 432.0480.
2.7. General procedure for Suzuki coupling (9)
A dioxane solution (5 mL per 1.0 mmol of triflate) of 8
(1.0 mmol), phenylboronic acid (1.3 mmol), K3PO4
(1.6 mmol) and Pd(PPh3)4 (0.03 mmol) was refluxed for
4 h. A saturated aqueous solution of ammonium chloride
was added. The organic and the aqueous layer was separated
and the latter was extracted (3ꢂ) with ether. The combined
organic layers were dried (Na2SO4), filtered and the filtrate
was concentrated in vacuo. The residue was purified by
chromatography (silica gel, 20% ethylacetate/heptane) to
give the product.
2.5.6. Compound 7f. Starting with 6f (45 mg, 0.2 mmol),
DDQ (46 mg, 0.2 mmol) and benzene (4 mL), 7f was iso-
1
ꢀ
lated as a yellow solid (38 mg, 86%), mp 40 C; H NMR
(300 MHz, CDCl3): d¼1.95 (s, 3H, CH3), 2.03 (s, 3H,
CH3), 3.90 (s, 3H, OCH3), 4.02 (s, 3H, OCH3); 13C NMR
(75 MHz, CDCl3): d¼8.9, 13.5, 52.7, 61.0 (CH3), 128.9,
135.4, 142.2, 154.7, 164.6, 180.0, 187.2 (C); IR (KBr):
n¼3430 (s), 2956 (w), 1740 (m), 1655 (s), 1616 (m), 1289
(m), 1225 (m), 1152 (w), 1056 (w), 950 (w), 728
(w) cmꢁ1; GC–MS (EI, 70 eV): m/z (%): 224 (M+, 22),
192 (65), 164 (64), 149 (19), 135 (16), 83 (66), 67 (100);
HRMS (EI): calcd for C11H12O5 [M]+: 224.0679; found:
224.0678.
2.7.1. Compound 9. Starting with 8 (255 mg, 0.5 mmol),
Phenylboronic acid (86 mg, 0.7 mmol), K3PO4 (188 mg,
0.8 mmol), Pd catalyst (21 mg, 0.017 mmol) and dioxane
(4 mL), 9 was isolated as a colourless solid (145 mg,
1
ꢀ
70%), mp 62 C; H NMR (300 MHz, CDCl3): d¼2.21 (s,
3H, CH3), 2.24 (s, 3H, CH3), 3.55 (s, 3H, OCH3), 7.14 (s,
1H, ArH), 7.24–8.27 (m, 10H, ArH); 13C NMR (75 MHz,
CDCl3): d¼13.6, 16.6, 51.8 (CH3), 127.3, 128.23 (2C),
128.26 (2C, CH), 128.3 (C), 128.7 (2C, CH), 128.9 (C),
129.9, 130.2 (2C, CH), 132.0, 132.4 (C), 133.8 (CH),
137.9, 140.5, 147.6, 164.2, 169.5 (C); IR (KBr): n¼3437
(m), 3061 (w), 3030 (w), 2949 (w), 1734 (s), 1451 (m),
1253 (s), 1235 (s), 1160 (s), 1082 (s), 1065 (s), 1024 (m),
709 (m) cmꢁ1; GC–MS (EI, 70 eV): m/z (%): 360.1 (M+,
2), 329.1 (1), 223.0 (2), 165.1 (5), 152.0 (6), 105.0 (100);
HRMS (EI): calcd for C23H20O4 [M]+: 360.1356; found:
360.1347.
2.5.7. Compound 7g. Starting with 6g (109 mg, 0.5 mmol),
DDQ (110 mg, 0.5 mmol) and benzene (10 mL), 7g was iso-
lated as a yellow oil (80 mg, 73%); H NMR (400 MHz,
1
CDCl3): d¼1.10 (t, 3H, J¼7.5 Hz, CH3), 1.13 (t, 3H,
J¼7.5 Hz, CH3), 2.42 (q, 2H, J¼7.5 Hz, CH2), 2.78 (dq,
2H, J¼7.4 Hz, 1.7 Hz, CH2), 3.89 (s, 3H, OCH3), 6.52 (t,
1H, J¼1.6 Hz, ArH); 13C NMR (100 MHz, CDCl3):
d¼11.5, 13.6 (CH3), 21.6, 22.2 (CH2), 52.6 (CH3), 130.8
(CH), 136.3, 146.8, 151.1, 164.5, 184.2, 186.7 (C); IR
(KBr): n¼2976 (m), 2941 (m), 1741 (s), 1653 (s), 1457
(w), 1338 (w), 1278 (m), 1223 (m), 1046 (m), 796
(w) cmꢁ1; GC–MS (EI, 70 eV): m/z (%): 222.1 (M+, 2),
190.1 (100), 147.1 (13), 91.1 (28); HRMS (EI): calcd for
C12H14O4 [M]+: 222.0887; found: 222.0886.
Acknowledgements
Financial support by the state of Mecklenburg-Vorpommern
(scholarship for Z.A. and Landesforschungsschwerpunkt
‘Neue Wirkstoffe und Screeningverfahren’) is gratefully
acknowledged.
2.6. General procedure for the synthesis of triflate (8)
The reaction was carried out analogously by a known proce-
dure. To a dichloromethane solution (10 mL/mmol) of 5i
(1.0 mmol) and triflic anhydride (1.2 mmol) was added
References and notes
ꢀ
pyridine (2.0 mmol) at ꢁ78 C. The solution was allowed
1. Ro€mpp Lexikon Naturstoffe; Steglich, W., Fugmann, B., Lang-
Fugmann, S., Eds.; Thieme: Stuttgart, 1997.
ꢀ
to warm to ꢁ10 C within 4 h. The product was isolated
by rapid chromatography (silica gel, dichloromethane) of
the reaction mixture as a colourless solid.
2. (a) Jinno, S.; Hata, K.; Shimidzu, N.; Okita, T. J. Antibiot. 1998,
51, 508; (b) Jinno, S.; Okita, T. Chem. Pharm. Bull. 1998, 46,
1688; (c) Papendorf, O.; Koenig, G. M.; Wright, A. D. Phyto-
chemistry 1998, 49, 2383; Methoxymicareic acid: (d) Elix,
J. A.; Jones, A. J.; Lajide, L.; Coppins, B. J.; James, P. W.
Aust. J. Chem. 1984, 37, 2349.
3. (a) Xanthanthusin E: Mei, S.-X.; Jiang, B.; Niu, X.-M.; Li,
M.-L.; Yang, H.; Na, Z.; Lin, Z.-W.; Li, C.-M.; Sun, H.-D.
J. Nat. Prod. 2002, 65, 633; blennione: (b) Spiteller, P.; Steg-
lich, W. J. Nat. Prod. 2002, 65, 725; lilacinone: (c) Spiteller, P.;
2.6.1. Compound 8. Starting with 5i (442 mg, 1.4 mmol),
Tf2O (0.29 mL, 1.7 mmol), pyridine (0.23 mL, 2.9 mmol)
and dichloromethane (15 mL), 8 ꢀwas isolated as a colourless
solid (628 mg, 98%), mp 74 C; 1H NMR (300 MHz,
CDCl3): d¼2.24 (s, 6H, CH3), 3.95 (s, 3H, OCH3), 7.10 (s,
1H, ArH), 7.52–8.24 (m, 5H, ArH); 13C NMR (75 MHz,
CDCl3): d¼14.1, 16.9, 52.7 (CH3), 118.5 (q, J¼318.2 Hz,

4806
Z. Ahmed et al. / Tetrahedron 62 (2006) 4800–4806
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11. Crystallographic data (excluding structure factors) for the
structure in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no.
CCDC-299670. Copies of this data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44(1223)336033 or e-mail: deposit@ccdc.
cam.ac.uk) or via http://www.ccdc.cam.ac.uk/conts/retrieving.
html.