Palladium-catalyzed coupling of vinyl tosylates with arylsulfinate salts

Article abstract of DOI:10.1016/j.tetlet.2009.03.174

The first palladium-catalyzed coupling of vinyl tosylates with arylsulfinate salts is described. A variety of cyclic and acyclic vinyl tosylates were coupled with aryl sulfinate salts using 2.5 mol % Pd₂(dba)₃/5.0 mol % XantPhos to g

Full text of DOI:10.1016/j.tetlet.2009.03.174

Tetrahedron Letters 50 (2009) 2870–2873
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Tetrahedron Letters
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Palladium-catalyzed coupling of vinyl tosylates with arylsulfinate salts
*
Diana C. Reeves , Sonia Rodriguez, Heewon Lee, Nizar Haddad, Dhileep Krishnamurthy,
Chris H. Senanayake
Department of Chemical Development, Boehringer Ingelheim Pharmaceuticals, Inc., 900 Ridgebury Road/PO Box 368, Ridgefield, CT 06877-0368, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The first palladium-catalyzed coupling of vinyl tosylates with arylsulfinate salts is described. A variety of
cyclic and acyclic vinyl tosylates were coupled with aryl sulfinate salts using 2.5 mol % Pd2(dba)3/
5.0 mol % XantPhos to give vinyl aryl sulfone products in good yields. The coupling was extended to
the preparation of a cyclopropyl vinyl sulfone.
Received 13 March 2009
Revised 24 March 2009
Accepted 25 March 2009
Available online 29 March 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Vinyl sulfones
Vinyl tosylates
Palladium
Cross-coupling
1. Introduction
Pd2(dba)3 as catalyst and XantPhos as ligand was found to be opti-
mal for the coupling. The choice of base proved to be critical, with
Aryl vinyl sulfones are useful synthetic intermediates,1 and
have been shown to have interesting biological activities.2 While
numerous procedures based on copper or palladium catalysis have
been reported for the synthesis of diaryl sulfones and aryl alkyl
sulfones, comparatively little work has been done on aryl vinyl sul-
fone preparations. In 1950 Baker reported the low yielding thermal
displacement of b-bromostyrene and b-bromo-4-nitrostyrene with
an arylsulfinate sodium salt.3 In 2004, Cacchi et al. described a Pd-
catalyzed coupling of vinyl triflates with arylsulfinate salts.4 Subse-
K3PO4 giving the best results. The catalyst loading could be low-
ered from 5.0 to 2.5 mol % Pd2(dba)3 with no reduction in yield.
Thus, the optimal reaction conditions (entry 4) were defined as
1.2 equiv of sulfinate salt, 1.5 equiv of K3PO4, 2.5 mol % Pd2(dba)3
and 5.0 mol % of XantPhos in toluene at 100 °C for 24 h.
The optimized reaction conditions were applied to the coupling
of a variety of ketone and aldehyde derived vinyl tosylates as
summarized in Table 2. A variety of cyclic (entries 1–6) vinyl sulf-
ones were prepared, including the functionalized N-Boc piperidine
substrate 12. Acyclic substrates could also be prepared (entries 7–
9, 11–12), and in the case of sulfones 14, 16, and 21 a mixture of
quently, Bao and Wang reported a CuI/L-proline-catalyzed coupling
of vinyl bromides with sulfinate salts in ionic liquids.5 Ma and co-
workers reported the CuI-catalyzed coupling of vinyl bromides
with sulfinate salts in anion-functionalized ionic liquids.6
The use of vinyl tosylates as electrophiles in palladium-cata-
lyzed cross-couplings is attractive due to their simple one-step
synthesis from ketone or aldehyde precursors, their lower cost of
preparation compared to vinyl triflates (compare the cost of Ts2O
and PhNTf2),7 and their generally high crystallinity which aids in
isolation and purification.8 Herein we describe our work on the
development of a palladium-catalyzed coupling of arylsulfinate
salts with vinyl tosylates.
Table 1
Catalyst, ligand, and base screening for the conversion of 1 to 2
Pd catalyst, ligand
p-TolylSO2Na (1.2 eq)
OTs
SO2Tolyl
base (1.5 eq)
º
toluene, 100 C, 24 h
1
2
Entry
Pd cat. (mol %)
Ligand (mol %)
Base
Yielda (%)
Initial screening experiments employed b-tetralone derived
vinyl tosylate 1 and p-toluenesulfinic acid sodium salt in toluene
solvent at 100 °C for 24 h. Key results for catalyst, ligand, and base
screening are summarized in Table 1. The combination of
1
2
3
4
5
6
Pd2(dba)3 (5.0)
Pd2(dba)3 (5.0)
Pd2(dba)3 (5.0)
Pd2(dba)3 (2.5)
Pd2(dba)3 (5.0)
PdCl2
XantPhos (10.0)
XantPhos (10.0)
XantPhos (10.0)
XantPhos (5.0)
dppf (10.0)
K2CO3
Cs2CO3
K3PO4
K3PO4
K3PO4
K3PO4
62
45
84
84
50
60
d(iPr)pf
* Corresponding author. Tel.: +1 203 791 6811; fax: +1 203 791 6130.
E-mail address: diana.reeves@boehringer-ingelheim.com (D.C. Reeves).
a
HPLC assay yield.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.03.174

D. C. Reeves et al. / Tetrahedron Letters 50 (2009) 2870–2873
2871
Table 2
successful coupling of cyclopropylsulfinic acid sodium salt
represents the first palladium-catalyzed coupling of an alkyl sulfi-
nate salt. The procedure described herein complements existing
copper (vinyl bromides) and palladium (vinyl triflates)-based
methods and provides an alternative starting material (vinyl tosy-
lates) for vinyl sulfone synthesis.
Pd-catalyzed coupling of vinyl tolsylates with arylsulfinate salts
Entry
Vinyl tosylatea
Vinyl sulfone
Yieldb (%)
59
SO2Tolyl
OTs
1
3
4
OTs
SO2Tolyl
2
3
4
5
6
7
69
53
63
75
70
65
2. Experimental
6
5
2.1. General procedure A for tosylate synthesis
OTs
SO2Tolyl
2.1.1. Cyclopentenyl 4-methylbenzenesulfonate 3
7
8
To a 200 mL round-bottomed flask under nitrogen atmosphere
and magnetic stirring was added cyclopentanone (2.00 g,
23.78 mmol) in tetrahydrofuran (100 mL) to give a yellow solution.
The reaction mixture was cooled to À15 °C. Potassium bis(trimeth-
ylsilyl)amide 0.5 M in toluene (52.3 mL, 26.20 mmol) was added in
one portion. The orange brown solution was stirred at À15 °C for
1 h. p-Toluenesulfonic anhydride (8.54 g, 26.20 mmol) was then
added at À15 °C. The resulting brown solution was stirred at
À15 °C for 30 min then allowed to reach room temperature and
stirred for 14 h, and finally quenched with aq NaHCO3 (20 mL).
MTBE was added (100 mL) and the organic phase was separated
and was washed with water (4 Â 100 mL), dried on MgSO4, and
concentrated. The residue was dissolved in dichloromethane
(2 mL) and purified by CombiFlash (hexane/EtOAc from 10% EtOAc
to 30% EtOAc). A tan oil (3.50 g, 62% yield) was obtained which
solidified under vacuum removal of solvent.
SO2Tolyl
10
OTs
OTs
9
BocN
SO2Tolyl
12
BocN
11
OTs
SO2Tolyl
1
2
OTs
SO2Tolyl
14 (Z:E = 3:1)
13 (Z:E = 3:1)
MeO
Ph
MeO
Ph
OTs
SO2Ph
2.2. General procedure B for tosylate synthesis
8
50
2.2.1. 1,1-Diphenylprop-1-en-2-yl 4-methylbenzenesulfonate 17
In a 200 mL round-bottomed flask under magnetic stirring and
nitrogen atmosphere was added 1,1-diphenylacetone (4.00 g,
19.02 mmol) in THF (47.6 mL) to give a pale yellow solution. The
reaction mixture was cooled to À20 °C. Sodium tert-butoxide
(2.01 g, 20.93 mmol) was added in one portion. The solution was
stirred at À5 °C for 1 h and then at room temperature for 30 min.
The solution was cooled to À15 °C. p-Toluenesulfonic anhydride
(6.83 g, 20.93 mmol) was added in one portion. The resulting solu-
tion was stirred at À15 to À5 °C for 1.5 h and quenched with aq NaH-
CO3 (20 mL). MTBE was added (100 mL) and the organic phase was
washedwithwater (4 Â 100 mL), driedon MgSO4, andconcentrated.
The white solid was separated by recrystallization from hexane/
EtOAc. Alternatively the residue was dissolved in dichloromethane
(2 mL) and purified by CombiFlash (hexane/EtOAc from 10% EtOAc
to 30% EtOAc). A white powder (5.20 g, 75% yield) was obtained.
15 (Z:E = 10:1)
16 (Z:E = 4:1)
SO2Tolyl
OTs
9
10
11
52
74
67
40
17
Ph
Ph
18
OTs
SO2
1
19
Ph
Ph
Ph
Ph
OTs
SO2Tolyl
21 (E:Z = 3:1)
20 (E:Z = 2.4:1)
OTs
SO2Tolyl
12
Ph
Ph
23
22
a
Vinyl tosylates were prepared from the corresponding ketones or aldehydes
using either NaOt-Bu or KHMDS and Ts2O. See Supplementary data for experi-
mental details.
2.2.2. 3,4-Dihydronaphthalen-2-yl 4-methylbenzenesulfonate 1
White solid mp 70–71 °C. Yield 73%, general procedure B. 1H
b
NMR (CDCl3, 400 MHz)
d 7.85 (d, J = 7.0 Hz, 2H), 7.37 (d,
Isolated yields after chromatography on SiO2.
J = 7.0 Hz, 2H), 7.29–7.09 (m, 3H), 7.00–6.93 (m, 1H), 6.13 (s, 1H),
2.91 (t, J = 6.9 Hz, 2H), 2.49 (s, 3H), 2.47 (t, J = 6.9 Hz, 2H). 13C
NMR (CDCl3, 100 MHz) d 149.7, 145.3, 133.1, 133.1, 132.3, 129.8,
128.4, 127.5, 127.3, 126.7, 126.7, 117.3, 28.5, 26.6, 21.7. HRMS
calcd for C17H20NO3S [M+NH4]+: 318.1159; found: 318.1169. HRMS
calcd for C17H17NO3S [M+H]+: 301.0892; found: 301.0896. IR (neat)
isomeric trans and cis olefins was produced with near retention of
the olefin geometries of the starting vinyl tosylates. While the
coupling of sodium methanesulfinate failed to give any coupling
products, we were gratified to find that sodium cyclopropylme-
thanesulfinate coupled with vinyl tosylate 1 to provide vinyl
cyclopropyl sulfone 19 in good yield (74%). To the best of our
knowledge, this is the first example of a palladium-catalyzed
coupling of an alkyl sulfinate salt with any type of electrophile.
In conclusion, a palladium-catalyzed cross-coupling of vinyl
tosylates with aryl and cyclopropyl sulfinate salts has been devel-
oped. The reaction gives moderate to good yields of vinyl sulfones
from a variety of ketone and aldehyde-derived vinyl tolsylates. The
m
1372.9, 1174.7, 1078.4, 888.4, 747.2 cmÀ1
.
2.3. General procedure for tosylate coupling
2.3.1. 3-(Cyclopropylsulfonyl)-1,2-dihydronaphthalene (19)
In a 10 mL sealed tube equipped with magnetic stirrer was
added 3,4-dihydronaphthalen-2-yl 4-methylbenzenesulfonate 1
(0.50 g, 1.67 mmol) in toluene (7 mL) to give a colorless solution.

2872
D. C. Reeves et al. / Tetrahedron Letters 50 (2009) 2870–2873
Cyclopropane sulfinic acid sodium salt (0.26 g, 2.00 mmol), potas-
sium phosphate (0.53 g, 2.50 mmol), Pd2(dba)3 (0.04 g, 0.04 mmol),
and XantPhos (0.05 g, 0.08 mmol) were added to give a brown sus-
pension. The reaction mixture was stirred at 110 °C for 18 h. After
cooling down to 22 °C the crude reaction mixture was filtered, the
solids were washed with ethyl acetate (10 mL), and the combined
organic solutions were concentrated under reduced pressure. The
residue was dissolved in dichloromethane (2 mL) and purified by
column chromatography on SiO2 (hexane/EtOAc from 10% EtOAc
to 30% EtOAc) to provide 0.29 g (74% yield) of 19 as an orange oil.
4.74 (m, 1H), 3.71–3.68 (m, 2H), 3.57–3.52 (m, 1H), 2.45 (s, 3H),
2.25 (br s, 1H), 2.00 (br s, 1H), 1.47 (s, 9H). 13C NMR (CDCl3,
100 MHz): d 1144.8, 134.4, 131.2, 129.7, 128.9, 94.9, 94.4, 81.5,
58.1, 39.1, 37.9, 28.1, 21.5. HRMS calcd for C17H27N2O4S
[M+NH4]+: 355.1686; found: 355.1688, difference = 0.5465 ppm.
HRMS calcd for C17H24NO4S [M+H]+: 338.1420; found: 355.1433,
difference = 3.6766 ppm. IR (neat)
m 1701.9, 1311.7, 1142.6,
715.0 cmÀ1
.
2.3.8. (Z)-1-Methyl-4-(non-4-en-5-ylsulfonyl)benzene
(Z:E = 3:1) (14)
2.3.2. 3-Tosyl-1,2-dihydronaphthalene (2)
Yellow oil. Yield 65%. 1H NMR (CDCl3, 400 MHz) d 7.71 (d,
J = 8.0 Hz, 2H), 7.28 (d, J = 8.0 Hz, 2H), 6.85 (t, J = 7.5 Hz, 1H), 2.40
(s, 3H), 2.20–2.13 (m, 4H), 1.53–1.46 (m, 2H), 1.32–1.18 (m, 4H),
0.94 (t, J = 7.3 Hz, 3H), 0.80 (t, J = 7.3 Hz, 3H). 13C NMR (CDCl3,
100 MHz) d 143.8, 141.5, 129.7, 129.0, 128.4, 128.0, 31.2, 30.3,
26.2, 22.7, 21.8, 21.5, 13.8, 13.6. HRMS calcd for C16H24O2S
[M+H]+: 281.1569; found: 281.1575, difference = 1.8548 ppm. IR
White solid mp 118–119 °C. Yield 70%. 1H NMR (CDCl3,
400 MHz)
d 8.00 (d, J = 8.4 Hz, 2H), 7.76 (s, 1H), 7.52 (d,
J = 8.3 Hz, 2H), 7.48–7.41 (m, 3H), 7.32 (d, J = 6.5 Hz, 1H), 3.06 (t,
J = 8.0 Hz, 2H), 2.69 (t, J = 8.0 Hz, 2H), 2.63 (s, 3H). 13C NMR (CDCl3,
100 MHz) d 144.3, 134.7, 130.4, 130.0, 129.1, 128.1, 127.8, 127.1,
27.6, 21.8, 21.6. HRMS calcd for C17H20NO2S [M+NH4]+:
302.1209; found: 302.1210, difference = 0.2406 ppm. HRMS calcd
for C17H17O2S [M+H]+: 285.0943; found: 285.0938, differ-
(neat)
m .
1287.8, 1134.7, 717.3 cmÀ1
ence = 2.0280 ppm. IR (neat)
m
1319.6, 1143.8, 1083.1, 759.8 cmÀ1
.
2.3.9. (Z)-1-Methoxy-4-(2-(phenylsulfonyl)prop-1-enyl)
benzene (Z:E = 4:1) (16)
2.3.3. 1-(Cyclopentenylsulfonyl)-4-methylbenzene (4)
Light yellow oil. Yield 50%. 1H NMR (CDCl3, 400 MHz) d 7.66 (d,
J = 8.4 Hz, 2H), 7.57–7.48 (m, 1H), 7.41–7.36 (m, 2H), 7.27 (d,
J = 8.4 Hz, 2H), 7.00 (s, 1H), 6.80 (d, J = 8.6 Hz, 2H), 3.82 (s, 3H),
2.19 (d, J = 1.5 Hz, 3H). 13C NMR (CDCl3, 100 MHz) d 159.8, 138.9,
133.2, 131.5, 131.0, 129.2, 128.6, 128.1, 127.7, 113.1, 55.3, 21.09.
HRMS calcd for C16H16O2S [M+H]+: 289.0892; found: 289.0890, dif-
Light orange solid mp 106–107 °C. Yield 59%. 1H NMR
(400 MHz, CDCl3) d 7.79 (d, J = 8.4 Hz, 2H), 7.34 (d, J = 8.4 Hz, 2H),
6.73–6.70 (m, 1H), 2.54–2.52 (m, 4H), 2.44 (s, 3H), 2.09–1.99 (m,
2H). 13C NMR (100 MHz, CDCl3) d 145.0, 144.2, 142.7, 133.0,
129.7, 128.0, 32.9, 30.9, 23.6, 21.6. HRMS calcd for C12H14SO2
(M+H)+: 223.0787, found 223.0785, difference = 1.0225 ppm. IR
ference = 1.0129 ppm. IR (neat)
m 1508.5, 1249.4, 1146.0,
(neat)
m
1288.8, 1147.1, 661.9 cmÀ1
.
728.3 cmÀ1
.
2.3.4. 1-(Cyclohexenylsulfonyl)-4-methylbenzene (6)
2.3.10. (2-Tosylprop-1-ene-1,1-diyl)dibenzene (18)
Yellow solid mp 64–65 °C. Yield 69%. 1H NMR (400 MHz, CDCl3)
d 7.74 (d, J = 8.3 Hz, 2H), 7.32 (d, J = 8.3 Hz, 2H), 7.04–7.01 (m, 1H),
2.43 (s, 3H), 2.29–2.23 (m, 2H), 2.19–2.14 (m, 2H),1.68–1.53 (m,
4H). 13C NMR (100 MHz, CDCl3) d 144.0, 140.0, 136.5, 129.7,
128.1, 25.5, 22.8, 21.8, 21.6, 20.8. HRMS calcd for C13H17SO2
(M+H)+: 237.0943, found 237.09331, difference = 4.5474 ppm. IR
White solid mp 141–142 °C. Yield 52%. 1H NMR (CDCl3,
400 MHz) d 7.72 (d, J = 8.2 Hz, 2H), 7.55–7.38 (m, 8H), 7.33–7.28
(m, 4H), 2.63 (s, 3H), 2.39 (s, 3H). 13C NMR (CDCl3, 100 MHz) d
143.4, 141.1, 139.5, 137.3, 129.7, 129.2, 128.5, 128.4, 128.3,
128.2, 127.8, 127.6, 127.4, 21.5, 18.0. HRMS calcd for C22H24NO2S
[M+NH4]+: 366.1522; found: 366.1532, difference = 2.6560 ppm.
HRMS calcd for C22H21O2S [M+H]+: 349.1256; found: 349.1280, dif-
(neat)
m .
1286.4, 1141.3, 814.2 cmÀ1
ference = 0.9213 ppm. IR (neat)
m 1312.6, 1156.1, 1077.2,
2.3.5. (E)-1-Tosylcyclooct-1-ene (8)
749.6 cmÀ1
.
White solid mp 76–77 °C. Yield 53%. 1H NMR (CDCl3, 400 MHz)
d 7.74 (d, J = 8.3 Hz, 2H), 7.31 (d, J = 8.3 Hz, 2H), 7.07 (t, J = 8.5 Hz,
1H), 2.42 (s, 3H), 2.40–2.36 (m, 2H), 2.33–2.26 (m, 2H), 1.67–
1.61 (m, 2H), 1.44–1.36 (m, 6H). 13C NMR (CDCl3, 100 MHz) d
143.9, 142.0, 137.0, 134.9, 129.7, 128.2, 29.1, 28.2, 26.5, 25.8,
25.6, 25.0, 21.6. HRMS calcd for C15H21O2S [M+H]+: 265.1256;
2.3.11. 3-(Cyclopropylsulfonyl)-1,2-dihydronaphthalene (19)
Orange oil. Yield 74%. 1H NMR (CDCl3, 400 MHz) d 7.38 (s, 1H),
7.32–7.14 (m, 4H), 2.98 (t, J = 8.0 Hz, 2H), 2.73 (t, J = 8.0 Hz, 2H),
2.45–2.39 (m, 1H), 1.28–1.24 (m, 2H), 1.06–1.01 (m, 2H). 13C
NMR (CDCl3, 100 MHz) d 137.6, 135.5, 134.8, 133.2, 130.2, 128.8,
127.7, 127.0, 29.8, 27.5, 22.1, 4.9. HRMS calcd for C13H14O2S
[M+H]+: 235.0787; found: 235.0783, difference = 1.8210 ppm. IR
found: 265.1249, difference = 2.9357 ppm. IR (neat)
m 1284.3,
1145.2, 713.3 cmÀ1
.
(neat)
m .
1288.5, 1132.4, 883.4 cmÀ1
2.3.6. (E)-1-Tosylcyclododec-1-ene (10)
Light yellow solid mp 101–102 °C. Yield 63%. 1H NMR (CDCl3,
400 MHz) d 7.75 (d, J = 8.3 Hz, 2H), 7.30 (d, J = 8.3 Hz, 2H), 6.95 (t,
J = 9.5 Hz, 1H), 2.43 (s, 3H), 2.32 (t, J = 6.0 Hz, 2H), 2.21 (t,
J = 8.4 Hz, 2H), 1.59–1.52 (m, 4H), 1.42–1.31 (m, 10H), 1.22–1.16
(m, 2H). 13C NMR (CDCl3, 100 MHz) d 143.4, 142.7, 137.6, 134.8,
129.7, 129.0, 128.4, 127.9, 26.5, 25.8, 25.6, 25.5, 25.2, 24.6, 23.5,
23.3, 23.1, 22.0, 21.6. HRMS calcd for C19H32NO2S [M+NH4]+:
338.2148; found: 338.2165, difference = 4.9441 ppm. HRMS calcd
for C19H29O2S [M+NH]+: 321.1882; found: 321.1884, differ-
2.3.12. (E)-1-Methyl-4-(styrylsulfonyl)benzene (E:Z = 3:1) (21)
White solid mp 98–99 °C. Yield 67%. 1H NMR (CDCl3, 400 MHz)
d 7.84 (d, J = 8.3 Hz, 2H), 7.67 (d, J = 15.4 Hz, 1H), 7.50–7.32 (m,
7H), 6.86 (d, J = 15.4 Hz, 1H), 2.45 (s, 3H). 13C NMR (CDCl3,
100 MHz) d 144.4, 141.9, 137.8, 133.0, 132.5, 131.1, 130.0, 129.9,
129.0, 127.7, 21.6. HRMS calcd for C15H18NO2S [M+NH4]+:
276.1052; found: 276.1047, difference = 2.0906 ppm. HRMS calcd
for C15H15O2S [M+H]+: 259.0787; found: 259.0781, differ-
ence = 2.4243 ppm. IR (neat)
m .
1303.3, 1141.8, 810.3 cmÀ1
ence = 0.3777 ppm. IR (neat)
m .
1286.8, 1134.3, 981.9, 690.2 cmÀ1
2.3.13. (2-Tosylethene-1,1-diyl)dibenzene (23)
2.3.7. tert-Butyl-4-tosyl-5,6-dihydropyridine-1(2H)-carboxylate
(12)
White solid, mp 94–95 °C. Yield 40%. 1H NMR (CDCl3, 400 MHz)
d 7.49 (t, J = 8.0 Hz, 2H), 2.73 (d, J = 8.3 Hz, 2H), 7.41–7.10 (m, 12H),
7.01 (s, 1H), 2.39 (s, 3H). 13C NMR (CDCl3, 100 MHz) d 154.7, 143.7,
139.3, 138.6, 135.6, 132.9, 130.22, 129.8, 129.4, 128.8, 128.6, 128.2,
Orange oil. Yield 75%. 1H NMR (CDCl3, 400 MHz): d 7.76 (d,
J = 7.2 Hz, 2H), 7.36 (d, J = 7.2 Hz, 2H), 7.19–7.06 (m, 1H), 4.76–

D. C. Reeves et al. / Tetrahedron Letters 50 (2009) 2870–2873
2873
127.8, 127.7, 21.5. HRMS calcd for C21H22NO2S [M+NH4]+:
352.1365; found: 352.1358, difference = 2.2076 ppm. HRMS calcd
for C21H19O2S [M+H]+: 335.1100; found: 335.1083, differ-
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ence = 5.1573 ppm. IR (neat)
m .
1299.6, 1134.8, 696.3 cmÀ1
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.03.174.
References and notes
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