Rongalite and base-promoted cleavage of disulfides and subsequent Michael addition to α,β-unsaturated ketones/esters: an odorless synthesis of β-sulfido carbonyl compounds

Article abstract of DOI:10.1016/j.tet.2010.02.001

A highly practical method to access β-sulfido carbonyl compounds was developed, which could be conducted without any expensive reagent, special apparatus/technique, and no requirement of metal catalysts. β-Sulfido carbonyl compounds were formed at room temperature, in short time and with high chemoselectivity in good to excellent yields. A plausible mechanism for the role of Rongalite, as a promoter for the cleavage of disulfides generating thiolate anions that then undergo facile thia-Michael addition to α,β-unsaturated ketones and esters is proposed.

Full text of DOI:10.1016/j.tet.2010.02.001

Tetrahedron 66 (2010) 2297–2300
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
RongaliteÒ and base-promoted cleavage of disulfides and subsequent Michael
addition to
a,
b-unsaturated ketones/esters: an odorless synthesis of
b-sulfido
carbonyl compounds
,
a
a
,
,
Wenxue Guo a, Guangshu Lv a, Jiuxi Chen a , Wenxia Gao , Jinchang Ding b, Huayue Wu a
*
*
a College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, PR China
b Wenzhou Vocational and Technical College, Wenzhou 325035, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly practical method to access
b-sulfido carbonyl compounds was developed, which could be
Received 15 December 2009
Received in revised form 27 January 2010
Accepted 1 February 2010
conducted without any expensive reagent, special apparatus/technique, and no requirement of metal
catalysts. b-Sulfido carbonyl compounds were formed at room temperature, in short time and with high
chemoselectivity in good to excellent yields. A plausible mechanism for the role of RongaliteÒ, as
a promoter for the cleavage of disulfides generating thiolate anions that then undergo facile thia-Michael
Available online 4 February 2010
addition to a,b-unsaturated ketones and esters is proposed.
Keywords:
Ó 2010 Elsevier Ltd. All rights reserved.
RongaliteÒ
Thia-Michael addition
Disulfides
a
a
b
,
,
b
b
-Unsaturated ketones
-Unsaturated esters
-Sulfido carbonyl compounds
1. Introduction
been reported. The use of long-chain alkyl thiols or substituted
thiophenols with a trimethylsilyl moiety as odorless substrates has
been reported for this purpose.6 The highlight of this methodology
is no free thiols involved in the reaction. However, these odorless
procedures often suffer from certain drawbacks, such as cost of
some catalysts, long reaction time, use of strong alkaline media, and
limited substrate scope.
The
blocks in the synthesis of natural and pharmacological com-
pounds.1 Various approaches toward the synthesis of
-sulfido
carbonyl compounds have been explored during the past years. One
of the most common approaches for the synthesis of -sulfido
carbonyl compounds is thia-Michael addition of thiols to -un-
b-sulfido carbonyl compounds are important building
b
b
a
,b
The cleavage of disulfide bond could lead to interesting products
by the reaction of the resulting nucleophilic sulfur7 species to
a variety of organic substrates. Studies directed toward the cleavage
of disulfide bonds in proteins is an important determinant in un-
derstanding the structural activity and structural domains of pro-
teins, which would help in the design and development of new
therapeutic agents and the conversion of large proteins into smaller
fragments that are more amenable for sequencing.8 The sulfur–
sulfur bond in organic disulfides may be cleaved by nucleophilic,
electrophilic, and radical processes. Several transition metal com-
plexes have also been shown to cleave sulfur–sulfur bond in organic
disulfides.9
saturated carbonyl compounds, which provides a great strategy for
chemoselective protection of the olefinic double bond of conju-
gated enones.2 Thus, there have been constant efforts toward the
development of methodologies for thia-Michael addition in the
presence of various promoting agents.3 Alternatively, the reactions
conducted in ionic liquids4 and using water as the medium5 have
been investigated. However, the use of highly volatile and un-
pleasant smelling free thiols leads to serious environmental, safety
problems and also limits the use of these methods for large-scale
operations in all the reported methods. Besides the drawback of
these methodologies are associated with undesirable side reactions
owing to the oxidation of thiols. In order to minimize or eliminate
the encountered problems, a range of odorless protocols has also
In 2000, Chandrasekaran and co-workers10 have discovered that
the tandem sulfur transfer/reduction/Michael addition strategy for
the synthesis of b-sulfido carbonyl compounds mediated by ben-
zyltriethylammonium tetrathiomolybdate for 4 h to 10 h in aceto-
* Correspondingauthors.Tel./fax:þ8657788368280.E-mail addresses: jiuxichen@
nitrile. Later, Ranu and Mandal have reported that InI-promoted11a,b
wzu.edu.cn (J. Chen), huayuewu@wzu.edu.cn (H. Wu).
0040-4020/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.02.001

2298
W. Guo et al. / Tetrahedron 66 (2010) 2297–2300
Table 1
Michael addition of thiolate anions to conjugated carbonyl com-
pounds for 1.5–5 h under reflux THF or In-TMSCl-promoted11c for
Screening conditions for the thia-Michael addition reaction of cyclohex-2-enone
with 1,2-diphenyldisulfidea
4.5–6 h in CH3CN under sonication. For the synthesis of
b-sulfido
carbonyl compounds from thia-Michael addition of disulfides to a,b-
unsaturated carbonyl compounds, very little attention was paid to
it.12 From a synthetic point of view, the development of an improved,
facile procedure using less expensive and more sustainable pro-
moter has remained a highly desirable goal.
As a continuing interest in developing novel synthetic routes for
the formations of carbon–carbon and carbon–heteroatom bonds,13
we expected to apply the sulfite/base system in the thia-Michael
Entry
RongaliteÒ (equiv)
Solvent
Base
Yieldb (%)
1
2
3
4
5
6
7
8
2
2
2
2
2
2
4
4
4
4
4
5
3
2
1
d
4
4
4
1,4-Dioxane
Toluene
HMPA
CH3CH2NO2
DMSO
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
KF$2H2O
Et3N
K3PO4
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
none
trace
trace
trace
trace
32
47
73
trace
12
trace
89
addition to a,b-unsaturated ketones and esters. Herein, we wish
to report a highly practical method to access b-sulfido carbonyl
compounds by RongaliteÒ (sodium formaldehyde sulfoxylate,
NaHSO2$CH2O$2H2O as an inexpensive reagent)-promoted cleav-
age of disulfides and subsequent thia-Michael addition to a,b-un-
saturated ketones and esters under mild reaction conditions
(Scheme 1).
9
10
11
12
13
14
15
16
17
18
19
O
89
77
67
13
O
R2
R2
R4
R1
R1
2O
K2CO3, DMF, rt, 10 min
NaHSO2
2
+
RSSR
R3
SR
NR
R3
R4
3
97c
97d
NR
1
2
DMF
1a: R1 + R3 = -(CH2)3- , R2 = R4 = H
1b: R1 = Me, R2 = R3 = H, R4 = Ph
1c: R1 = OMe, R2 = R3 = R4 = H
2a: R = Ph
NR¼No reaction.
2b: R = p-CH3C6H4
2c: R = p-ClC6H4
2d: R = p-FC6H4
a
All reactions were run with cyclohex-2-enone 1a (0.4 mmol), phenyl disulfide
2a (0.2 mmol), NaHSO2$CH2O$2H2O, and base (1 equiv) in 3 mL of solvent at room
temperature for 10 min.
1d: R1 = OMe, R2 = Me, R3 = R4 = H
1e: R1 = Me, R2 = R4 = H, R3 = n-C4H9
b
Isolated yields.
2 equiv of K2CO3.
3 equiv of K2CO3.
c
d
Scheme 1. RongaliteÒ-promoted thia-Michael addition.
2. Results and discussion
As shown in Table 2, the substitution groups on the aromatic
ring associated with disulfides had little effect on the yields. It is
observed that the corresponding products were obtained in
excellent yields by the reaction of electron-rich disulfide, such as
p-tolyl disulfide 2b with various Michael acceptor 1(a–e) (Table 2,
entries 2, 6, 10, 14, and 18) under optimal conditions. In contrast,
the yields were decreased slightly when less nucleophilic disul-
fide, such as p-fluorophenyl disulfide 2e was used under the
same conditions (Table 2, entries 4, 8, 12, 16, and 20). A little
decrease in the product yield was observed in reaction with
various using benzalacetone (1b) as Michael acceptor (Table 2,
entries 5–8). In the case of Michael acceptor, such as methyl
acrylate (1c), the product was formed in good yields (Table 2,
entries 9–12).
Initial studies focused on the screening of the solvents, bases as
well as RongaliteÒ loading with the reaction of cyclohex-2-enone
1a and phenyl disulfide 2a as the model reaction at room temper-
ature. The results are listed in Table 1.
Through the screening process, trace target product was
detected in the presence of a series of solvents such as 1,4-dioxane,
toluene, HMPA, and CH3CH2NO2. However, we were delighted to
find that the yield could be improved to 32% or 47% when the
combination of Cs2CO3 and DMSO or DMF was employed at room
temperature, respectively (Table 1, entries 1–6). Encouraged by
these promising results, we further optimized the reaction condi-
tions, such as bases, the amount of RongaliteÒ loading, and bases
(Table 1, entries 7–19).
Next, different experiments were carried out by varying the
amount of RongaliteÒ (Table 1, entries 11–15). Increasing the
amount of RongaliteÒ to 4 equiv resulted in excellent yield. No
target product was detected in the absence of RongaliteÒ and both
starting materials were recovered in quantitative, which also fur-
ther proved that RongaliteÒ do play an important role in this re-
action (Table 1, entry 16). On the other hand, among the bases
tested, K2CO3 afforded best yield under these conditions (Table 1,
entries 7–11). We also examined the effect of the amount of K2CO3,
the target product was not observed without K2CO3, increasing the
amount of K2CO3 to 2 equiv resulted in excellent yield.
Moreover, we further investigated
ceptor methyl methacrylate (1d) (Table 2, entries 13–16) and
-substituted Michael acceptor oct-3-en-2-one (1e) (Table 2, en-
a-substituted Michael ac-
b
tries 17–20) could react with various disulfides rapidly afford good
yields.
Next, the chemoselective addition reaction in the presence of
unprotected reactive functional groups, such as –NH2 also proved to
be successful. The corresponding product of 3ce was obtained in
94% yield (Scheme 2).
Furthermore, the present route to
b-sulfido carbonyl com-
pounds was successfully applied to a large scale reaction. For in-
stance, the thia-Michael addition of cyclohex-2-enone 1a (1.92 g) to
phenyl disulfide 2a (2.18 g) promoted by RongaliteÒ provided the
desired product 3aa in 98% yield (Table 2, entry 1).
With optimal conditions in hand, the reaction of various cyclic
and acyclic
a,b-unsaturated carbonyl compounds with disulfides
was examined to explore the scope of the reaction and the results
are summarized in Table 2. In all cases, RongaliteÒ-promoted re-
actions proceeded smoothly and gave the corresponding products
in good to excellent yields.
According to the previous proposed mechanism,14 a tentative
mechanism for the formation of b-sulfido carbonyl compounds was
proposed in Scheme 3. RongaliteÒ can be readily decomposed into
HCHO and HSOꢀ2 anion (A). Intermediate (A) then reacts with RSSR

W. Guo et al. / Tetrahedron 66 (2010) 2297–2300
2299
(1)
Table 2
OCH2SO2H
HCHO
+
HSO2
RongaliteÒ-promoted cleavage of disulfides followed by thia-Michael additiona
A
+ HSO2
C
D
O
O
HSO2
A
RSSR +
2
+
(2)
RS
RS
B
R2
SR
R2
R4
R1
R1
+
RSSR
R3
K2CO3, DMF, rt, 10 min
R3
+
HSO2
D
+ SO2
+
(3)
(4)
RS
C
RS
B
H
R4
3
1
2
SR
R1
O
O
Entry
Acceptor 1
R (Disulfide 2)
Product
Yieldb (%)
97 (98)c
98
94
87
+
RS
R2
1
2
3
4
6C6H5 (2a)
3aa
3ab
3ac
3ad
R1
R2
p-(CH3)C6H4 (2b)
p-(Cl)C6H4 (2c)
p-(F)C6H4 (2d)
1
C
4
SR
O
SR
R1
O
H
(5)
R1
R2
R2
5
6
7
8
C6H5 (2a)
3ba
3bb
3bc
3bd
92
93
90
83
p-(CH3)C6H4 (2b)
p-(Cl)C6H4 (2c)
p-(F)C6H4 (2d)
3
5
Scheme 3. A tentative mechanism for the formation of b-sulfido carbonyl compounds.
synthesis. Efforts to explore the detailed mechanism and further ap-
plications of the present system in other transformations using
disulfide as a reaction partner are ongoing in our group.
9
C6H5 (2a)
3ca
3cb
3cc
3cd
96
98
95
88
10
11
12
p-(CH3)C6H4 (2b)
p-(Cl)C6H4 (2c)
p-(F)C6H4 (2d)
4. Experimental section
4.1. General
O
13
14
15
16
C6H5 (2a)
3da
3db
3dc
3dd
94
96
93
88
p-(CH3)C6H4 (2b)
p-(Cl)C6H4 (2c)
p-(F)C6H4 (2d)
MeO
Chemicals and solvents were either purchased or purified by
standard techniques. Melting points were uncorrected and recor-
ded on Digital Melting Point Apparatus WRS-1B. IR spectra were
recorded on a AVATAR 370 FI-Infrared Spectrophotometer. NMR
spectroscopy was performed on both a Bruck-300 spectrometer
operating at 300 MHz (1H NMR) and 75 MHz (13C NMR). TMS
(tetramethylsilane) was used as an internal standard and CDCl3 was
used as the solvent. Mass spectrometric analysis was performed on
GC–MS analysis (SHIMADZU GCMS-QP2010). Elemental analysis
was determined on a Carlo-Erba 1108 instrument.
17
18
19
20
C6H5 (2a)
3ea
3eb
3ec
3ed
94
93
90
84
O
p-(CH3)C6H4 (2b)
p-(Cl)C6H4 (2c)
p-(F)C6H4 (2d)
3
a
All reactions were run with
a
,
b
-unsaturated ketones/esters
1
(0.4 mmol),
disulfides 2 (0.2 mmol), NaHSO2$CH2O$2H2O (0.8 mmol), and K2CO3 (0.4 mmol) in
2 mL of DMF at room temperature for 10 min.
b
Isolated yields.
c
4.2. General procedure for synthesis of synthesis of
carbonyl compounds
b-sulfido
The reaction was run with cyclohex-2-enone 1a (20 mmol, 1.92 g), phenyl
disulfide 2a (10 mmol, 2.18 g), NaHSO2$CH2O$2H2O (40 mmol), and K2CO3
(20 mmol) in 10 mL of DMF at room temperature for 10 min.
A mixture of
a,b-unsaturated ketones/esters 1 (0.4 mmol),
disulfide 2 (0.2 mmol), RoganliteÒ (4 equiv), and K2CO3 (2 equiv) in
DMF (2 mL) was stirred at room temperature for 10 min. The
mixture washed with water and extracted with ethyl acetate. The
organic phase was separated and dried over anhydrous sodium
sulfate, filtered and the solvent was evaporated under vacuum. The
residue was purified by flash column chromatography (ethyl
acetate or hexane/ethyl acetate) to afford the desired product 3.
(2) to generate two radical intermediates (B and D) and an anion
(C). The radical (B) can also be converted into thiolate anion (C) by
reacting with intermediate (D). Finally, the 1,4-addition of the
thiolate anion (C) to the
b-position of the Michael acceptor (1)
affords the target product.
3. Conclusion
In summary, we have developed a highly practical and opera-
tionally simple method by the RongaliteÒ and base-promoted cleav-
age of disulfides and subsequent odorless thia-Michael addition in
good to excellent yields under mild reaction conditions. Compare to
reported methodologies, the important features of this methodology
are broad substrates scope, high yielding, no requirement of metal
catalysts, reasonably rapid reaction rate, and especially in a scaled-up
4.2.1. 4-(p-Fluorophenylthio)-4-phenylbutan-2-one (3bd). (Table 2,
entry 8). 1H NMR (30 MHz, CDCl3)
d: 7.28–7.18 (m, 7H), 6.94–6.88
(m, 2H), 4.60 (t, J¼7.3 Hz, 1H), 3.04 (dd, J¼7.8 Hz, J¼1.0 Hz, 2H), 2.09
(s, 3H); 13C NMR (75 MHz, CDCl3)
d
: 205.5, 162.9 (d, 1JC–F¼246.6 Hz),
3
141.0, 136.2 (d, JC–F¼8.3 Hz), 128.9, 128.6, 127.8, 127.6, 116.1 (d,
2JC–F¼21.6 Hz), 49.3, 49.0, 30.9; MS (EI, 70 eV) m/z (%): 274(Mþ, 5),
147 (22), 43 (100). Anal. Calcd for C16H15FOS: C, 70.05; H, 5.51.
Found: C, 70.11; H, 5.46.
O
S
OMe
NaHSO2·CH2O·2H2O
K2CO3, DMF, rt, 10 min
94% yield
S
+
4.2.2. Methyl-3-(p-fluorophenylthio)propanoate (3cd). (Table 2, en-
MeO
O
2
try 12). 1H NMR (300 MHz, CDCl3)
d: 7.41–7.36 (m, 2H), 7.03–6.98 (m,
NH2
NH2
2e
2H), 3.67 (s, 3H), 3.11 (t, J¼7.3 Hz, 2H), 2.59 (t, J¼7.4 Hz, 2H); 13C NMR
1c
3ce
1
3
(75 MHz, CDCl3)
d
: 172.2, 162.3 (d, JC–F¼245.6 Hz), 133.5 (d, JC–
Scheme 2. RongaliteÒ-promoted chemoselective addition reaction.
¼8.0 Hz), 130.1 (d, 4JC–F¼3.2 Hz), 116.3 (d, 2JC–F¼21.8 Hz), 51.9, 34.3,
F

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W. Guo et al. / Tetrahedron 66 (2010) 2297–2300
30.5; MS (EI, 70 eV) m/z (%): 214(Mþ, 100), 127 (53), 141 (69), 154 (46).
Anal. Calcd for C10H11FO2S: C, 56.06; H, 5.17. Found: C, 56.10; H, 5.22.
Cohen, J.; Perun, T. J.; Plattner, J. J. J. Med. Chem. 1987, 30, 1609; (d) Cherkauskas,
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38, 6557; (b) CdI2: Saito, M.; Nakajima, M.; Hashimoto, S. Tetrahedron 2000, 56,
9589; (c) Hf(OTf)3: Kobyashi, S.; Ogawa, C.; Kawamura, M.; Sugiura, M. Synlett
2001, 983; (d) InBr3: Bandini, M.; Cozzi, P. G.; Giacomini, M.; Melchiorre, P.;
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Tetrahedron 2004, 60, 4183.
4.2.3. Methyl-3-(p-fluorophenylthio)butanoate (3dd). (Table 2, entry
16). 1H NMR (30 MHz, CDCl3)
d: 7.40–7.36 (m, 2H), 7.03–6.97 (m, 2H),
3.66 (s, 3H), 3.23–3.16 (m, 1H), 2.91–2.84 (m, 1H), 2.68–2.61 (m, 1H),
1.25 (d, J¼7.0 Hz, 3H); 13C NMR (75 MHz, CDCl3)
d: 175.4, 162.2 (d,
3
2
1JC–F¼245.5 Hz), 133.4 (d, JC–F¼8.0 Hz), 130.6, 116.2 (d, JC–
¼21.8 Hz), 52.0, 39.8, 38.9, 16.9; MS (EI, 70 eV) m/z (%): 228(Mþ, 57),
F
141 (100), 45 (42). Anal. Calcd for C11H13FO2S: C, 57.87; H, 5.74.
Found: C, 57.74; H, 5.77.
4.2.4. 4-(p-Tolylthio)octan-2-one (3eb). (Table 2, entry 18). 1H NMR
(30 MHz, CDCl3)
d
: 7.31 (d, J¼8.1 Hz, 2H), 7.11 (d, J¼8.0 Hz, 2H),
3.52–3.48 (m, 1H), 2.68–2.61 (m, 2H), 2.33 (s, 3H), 2.12 (s, 3H), 1.56–
1.26 (m, 6H), 0.89 (t, J¼7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3)
d:
207.1, 137.6, 133.3, 130.6, 129.9, 49.2, 44.2, 34.5, 30.8, 29.2, 22.6, 21.3,
14.1; MS (EI, 70 eV) m/z (%): 250(Mþ, 30), 124 (100), 43 (87). Anal.
Calcd for C15H22OS: C, 71.95; H, 8.86. Found: C, 72.01; H, 8.89.
4.2.5. 4-(p-Chlorophenylthio)octan-2-one (3ec). (Table 2, entry 19).
1H NMR (30 MHz, CDCl3)
d
: 7.35–7.24 (m, 4H), 3.56–3.52 (m, 1H),
2.67–2.64 (m, 2H), 2.13 (s, 3H), 1.57–1.25 (m, 6H), 0.87 (t, J¼7.2 Hz,
3H); 13C NMR (75 MHz, CDCl3)
: 206.7, 133.7, 133.4, 133.3, 129.2,
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655; (c) Krishnaveni, N. S.; Surendra, K.; Rao, K. R. Chem. Commun. 2005, 669;
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6. Nishide, K.; Miyamoto, T.; Kumar, K.; Ohsugi, S.; Node, M. Tetrahedron Lett.
2002, 43, 8569.
d
49.0, 44.1, 34.6, 30.8, 29.1, 22.6, 14.1; MS (EI, 70 eV) m/z (%): 270
(Mþ, 12), 144 (32), 109 (16), 43 (100). Anal. Calcd for C14H19ClOS: C,
62.09; H, 7.07. Found: C, 62.12; H, 7.13.
7. (a) Kondo, T.; Uenoyama, S.; Fujita, K.; Mitsudo, T. J. Am. Chem. Soc. 1999, 121,
482; (b) Chakraborti, A. K.; Nayak, M. K.; Sharma, L. J. Org. Chem. 2002, 67, 1776;
(c) Gavande, N. S.; Kundu, S.; Badgujar, N. S.; Kaur, G.; Chakraborti, A. K.
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references therein.
9. (a) Taniguchi, Y.;Maruo, M.;Takaki, K.;Fujiwara, Y. Tetrahedron Lett.1994, 35, 7789; (b)
Ogawa, A.; Nishiyama, Y.; Kambe, N.; Murai, S.; Sonada, N. Tetrahedron 1987, 28, 3271.
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4316.
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Chem. 2006, 84, 762; (c) Ranu, B. C.; Mandal, T. Synth. Commun. 2007, 37, 1517.
12. (a) Zheng, X.; Xu, X.; Zhang, Y. Synlett 2003, 2062; (b) Bartolozzi, A.; Foudou-
lakis, H. M.; Cole, B. M. Synthesis 2008, 2023.
4.2.6. 4-(p-Fluorophenylthio)octan-2-one (3ed). (Table 2, entry 20).
1H NMR (30 MHz, CDCl3)
d: 7.42–7.37 (m, 2H), 7.01–6.95 (m, 2H),
3.47–3.43 (m, 1H), 2.65–2.60 (m, 2H), 2.11 (s, 3H), 1.53–1.24 (m, 6H),
0.86 (t, J¼7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3)
d: 206.8, 162.6 (d,
3
2
1JC–F¼246.1 Hz), 135.5 (d, JC–F¼8.1 Hz), 129.3, 116.1 (d, JC–
¼21.5 Hz), 49.1, 44.8, 34.5, 30.8, 29.1, 22.6, 14.1; MS (EI, 70 eV) m/z
F
(%): 254(Mþ, 18), 128 (44), 43 (100). Anal. Calcd for C14H19FOS: C,
66.11; H, 7.53. Found: C, 62.08; H, 7.59.
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We thank the National Key Technology R&D Program (No.
2007BAI34B00), the National Natural Science Foundation of China
(No. 20471043) and Natural Science Foundation of Zhejiang Prov-
ince (No. Y4080107) for financial support.
Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2010.02.001.
References and notes
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