'Green' iodination of dimethoxy- and trimethoxy-substituted aromatic compounds using an iodine-hydrogen peroxide combination in water

Article abstract of DOI:10.1055/s-2006-942470

Mild iodination using iodine and a 30% solution of hydrogen peroxide as oxidant was performed in water. The method proved to be efficient and selective for the introduction of iodine into dimethoxybenzenes, trimethoxybenzenes, and dimethoxy- and trimethox

Full text of DOI:10.1055/s-2006-942470

PAPER
2603
‘Green’ Iodination of Dimethoxy- and Trimethoxy-Substituted Aromatic
Compounds Using an Iodine–Hydrogen Peroxide Combination in Water
‘G
reen’ Io
a
dination sminka Pavlinac,a Marko Zupan,a,b Stojan Stavber*a
a
Laboratory for Organic and Bioorganic Chemistry, ‘Jozef Stefan’ Institute, Jamova 39, 1000 Ljubljana, Slovenia
b
Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, 1000 Ljubljana, Slovenia
Fax +386(1)4773822; E-mail: Stojan.stavber@ijs.si
Received 2 March 2006; revised 11 April 2006
Dedicated to Professor Miha Tišler on the occasion of his 80th birthday
emerged a decade ago and it is becoming increasingly im-
portant to apply the principles of green chemistry to every
area of science.11 Within organic chemistry, efficient cat-
alytic methodologies compared to stochiometric reagents,
Abstract: Mild iodination using iodine and a 30% solution of hy-
drogen peroxide as oxidant was performed in water. The method
proved to be efficient and selective for the introduction of iodine
into dimethoxybenzenes, trimethoxybenzenes, and dimethoxy- and
trimethoxy-substituted acetophenones, thus significantly contribut- utilization of renewable raw materials rather than fossil
ing toward a more environmentally friendly procedure than that pre-
viously reported.
fuels as the basic feedstock, reactions involving the prin-
ciple of atom economy, and the suitability of a safer alter-
Key words: green chemistry, iodine, halogenation, hydrogen per-
oxide, iodination in water
native reaction medium in place of volatile organic
solvents are some of the major and still challenging issues
driven by the pressing need for environmentally more ac-
ceptable processes. Bearing this in mind and our contin-
Iodo-substituted organic compounds have received signif- ued interest in the development of ‘greener’ halogenation
icant attention in the scientific community.1 Iodo-deriva- synthetic methods12 we now report the application of the
tives play an important role as synthons or as valuable reaction system I2/H2O2/H2O12b,13 for selective and effi-
precursors in organic synthesis,2 mainly in C–C and C–N cient iodination of di- and trimethoxy substituted aromatic
bond formation, which has been used for the synthesis of compounds. The chosen substrates either possess bioac-
several natural products and bioactive compounds. More- tive properties (anti-tumor or antioxidant activity), or
over, iodinated compounds can be used as radioactively mimic the structural fragment often present in bioactive
labelled markers or contrastors in medical diagnosis.3
compounds.14 It is known that the introduction of a halo-
gen atom can even enhance bioactivity in many cases.
Due to the low reactivity of molecular iodine, consider-
able efforts have been put into the development of an ef- Water, the most readily available medium, possesses sev-
ficient and mild method for direct introduction of iodine eral advantages over traditional organic solvents. Due to
into organic molecules through an electrophilic reaction its non-toxicity and non-flammability, it is considered as
process. Intensive research over the years resulted in nu- the most benign reaction medium. In addition to its inex-
merous diverse procedures using iodonium donating pensiveness and abundance, water as a reaction medium
agents. In some of them rigorous conditions are applied, corresponds well to the current trends of green chemis-
such as, extensive use of strong acids in combination with try.15 Although its application was much underestimated
an iodinating agent (HNO3/H2SO4, HIO3, H2SO4, HIO4/ in the last century compared to the use of organic solvents,
H2SO4, CF3SO3H),4 or the use of heavy metal salts or ox- water is again becoming an increasingly important medi-
ides as activators of iodine (I2/HgX2, I2/HgO, I2/Ag2SO4, um for organic reactions.16 It is far from unusual to per-
I2/Pb(OAc)4/HOAc).5 Profound concerns over environ- form organometallic chemistry17 or free-radical
mental issues, as well as high health risks are associated functionalization18 of organic molecules in aqueous me-
with the mentioned procedures. Some other reported dia. Also some mechanistic studies of the influence of wa-
methods employing less toxic reagents and milder condi- ter on the reactivity of organic molecules have been
tions include I2/F–TEDA–BF4,6 ICl,7 NIS,8 NaOCl–NaI,9 carried out.19 However, iodination methods reported in
I2/PhI(OAc)2,10 but organic solvents are still required as water are very few.12b,e,20 In our research water proved to
reaction media.
be a suitable medium for the introduction of iodine into
the studied substrates using iodine and a 30% aqueous so-
lution of H2O2 as oxidizer in the presence of a catalytic
amount of H2SO4. H2O2, widely accepted as a green oxi-
dant,21 was used to activate elemental iodine. H2O2 has
several advantages over other oxidants: it is relatively
cheap, relatively non-toxic, and breaks down readily to
benign byproducts, thus avoiding environmental prob-
lems. However, its high activation potential (E° = 1.77 V)
has to be overcome in order to successfully accomplish
Continuous growing concern over environmental pollu-
tion, health risks, and sustainable development has
prompted chemists to search for greener methods to re-
place traditional ones. The concept of green chemistry
SYNTHESIS 2006, No. 15, pp 2603–2607
Advanced online publication: 04.07.2006
DOI: 10.1055/s-2006-942470; Art ID: T03206SS
© Georg Thieme Verlag Stuttgart · New York
x
x.
x
x
.
2
0
0
6

2604
J. Pavlinac et al.
PAPER
oxidation for less activated substrates. Therefore, an ap- tones.12d It should also be noted that a high concentration
propriate catalyst is of crucial importance for activation of of H2O2 can represent a certain safety risk during transpor-
H2O2 in such cases. In one of our previous studies a cata- tation, use, and storage, however, this can be easily avoid-
12a,12b,12d,12e
lytic amount of H2SO4 was found to be a sufficient and ef- ed by the use of a 30% aqueous solution of H2O2
ficient catalyst for the activation of H2O2 and for the or by the use of a ‘dry carrier’ of hydrogen peroxide, e.g.
further transformation of the iodinated products to ke- a urea–H2O2 adduct or sodium percarbonate.22
Table 1 Iodination of Dimethoxy- and Trimethoxybenzenes in Water Using I2/H2O2/H+
OMe
OMe
I2/H2O2/H+
cat.
H2O, 50 °C
(OMe)n
m(I)
(OMe)n
n = 1,2
Entry
Substrate
Ratioa
Time (h)
18
Product
Conversionb (%) Yieldc (%)
OMe
OMe
OMe
OMe
1
2
3
4
5
1:1:1
88
100
100
94
6813
I
1a
1
2
2
OMe
OMe
1:0.5:0.6
1:1.5:1.5
1:1:1
2
15
27
3
89
OMe
OMe
I
2a
OMe
OMe
I
85
OMe
OMe
I
2b
OMe
OMe
I
6713
OMe
OMe
OMe
OMe
3
4
3a
4a
OMe
OMe
OMe
OMe
1: 1: 1
100
89
I
OMe
OMe
OMe
OMe
OMe
6
7
1:0.5:0.6
1:0.5:0.6
4
4
95
83
81
I
OMe
5
5a
OMe
OMe
I
100
MeO
OMe
MeO
OMe
6a
6
a Substrate/I2/H2O2 ratio.
b Conversion of substrate determined from 1H NMR spectra.
c Isolated pure product.
Synthesis 2006, No. 15, 2603–2607 © Thieme Stuttgart · New York

PAPER
‘Green’ Iodination
2605
Among the methoxy-substituted benzene derivatives, 1,3- 0.5 equivalents of the reagent proved to be sufficient for
dimethoxybenzene (2) and 1,3,5-trimethoxybenzene (6), the introduction of one iodine atom into the a-alkyl posi-
the most activated toward electrophilic substitution, were tion of the substrates under investigation, thus substantial-
chosen as trial substrates.12b We found that 0.5 equivalent ly contributing to the high atom economy of the reaction
of iodine and 0.6 equivalent of H2O2 in the presence of a with regard to iodine. Attempts to introduce two iodine at-
catalytic amount of H2SO4 were sufficient for the com- oms into the molecule were successful in the case of 1-
plete conversion to 1-iodo-2,4-dimethoxybenzene (2a) or (1,6-dimethoxyphenyl)-1-ethanone (8) with one equiva-
2-iodo-1,3,5-trimethoxy benzene (6a) at 50 °C after a few
hours in aqueous medium. Encouraged by these results,
we extended the method to 1,2-dimethoxybenzene (1),
1,4-dimethoxybenzene (3),13 1,2,3-trimethoxybenzene
(4), and 1,2,4-trimethoxybenzene (5). Moreover, we at-
tempted to achieve functionalization to diiodo derivatives
by increasing the amount of reagent added. In the case of
1,2-dimethoxybenzene (1) and 1,4-dimethoxybenzene (3)
the amount of reagent and the reaction time had to be in-
creased to achieve comparable efficiency. With 0.5 equiv-
alents of the reagent, the iodo-transformation was
achieved with 51% yield of the iodinated product in the
case of 1,2-dimethoxybenzene and even less in the case of
1,4-dimethoxybenzene. However, the yield was improved
when one equivalent of the reagent was employed
(Table 1, entries 1 and 4). The introduction of two iodine
atoms was successfully achieved in the case of 1,3-
dimethoxybenzene (Table 1, entry 3), while it failed for
1,2-dimethoxybenzene and 1,4-dimethoxybenzene, even
with a huge excess of the added reagent. The reaction of
1,3-dimethoxybenzene and 1 equivalent of the reagent in
water at 50 °C after 15 hours resulted in the formation of
1-iodo-2,4-dimethoxybenzene (2a) and 1,5-diiodo-2,4-
dimethoxy benzene (2b) in a ratio of 1:1, while complete
conversion to 1,5-diiodo-2,4-dimethoxybenzene (2b) was
achieved with 1.5 equivalents of the reagent at 50 °C after
15 hours (Table 1, entry 3). In the case of trimethoxyben-
zenes water proved to be a suitable medium for the intro-
duction of one iodine atom using the I2/H2O2/H+ catalyst
system with high efficiency and atom economy (Table 1,
entry 5–7). On the other hand, the introduction of two io-
dine atoms was found to be more difficult. Even with a
large excess of the reagent complete conversion to diiod-
inated product was not achieved for any of the tri-
methoxy-substituted substrates studied in water. Still,
1,3,5-trimethoxybenzene is worth mentioning, the diiodi-
nated product and monoiodinated product (6a) were
present in a 88:12 ratio after a reaction time of 20 hours at
50 °C with 1.5 equivalents of reagent.
Table 2 Iodination of Methoxy-Substituted Acetophenones in Wa-
ter Using I2/H2O2/H+
O
Me
O
CH2I
0.5I2/0.6H2O2/H+
H2O, 50 °C
cat.
(OMe)n
(OMe)n
n = 2, 3
7–12
7a–12a
Entry Substrate
Time
(h)
Conversiona Yieldb
(%)
(%)
O
CH3
OMe
1
2
3
4
5
19
18
18
18
19
84
71
OMe
7
O
CH3
OMe
MeO
72
81
86
87
54
76
76
74
8
O
CH3
OMe
OMe
9
O
CH3
OMe
MeO
10
O
CH3
OMe
We then extended our research to dimethoxy- and tri-
methoxyacetophenones, where not only the suitability of
water as a reaction medium for iodination with the I2/
OMe
OMe
11
H2O2/H+ system, but also the regioselectivity of the
O
CH3
cat.
transformation for selected substrates was of interest. Be-
side the aromatic ring, the a-alkyl position can also under-
go electrophilic functionalization. In our previous study it
was found that the regioselectivity in the functionalization
of acetophenones could be efficiently regulated with the
solvent used.23 The iodinating system I2/H2O2/H+ in water
showed a-alkyl regioselectivity for the dimethoxy- and
trimethoxyacetophenones studied (Table 2). Moreover,
6
18
59
48
MeO
OMe
OMe
12
a Conversion of substrate determined from 1H NMR spectra.
b Isolated pure product.
Synthesis 2006, No. 15, 2603–2607 © Thieme Stuttgart · New York

2606
J. Pavlinac et al.
PAPER
tures were analyzed by TLC and 1H NMR spectroscopy. Pure sam-
ples of products were isolated by column chromatography (SiO2;
CH2Cl2 for 1a–6a; 1% EtOH in CH2Cl2 for 7a–12a), followed by
crystallization in the case of solid products, and were identified on
the basis of comparison of their spectroscopic data with the
literature5b,6b,23 or characterized by standard methods.
lent of the reagent. However, conversion to 2,2-diiodo-1-
(1,6-dimethoxyphenyl)-1-ethanone was not complete and
14% of monoiodinated product 8a was still present in the
reaction mixture after 18 hours at 50 °C in water. More-
over, difficulties during the purification process were en-
countered.
After
two
consecutive
column
chromatographies (SiO2, 1% EtOH in CH2Cl2), 10% of
monoiodinated product 8a was still present along with the
major diiodinated product.
1-(2,5-Dimethoxyphenyl)-2-iodoethanone (10a)
Yield: 76%; white crystals (MeOH); mp 48.5–49.5 °C.
IR (KBr): 2995, 2940, 1660, 1490, 1460, 1405, 1325, 1280, 1250,
1220, 1100, 1040, 1010, 880, 840, 815, 730 cm–1.
In conclusion, we feel that several aspects of the method
presented for mild and efficient iodination of dimethoxy-
and trimethoxybenzenes, as well as dimethoxy- and tri-
methoxy-substituted acetophenones, should be stressed.
Firstly, water proved to be suitable for these transforma-
tions. Moreover, the reactivity of otherwise poorly reac-
tive iodine was considerably enchanced by the use of a
30% aqueous solution of H2O2, widely recognized as a
green oxidant, thus avoiding toxic waste as a side-product
of the reaction. Only a catalytic amount of H2SO4 was
needed to activate the oxidizing power of H2O2. In most
cases a high atom efficiency for iodine was observed since
only 0.5 equivalents of elemental iodine was needed for
efficient transformation. Therefore, comparing the I2/
H2O2/H+ system in water with other previously reported
methods, which either employ large amounts of strong ac-
ids or heavy metal salts for enhancing the reactivity of io-
dine, we believe that the presented method makes a
substantial contribution from the green chemistry point of
view.
1H NMR (300 MHz, CDCl3): d = 3.80 (s, 3 H, OCH3), 3.92 (s, 3 H,
OCH3), 4.51 (s, 2 H, CH2I), 6.94 (d, J = 9.0, 1 H, Ar), 7.08 (dd,
J = 9.0 Hz, 3.2 Hz, 1 H, Ar), 7.37 (d, J = 3.2 Hz, 1 H, Ar).
13C (76 MHz, CDCl3): d = 9.5, 55.8, 56.1, 113.0, 114.6, 121.7,
124.2, 153.2, 153.5, 193.4.
MS (EI, 70 eV): m/z (%) = 306 (M+, 55), 165 (100), 151 (7), 121
(18), 107 (10), 92 (12), 77 (20).
Anal. Calcd for C10H11IO3: C, 39.24; H, 3.62. Found: C, 39.18; H,
3.62.
Acknowledgment
The authors are grateful to A. Podgoršek and A. Gačeša for assi-
stance in recording NMR spectra, to T. Stipanovič and Prof. B. Sta-
novnik for elemental combustion analysis, to B. Kralj and D. Žigon
for MS, and to the Slovenian Research Agency for financial sup-
port.
References
(1) (a) Merkushev, E. B. Synthesis 1988, 923. (b) Sasson, Y. In
The Chemistry of Functional Groups,: The Chemistry of
Halides, Pseudo Halides and Azides, Supplement D2, Part 2;
Patai, S.; Rappoport, Z., Eds.; Wiley: Chichester, 1995, 535–
620.
(2) Diederich, F.; Stang, P. J. Metal-Catalysed Cross-Coupling
Reactions; Wiley-VCH: Weinheim, 1998.
(3) (a) Seevers, R. H.; Counsell, R. E. Chem. Rev. 1982, 82,
575. (b) Sovak, M. Radiocontrast Agents, In Handbook of
Experimental Pharmacology, Vol. 73; Springer: Berlin,
1993.
(4) (a) Kraszkiewicz, L.; Sosnowski, M.; Skulski, L.
Tetrahedron 2004, 60, 9113. (b) Suzuki, H.; Nakamura, K.;
Goto, R. Bull. Chem. Soc. Jpn. 1966, 39, 128. (c) Patil, B.
R.; Bhusare, S. R.; Pawar, R. P.; Vibhute, Y. B. Tetrahedron
Lett. 2005, 46, 7179. (d) Olah, G. A.; Qi, W.; Sandford, G.;
Prakash, G. K. S. J. Org. Chem. 1993, 58, 3194.
(5) (a) Bachki, A.; Foubelo, F.; Yus, M. Tetrahedron 1994, 50,
5139. (b) Orito, K.; Hatakeyama, T.; Takeo, M.; Suginome,
H. Synthesis 1995, 1273. (c) Sy, W.-W.; Lodge, B. A.; By,
A. W. Synth. Commun. 1990, 20, 877. (d) Sy, W.-W.
Tetrahedron Lett. 1993, 34, 6223. (e) Krassowska-
Swiebocka, B.; Luliꢀski, P.; Skulski, L. Synthesis 1995, 926.
(6) (a) Zupan, M.; Iskra, J.; Stavber, S. Tetrahedron Lett. 1997,
38, 6305. (b) Jereb, M.; Stavber, S.; Zupan, M. Synthesis
2003, 853.
Iodine was purchased from Sigma Aldrich and used as received. A
30% aq solution of H2O2 was purchased from Merck; the precise
content of active H2O2 was determined by the iodometric method;
H2O2 was reacted with an acidic solution of a known, precisely
weighed amount of KI, the amount of iodine released was calculat-
ed by titration with Na2S2O3. Substrates were purchased from Sig-
ma Aldrich and used as received, other chemicals (Na2SO4,
Na2S2O3, CH2Cl2) were purchased from Merck. Mps were deter-
mined on a Büchi apparatus. 1H NMR spectra of crude reaction mix-
tures were recorded on a Varian EM 360L spectrometer at 60 MHz
and spectra of purified products on a Varian INOVA 300 spectrom-
eter at 300 MHz and 13C NMR spectra on the same instrument at 76
MHz. Chemical shifts are reported in ppm from TMS as the internal
standard. IR spectra were recorded on a Perkin–Elmer 1310 spec-
trometer. Standard KBr pellet procedures were used to obtain IR
spectra of solids, while a film of neat material was used to obtain IR
spectra of liquid products. MS were obtained on an Autospec Q in-
strument under EI conditions at 70 eV. Elemental analyses were
carried out on a Perkin-Elmer 2400 CHN analyzer.
General Procedure
Substrate (1 mmol) was added to H2O (10 mL), followed by the ad-
dition of finely powdered I2 (127 mg, 0.5 mmol; 254 mg, 1.0 mmol;
381 mg, 1.5 mmol; see Tables 1 and 2). After the mixture was
stirred at 50 °C for a few minutes a 30% aq solution of H2O2 (con-
taining 0.6 mmol, 20.4 mg or 1 mmol, 34.0 mg of active oxidant, see
Tables 1 and 2) and a drop of 40% H2SO4 were added. The reaction
mixture was vigorously stirred at 50 °C for various times (2–27 h,
see Tables 1 and 2). When the reaction was complete the product
was extracted with CH2Cl2 (20 mL), the organic phase was washed
an aq solution of Na2S2O3 (10%, 20 mL), H2O (20 mL), dried over
anhyd Na2SO4, and concentrated in vacuo. The crude reaction mix-
(7) Hubig, S. M.; Jung, W.; Kochi, J. K. J. Org. Chem. 1994, 59,
6233.
(8) Carreño, M. C.; Ruano, J. L. G.; Sanz, G.; Toledo, M. A.;
Urbano, A. Tetrahedron Lett. 1996, 37, 4081.
(9) Edgar, K. J.; Falling, S. N. J. Org. Chem. 1990, 55, 5287.
(10) Kryska, A.; Skulski, L. J. Chem. Res., Synop. 1999, 590.
Synthesis 2006, No. 15, 2603–2607 © Thieme Stuttgart · New York

PAPER
‘Green’ Iodination
2607
(11) (a) Anastas, P. T.; Williamson, T. C. Green Chemistry,
Frontiers in Benign Chemical Syntheses and Processes;
Oxford University Press: New York, 1998. (b) Anastas, P.
T.; Warner, J. C. Green Chemistry: Theory and Practice;
Oxford University Press: New York, 1998. (c) Ritter, S. K.
Chem. Eng. News 2001, 79 (29), 27. (d) Anastas, P. T.;
Kirchhoff, M. M. Acc. Chem. Res. 2002, 35, 686.
(12) (a) Iskra, J.; Stavber, S.; Zupan, M. Synthesis 2004, 1869.
(b) Jereb, M.; Zupan, M.; Stavber, S. Chem. Commun. 2004,
2614. (c) Stavber, G.; Zupan, M.; Jereb, M.; Stavber, S. Org.
Lett. 2004, 6, 4973. (d) Jereb, M.; Iskra, J.; Zupan, M.;
Stavber, S. Lett. Org. Chem. 2005, 2, 465. (e) Jereb, M.;
Zupan, M.; Stavber, S. Green Chem. 2005, 7, 100.
(f) Podgoršek, A.; Stavber, S.; Zupan, M.; Iskra, J.
Tetrahedron Lett. 2006, 47, 1097. (g) Podgoršek, A.;
Stavber, S.; Zupan, M.; Iskra, J. Eur. J. Org. Chem. 2006,
483.
(15) Adams, D. J.; Dyson, P. J.; Tavener, S. J. Chemistry in
Alternative Reaction Media; Wiley: New York, 2004.
(16) (a) Li, C.-J.; Chan, T.-H. Organic Reactions in Aqueous
Media; Wiley: New York, 1997. (b) Lubineau, A.; Augé, J.;
Queneau, Y. Synthesis 1994, 741. (c) Lindström, U. M.
Chem. Rev. 2002, 102, 2751. (d) Li, C.-J. Chem. Rev. 2005,
105, 3095.
(17) (a) Li, C.-J. Tetrahedron 1996, 52, 5643. (b) Li, C.-J.
Tetrahedron 1999, 55, 11149. (c) Li, C.-J. Green Chem.
2002, 4, 1. (d) Okuhara, T. Chem. Rev. 2002, 102, 3641.
(18) (a) Miyabe, H.; Naito, T. Org. Biomol. Chem. 2004, 2, 1267.
(b) Yorimitsu, H.; Shinokubo, H.; Oshima, K. Synlett 2002,
674.
(19) (a) Breslow, R.; Groves, K.; Mayer, M. U. J. Am. Chem. Soc.
2002, 124, 3622. (b) Breslow, R. Acc. Chem. Res. 2004, 37,
471. (c) Pirrung, M. C.; Sarma, K. D. Tetrahedron 2005, 61,
11456.
(13) Pavlinac, J.; Zupan, M.; Stavber, S. J. Org. Chem. 2006, 71,
1027.
(20) (a) Barluenga, J.; Marco-Arias, M.; González-Bobes, F.;
Ballesteros, A.; González, J. M. Chem. Commun. 2004,
2616. (b) Higgs, D. E.; Nelen, M. I.; Detty, M. R. Org. Lett.
2001, 3, 349.
(21) (a) Noyori, R.; Aoki, M.; Sato, K. Chem. Commun. 2003,
1977. (b) Hâncu, D.; Green, J.; Beckman, E. J. Acc. Chem.
Res. 2002, 35, 757.
(22) (a) Lulinski, P.; Kryska, A.; Sosnowski, M.; Skulski, L.
Synthesis 2004, 441. (b) Zielinska, A.; Skulski, L.
Molecules 2005, 10, 1307.
(23) Stavber, S.; Jereb, M.; Zupan, M. Chem. Commun. 2002,
488.
(14) (a) Uchida, M.; Nakajin, S.; Toyoshima, S.; Shinoda, M.
Biol. Pharm. Bull. 1996, 19, 623. (b) Masuda, M.;
Kishimoto, D.; Kurihara, N. Biosci. Biotechnol. Biochem.
1996, 60, 806. (c) Lopes, N. P.; Chicaro, P.; Kato, M. J.;
Albuquerque, S.; Yoshida, M. Planta Med. 1998, 64, 667.
(d) Scherrmann, J. M.; Boudet, L.; Pontikis, R.; Nguyen-
Hoang-Nam, ; Fournier, E. J. Pharm. Pharmacol. 1980, 32,
800. (e) Schmitt, J. A.; Klose, W. Liebigs Ann. Chem. 1973,
544.
Synthesis 2006, No. 15, 2603–2607 © Thieme Stuttgart · New York