Full text of DOI:10.1021/op1002177

Organic Process Research & Development 2010, 14, 1457–1463
Green Progression for Synthesis of Regioselective ꢀ-Amino Alcohols and
Chemoselective Alkylated Indoles
Boningari Thirupathi, Rapelli Srinivas, Avvari N. Prasad, J. K. Prashanth Kumar, and Benjaram M. Reddy*
Inorganic and Physical Chemistry DiVision, Indian Institute of Chemical Technology, Uppal Road, Hyderabad - 500 607, India
Abstract:
soluble steroidal intravenous anaesthetics.5 Recently, a steroid
possessing a methylpiperazine nucleus was reported to inhibit
the proliferation of HL-60 or WEHI-3B cell lines and is a
promising potential new drug for the treatment of leukemia.6
The significant biological properties of A-ring amino moiety
have urged medicinal chemists to synthesize and test a great
number of novel molecules. Therefore, insertion of the amino
moiety into new positions of a steroid skeleton is an important
synthetic strategy in drug discovery.
Solid acid catalysts based on zirconia materials were investigated
for the first time as catalysts for regioselective organic synthesis
under environmentally benign and mild conditions. The novel
TiO2-ZrO2 mixed oxide catalyst led to two distinct products by
the formation of an N-C bond (ꢀ-amino alcohols) and a C-C
bond (Friedel-Crafts alkylation).
Amino propanols constitute one of the most common central
units of various protease inhibitors such as ꢀ- or γ-secretase,7
HIV protease,8 cathepsins,9 or plasmepsins.10 The scaffold (3,
Figure 1) is a typical example of the 1,3-diaminoalkan-2-ol-
based structure of inhibitors.11 An intermediate in the preparation
of ꢀ-secretase (4, Figure 1) is used in applications such as the
treatment of Alzheimer’s disease and as peptidomimetic inhibi-
tors of human ꢀ-secretase.12 A notable ꢀ-amino alcohol is
propranolol (5, Figure 1), one of the first nonselective ꢀ-blockers
developed, used extensively in the treatment of hypertension
(6, Figure 1) and as an intermediate in the synthesis of an
antitubercular drug,13 which can be obtained by the reaction of
a chlorohydrin with a nucleophile in the presence of stoichio-
metric amounts of a base or by the ring-opening of an epoxide,
1. Introduction
Green chemistry is an important challenge in the battle to
produce biologically active compounds and to replace current
organic chemical processes with more environmentally benign
alternatives. It is also an important task from an ecological point
of view.1 A great deal of interest has been devoted to the use
of aqueous media in organic transformations because water is
nontoxic, nonflammable, inexpensive, and environmentally
benign.2 Other benefits from the use of water as a medium
include replacement of environmentally unfriendly, potentially
dangerous, and expensive organic solvents, thereby reducing
the volatile organic compounds (VOCs) released into the
atmosphere.
ꢀ-Amino alcohols are known for their biological activity.
They find numerous applications as amino alcohol antibiotics
(1, Figure 1) and antibacterial drugs, which function by
specifically binding to prokaryotic 16S rRNA, causing mis-
translation and premature termination of mRNA translation. The
targeted binding site for aminoglycosides is the A-site decoding
region of prokaryotic 16S rRNA.3 Amino alcohol or amino
steroid motifs could be easily obtained through the ring-opening
of the epoxides by amines. ꢀ-Amino alcohol steroids (2, Figure
1) serve as the basis for a wide range of biologically active
natural and synthetic products.4 A series of 2ꢀ-morpholinyl
steroids have been shown to exhibit anaesthetic activity, and
some have been selected for development as potential water-
(5) (a) Hewett, C. L.; Savage, D. S.; Lewis, J. J.; Sugrue, M. F. J. Pharm.
Pharmacol. 1964, 16, 765. (b) Daniell, C. J.; Cross, M.; Sneyd, J. R.;
Thompson, P.; Andrews, C. J. Br. J. Anaesth. 1996, 76, 586. (c) Hill-
Venning, C.; Peters, J. A.; Callachan, H. Neuropharmacology 1996,
35, 1209. (d) Anderson, A.; Boyd, A. C.; Byford, A.; Gemmell, D. K.
J. Med. Chem. 1997, 40, 1668.
(6) (a) He, Q.; Jiang, D. Leuk. Res. 1999, 23, 369. (b) He, Q.; Na, X.
Leuk. Res. 2001, 25, 455. (c) Roy, J.; Deroy, P.; Poirier, D. J. Comb.
Chem. 2007, 9, 347.
(7) (a) Liao, Y. F.; Wang, B.-J.; Hsu, W. M.; Lee, H.; Liao, C. Y.; Wu,
S. Y.; Cheng, H. T.; Hu, M. K. Mol. Pharmacol. 2007, 71, 588. (b)
Bakshi, P.; Wolfe, M. S. J. Med. Chem. 2004, 47, 6485.
(8) (a) Beaulieu, P. L.; Wernic, D.; Abraham, A.; Anderson, P. C.; Bogri,
T.; Bousquet, Y.; Croteau, G.; Guse, I.; Lamarre, D.; Liard, F.; Paris,
W.; Thibeault, D. J. Med. Chem. 1997, 40, 2164. (b) Kempf, D. J.;
Codavoci, L.; Wang, X. C.; Kohlbrenner, W. E.; Wideburg, N. E.;
Saldivar, A.; Vasavanonda, S.; Marsh, K. C.; Bryant, P.; Sham, H. L.;
Green, B. E.; Betebenner, D. A.; Erickson, J.; Norbeck, D. W. J. Med.
Chem. 1993, 36, 320. (c) Medou, M.; Priem, G.; Rocheblave, L.; Pepe,
G.; Meyer, M.; Chermann, J. C.; Kraus, J. L. Eur. J. Med. Chem.
1999, 34, 625.
(9) (a) Lee, C. E.; Kick, E. K.; Ellman, J. A. J. Am. Chem. Soc. 1998,
120, 9735. (b) Kick, E. K.; Roe, D. C.; Skillman, A. G.; Liu, G.;
Ewing, T. J. A.; Sun, Y.; Kuntz, I. D.; Ellman, J. A. Chem. Biol.
1997, 4, 297.
* Corresponding author. Telephone: +91-40-27191714. Fax: +91-40-
27160921. E-mail: bmreddy@iict.res.in; mreddyb@yahoo.com.
(1) (a) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and
Practice; Oxford University Press: New York, 1998. (b) Eissen, M.;
Metzger, J. O.; Schmidt, E.; Schneidewind, V. Angew. Chem., Int.
Ed. 2002, 41, 414. (c) Special Topic Issue on Green Chemistry: Acc.
Chem. Res. 2002, 35, 685.
(2) (a) Li, C.-J.; Chan, T.-H. Organic Reactions in Aqueous Media; Wiley:
Chichester, UK, 1997. (b) Organic Synthesis in Water: Grieco, P. A.,
Ed.; Blake Academic and Professional: London, 1998.
(10) Haque, T. S.; Skillman, A. G.; Lee, C. E.; Habashita, H.; Gluzman,
I. Y.; Ewing, T. J. A.; Goldberg, D. E.; Kuntz, I. D.; Ellman, J. A.
J. Med. Chem. 1999, 42, 1428.
(11) Santos, M. D.; Crousse, B.; Bonnet-Delpon, D. Tetrahedron Lett. 2009,
50, 857.
(12) Maillard, M. C.; Hom, R. K.; Benson, T. E.; Moon, J. B.; Mamo, S.;
Bienkowski, M.; Tomasselli, A. G.; Woods, D. D.; Prince, D. B.;
Paddock, D. J.; Emmons, T. L.; Tucker, J. A.; Dappen, M. S.; Brogley,
L.; Thorsett, E. D.; Jewett, N.; Sinha, S.; John, V. J. Med. Chem.
2007, 50, 776.
(3) Tok, J. B.-H.; Rando, R. R. J. Am. Chem. Soc. 1998, 120, 8279.
(4) Ke, X.; Hu, H.; Zhang, K.; Xu, W.; Zhu, Q.; Wu, L.; Hu, X. Chem.
Commun. 2009, 1037.
10.1021/op1002177  2010 American Chemical Society
Published on Web 10/05/2010
Vol. 14, No. 6, 2010 / Organic Process Research & Development
•
1457

Figure 1. Biologically active ꢀ-amino alcohols useful for the treatment of different diseases.
Table 1. Screening of various solid acid catalysts for the
geneous mixed oxide-based solid acid catalysts such as
synthesis of ꢀ-amino alcoholsa
TiO2-ZrO2, supported or not, have been widely used in organic
syntheses. We have reported recently the oxydehydrogenation
of ethylbenzene to styrene over TiO2-ZrO2 mixed oxide
catalysts, supported or not.16
entry
catalyst
time (min)
yield %b
1
2
3
4
5
6
7
8
WOx/ZrO2
180
180
50
150
150
180
180
12c
82
54
91
60
44
56
38
17d
MoOx/ZrO2
TiO2-ZrO2
WOx/TiO2-ZrO2
MoOx/TiO2-ZrO2
WOx/CeO2-ZrO2
MoOx/CeO2-ZrO2
-
2. Results and Discussion
It is a gigantic challenge to treat water-insoluble organic
materials in water as well as to treat water-unstable materials
in water.17 In order to find out the best catalytic system suited
to the synthesis of ꢀ-amino alcohols, we screened typical
reaction parameters including catalysts and solvents, using
cyclohexene oxide (1.0 equiv) and aniline (1.2 equiv) as model
substrates, and the results are summarized in Table 1 and Table
2. The preliminary exploration was carried out for the selection
of a catalyst for the above reaction in solvent-free conditions.
We have systematically investigated the influence of solid acids
and a series of zirconia-based catalysts, metal oxides, and mixed
oxides and compared them as catalysts for this reaction (Table
1). Among various catalysts we have screened, the TiO2-ZrO2
mixed oxide catalyst exhibited better activity and enriched
regioselectivity towards desired products in these reactions.
Other catalysts, namely, WOx/ZrO2, WOx/TiO2-ZrO2, and
WOx/CeO2-ZrO2, also showed good activity, but the consump-
tion of time was longer to get the desired products, and also
the yields of the products were less than that of TiO2-ZrO2
mixed oxide.
a Reagents and reaction conditions: cyclohexene oxide (1 mmol), aniline (1.2
mmol), catalyst (50 mg); unless otherwise mentioned, stirred at room temperature
for an appropriate time. b Yields of isolated pure regeoisomers. c Time in hours.
d Side products are also observed.
Table 2. Search for optimal solvents and green conditionsa
entry
catalyst
solvent
time (min)
yield %b
1
2
3
4
5
6
TiO2-ZrO2
TiO2-ZrO2
TiO2-ZrO2
TiO2-ZrO2
TiO2-ZrO2
TiO2-ZrO2
DCM
THF
ACN
toluene
-
240
120
120
120
50
68
76
61
55
89
91
H2O
50
a Reagents and reaction conditions: cyclohexene oxide (1 mmol), aniline
(1.2 mmol), TiO2-ZrO2 (50 mg), solvent (2 mL), unless otherwise mentioned,
stirred at room temperature for an appropriate time. b Yields of isolated pure
regeoisomers.
where the epoxide is heated in the presence of the nucleophile.14
We herein report the synthesis of ꢀ-amino alcohols using mild
and environmentally friendly conditions. An obvious advantage
of using an epoxide as a substrate in the production of ꢀ-amino
alcohols is the general atom efficiency of the reaction.
Ecological and economic considerations have recently raised
a strong interest in redesigning commercially important pro-
cesses so that the use of harmful substances and the generation
of toxic wastes could be avoided.15 In this respect, heteroge-
neous catalytic processes are more favorable over their homo-
geneous counterparts in the production of fine chemicals and
biological active molecules, owing to their ease of handling,
simple workup procedures, and most importantly, their reus-
ability and catalytic activity in water (green solvent). Hetero-
(13) Tewari, N.; Tiwari, V. K.; Tripathi, R. P.; Chaturvedi, V.; Srivastava,
A.; Srivastava, R.; Shukla, P. K.; Chaturvedi, A. K.; Gaikwad, A.;
Sinha, S.; Srivastava, B. S. Bioorg. Med. Chem. Lett. 2004, 14, 329.
(14) (a) Press, J. B.; Falotico, R.; Hajos, Z. G.; Sawyers, R. A.; Kanojia,
R. M.; Williams, L.; Haertlein, B.; Kauffman, J. A.; Lakas-Weiss,
C.; Salata, J. J. J. Med. Chem. 1992, 35, 4509. (b) Barton, D. L.;
Press, J. B.; Hajos, Z. G.; Sawyers, R. A. Tetrahedron: Asymmetry
1992, 3, 1189. (c) Butrous, G. S.; Debbas, N. M. G.; Erwin, J.; Davies,
D. W.; Keller, H. P.; Lunnon, M. W.; Nathan, A. W.; Camm, A. J.
Eur. Heart J. 1988, 9, 489. (d) Sircar, I.; Haleen, S. J.; Burke, S. E.
J. Med. Chem. 1992, 35, 4442. (e) Suzuki, T.; Fukazawa, N.; San-
nohe, K. J. Med. Chem. 1997, 40, 2047.
(15) Taguchi, A.; Schuth, F. Microporous Mesoporous Mater. 2005, 77,
1.
(16) (a) Rao, K. N.; Reddy, B. M.; Abhishek, B.; Seo, Y.-H.; Jiang, N.;
Park, S.-E. Appl. Catal., B 2009, 91, 649. (b) Reddy, B. M.; Lee,
S.-C.; Han, D.-S.; Park, S.-E. Appl. Catal., B 2009, 87, 230.
(17) Azoulay, S.; Manabe, K.; Kobayashi, S. Org. Lett. 2005, 7, 4593.
1458
•
Vol. 14, No. 6, 2010 / Organic Process Research & Development

Scheme 1. Synthesis of ꢀ-amino alcohols catalysed by TiO2-ZrO2 mixed oxide in water
Scheme 2. Friedel-Crafts alkylation of nitrogen heterocycles with epoxides
Use of mixed oxides such as TiO2-ZrO2 in place of ZrO2
is very interesting. Mixed oxides exhibit relatively high BET
surface area, high thermal stability, and high mechanical
strength.18 The TiO2-ZrO2 mixed oxide has also been reported
to exhibit high surface acidity by a charge imbalance based on
the generation of Ti-O-Zr bonding.19 The ZrTiO4 compound
formation was discovered while studying phase equilibria in
ZrO2 and TiO2 systems. The ZrTiO4 belongs to the space group
Pbcn and has an orthorhombic structure (R-PbO2) with complete
disorder of the Zr4+ and Ti4+ ions on the metal sites.20 The
crystal structure consists of an edge-shared metal-oxygen
octahedral [MO6], which is quite distorted.
After selection of the most efficient catalyst, we screened
different solvents to adjust the reaction conditions (Table 2).
Among the reactions screened, the only ones where reactions
progressed without detectable formation of byproducts used
water as the solvent or no solvent at all (entries 5 and 6). The
increased rates and yields are probably due to the hydrophobic
nature of the reagents towards water, since their repulsion from
water would enhance the collisions between the organic
molecules and increase their ground-state energies, leading to
an increase in the rate of the reaction.18 To be as “green” as
possible we selected water as the reaction medium and ambient
temperature for mildness and economy.
With these “environmentally benign” conditions in hand, in
addition to our employment of solid acid catalysts for green
synthesis,19,20 we then explored the scope of this TiO2-ZrO2
mixed oxide-catalysed regioselective ring-opening of diverse
epoxide reactions with various amines (C-N bond formation)
by using water as the solvent (Scheme 1). In addition to these
reactions, we also explored ring-opening of epoxides with
indoles. To our surprise, we ended up with regio-enriched
Friedel-Crafts alkylation (C-C bond formation) products. To
evaluate the probability of Friedel-Crafts alkylation, we
extended this reaction also to the synthesis of alkylated nitrogen
heterocycles by Friedel-Crafts alkylation of epoxides with
indoles (Scheme 2).
epoxides to form the corresponding ꢀ-amino alcohols and
Friedel-Crafts alkylated products using TiO2-ZrO2 mixed
oxide catalyst in water. The variation in the yields of the
corresponding ꢀ-amino alcohols and alkylated products formed
are found to depend on the intrinsic properties of the substituted
amines and indoles employed.
Electronic effects are known to influence the reaction rates
and product yields. In the case of the present reactions
investigated, it is observed that the amines and indoles bearing
electron-donating groups afforded the corresponding products
in good yields (Table 3, entries 7, 9, 12, 14, and 18). Conversely,
electron-withdrawing groups being present on the nucleophile
moiety drastically decreased the rate of the reaction, and the
yields of the products formed are also very low, even after
extended reaction times (Table 3, entries 4 and 11).
Cyclohexene oxide with a variety of amines (Scheme 3)
afforded diastereo-enriched trans-regioisomers (Table 3, entries
10-14) and the resultant racemic 2-amino cyclohexanol
products were recognized as the trans-diastereoisomers on the
basis of NMR spectral data. Reaction of aliphatic oxirane
(propylene oxide, epichlorohydrin) with various amines and
indoles afforded the major secondary alcohol regioisomer (Table
3, entries 6-9 and 15-17) by nucleophilic attack at a less
sterically hindered carbon, whereas in the case of styrene oxide
electronic factors predominate over the steric factors to produce
a major primary alcohol regioisomer by nucleophilic attack at
the more stable benzylic carbon (Table 3, entries 1-5, 18, and
19), and also the reaction is regio-enriched. In aryl oxirane, the
positive charge on oxygen appears to be localized on the highly
substituted benzylic carbon leading to the major product
(Scheme 4). We strongly believe that rendering of the nucleo-
phile is governed by the group environment of the oxirane and
the stability of the carbonium ion.
Rationale to exhibit better catalytic activity of the TiO2-ZrO2
mixed oxide for the above reactions could be due to the
activation of epoxide by adsorption on the surface Lewis acidic
site, which is electron deficient in TiO2-ZrO2 mixed oxide.
Epoxides are able to act as Lewis bases directly through their
nonbonded electron pairs, confirming that the presence of Lewis
acids in these reactions enhances the rate of ring-opening
processes, primarily by weakening the already strained C-O
bond. On the other hand, the surface Lewis basic site, which is
created by surface Ti-O-, is specific for the generation of a
nucleophile by proton abstraction from amines or indoles. This
suggests that the surface Lewis acidic site is responsible for
Both types of reactions were carried out using a wide variety
of structurally diverse amines and indoles with quite a lot of
(18) (a) Fung, J.; Wang, I. J. Catal. 1991, 130, 577. (b) Reddy, B. M.;
Manohar, B.; Mehdi, S. J. Solid State Chem. 1992, 97, 233.
(19) (a) Reddy, B. M.; Chowdhury, B.; Smirniotis, P. G. Appl. Catal., A
2001, 211, 19. (b) Mullins, W. M.; Averbach, B. Surf. Sci. 1988, 206,
29.
(20) (a) Newnham, R. E. J. Am. Ceram. Soc. 1967, 50, 216. (b) Park, Y.
Mater. Res. Bull. 1998, 33, 1325.
Vol. 14, No. 6, 2010 / Organic Process Research & Development
•
1459

Table 3. Amination of epoxides and alkylation of indoles over TiO2-ZrO2 in watere
1460
•
Vol. 14, No. 6, 2010 / Organic Process Research & Development

Table 3. Continued
a Yields of isolated pure regeoisomers A:B. b Isolated yield of the products. c Time in days. d Starting materials are recovered. e Reagents and reaction conditions: epoxide 1
(1 mmol), 2 amine/indole (1.2 mmol), TiO2-ZrO2 (50 mg), water (2 mL) stirred at room temperature for an appropriate time.
Scheme 3. Highly regio-enriched ring-opening of
symmetrical epoxide
the activation of epoxide, and the Lewis basic site is responsible
for the generation of a nucleophile (Figure 2).
3. Conclusions
In conclusion, we have reported for the first time an
original use of TiO2-ZrO2 mixed oxide as catalyst for
regioselective organic synthesis in water. With such a
Scheme 4. Desired product differentiations by changing the nature of unsymmetrical epoxide
Vol. 14, No. 6, 2010 / Organic Process Research & Development
•
1461

(Fluka, AR grade) were dissolved separately in double
distilled water and mixed together. Cold titanium(IV)
chloride was digested first in cold concentrated HCl and
diluted with deionized water. To the solution mixture was
added dropwise aqueous NH3 with vigorous stirring until
the pH of the solution reached 8.The resulting precipitate
was allowed to settle down for 2 days. The obtained
precipitate was then filtered off, washed several times with
deionized water and hot distilled water until free from
(chloride) anion impurities, and oven-dried at 393 K for
12 h. A portion of the oven-dried material was calcined
at 923 K for 5 h in air atmosphere. The prepared catalyst
was characterized by various physicochemical techniques,
namely, X-ray diffraction, Raman spectroscopy, X-ray
photoelectron spectroscopy, and other methods. The
characteristic features of the synthesized TiO2-ZrO2
mixed oxide catalyst are included in the Supporting
Information.
4.2. Typical Experimental Procedure and Reus-
ability. All chemicals employed in this study were
commercially available and used without further purifica-
tion. A mixture of epoxide (1 mmol), amine or indole (1.2
mmol), and TiO2-ZrO2 mixed oxide catalyst (50 mg) in
water (2 mL) was stirred at room temperature for an
appropriate time. After completion of the reaction, as
indicated by TLC, the reaction mixture was filtered and
washed with ethyl acetate (3 × 5 mL), and the combined
layers were dried over anhydrous Na2SO4, concentrated
in vacuum, and purified by column chromatography on
silica gel using ethyl acetate and hexane as an eluent to
afford pure ꢀ-amino alcohols and Friedel-Crafts alkylated
Figure 2. Exact catalyst surface sites for the activation of
epoxide and generation of nucleophile.
catalyst, we have developed a simple and efficient method
for the regio-enriched ring-opening of epoxides (C-N
bond construction) and chemoselective Friedel-Crafts
alkylation of indoles with epoxides (C-C bond construc-
tion). Moreover, the same TiO2-ZrO2 mixed oxide
catalyst has been used for both reactions (see Figure 3).
These results thus broaden the scope of the so-called
synthesis of ꢀ-amino alcohols and Friedel-Crafts alkyla-
tion.
4. Experimental Section
4.1. Catalyst Preparation. TiO2-ZrO2 (1:1 mol ratio
based on oxides) mixed oxide was prepared by adopting
a coprecipitation method by hydrolysis with dilute aqueous
ammonia solution. For this purpose, the requisite quantity
of ZrOCl2 · 8H2O (Loba Chime, GR grade) and TiCl4
Figure 3. Plausible reaction mechanism for both syntheses catalysed by TiO2-ZrO2.
1462
•
Vol. 14, No. 6, 2010 / Organic Process Research & Development

indoles. All products were identified by comparing their
spectral data with literature.21-25
for the reaction, and there was no big change in the catalytic
activity in the next five cycles.
The solid catalyst was conveniently removed by simple
filtration from the reaction mixture. The wet catalyst was reused
Acknowledgment
B.T. thanks CSIR for a Senior Research Fellowship, New
Delhi. R.S., A.N.P., and J.K.P.K. thank the CSIR, New Delhi,
for Junior Research Fellowships.
(21) (a) Narayan, S.; Muldoon, J.; Finn, M. G.; Fokin, V. V.; Kolb, H. C.;
Sharpless, K. B. Angew. Chem., Int. Ed. 2005, 44, 3275. (b) Klijn,
J. E.; Engberts, J. B. N. Nature 2005, 435, 746. (c) Pirrung, M. C.
Chem.sEur. J. 2006, 12, 1312.
Supporting Information Available
(22) Recently, we introduced zeo-click-MCR concept for organic cascade
syntheses by using zeolites in water: (a) Thirupathi, B.; Olmos, A.;
Reddy, B. M.; Pale, P.; Sommer, J. Eur. J. Org. Chem. 2010, DOI:
10.1002/ejoc.200. (b) Chassaing, S.; Alix, A.; Thirupathi, B.; Karim,
S. S. S.; Keller, M.; Kuhn, P.; Louis, B.; Sommer, J.; Pale, P. Synthesis
2010, 1557. (c) Be´neteau, V.; Olmos, A.; Thirupathi, B.; Sommer, J.;
Pale, P. Tetrahedron Lett. 2010, 51, 3673.
1H NMR, FTIR, and MS data of all isolated products, and
X-ray powder diffraction, Raman, and XPS analysis of the
catalyst. This material is available free of charge via the Internet
at http://pubs.acs.org/.
(23) For recent reviews on solid acid activity under green conditions: (a)
Reddy, B. M.; Patil, M. K. Chem. ReV. 2009, 109, 2185. (b) Reddy,
B. M.; Patil, M. K. Curr.Org. Chem. 2008, 12, 118. (c) Arata, K.
Green Chem. 2009, 11, 1719.
Received for review August 6, 2010.
OP1002177
(24) (a) Sekar, G.; Singh, V. K. J. Org. Chem. 1999, 64, 287. (b) Chini,
M.; Crotti, P.; Macchia, F. J. Org. Chem. 1991, 56, 5939. (c)
Yamamoto, Y.; Asao, N.; Meguro, M.; Tsukada, N.; Nemoto, H.;
Sadayori, N.; Wilson, J. G.; Nakamura, H. J. Chem. Soc., Chem.
Commun. 1993, 1201. (d) Meguro, M.; Asao, N.; Yamamoto, Y.
J. Chem. Soc., Perkin Trans. 1 1994, 2597. (e) Mojtahedi, M. M.;
Saidi, M. R.; Bolourtchian, M. J. Chem. Res. (S) 1999, 22, 128.
(25) (a) Kotsuki, H.; Teraguchi, M.; Shimomoto, N.; Ochi, M. Tetrahedron
Lett. 1996, 37, 3727. (b) Bandini, M.; Cozzi, P. G.; Melchiorre, P.;
Umani-Ronchi, A. J. Org. Chem. 2002, 67, 5386. (c) Yadav, J. S.;
Reddy, B. V. S.; Abraham, S.; Sabitha, G. Synlett 2002, 1550. (d)
Bandgar, B. P.; Patil, A. V. Tetrahedron Lett. 2007, 48, 173.
Vol. 14, No. 6, 2010 / Organic Process Research & Development
•
1463