Gallium(III) triflate catalyzed direct reductive amination of aldehydes

Article abstract of DOI:10.1007/s10562-010-0347-7

Direct hydroamination of aldehydes and ketones provides one-step entry into desired α-aminoalkane derivatives which are important synthons for many biologically active molecules. The reductive amination of aldehydes in the presence of silanes has been eff

Full text of DOI:10.1007/s10562-010-0347-7

Catal Lett (2010) 137:111–117
DOI 10.1007/s10562-010-0347-7
Gallium(III) Triflate Catalyzed Direct Reductive Amination
of Aldehydes
•
G. K. Surya Prakash Clement Do
•
•
Thomas Mathew George A. Olah
Received: 18 March 2010 / Accepted: 9 April 2010 / Published online: 28 April 2010
Ó Springer Science+Business Media, LLC 2010
Abstract Direct hydroamination of aldehydes and
ketones provides one-step entry into desired a-aminoalkane
derivatives which are important synthons for many bio-
logically active molecules. The reductive amination of
aldehydes in the presence of silanes has been effectively
promoted by Ga(OTf)3 as a catalyst. Mild conditions, easy
work-up and high purity of products with excellent yields
are the major advantages of this method.
preparation of secondary or tertiary amines and related
functional compounds in biological and chemical systems
[3–6]. The major advantage of this reaction is that there is no
need to isolate intermediate imines, in particular, in cases
where condensation of aromatic amines with aliphatic car-
bonyls would give expectedly unstable imines. The choice of
the reducing agent is very crucial for the success of the
reaction, since the selective reduction of imines (or iminium
ions) by the reducing agent over aldehydes or ketones is
required under the reaction conditions [7, 8].
Keywords Gallium triflate Á Hydroamination Á
Organosilanes Á Green chemistry Á
The two most commonly used direct reductive amination
methods differ in the nature of the reducing agent. The first
method is catalytic hydrogenation with platinum, palladium,
or nickel catalysts. This is an economical and effective
reductive amination method, particularly in large scale
reactions. However, hydrogenation has limited use with
compounds containing carbon–carbon multiple bonds and in
the presence of reducible functional groups such as nitro and
cyano groups. Inhibition of the catalytic activity of the cat-
alyst by sulfur compounds has also been observed. The
second method utilizes hydride based reducing agents [8].
Among the hydride based reducing agents [5–9], different
borohydrides are frequently used to carry out this transfor-
mation, mainly sodium cyanoborohydride (NaBH3CN) and
sodium triacetoxyborohydride {NaBH(OAc)3} [8, 10, 11].
However, most of these reagents have one drawback or
another. For example, cyanoborohydride and tin hydride are
highly toxic and generate toxic byproducts such as HCN,
NaCN, or organotin compounds [12]. Upon workup con-
tamination of the product with the toxic compounds has been
observed in many cases. Other hydrides such as sodium
triacetoxyborohydride also involve the use of corrosive
acetic acid [13]. The processes based on such reducing
agents are not environmentally friendly and do not conform
with green chemistry and sustainable development. Hence,
Organic synthetic transformations
1 Introduction
Amines and their derivatives act as important synthetic
precursors in organic chemistry. They are highly versatile
building blocks for various organic target molecules and are
essential precursors to a variety of biologically active com-
pounds, such as pharmaceuticals [1] and agrochemicals [2].
Due to their importance, numerous methods have been
developed for the preparation of amines. Reductive amina-
tion of aldehydes and ketones, in which a mixture of an
aldehyde or ketone and an amine is treated with a reductant in
a one-pot fashion, is one of the most useful methods for the
G. K. Surya Prakash (&) Á C. Do Á T. Mathew (&) Á
G. A. Olah
Department of Chemistry, Loker Hydrocarbon Research
Institute, University of Southern California, Los Angeles,
CA 90089-1661, USA
e-mail: gprakash@usc.edu
T. Mathew
e-mail: tmathew@usc.edu
123

112
G. K. Surya Prakash et al.
the developments of improved protocols are still in demand.
Consequently, there is a need for the development of an
easily tunable and environmentally friendly method for the
reduction of Schiff’s bases generated in situ [2].
standard CFCl3 at d 0.00 ppm. Mass spectra were recorded
on Agilent HP 5973 Mass Spectrometer in the EI mode.
2.2 General Procedure for Ga(OTf)3-Catalyzed
Reductive Amination of Aldehydes
Reductive amination of aldehydes using various reducing
agents were recently reported [14–17]. There are however
only few reports on the direct catalytic reductive amination
of carbonyl compounds using organosilane as a reducing
reagent. Hydrosilanes such as triethylsilane in the presence
of acid catalysts [18] and molecular hydrogen in the presence
of water-soluble transition metal catalysts [19] are mild and
useful reagent systems for reduction. Apodaca and
coworkers [20] showed that the direct reductive amination
with carbonyl compounds and phenylsilane (PhSiH3) was
achieved in the presence of catalytic amount of dibutyltin
dichloride. Titanium catalyzed reduction of imines, prepared
independently from the reaction of amines with aromatic and
nonaromatic ketones, with PhSiH3 and polymethylhydrosi-
loxane (PMHS) was also reported by Buchwald and
coworkers [21]. Recently, reductive amination of aldehydes
and ketones using PMHS/trifluoroacetic acid system has
been reported [22]. On the other hand, we have shown that
gallium (III) trifluoromethanesulfonate {Ga(OTf)3, gallium
triflate}, acts as an effective but mild and water tolerant
Lewis acid catalyst for many organic synthetic transforma-
tions such as Friedel–Crafts alkylations, dehydration of
oximes to the corresponding nitriles, Beckman rearrange-
ment [23–30] etc. It was found that gallium triflate offers the
optimum acidity required for ketonic Strecker reaction [31]
and the synthesis of various heterocycles such as dihydro-
benzimidazolines, benzothiazolines, benzoxazinones [32]
etc. Based on these results, we decided to carry out direct
hydroamination in a three component fashion using alde-
hydes and amines in the presence of hydrosilanes using
Ga(OTf)3 as catalyst.
Aldehyde (2.04 mmol) and amine (2 mmol) dissolved in
4 mL of CH2Cl2 was added to Ga(OTf)3 (52 mg, 5 mol%)
in a pressure tube. Organosilane in excess (6 mmol) was
then added to the reaction mixture and the pressure tube
was closed. The mixture was stirred at 100 °C until the
completion of the reaction (monitored at different time
intervals by TLC and NMR). The mixture was then filtered
and the residue was washed with CH2Cl2 (2 9 5 mL). The
combined filtrate was concentrated under reduced pressure
and the product obtained was further purified by trituration
of the residue with excess hexane followed by evaporation
of hexane.
2.3 Spectral Data
The 1H and 13C spectra of the products 3a [33], 3b [34], 3c
[35], 3d [36], 3e [37], 3f [38], 3j [39], 3m [40] and 3o [41],
3r [42, 43] and 3s [22] were consistent with those reported.
2.4 4-Chloro-N-(2,5-Dimethoxybenzyl)Aniline (3g)
1H NMR (400 MHz, CDCl3): d 7.10–7.15 (m, 2H), 6.89–
6.76 (m, 3H), 6.68–6.63 (m, 2H), 5.46 (bs, 1H), 4.30 (s,
2H), 3.82 (s, 3H), 3.72 (s, 3H). 13C NMR (100.63 MHz,
CDCl3): d 153.67, 151.58, 146.90, 129.07, 128.15, 122.01,
115.35, 114.26, 112.25, 111.24, 55.93, 55.79, 43.70. MS
(m/z): 277 (M?), 151 (100, M?-NHC6H4Cl).
2.5 N-(4-Ethylbenzyl)Aniline (3h)
2 Experimental
1H NMR (400 MHz, CDCl3): d 7.26–7.06 (m, 6H), 6.81–
6.70 (m, 1H), 6.65 (dt, J = 9.11, 8.88, 3.99 Hz, 2H), 5.47
(bs, 1H), 4.22 (s, 2H), 2.58 (q, J = 7.59 Hz, 2H), 1.19
(t, J = 7.61 Hz, 3H). 13C NMR (100.63 MHz, CDCl3):
d 145.98, 143.63, 134.99, 129.31, 128.13, 128.05, 119.29,
114.44, 49.32, 28.53, 15.62. MS (m/z): 211 (M?), 119 (100,
M?-CH3CH2C6H4CH2).
2.1 Materials and Methods
Unless otherwise mentioned, all the chemicals were pur-
chased from commercial sources. Ga(OTf)3 was prepared
by following a reported procedure [25]. The products were
identified by 1H, 13C, 19F NMR and GC–MS spectral
1
analysis. H, 13C, and 19F NMR spectra were recorded on
400 MHz Varian NMR spectrometer using CDCl3 as sol-
1
vent. H and 13C NMR chemical shifts were determined
2.6 4-Chloro-N-(4-Ethylbenzyl)Aniline (3i)
relative to tetramethylsilane (TMS) at d 0.0 ppm or to
residual solvent peak at d 7.26 ppm for CDCl3. 13C NMR
shifts were determined relative to TMS at d 0.00 ppm or to
the residual solvent peak (at d 77.16 ppm for CDCl3). 19F
NMR chemical shifts were determined relative to internal
1H NMR (400 MHz, CDCl3): d 7.29–7.24 (m, 2H), 7.13–
7.10 (m, 4H), 7.08–7.03 (m, 2H), 4.38 (s, 2H), 2.58
(q, J = 7.61 Hz, 2H), 1.17 (t, J = 7.61 Hz, 3H). 13C NMR
(100.63 MHz, CDCl3): d = 146.20, 134.40, 133.79,
123

Gallium(III) Triflate Catalyzed Direct Reductive Amination of Aldehydes
113
130.41, 130.07, 128.63, 126.85, 123.63, 55.91, 28.61,
15.33. MS (m/z): 245 (M?), 119 (100, M?-
CH3CH2C6H4CH2).
3 Results and Discussion
We found that gallium triflate can efficiently catalyze the
direct reductive amination of aldehydes under mild con-
ditions (Scheme 1). As shown in the previous cases
[23–31], gallium triflate can be easily recovered from the
reaction mixture and reused, showing its significant
potential as a safe and environmentally benign catalyst.
The procedure offers broad application and the synthesis of
diverse higher secondary amines with a high degree of
functional group tolerance.
2.7 4-Chloro-N-(Naphthalen-2-ylmethyl)Aniline (3k)
1H NMR (400 MHz, CDCl3): d 7.82–7.72 (m, 4H), 7.48–
7.41 (m, 2H), 7.40–7.36 (m, 1H), 7.16–7.04 (m, 2H), 6.69–
6.58 (m, 2H), 5.60 (bs, 1H), 4.42 (s, 2H). 13C NMR
(100.63 MHz, CDCl3): d 144.05, 134.54, 133.42, 132.99,
129.37, 128.66, 127.92, 127.80, 126.88, 126.45, 126.20,
125.86, 124.74, 116.01, 50.13. MS (m/z): 267 (M?), 141
(100, M?-NHC6H4Cl).
The direct reductive amination of 4-methoxybenzalde-
hyde with 4-chloroaniline in the presence of 5 mol% of
Ga(OTf)3 was selected as a model reaction and was
examined under the influence of several organosilanes
reductants (Table 1). Reductive amination was found to be
feasible with triethylsilane, phenylsilane, and polymethyl-
hydrosiloxane (PMHS). However, sterically hindered
organosilanes, such as, triisopropylsilane (entries 2, 4 and
5) were ineffective. The steric factors associated with dif-
ferent organosilane reductants seem to be important in
determining the fate of the reaction. Of these three effec-
tive reductants, triethylsilane was the most suitable (entries
2.8 4-Chloro-N-(4-Nitrobenzyl)Aniline (3l)
1H NMR (400 MHz, CDCl3): d = 8.15–8.10 (m, 2H),
7.54–7.41 (m, 2H), 7.15–7.02 (m, 2H), 6.60–6.46 (m, 2H),
5.03 (bs, 1H), 4.44 (s, 2H). 13C NMR (100.63 MHz,
CDCl3): d = 147.19, 146.27, 144.90, 129.25, 127.97,
123.89, 123.55, 114.77, 48.09. MS (m/z): 262 (100, M?).
2.9 4-{(4-Chlorophenylamino)Methyl}Benzonitrile (3n)
Organosilane
R1
1H NMR (400 MHz, CDCl3): d 7.57 (d, J = 8.28 Hz, 2H),
7.43 (d, J = 8.04 Hz, 2H), 7.12–7.06 (m, 2H), 6.58–6.52 (m,
2H), 5.45 (bs, 1H), 4.40 (s, 2H). 13C NMR (100.63 MHz,
CDCl3): d 144.40, 143.69, 132.44, 129.23, 128.06, 127.17,
118.77, 115.12, 111.11, 48.55. MS (m/z): 242 (100, M?).
NH2
CHO
+ R
Ga(OTf)3 (5mol%)
N
H
1
R
R
°
CH2Cl2,100
2.5-12h
C
1
2
3
Scheme 1 Gallium triflate catalyzed reductive amination of
aldehydes
2.10 4-Chloro-N-(2-Methylbenzyl)Aniline (3o)
Table 1 Screening of organosilane reductants for gallium triflate
catalyzed reductive amination of aldehydes
1H NMR (400 MHz, CDCl3): d 7.23 (d, J = 7.34 Hz, 1H),
7.19–7.09 (m, 3H), 7.08–7.03 (m, 2H), 6.48–6.43 (m, 2H),
4.14 (s, 2H), 4.10 (bs, 1H), 2.29 (s, 3H). 13C NMR
(100.63 MHz, CDCl3): d 146.38, 136.29, 136.27, 130.50,
129.09, 128.10, 127.60, 126.21, 122.24, 114.04, 46.55,
18.92. MS (m/z): 231 (M?), 105 (100, M?-CH3, -C6H5Cl).
Cl
NH2
CHO
Organosilane
Ga(OTf)3 (5 mol%)
N
H
+
CH2Cl2, 100 °C, 12h
MeO
Cl
OMe
1b
2c
3e
2.11 4-Chloro-N-(3-Fluorobenzyl)Aniline (3q)
Entry
Organosilane (equiv.)
Conversion (%)
Yield (%)a
1H NMR (400 MHz, CDCl3): d 7.31–7.24 (m, 1H), 7.11–
7.08 (m, 3H), 7.05–7.03 (d, J = 9.78 Hz 1H), 6.97–6.91
1
2
3b
4
5
6
7
a
Et3SiH (1.1)
48
100
0
–
98
–
Et3SiH (3.0)
(m, 1H), 6.49–6.54 (m, 2H), 4.30 (s, 2H), 4.25 (bs, 1H). 13
C
Et3SiH (3.0)
NMR (100.63 MHz, CDCl3): d 161.63 (d, J = 246.09 Hz),
146.17, 141.69 (d, J = 6.73 Hz), 130.20 (d, J = 8.23 Hz),
129.12, 122.71 (d, J = 2.24 Hz), 122.49, 114.27 (d,
J = 11.97 Hz), 114.07 (d, J = 8.98 Hz), 113.99, 47.83 (d,
J = 2.24 Hz). 19F NMR (376.78 MHz, CDCl3): d ppm -
113.28 (dd, J = 9.16, 6.10 Hz, 1F). MS (m/z): 235 (M?),
109 (100, M?-NHC6H4Cl).
((CH3)2CH)3SiH (3.0)
PhSiH3 (3.0)
PhSiH3 (1.1)
PMHS (2.8)
0
–
100
25
55
97
–
–
Isolated yield
b
Reaction was performed in the absence of Ga(OTf)3
123

114
G. K. Surya Prakash et al.
Time (h) Yield (%)a
Table 2 Ga(OTf)3 catalyzed reductive amination of aldehydes with triethylsilane
Entry
Aldehydes (1)
Amines (2)
Product (3)
CHO
N
2.5
2.5
87
88
1
H
Cl
Cl
H2N
H2N
Cl
Cl
3a
CH3
1a
2a
CH3
CHO
N
H
2
3b
2b
2c
Cl
Cl
CHO
CHO
3
3
97
78
N
H
3
4
Cl
H2N
H2N
Cl
3c
N
H
MeO
MeO
3d
Cl
1b
Cl
CHO
CHO
4.5
98
N
H
5
6
7
8
H2N
MeO
MeO
3e
CH3
Cl
CH3
3
94
95
N
H
H2N
H2N
MeO
MeO
MeO
MeO
3f
CHO
OMe
Cl
2.5
N
H
OMe
1c
1d
3g
CHO
3
89
95
N
H
Et
Et
H2N
H2N
Et
Et
3h
Cl
Cl
CHO
3.5
N
H
3i
9
123

Gallium(III) Triflate Catalyzed Direct Reductive Amination of Aldehydes
115
Table 2 continued
Entry
Aldehydes (1)
Amines (2)
Product (3)
Time (h) Yield (%)a
CHO
N
H
10
4
93
97
H2N
H2N
H2N
3j
Cl
Cl
1e
CHO
CHO
Cl
Cl
N
H
4.5
11
12
3k
N
H
5
95
O2N
O2N
NC
O2N
3i
OCH3
1f
CHO
CHO
OCH3
N
H
13
14
4.5
4.5
98
91
H2N
H2N
O2N
NC
3m
Cl
2d
Cl
Cl
N
H
1g
3n
Cl
CH3
CH3
CHO
N
H
15
16
17
3
87
H2N
3o
Cl
Cl
1h
1i
CHO
CHO
Cl
Cl
N
H
73
68
2.5
2.5
F
F
H2N
H2N
3p
N
H
3q
F
F
1j
OCH3
NO2
OCH3
O
H
H3C
H3C
12
94
85
18
N
H
H3C
H3C
H2N
H3C
H3C
H
H
3r
1k
CHO
NO2
N
H
19
2.5
Cl
H2N
Cl
3s
2e
a
Isolated yield
123

116
G. K. Surya Prakash et al.
H
2 and 5) and PMHS produced an insoluble precipitate
which required tedious purification procedure (entry 7).
Reactions performed with substoichiometric phenylsilane
or triethylsilane suggest that excess silane was required as a
hydrogen source for 100% conversion (entries 1, 2, 5, and
6). No reductive amination was observed with triethylsi-
lane in the absence of Ga(OTf)3 (entry 3a), demonstrating
the role of Ga(OTf)3 as the catalyst [15].
R2
H
R2
H
Ga(OTf)3
H
O
H
H
N
N
O
R2
N
+
H
OH
-H2O
R1
H
Ga(OTf)3
R1
R1
1
2
H
H
R2
Si(Et)3
R2
H
R2
H
R2
H
Ga(OTf)3
Si(Et)3
N
H2O
N
N
N
-Et3SiOH
H
H
R1
R1
H
R1
R1
(Et3Si)2O
4
From the screening of different organosilanes in varying
amounts and studying their efficiency as effective reducing
agents (Table 1), it has been found that triethylsilane and
phenylsilane are the most efficient reducing agents under
the reaction conditions when they are used in excess (3 M
equivalents). Use of dichloromethane minimizes solvent
interaction with the catalyst resulting in enhanced catalytic
activity of the catalyst towards aldehydes and the resultant
imines providing suitable environment for further reduc-
tion step.
3
Scheme 2 Proposed mechanism of the Ga(OTf)3 catalyzed direct
reductive amination
amination of aldehydes and ketones, was found to be a
sluggish and inefficient reagent for this purpose [8].
However, Ga(OTf)3 catalyzes the reaction of both 4-nitro-
and 4-chloroaniline with aromatic aldehydes and the sub-
sequent triethylsilane reduction to the corresponding sec-
ondary amines very effectively (entries 9, 19) [3].
Additionally, it is worth noticing that no reduction of
aldehyde 1 to alcohol or the corresponding silyl ether
occurred during the reaction. It shows that in the presence
of Ga(OTf)3, chemoselective reduction of aldimine 4 is
preferred to the reduction of aldehydes 1 under the reaction
conditions. A recent study by Laali et al. [44] showed that
formation of the corresponding benzyl ether was predom-
inant during the Lewis acid catalyzed hydrosilylation
of aldehydes and ketones using various metal triflates
including Ga(OTf)3. Therefore, we also examined the
chemoselective reductive amination of functionalized
benzaldehydes bearing other reducible functional groups
employing the same methodology. As indicated in Table 2,
aromatic aldehydes having chloro, fluoro, methoxy, nitro
and cyano groups smoothly underwent reductive amination
to give the corresponding N-phenyl amines in good yields
without affecting other functional groups (entries 1–3,
12–14, 16 and 17). Similarly aliphatic aldehydes such as
pivalaldehyde underwent reductive amination successfully
with 4-chloroaniline to give the corresponding amines in
high yield (entry 18).
Based on the efficient reductive amination protocol for
aldehydes catalyzed by Ga(OTf)3, we decided to extend
this methodology to ketones also. However, it was found
that the reactions were not feasible and clean under similar
conditions. Several modifications of the reaction conditions
(changing temperature, reaction time, catalyst loading etc.)
did not result in any significant improvement.
The suggested mechanism, involved in this reaction is
schematically shown in Scheme 2. The first step involves
the formation of the aldimine intermediate 4 from the
Ga(OTf)3 catalyzed condensation reaction between alde-
hyde and amine. Aldimine 4 activated by coordination with
the Lewis acid Ga(OTf)3 undergoes subsequent silane
addition followed by hydrolysis to yield the secondary
amines 3 as the reduction product. Conversion and yield of
the secondary amines 3 depend on the amounts of orga-
nosilane used. Absence of organosilane stops the reaction
at aldimine stage manifesting its role as the hydrogen
source under gallium triflate catalysis. When reduction has
been attempted in the absence of Ga(OTf)3, no reduction
product has been observed. When the imine prepared
separately was subjected to reduction conditions in the
absence of Ga(OTf)3, no change was observed, further
emphasizing the catalytic role of Ga(OTf)3.
Aromatic amines containing electron withdrawing sub-
stituents, such as nitro and chloroanilines, are poor nucle-
ophiles as well as weak bases (e.g., pKa 3.98 for
4-chloroaniline, 1.02 for 4-nitroaniline) [8]. This slows
down the initial nucleophilic attack on the carbonyl carbon
by the amine and leads to slower overall reaction rates
(Scheme 2). In addition, the carbonyl group now competes
effectively with the less basic intermediate imine for pro-
tonation (or complexation with Lewis acid) and the sub-
sequent hydride reduction step [45]. This may lead to a
significant carbonyl reduction, consumption of both the
carbonyl compound and the reducing agent, and low yields
of the reductive amination products. Hence, NaBH3CN, the
most widely used hydride reagent for the reductive
4 Conclusion
In summary, we have developed a simple direct reductive
amination procedure for aldehydes, which employs trieth-
ylsilane as an effective reductant and Ga(OTf)3 as a cata-
lyst. These reducing systems are superior due to their broad
application in the synthesis of diverse higher secondary
amines with a high degree of functional group tolerance.
123

Gallium(III) Triflate Catalyzed Direct Reductive Amination of Aldehydes
117
19. Robichaud A, Ajjou AN (2006) Tetrahedron Lett 47:3633
20. Apodaca R, Xiao W (2001) Org Lett 3:1745
21. Hansen MC, Buchwald SL (2000) Org Lett 2:713
22. Patel JP, Li A, Don H, Korlipara VL, Mulvihill MJ (2009) Tet-
rahedron Lett 50:5975
23. Prakash GKS, Yan P, Torok B, Bucsi I, Tanaka M, Olah GA
(2003) Catal Lett 85:1
24. Prakash GKS, Yan P, Torok B, Olah GA (2003) Synlett 4:527
25. Prakash GKS, Yan P, Torok B, Olah GA (2003) Catal Lett 87:109
26. Yan P, Batamack P, Prakash GKS, Olah GA (2005) Catal Lett
101:141
The simple experimental protocol, involving easy product
isolation procedures without chromatographic separations,
excellent yields combined with effective regeneration of
the catalyst, is expected to contribute significantly towards
efficient green synthesis of highly functionalized amino
compounds.
Acknowledgment Support by the Loker Hydrocarbon Research
Institute is gratefully acknowledged.
27. Yan P, Batamack P, Prakash GKS, Olah GA (2005) Catal Lett
103:165
28. Deng X-M, Tang X-L, Sun Y (2005) J Org Chem 70:6537
29. Lacey JR, Anzalone PW, Duncan CM, Hackert MJ, Mohan RS
(2005) Tetrahedron Lett 46:8507
30. Li R-V, Nguyen C-J (2005) J Am Chem Soc (127):17184
31. Prakash GKS, Mathew T, Panja C, Alconcel S, Vaghoo H, Do C,
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32. Prakash GKS, Mathew T, Panja C, Vaghoo H, Venkataraman K,
Olah GA (2007) Org Lett 9:179
33. Heydari A, Tavakol H, Aslanzadeh S, Azarnia J, Ahmadi N
(2005) Synthesis 4:627
34. Zhang Z, Gao J, Xia JJ, Wang GW (2005) Org Biomol Chem
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