Chemoselective CaO-mediated acylation of alcohols and amines in 2-methyltetrahydrofuran

Article abstract of DOI:10.1002/cssc.201200922

Calcium oxide is proposed as an innocuous acid scavenger for the chemoselective synthesis of amide- and ester-type compounds. Although these molecules have wide spread applications in organic and pharmaceutical chemistry, and a large number of routes have been designed for their synthesis, the development of more efficient and environmentally friendly acylation strategies remains an ongoing challenge. The use of CaO allows for the stoichiometric acylation of primary alcohols in the presence of phenols or tertiary alcohols; amines can also be subjected to acylation reactions in the presence of hydroxyl groups. Chirality is obtained through acylation if the starting material is an optically pure alcohol or if a chiral acylating agent is used. Furthermore, the use of 2-methyltetrahydrofuran (2-MeTHF), a more ecofriendly solvent, leads to maximized yields. This protocol is successfully applied to the synthesis of an interesting N-aryloxazolidin-2-one intermediate for the preparation of linezolid-type compounds.

Full text of DOI:10.1002/cssc.201200922

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DOI: 10.1002/cssc.201200922
Chemoselective CaO-Mediated Acylation of Alcohols and
Amines in 2-Methyltetrahydrofuran
Vittorio Pace,*[a] Pilar Hoyos,[b] Andrꢀs R. Alcꢁntara,[b] and Wolfgang Holzer[a]
Calcium oxide is proposed as an innocuous acid scavenger for
the chemoselective synthesis of amide- and ester-type com-
pounds. Although these molecules have wide spread applica-
tions in organic and pharmaceutical chemistry, and a large
number of routes have been designed for their synthesis, the
development of more efficient and environmentally friendly
acylation strategies remains an ongoing challenge. The use of
CaO allows for the stoichiometric acylation of primary alcohols
in the presence of phenols or tertiary alcohols; amines can
also be subjected to acylation reactions in the presence of hy-
droxyl groups. Chirality is obtained through acylation if the
starting material is an optically pure alcohol or if a chiral acylat-
ing agent is used. Furthermore, the use of 2-methyltetrahydro-
furan (2-MeTHF), a more ecofriendly solvent, leads to maxi-
mized yields. This protocol is successfully applied to the syn-
thesis of an interesting N-aryloxazolidin-2-one intermediate for
the preparation of linezolid-type compounds.
Introduction
Ester and amide moieties are ubiquitous in nature;[1,2] they are
often encountered both in physiologically relevant structures
(e.g., acetylcholine, peptides) and in biologically active mole-
cules (e.g., acetylsalicylic acid, paracetamol, and dextro-
moramide).[3] Because of this widespread importance, acyla-
tion-type reactions continue to be an exciting research field in
view of designing efficient and environmentally acceptable
synthetic processes that can be easily scaled-up for industrial
purposes.[4,5] Generally, acylations are performed by treating an
appropriate carboxylic acid derivative (acyl halide, anhydride,
ester) with alcohols or amines in the presence of a suitable
activator (Scheme 1).[1]
Scheme 1. Acylation procedure for alcohols and amines.
The procedure involving acylation through acid halides
offers the clear advantage of atom efficiency compared to the
use of other acylating agents (e.g., anhydrides). As observed
by Strazzolini et al., the need for such an activator in esterifica-
tion reactions is explained by competition between alkyl
halide and ester formation.[6] Esterification reactions in the ab-
sence of activators are not fully understood and have limited
substrate scope (aromatic acid chlorides or compounds with
a-acidic hydrogens are not used); this may be attributed to
a considerable excess of the required acyl donor.[7] These limi-
tations stimulated efforts to develop activators for improving
the reactivity, among the most common are Brønsted acids
(e.g., HCl,[8] p-TsOH[9]) and Lewis acids (e.g., CeCl3,[10] MgBr2,[11]
ZrCl4,[12] Sc(OTf)3,[13] LiClO4,[14] Mg(ClO4)2[15]). These acids cannot
be used in the presence of acid-sensitive substrates, thus re-
ducing their real usefulness. In addition, nucleophilic entities
such as DMAP (4-N,N-dimethylpyridine)[17,18] and P(nBu)3 are
often used to accelerate the acyl transfer.[19] However, their use
is in contrast with Anastas’s Principles of Green Chemistry[20]
because of the exceptionally high toxicity of the former (lethal
dosage, LD50 =56 mgkgÀ1)[21] and the high flammability of the
latter (flash point=378C).[22] Alternatively, the hydrohalic acid
released upon formation of the ester (or amide) bond is fre-
quently trapped with aminic bases (e.g., pyridine or triethyl-
amine).[1] However, this procedure suffers from several chemo-
selectivity issues, such as the possible intramolecular migration
of the acyl group if different hydroxyl functions are simulta-
neously present[23] or the discrimination between those alco-
holic functions.[24] Interestingly, the use of an amine to facilitate
an acylation is severely limited when the acid halide bears
acidic a-hydrogens because of the concomitant formation of
the corresponding ketene (Scheme 1, compound C).[25] Another
important general limitation of this type of esterification reac-
[a] Dr. V. Pace, Prof. Dr. W. Holzer
Department of Drug and Natural Product Synthesis
Faculty of Life Sciences, University of Vienna
Althanstrasse 14, 1090 Vienna (Austria)
Fax: (+43)1-4277-9556
E-mail: vpace@farm.ucm.es
[b] Dr. P. Hoyos, Prof. Dr. A. R. Alcꢀntara
Organic and Pharmaceutical Chemistry Department
Faculty of Pharmacy, Complutense University of Madrid
Plaza Ramꢁn y Cajal s/n, 28049 Madrid (Spain)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200922.
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tion is rapid catalyst deactivation by deposition of char or coke
on its surface, which is undesired.[16]
Table 1. Optimization of the acylation of methanol with 2.[a]
In this sense, the development of a simple acylation proto-
col for both alcohols and amines in the absence of any activa-
tor (base, acid, or catalyst) is highly desirable. For this purpose,
performing reactions on the surface of solids instead of using
solutions or gas phases could be a solution.[26–27] Among the
inherent advantages of this strategy, we must remark on the
easy isolation of products, the possibility of using mild reaction
conditions (compatible with other labile groups, which could
be present in the substrate), the high chemo- and regioselec-
tivity, and the high yields obtained with almost complete sup-
pression of by-products. Hence, the use of metal oxides has
been reported; in fact, Yadav and co-workers evaluated the
Entry
Acid
scavenger
Eq.
Solvent
T
[8C]
Reaction
time [h]
Yield of
2 [%][b]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
CaO
CaO
CaO
–
1.1
2.0
0.5
–
Et2O
Et2O
Et2O
Et2O
0
0
0
12
10
16
24
8
61
70
52
39
0
CaO
CaO
CaO
CaO
CaO
CaO
CaO
MgO
K2CO3
KF-Celite
NEt3
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
Et2O
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
82
2-MeTHF
CH2Cl2
TBME
CH3CN
toluene
THF
2-MeTHF
2-MeTHF
2-MeTHF
2-MeTHF
6
97(91)[c,d]
65
10
12
8
8
8
10
8
8
77
81
72
85
74
57
62
73
[28]
use of basic KF–Al2O3 and Al2O3 for promoting these reac-
tions, mainly in the presence of nonphenolic systems.[29]
Herein, we present an effective approach to promote these
acylation reactions, inspired by the precedents mentioned
above and by our previous studies on the preparation of a-di-
azocarbonyl compounds from acyl halides and diazomethane,
in the presence of CaO as an excellent hydrohalic acid scav-
enger, which allows the use of only a stoichiometric amount of
the dangerous CH2N2.[30]
8
[a] Reaction conditions: acyl halide (1.05 eq.) and alcohol (1.00 eq.).
[b] 1H NMR calculated yields. [c] Isolated yield. [d] 88% isolated yield on
the 0.1 mol scale.
cant improvement when switching from Et2O to 2-MeTHF;
maximizing the reaction yield to 97% (entry 6). Other solvents
such as dichloromethane, tert-butylmethyl ether, acetonitrile,
toluene, or THF, did not match the performance of 2-MeTHF
(entries 7–11). Preliminary studies on analogous O-acylation re-
actions in the presence of catalytic CaO resulted in large
amounts of carboxylic acid (originating from the hydrolysis of
the acyl halide) under solvent-free conditions; this suggested
the demand for a solvent in this type of reaction.[42]
Results and Discussion
Commercially available 2-phthalimidoacetyl chloride (1,
Scheme 2) bearing two acidic a-hydrogens (susceptible to ab-
straction and thus, ketene formation) was selected as the
model substrate for the acylation of methanol.
Using the optimized conditions (CaO, rt, and 2-MeTHF), an
ecofriendly process was designed; the innocuous CaO was
withdrawn by using suction and the excess acyl halide
(0.05 eq.) was removed through alkaline (NaHCO3) washing.
Interestingly, because of the low mutagenicity of 2-MeTHF[43]
and its immiscibility with water,[44] the separation of the organ-
ic phase did not require any addition of noxious or environ-
mental contaminating organic solvents (e.g., ethyl acetate, di-
ethyl ether, hydrocarbons, or halogenated solvents), thus
rendering the experimental procedure extremely simple.
Importantly, the optimized reaction did not require chromato-
graphic purification of the product; a big advancement
towards the concept of ideal synthesis.
Scheme 2. Model acylation of methanol with acid chloride 1.
As shown in Table 1, formation of ester 2 is clearly improved
by the presence of an acid scavenger. Interestingly, CaO was
the most effective compared to other alkaline-earth metal
oxide such as MgO (entry 12) or to different bases such as
K2CO3, KF-Celite, or triethylamine (entries 13–15). The presence
of 2.0 eq. CaO provided the best result, and the optimal reac-
tion was achieved at room temperature (rt, entries 5–6). The
solvent effect was prominent; initial optimization experiments
were performed in diethyl ether (analogous to our previous
findings regarding CaO-mediated a-diazocarbonyl prepara-
tion).[30] Evidently, an organic process performed in a toxic sol-
vent, Et2O, does not meet the strict safety requirements of
pharmaceutical industry applications and the search for alter-
native, non-contaminating solvents is of great importance.[31]
Therefore, in recent years we[32–35] and others[36–40] have demon-
strated the beneficial effect of replacing ether-type solvents
with biomass-derived (corncobs or bagasse) solvents such as
2-methyltetrahydrofuran (2-MeTHF).[41] We observed a signifi-
We next evaluated the substrate scope, as shown in Table 2.
A series of different alcohols were efficiently acylated with dif-
ferent acyl halides. Aliphatic primary alcohols reacted smoothly,
affording the desired esters in excellent yields; the protocol
allows the incorporation of acid-sensitive functionalities such
as double or triple CÀC bonds, chiral epoxides, halogens, aro-
matic ethers, or esters. A wide range of acyl halides may be
used, including highly electrophilic acryloyl-type moieties (en-
tries 1–4, 7, 10) or sterically hindered pivaloyl chloride (entry 6).
p-Nitrophenol reacted slower compared to aliphatic alcohols,
in agreement with its lower nucleophilicity; however, this reac-
tion did reach completion after prolonged reaction times
(entry 9, 12 h). The different reactivities that were obtained as
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Table 2. CaO-mediated acylation of different alcohols in 2-MeTHF.[a]
Entry
Alcohol
Acylating reagent
acryloyl chloride
acryloyl chloride
acryloyl chloride
Reaction time [h]
Product
Yield [%][b]
1
2
3
6
6
6
3
4
5
96
90
86
4
5
6
(E)-crotonyl chloride
butyryl chloride
6
6
9
6
7
8
94
89
82
EtOH
pivaloyl chloride
7
8
MeOH
cinnamoyl chloride
acetyl chloride
6
6
9
95
91
10
9
p-NO2C6H4OH
bromoacetyl bromide
acryloyl chloride
12
9
11
12
95
89
10
11
12
benzoyl chloride
acetyl chloride
9
6
13
14
94
96
[a] Reaction conditions: acyl halide (1.05 equiv); alcohol (1.00 equiv), rt. [b] Isolated yield.
a consequence of the diverse nucleophilicity of aliphatic and
aromatic alcohols were advantageously exploited for the
chemoselective acylation of an aliphatic alcohol in the pres-
half of the molecule); therefore, the use of acyl halides would
be beneficial.
The treatment of different aliphatic amines cleanly afforded
the desired amides or carbamates in high yields both in the
case of primary and secondary aminic functionalities. Impor-
tantly, the chemoselective benzoylation of 3-aminopropan-1-ol
(entry 4) demonstrated functionalization exclusively on the ni-
trogen atom leaving the primary hydroxyl group unaltered, in
agreement with higher nucleophilicity of an amine compared
to an alcohol. Analogously, an aminophenol (entry 9) under-
went acylation exclusively at the aminic site. Aromatic amines
were also effective substrates for this CaO-mediated transfor-
mation, which resulted in the desired acylated products in ex-
cellent yields after only slightly increased reaction times, as
a consequence of their low nucleophilicity compared to ali-
phatic ones. Remarkably, steric congestion in the vicinal posi-
tion of nitrogen (entry 7) did not impede an efficient
carbamoylation.
1
ence of an aromatic amine. As indicated by the crude H NMR
spectrum of the acetylation of 4-(hydroxymethyl)-2-methoxy-
phenol (entry 8), no modification occurred at the phenolic OH-
group under the classical reaction conditions (AcCl, 1.0 eq.,
6 h, rt).
The procedure was also applied to secondary alcohols, af-
fording the desired esters in high yields after slightly longer re-
action times (entry 10, 9 h). Notably, tertiary alcohols were
completely inert under the reaction conditions. Again, this fact
was exploited in the chemoselective acylation of 2-methyl-2,4-
butanediol (entry 12), which led to exclusive acylation at the
primary alcohol site and afforded ester 14 in a 96% yield.
Based on these results, with the aim to develop a widely ap-
plicable acylation strategy, we focused on the acylation of
amines (Table 3). This reaction is usually performed with acid
anhydrides as the acylating agents; however, the use of anhy-
drides leads to a higher generation of waste (i.e., the other
To study the stereochemical influence of the process, we
subjected a series of optically active acyl chlorides to our stan-
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Table 3. CaO-mediated acylation of different amines in 2-MeTHF.[a]
Entry
1
Amine
BnNH2
Acylating reagent
acryloyl chloride
Reaction time [h]
3
Product
Yield [%][b]
15
16
95
91
2
3
methyl chloroformate
methyl chloroformate
6
5
17
97
4
5
benzoyl chloride
acetyl chloride
3
4
18
19
88
93[c]
6
6
20
82
7
8
methyl chloroformate
acetyl chloride
6
4
21
22
87
96
PhNH2
9
acetyl chloride
4
23
93
[a] Reaction conditions: acyl halide (1.05 eq.) and alcohol (1.00 eq.), rt. [b] Isolated yield. [c] 92% isolated yield on the 0.1 mol scale.
dard reaction conditions. Excellent enantioselectivities were
observed both in the case of esterification and amidation of
(R)-Mosher acid chloride 24 (Scheme 3, paths 1 and 2). Remark-
ably, the pharmaceutically relevant (S)-naproxene chloride 27,
which bears an a-acidic proton susceptible to base-mediated
abstraction, also afforded the desired methyl ester 28 with an
excellent enantiomeric ratio (97:3 e.r.; Scheme 3, path 3). This
result confirmed that CaO does not act as a base, it is exclu-
sively an acid trapping agent. Finally, no loss of optical purity
was observed in the acylation of enantiopure alcohol 29
(Scheme 3, path 4).
mal and involved the use of a large excess of methyl chlorofor-
mate, which acylated both the alcohol and the amine at the
same time. This reaction afforded the bis-protected aminoalco-
hol, which was hydrolyzed to the mono N-protected aminoal-
cohol by using aqueous methanolic potassium carbonate.[45]
Herein, an improvement in obtaining compound 32 is demon-
strated, which upon treatment with NaH in 2-MeTHF produced
the N-aryloxazolidin-2-one 33 in a very good isolated yield
(78%). N-aryloxazolidin-2-one represents the core of linezolid-
type drugs.[46,47] Interestingly, the choice of base (NaH) for the
second step was crucial to avoid possible concomitant epoxide
ring formation.[48]
The high chemoselectivity of this protocol can be exploited
for the preparation of interesting synthetic scaffolds, such as
ozazolidin-2-one 33 (Scheme 4). The selective acylation of the
secondary aminic function of chloroalcohol 31 with methyl
chloroformate allowed the preparation of carbamate 32 in an
excellent 92% yield. In a previous study we were able to
obtain carbamate 32 by using a procedure that was not opti-
Conclusions
We demonstrate a new, efficient, chemoselective, and sustain-
able protocol for the stoichiometric acylation of alcohols and
amines by use of CaO as an innocuous acid scavenger.
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tion was washed with aqueous saturated NaHCO3 (10 mL) and the
two resulting phases were separated; the organic phase (2-MeTHF)
was dried over anhydrous MgSO4, filtered, and dried in vacuo to
give 2 as a colorless solid (mp 1138C) in 96% yield (2.63 g), which
1
did not require further purification. H NMR (500 MHz, CDCl3, 258C,
TMS): d=7.79 (dd, J=5.5, 3.1 Hz, 2H, CH-Ar), 7.66 (dd, J=5.5,
3.1 Hz, 2H, CH-Ar), 4.36 (s, 2H, CH2), 3.68 ppm (s, 3H, CH3);
13C NMR (126 MHz, CDCl3, 258C): d=167.6 (CO), 167.3 (CO), 134.2
(CH), 131.9 (C), 123.5 (CH), 52.6 (CH2), 38.6 ppm (CH3); IR (KBr):
n˜ =1731, 1694, 1595, 1208, 909 cmÀ1; elemental analysis (%) for
C11H9NO4: calculated: C, 60.27; H, 4.14; N, 6.39, and measured:
C, 60.51; H, 4.01; N, 6.19.
Synthesis of amide 23
To a solution of 4-aminophenol (400 mg, 3.7 mmol, 1.0 eq.) in
2-MeTHF (9 mL), CaO (415 mg, 7.4 mmol, 2.0 eq., dried over night
over P2O5 in a desiccator) was added and the resulting suspension
was stirred at rt for 5 min. Afterwards, a solution of acetyl chloride
(306 mg, 3.9 mmol, 1.05 eq.) in 2-MeTHF (4 mL) was added drop-
wise over 5 min. The mixture was stirred for 4 h and subsequently
filtered in vacuo and washed with 2-MeTHF (12 mL). The solution
was washed with aqueous saturated NaHCO3 (8 mL) and the two
resulting phases were separated; the organic phase (2-MeTHF) was
dried over anhydrous MgSO4, filtered, and dried in vacuo to give
1
23 as a colorless solid (mp 1708C) in 93% yield (548 mg). H NMR
(300 MHz, [D6]DMSO, 258C, TMS): d=9.65 (s, 1H, NH), 9.13 (s, 1H,
OH), 7.34 (d, J=8.8 Hz, 2H, CH-Ar), 6.68 (d, J=8.8 Hz, 2H, CH-Ar),
1.98 ppm (s, 3H, CH3); 13C NMR (75 MHz, [D6]DMSO, 258C):
d=167.5 (CO), 153.1 (C), 131.0 (C), 120.8 (CH), 115.0 (CH), 23.7 ppm
(CH3); IR (KBr): n˜ =3430, 1662, 1591 cmÀ1; elemental analysis (%) for
C8H9NO2: calculated: C, 63.56; H,
Scheme 3. Acylation of alcohols and amines with enantiopure acyl chlorides
(paths 1–3) and the acylation of an enantiopure alcohol (path 4).
6.00; N, 9.27, and measured:
C, 63.70; H, 6.13; N, 9.12.
(See the Supporting Information
for full experimental procedures,
Scheme 4. Chemoselective protection of a secondary amine in the presence of a secondary alcohol during the
preparation of an N-aryloxazolidin-2-one.
compound characterization, and
1
copies of H and 13C NMR spectra).
No competitive ketene formation is observed for acylating
Acknowledgements
agents bearing acidic a-hydrogens. Furthermore, the proce-
dure allows the acylation of primary alcohols in a chemoselec-
Financial support from the University of Vienna and Austrian
Education Federal Ministry is gratefully acknowledged. Projects
CTQ2009-11801 from MICINN (Spanish Ministry of Science and In-
novation) and CTQ2012-32042 from MINECO (Spanish Ministry of
Economic Affairs and Competitiveness) are also acknowledged
for partial financial support.
tive manner in the presence of phenols or tertiary alcohols.
The proposed technique can also be used to acylate amines in
the presence of alcohol functionalities. The use of 2-MeTHF as
the solvent is crucial and it leads to maximized yields; it can
also prevent the need for chromatographic purifications.
Keywords: acylation · alcohols · amines · chemoselectivity ·
solvent effects
Experimental Section
Synthesis of ester 2
[1] J. Otera, J. Nishikido, Esterification: Methods, Reactions, and Applications,
Wiley-VCH, Weinheim, Germany, 2010.
[2] The Amide Linkage. Selected Structural Aspects in Chemistry, Biochemistry
and Materials Science (Eds.: A. Greenberg, C. M. Breneman, J. F. Lieb-
man), Wiley, New York, 2000.
[3] D. J. C. Constable, P. J. Dunn, J. D. Hayler, G. R. Humphrey, J. J. L. Leazer,
R. J. Linderman, K. Lorenz, J. Manley, B. A. Pearlman, A. Wells, A. Zaks,
T. Y. Zhang, Green Chem. 2007, 9, 411–420.
To a solution of methanol (400 mg, 12.5 mmol, 1.0 eq.) in 2-MeTHF
(25 mL), CaO (1.40 g, 25.0 mmol, 2.0 eq., dried over night over P2O5
in a desiccator) was added and the resulting suspension was
stirred at rt for 5 min. Afterwards, a solution of phthaloyl chloride
(2.93 g, 13.1 mmol, 1.05 eq.) in 2-MeTHF (10 mL) was added drop-
wise over 5 min. The mixture was stirred for 6 h and subsequently
filtered in vacuo before washing with 2-MeTHF (20 mL). The solu-
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[4] S. D. Roughley, A. M. Jordan, J. Med. Chem. 2011, 54, 3451–3479.
[5] V. R. Pattabiraman, J. W. Bode, Nature 2011, 480, 471–479.
[6] P. Strazzolini, A. G. Giumanini, G. Verardo, Tetrahedron 1994, 50, 217–
254.
[7] B. C. Ranu, S. S. Dey, A. Hajra, Green Chem. 2003, 5, 44–46.
[8] C. McNicholas, T. J. Simpson, N. J. Willett, Tetrahedron Lett. 1996, 37,
8053–8056.
[9] A. N. Dixit, S. K. Tandel, S. Rajappa, Tetrahedron Lett. 1994, 35, 6133–
6134.
[10] E. Torregiani, G. Seu, A. Minassi, G. Appendino, Tetrahedron Lett. 2005,
46, 2193–2196.
[11] E. Vedejs, O. Daugulis, J. Org. Chem. 1996, 61, 5702–5703.
[12] A. K. Chakraborti, R. Gulhane, Synlett 2004, 627–630.
[13] K. Ishihara, M. Kubota, H. Kurihara, H. Yamamoto, J. Am. Chem. Soc.
1995, 117, 4413–4414.
[14] B. P. Bandgar, V. T. Kamble, V. S. Sadavarte, L. S. Uppalla, Synlett 2002,
0735–0738.
[15] G. Bartoli, M. Bosco, R. Dalpozzo, E. Marcantoni, M. Massaccesi, S. Rinal-
di, L. Sambri, Synlett 2003, 39–42.
[16] A. Corma, H. Garcꢃa, Chem. Rev. 2003, 103, 4307–4366.
[17] K. Ishihara, H. Kurihara, H. Yamamoto, J. Org. Chem. 1993, 58, 3791–
3793.
[30] V. Pace, G. Verniest, J. V. Sinisterra, A. R. Alcꢁntara, N. De Kimpe, J. Org.
Chem. 2010, 75, 5760–5763.
[31] P. G. Jessop, Green Chem. 2011, 13, 1391–1398.
[32] V. Pace, P. Hoyos, M. Fernꢁndez, J. V. Sinisterra, A. R. Alcꢁntara, Green
Chem. 2010, 12, 1380–1382.
[33] V. Pace, A. R. Alcꢁntara, W. Holzer, Green Chem. 2011, 13, 1986–1989.
[34] V. Pace, L. Castoldi, P. Hoyos, J. V. Sinisterra, M. Pregnolato, J. M.
Sꢁnchez-Montero, Tetrahedron 2011, 67, 2670–2675.
[35] V. Pace, L. Castoldi, A. R. Alcꢁntara, W. Holzer, Green Chem. 2012, 14,
1859–1863.
[36] T. vom Stein, P. M. Grande, H. Kayser, F. Sibilla, W. Leitner, P. Domꢃnguez
de Marꢃa, Green Chem. 2011, 13, 1772–1777.
[37] S. Shanmuganathan, D. Natalia, A. Van den Wittenboer, C. Kohlmann, L.
Greiner, P. Domꢃnguez de Marꢃa, Green Chem. 2010, 12, 2240–2245.
[38] S. Shanmuganathan, L. Greiner, P. Domꢃnguez de Marꢃa, Tetrahedron
Lett. 2010, 51, 6670–6672.
[39] E. J. Garcia-Suꢁrez, A. M. Balu, M. Tristany, A. B. Garcia, K. Philippot, R.
Luque, Green Chem. 2012, 14, 1434–1439.
[40] U. Azzena, F. Kondrot, L. Pisano, M. Pittalis, Appl. Organomet. Chem.
2012, 26, 180–184.
[41] V. Pace, P. Hoyos, L. Castoldi, P. Domꢃnguez de Marꢃa, A. R. Alcꢁntara,
ChemSusChem 2012, 5, 1369–1379.
[18] S. Xu, I. Held, B. Kempf, H. Mayr, W. Steglich, H. Zipse, Chem. Eur. J.
2005, 11, 4751–4757.
[42] F. Tamaddon, M. A. Amrollahi, L. Sharafat, Tetrahedron Lett. 2005, 46,
7841–7844.
[19] E. Vedejs, S. T. Diver, J. Am. Chem. Soc. 1993, 115, 3358–3359.
[20] P. Anastas, N. Eghbali, Chem. Soc. Rev. 2010, 39, 301–312.
[21] M. Hosseini Sarvari, H. Sharghi, Tetrahedron 2005, 61, 10903–10907.
[22] S. A. Buckler, J. Am. Chem. Soc. 1962, 84, 3093–3097.
[23] P. Allevi, M. Anastasia, A. Fiecchi, A. M. Sanvito, A. Scala, Synthesis 1991,
438–440.
[24] M. Takahashi, T. Kuroda, T. Ogiku, H. Ohmizu, K. Kondo, T. Iwasaki, Tetra-
hedron Lett. 1991, 32, 6919–6922.
[25] A. E. Taggi, A. M. Hafez, H. Wack, B. Young, D. Ferraris, T. Lectka, J. Am.
Chem. Soc. 2002, 124, 6626–6635.
[43] V. Antonucci, J. Coleman, J. B. Ferry, N. Johnson, M. Mathe, J. P. Scott, J.
Xu, Org. Proc. Res. Dev. 2011, 15, 939–941.
[44] V. Pace, Aust. J. Chem. 2012, 65, 301–302.
[45] V. Pace, ꢄ. Cortꢀs-Cabrera, M. Fernꢁndez, J. V. Sinisterra, A. R. Alcꢁntara,
Synthesis 2010, 3545–3555.
[46] S. J. Brickner, M. R. Barbachyn, D. K. Hutchinson, P. R. Manninen, J. Med.
Chem. 2008, 51, 1981–1990.
[47] M. R. Barbachyn, C. W. Ford, Angew. Chem. 2003, 115, 2056–2070;
Angew. Chem. Int. Ed. 2003, 42, 2010–2023.
[48] V. Pace, P. Hoyos, J. V. Sinisterra, A. R. Alcꢁntara, W. Holzer, Synlett 2011,
1831–1834.
[26] A. McKillop, D. W. Young, Synthesis 1979, 401–422.
[27] G. W. Kabalka, R. M. Pagni, Tetrahedron 1997, 53, 7999–8065.
[28] V. K. Yadav, K. Ganesh Babu, M. Mittal, Tetrahedron 2001, 57, 7047–
7051.
Received: December 3, 2012
[29] V. K. Yadav, K. Ganesh Babu, J. Org. Chem. 2004, 69, 577–580.
Published online on March 26, 2013
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