Robust eco-friendly protocol for the preparation of γ-hydroxy- α,β-acetylenic esters by sequential one-pot elimination-addition of 2-bromoacrylates to aldehydes promoted by LTMP in 2-MeTHF

Article abstract of DOI:10.1039/c2gc35305a

An efficient and widely applicable preparation of γ-hydroxy-α, β-acetylenic esters is described by means of a one-pot dehydrobromination of a 2-bromoacrylate ester with LTMP followed by the electrophilic addition of the transient propiolate to different aldehydes in the eco-friendly solvent 2-MeTHF. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2gc35305a

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Green Chemistry
Cite this: Green Chem., 2012, 14, 1859
www.rsc.org/greenchem
COMMUNICATION
Robust eco-friendly protocol for the preparation of γ-hydroxy-α,β-acetylenic
esters by sequential one-pot elimination–addition of 2-bromoacrylates to
aldehydes promoted by LTMP in 2-MeTHF†
Vittorio Pace,*a,b Laura Castoldi,c Andrés R. Alcántarab and Wolfgang Holzera
Received 29th February 2012, Accepted 24th April 2012
DOI: 10.1039/c2gc35305a
An efficient and widely applicable preparation of γ-hydroxy-
α,β-acetylenic esters is described by means of a one-pot
dehydrobromination of a 2-bromoacrylate ester with LTMP
followed by the electrophilic addition of the transient propio-
late to different aldehydes in the eco-friendly solvent 2-
MeTHF.
The introduction of a functionalised fragment into a molecular
scaffold by means of a one-pot reaction represents an extremely
Scheme 1 Classical and improved strategy to obtain γ-hydroxy-
α,β-acetylenic esters.
versatile procedure, being in agreement with the 2nd Principle of
the Green Chemistry philosophy, which demands minimisation
of the synthetic operations within a chemical process.1 In such a
context, the preparation of γ-hydroxy-α,β-acetylenic esters rep-
resents an intriguing challenge because of the high versatility
that such structures possess in organic synthesis.2–4 In fact, they
contain three adjacent functional groups exhibiting different
reactivity, namely the alkoxycarbonyl group, the acetylenic unit
and the alcoholic moiety, all of them able to undergo different
functionalisations. For example, such complex propargylic alco-
hols are key intermediates for the preparation of 4-alkoxy-2(5H)-
furanones,5 4-amino-2(5H)-furanones,6 γ-acetoxy dienoates,7
and gem-difluoro-bisarylic derivatives.8 Because of their useful-
ness in the preparation of natural products such as NP25302,9
Bipinnatin J,10 Merrilactone A10 or Pregaliellalactone,11 their
synthesis has been recently studied, mainly focussing on the
development of enantioselective strategies. Thus, the most direct
approach to the title compounds is represented by the generation
of the alkynoic carbanion and subsequent trapping with the elec-
trophilic aldehyde (Scheme 1).12 However, this procedure suffers
two main drawbacks, namely the low nucleophilicity of the same
carbanions and the considerable electrophilicity of the conju-
gated triple bond.2 As a consequence, the correct choice of the
metal employed in the generation of the carbanion is crucial for
the successful outcome of the reaction. In this context, the
introduction of catalytically generated zinc acetylides by Carreira
and co-workers13,14 constituted a significant breakthrough for
the preparation of such compounds. Disappointingly, the zinca-
tion of an alkynoate often requires the presence of HMPA as a
Lewis base additive jointly with a Lewis acid (e.g. Zn(OTf)2) to
take place.15 In fact, attempts to perform the reaction by simply
refluxing diethylzinc in toluene and the propiolate may lead to
the decomposition of the same methyl propiolate.16 Unfortu-
nately, HMPA is well known to be a carcinogenic, antispermato-
genic and mutagenic compound as a consequence of the in vivo
biotransformation into formaldehyde.17–20 Thus, the use of
HMPA-free protocols has been recently developed.21–26 An
alternative approach to γ-hydroxy-α,β-acetylenic esters has been
discovered by Koide, who introduced silver acetylides for this
purpose: again, their low nucleophilicity requires a stoichio-
metric amount of expensive Cp2ZrCl2 and a catalytic amount of
AgOTf.27 Although the procedure is chemoselective, it depends
on the electrophilicity of the aldehyde and thus aliphatic ones
react with only moderate yields. At first glance, the key to
solving the problem may be represented by a reactivation of the
use of the nucleophilic lithium alkynoates reported by Midland
and co-workers in 1980.12 These authors simply generated the
alkynoate from the corresponding propiolate in the presence of
n-BuLi followed by quenching with the carbonyl compounds.
Although ideally simple, this strategy suffers from a series of
drawbacks. Firstly, a concomitant addition of RLi to the ester
carbonyl may take place and secondly, due to the high instability
of the generated lithium alkynoates, reactions must be conducted
at very low temperatures (−78 °C starting from ethyl propiolate
and −120 °C with methyl propiolate).2 As a consequence, this
protocol is not optimal for a possible scale-up and establishing
aDepartment of Drug and Natural Product Synthesis, University of
Vienna, Althanstrasse 14, 1090, Vienna, Austria. E-mail: vpace@
farm.ucm.es; Fax: +43-1-4277-9556; Tel: +43-1-4277-55623
bDepartment of Organic and Pharmaceutical Chemistry, Complutense
University of Madrid, Pza. Ramon y Cajal s/n, 28040, Madrid, Spain
cDepartment of Drug Sciences, University of Pavia, V.le Taramelli 12,
27100, Pavia, Italy
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2gc35305a
This journal is © The Royal Society of Chemistry 2012
Green Chem., 2012, 14, 1859–1863 | 1859

View Online
such prohibitive temperatures is highly expensive and energeti-
cally not efficient (6th Green Chemistry Principle).1
For this reason, we decided to carry out the reaction in the
biomass-derived solvent 2-MeTHF, and therefore obeying the
7th Principle of Green Chemistry.1 This solvent is being increas-
ingly used both in organic chemistry34–42 and biocatalysis43–45
and, although it is an ether-type solvent, it resembles toluene in
terms of physical properties.46 Effectively, we could observe
both a huge improvement of the yield of 2a, with a concomitant
and significant decrease of the side-product 3d (16%, entry 9).
Moreover, carrying out the reaction at higher temperature
(−40 °C) it was possible to considerably reduce the reaction time
down to 1.5 h, without any influence on the formation of the
side-product 3d (11%, entry 10). With the aim to eliminate,
whenever possible, the formation of the side-product, different
sterically hindered bases (alkali amides) were evaluated. Effec-
tively, the use of LDA afforded a clean reaction giving the
desired alcohol 2a in 69% yield without any detection of the car-
bonyl addition product 3a (entry 11), while when using
LHDMS, NaHDMS and KHDMS – although a complete sup-
pression of this undesired compound 3 was detected – a con-
siderable decrease in the formation of the target alcohol 2a was
observed, regardless of the temperature employed (entries
12–15). This observation suggests that the chemoselectivity of
the process arises from the appropriate steric hindrance of the
base used for the dehydrobromination. As a consequence, we
tested the reaction in the presence of the sterically very hindered
and non-nucleophilic base LTMP: effectively, it afforded the best
result giving an excellent 94% yield of the desired propargylic
alcohol 2a (entry 16). It should be emphasized that such a
highly sterically hindered base was prepared by deprotonation of
As part of our research aimed to design synthetic strategies
based on the robustness of the process employed,28–32 we envi-
saged the possibility to generate the lithium alkynoates under
milder conditions in such a way that the carbanions may react
instantaneously with the electrophiles, without needing to
prepare them prior to the quenching with the aldehyde: in this
sense, we argued that methyl 2-bromoacrylate (1a) would have
represented the optimal candidate for the generation of the pro-
piolate carbanion upon dehydrobromination. Such planned strat-
egy would avoid the aforementioned drawback concerning the
high instability of the propiolate nucleophile (Scheme 1).
Indeed, treatment of 1a with n-BuLi at −78 °C in the presence
of benzaldehyde gave only a 6% yield of the desired benzylic
alcohol 2a, jointly with the product 3a, generated by the addition
of n-BuLi to the aldehyde in 76% yield (entry 1, Table 1). Ana-
logous results were observed with the less hindered MeLi (entry
2), while the use of sterically demanding s-BuLi and t-BuLi
improved the yields of the desired product up to 18% and 26%,
respectively (entries 3–4), jointly with a significant reduction of
the addition of these organolithiums to benzaldehyde. The reac-
tion seems to be dependent on the solvent polarity: in THF the
formation of the side-product was higher, while in more apolar
solvents such as diethyl ether (entry 6), toluene (entry 7) and
pentane (entry 8), a significant increase of the yield of the
desired alcohol was observed. Significantly, the use of the so-
called Trapp-mixture33 (THF–Et2O–pentane 4 : 1 : 1, entry 5)
afforded an analogous result to that observed when using THF.
Table 1 Optimization of the reaction conditions considering temperature, base and solventa
Entry
Base
Solvent
Temp. [°C]
Reaction time [h]
Yield of 2ab [%]
Yield of 3b [%]
1
2
3
4
5
6
7
8
n-BuLi
MeLi
THF
THF
THF
THF
Trapp
Et2O
Toluene
Pentane
MeTHF
MeTHF
MeTHF
MeTHF
MeTHF
MeTHF
MeTHF
MeTHF
MeTHF
THF
−78
−78
−78
−78
−78
−78
−78
−78
−78
−40
−40
−40
−40
−40
−78
−40
−20
−40
−78
−40
−40
0
3
3
4
4
3
2
3
4
6
4
18
26
8
76
79
64
58
72
55
49
51
16
11
—
—
—
—
—
—
—
—
—
—
—
—
s-BuLi
t-BuLi
t-BuLi
t-BuLi
t-BuLi
t-BuLi
t-BuLi
t-BuLi
LDA
LHDMS
NaHDMS
KHDMS
LHDMS
LTMP
LTMP
LTMP
LTMP
LTMP
LTMP
LiOBut
33
37
35
54
57
69
48
46
43
45
94
63
67
55
62
58
—
9
3
10
11
12
13
14
15
16
17
18
19
20
21
22
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1
1.5
1.5
1.5
3
THF
Et2O
Toluene
MeTHF
12
a Conditions: methyl-2-bromoacrylate (1 equiv.); base (2 equiv.). b Isolated yields.
1860 | Green Chem., 2012, 14, 1859–1863
This journal is © The Royal Society of Chemistry 2012

View Online
Table 2 Synthesis of γ-hydroxy-α,β-acetylenic esters starting from
2,2,6,6-tetramethylpiperidine with MeLi in the same 2-MeTHF
prior to its transfer via cannula to the reaction flask: due to the
high stability of organolithiums in 2-MeTHF,34 the 1.2 M sol-
ution of LTMP could be used without any detrimental effect.
When the reaction was performed at −20 °C a significant
decrease of the yield was observed (entry 17). This temperature
effect may be related to the well known lability of the intermedi-
ate propiolate nucleophile.2 The use of 2-MeTHF was crucial for
the success of the procedure, leading to best results compared to
both toluene (entry 21) and other ether-type solvents (entries
18–20). Under the optimized reaction conditions, THF afforded
the desired alcohol 2a in only 67% yield, and moreover the reac-
tion crude was contaminated by the presence of impurities prob-
ably formed by the lithiation processes on THF. In fact, as
previously reported, among the several advantages that 2-
MeTHF shows compared to THF, there is the possibility to
perform organometallic reactions at higher temperatures in the
former, without observing decomposition of the solvent, as a
consequence of its improved stability towards organolithiums.34
It is worth mentioning the straight relationship existing among
base employed, solvent and temperature: in fact, as reported very
recently by Eisch and co-workers47 the treatment of benzal-
dehyde with LTMP in hexane at 0 °C affords benzyl benzoate as
a consequence of carbene formation (generated by the C–H
lithiation of the formyl group), followed by the addition of this
carbene to a second molecule of benzaldehyde. In contrast, by
using our conditions (2-MeTHF at −40 °C) no lithiation of ben-
zaldehyde was observed whatsoever, and thus the procedure
shows an excellent chemoselective profile by directing the action
of LTMP exclusively on the 2-bromoacrylic ester. Furthermore,
no other side-reactions (metal–halogen exchange, addition to the
electrophilic alkene) were observed. This latter point was funda-
mental for the development of our synthetic strategy: in fact, as
observed by Marino and Linderman48 such substrates may
undergo lithiation–Cu-transmetallation reactions, or may afford
cyclopropanes as a consequence of the addition of the organo-
metallic (Li or Mg) to 2 equiv. of 2-haloacrylic ester.49 Further-
more, the use of non-organometallic bases such as lithium tert-
butoxide did not afford any product even at 0 °C (entry 22).
Thus, these preliminary results of our investigations confirm
the effectiveness of replacing normally used ether-type solvents
like THF with its greener substitute 2-MeTHF. In particular, we
were delighted to confirm its excellent stability in a high basic
environment at temperatures that were not prohibitive (<−78 °C)
for industrial purposes. Remarkably, it is now recognized as a
non-genotoxic and non-mutagenic solvent,50 so that it is ideal
for scale-up processes.51 The use of 2-MeTHF enormously
improves the work-up of the reactions; in fact, its limited solubi-
lity in water allows the recovery of the reaction product without
adding any other organic solvent. In the procedure described
herein, we used 2-MeTHF distilled under simple atmospheric
conditions, thus avoiding the use of hazardous methodologies
based on ketyl formation (Na or K, benzophenone).
different alkyl 2-haloacrylic estersa
Entry
X
R
Reaction time [h]
Productb [%]
1
2
3
Br
Br
Br
Cl
Cl
Et (1b)
tBu (1c)
Bn (1d)
Me (1e)
Me (1e)
1.5
2.5
1.5
6
2b (93)
2c (91)
2d (94)
2a (49)
2a (54)
4
5c
6
a Conditions: alkyl-2-haloacrylate (1 equiv.); LTMP (2 equiv.). b Isolated
yields. c Reaction run at −20 °C.
In fact, our system showed significant superiority compared to
previously described protocols: in fact, Villeneuve and Tam
could not obtain more than 45% using LiHMDS in the deproto-
nation of ethyl propiolate at −70 °C.52 Remarkably, the dehydro-
halogenation–addition reaction dramatically decreased when
using the corresponding methyl 2-chloroacrylate because of the
lower tendency of a chloroalkene to undergo an elimination reac-
tion, even running the reaction at a higher temperature (entries
4–5).
Subsequently, a series of different aldehydes were used to
check the scope of the reaction on the aldehyde side. As shown
in Table 3, aromatic aldehydes bearing electron withdrawing
groups gave the best yields in the shortest reaction times (entries
1–4, 7–8). In fact, our procedure led to better yields compared to
previous reported methods; for example, the use of silver acety-
lide afforded compound 4a in only 61% yield,53 while our proto-
col gave 93% after only 1 h.
On the other hand, the presence of an electron donating group
(entry 5) increased the reaction times up to 6 h, but no loss of
efficiency of the protocol was observed even in the presence of a
sterically demanding aldehyde (entry 6). Again, the method-
ology confirmed its chemoselectivity also in the presence of a
bis-electrophilic substrate, providing compound 4b in 94% yield,
without any detection of the product derived by the attack of the
transient propiolate on the carboxylic ester (entry 2). The system
performed very well with heteroaromatic aldehydes, thus
expanding the substrate scope (entries 9–10). Moreover, the pro-
tocol can also be used with a non-enolizable aliphatic aldehyde,
although in this case a longer reaction time was required (entry
11).
To conclude our study on the applicability of this strategy, we
turned our attention to an α,β-unsaturated aldehyde like cinna-
maldehyde: the first attempt under our classical conditions
afforded 4l in only 67% yield after 12 h. Pleasingly, as shown in
entry 12, we could improve the yield up to 95% simply by
adding LiBr (1.50 equiv.) to the reaction mixture. As previously
reported by our group,54 this task strongly enhances the attack of
an organolithium on the carbonylic carbon of this type of
aldehyde.
With the optimized conditions in hand, we turned our atten-
tion to different 2-bromoacrylate esters to evaluate the scope of
the reaction. As shown in Table 2, the protocol is widely appli-
cable to such esters giving excellent yields in all cases, including
those substrates bearing either a sterically hindered tert-butyl
ester or a benzyl ester (entries 2–3).
A possible mechanistic pathway that accounts for the observed
reaction is shown in Scheme 2. Thus, the first equivalent of
This journal is © The Royal Society of Chemistry 2012
Green Chem., 2012, 14, 1859–1863 | 1861

View Online
Table 3 Scope of the reaction with different aldehydesa
of the base, leading to the transient lithium acetylide (c) that
attacks the aldehyde, furnishing the desired acetylenic ester d.
Conclusions
In summary, we have developed a straightforward route to
γ-hydroxy-α,β-acetylenic esters, through a one-pot dehydrobro-
mination–addition of a 2-haloacrylic ester with LTMP in the eco-
friendly solvent 2-MeTHF. The protocol is based on the gener-
ation of the propiolate nucleophile by means of a dehydrohalo-
genation performed in the presence of the corresponding
aldehyde: in fact, this strategy allows one to overcome the well-
known drawbacks associated with the preparation of lithium
acetylides directly starting from the parent propiolates. The use
of the biomass-derived solvent 2-MeTHF was shown to be
optimal for the development of the protocol, thus highlighting its
excellent properties as a solvent medium to carry out organome-
tallic reactions.
Entry Aldehyde
1
Reaction time [h] Product Yieldb [%]
1
4a
4b
4c
4d
4e
4f
96
94
87
85
88
84
2
3
4
5
6
1
1.5
1
6
Notes and references
6
1 P. Anastas and N. Eghbali, Chem. Soc. Rev., 2010, 39, 301–312.
2 D. Tejedor, S. López-Tosco, F. Cruz-Acosta, G. Méndez-Abt and
F. García-Tellado, Angew. Chem., Int. Ed., 2009, 48, 2090–2098.
3 B. M. Trost and A. H. Weiss, Adv. Synth. Catal., 2009, 351, 963–983.
4 G. Gao and L. Pu, Sci. China: Chem., 2010, 53, 21–35.
5 R. S. Ramón, C. Pottier, A. Gómez-Suárez and S. P. Nolan, Adv. Synth.
Catal., 2011, 353, 1575–1583.
7
8
2
2
4g
4h
86
91
6 L.-H. Zhou, X.-Q. Yu and L. Pu, J. Org. Chem., 2009, 74, 2013–2017.
7 Y. Yue, X.-Q. Yu and L. Pu, Chem.–Eur. J., 2009, 15, 5104–5107.
8 A. Khalaf, D. Grée, H. Abdallah, N. Jaber, A. Hachem and R. Grée, Tet-
rahedron, 2011, 67, 3881–3886.
9
4
4i
93
9 K. Stevens, A. J. Tyrrell, S. Skerratt and J. Robertson, Org. Lett., 2011,
13, 5964–5967.
10 P. A. Roethle and D. Trauner, Org. Lett., 2005, 8, 345–347.
11 M. Johansson, B. Köpcke, H. Anke and O. Sterner, Tetrahedron, 2002,
58, 2523–2528.
12 M. M. Midland, A. Tramontano and J. R. Cable, J. Org. Chem., 1980,
45, 28–29.
13 N. K. Anand and E. M. Carreira, J. Am. Chem. Soc., 2001, 123, 9687–
9688.
14 D. E. Frantz, R. Fässler and E. M. Carreira, J. Am. Chem. Soc., 2000,
122, 1806–1807.
10
11
12
3
8
3
4j
4k
4l
82
80
95c
15 G. Gao, R.-G. Xie and L. Pu, Proc. Natl. Acad. Sci. U. S. A., 2004, 101,
5417–5420.
16 G. Gao, Q. Wang, X.-Q. Yu, R.-G. Xie and L. Pu, Angew. Chem., Int.
Ed., 2006, 45, 122–125.
17 A. E. Harman, J. M. Voigt, S. R. Frame and M. S. Bogdanffy, Mutat.
Res., Fundam. Mol. Mech. Mutagen., 1997, 380, 155–165.
18 D. A. Keller, C. E. Marshall and K. P. Lee, Fundam. Appl. Toxicol.,
1997, 40, 15–29.
a Conditions: methyl-2-bromoacrylate (1 equiv.); LTMP (2 equiv.).
b Isolated yields. c Reaction carried out in the presence of 1.5 equiv. of
LiBr.
19 J. R. Thornton-Manning, K. J. Nikula, J. A. Hotchkiss, K. J. Avila, K.
D. Rohrbacher, X. Ding and A. R. Dahl, Toxicol. Appl. Pharmacol.,
1997, 142, 22–30.
20 J. Hernando, L. Álvarez, J. A. Ferreiro, I. Sancho, M. A. Comendador
and L. M. a. Sierra, Mutat. Res., Fundam. Mol. Mech. Mutagen., 2004,
545, 59–72.
21 B. M. Trost, A. H. Weiss and A. Jacobi von Wangelin, J. Am. Chem.
Soc., 2005, 128, 8–9.
22 B. M. Trost and A. H. Weiss, Org. Lett., 2006, 8, 4461–4464.
23 L. Lin, X. Jiang, W. Liu, L. Qiu, Z. Xu, J. Xu, A. S. C. Chan and
R. Wang, Org. Lett., 2007, 9, 2329–2332.
24 T. Xu, C. Liang, Y. Cai, J. Li, Y.-M. Li and X.-P. Hui, Tetrahedron: Asym-
metry, 2009, 20, 2733–2736.
Scheme 2 Proposed mechanistic pathway.
25 X. Chen, W. Chen, L. Wang, X.-Q. Yu, D.-S. Huang and L. Pu, Tetrahe-
dron, 2010, 66, 1990–1993.
26 N. Kojima, S. Nishijima, K. Tsuge and T. Tanaka, Org. Biomol. Chem.,
2011, 9, 4425–4428.
LTMP would promote the dehydrobromination on the 2-bromo-
acrylic ester (a) affording the corresponding propiolate ester (b),
which is instantaneously deprotonated by the second equivalent
27 S. P. Shahi and K. Koide, Angew. Chem., Int. Ed., 2004, 43, 2525–2527.
1862 | Green Chem., 2012, 14, 1859–1863
This journal is © The Royal Society of Chemistry 2012

View Online
28 V. Pace, F. Martínez, M. Fernández, J. V. Sinisterra and A. R. Alcántara,
Org. Lett., 2007, 9, 2661–2664.
41 S. Shanmuganathan, L. Greiner and P. Domínguez de María, Tetrahedron
Lett., 2010, 51, 6670–6672.
29 V. Pace, F. Martínez, M. Fernández, J. V. Sinisterra and A. R. Alcántara,
Adv. Synth. Catal., 2009, 351, 3199–3206.
30 V. Pace, Á. Cortés-Cabrera, M. Fernández, J. V. Sinisterra and A.
R. Alcántara, Synthesis, 2010, 3545–3555.
42 V. Pace, Aust. J. Chem., 2012, 65, 301–302.
43 S. Shanmuganathan, D. Natalia, A. van den Wittenboer, C. Kohlmann,
L. Greiner and P. Dominguez de Maria, Green Chem., 2010, 12, 2240–
2245.
31 V. Pace, G. Verniest, J.-V. Sinisterra, A. R. Alcántara and N. De Kimpe,
J. Org. Chem., 2010, 75, 5760–5763.
44 Y. Simeó, J. V. Sinisterra and A. R. Alcántara, Green Chem., 2009, 11,
855–862.
32 V. Pace, F. Martínez, M. Fernández, C. I. Nova, J. V. Sinisterra and A.
R. Alcántara, Tetrahedron Lett., 2009, 50, 3050–3053.
33 G. Köbrich and H. Trapp, Chem. Ber., 1966, 99, 670–679.
34 V. Pace, P. Hoyos, L. Castoldi, P. Dominguez de María and A.
R. Alcántara, ChemSusChem, 2012, DOI: 10.1002/cssc.201100780.
35 V. Pace, A. R. Alcántara and W. Holzer, Green Chem., 2011, 13, 1986–
1989.
36 V. Pace, P. Hoyos, M. Fernández, J. V. Sinisterra and A. R. Alcántara,
Green Chem., 2010, 12, 1380–1382.
37 T. Robert, J. Velder and H. G. Schmalz, Angew. Chem., Int. Ed., 2008,
47, 7718–7721.
45 P. Hoyos, M. A. Quezada, J. V. Sinisterra and A. R. Alcántara, J. Mol.
Catal. B: Enzym., 2011, 72, 20–24.
46 D. F. Aycock, Org. Process Res. Dev., 2007, 11, 156–159.
47 J. J. Eisch, J. N. Gitua and K. Yu, Eur. J. Org. Chem., 2011, 3523–
3530.
48 J. P. Marino and R. J. Linderman, J. Org. Chem., 1981, 46, 3696–3702.
49 C. Chen, C. Xi, Y. Jiang and X. Hong, Tetrahedron Lett., 2004, 45,
6067–6069.
50 V. Antonucci, J. Coleman, J. B. Ferry, N. Johnson, M. Mathe, J. P. Scott
and J. Xu, Org. Process Res. Dev., 2011, 15, 939–941.
51 R. K. Henderson, C. Jimenez-Gonzalez, D. J. C. Constable, S. R. Alston,
G. G. A. Inglis, G. Fisher, J. Sherwood, S. P. Binks and A. D. Curzons,
Green Chem., 2011, 13, 854–862.
38 N. Slavov, J. Cvengroš, J.-M. Neudörfl and H.-G. Schmalz, Angew.
Chem., Int. Ed., 2010, 49, 7588–7591.
39 N. Kausch-Busies, B. Kater, J. M. Neudörfl, A. Prokop and H.-
G. Schmalz, Eur. J. Org. Chem., 2011, 1133–1139.
40 Q. Naeemi, T. Robert, D. P. Kranz, J. Velder and H.-G. Schmalz, Tetrahe-
dron: Asymmetry, 2011, 22, 887–892.
52 K. Villeneuve and W. Tam, Organometallics, 2007, 26, 6082–6090.
53 J. P. Sonye and K. Koide, J. Org. Chem., 2006, 71, 6254–6257.
54 V. Pace, L. Castoldi, P. Hoyos, J. V. Sinisterra, M. Pregnolato and J.
M. Sánchez-Montero, Tetrahedron, 2011, 67, 2670–2675.
This journal is © The Royal Society of Chemistry 2012
Green Chem., 2012, 14, 1859–1863 | 1863