Direct room-temperature lactonisation of alcohols and ethers onto amides: An "amide strategy" for synthesis

Article abstract of DOI:10.1002/chem.201203906

Last-minute deal: A direct lactonisation of ethers and alcohols onto amides that proceeds at room temperature under mild conditions is reported (see scheme). This allows the effective saving of up to two unproductive, sequential deprotection operations in synthetic sequences. Mechanistic studies are described, and a new "amide strategy" that exploits the dual robustness/late-stage selective activation properties of this functional group is outlined. Copyright

Full text of DOI:10.1002/chem.201203906

DOI: 10.1002/chem.201203906
Direct Room-Temperature Lactonisation of Alcohols and Ethers onto
Amides: An “Amide Strategy” for Synthesis
Viviana Valerio, Desislava Petkova, Claire Madelaine, and Nuno Maulide*[a]
The amide functional group is usually considered as the
most robust and resistant of the carboxylic acid derivatives.
As taught in standard organic chemistry undergraduate text-
books, the reduced electronegativity of the nitrogen atom in
an amide (as compared to oxygen in esters or acids) results
in more effective orbital overlap with the adjacent carbonyl,
thus markedly reducing its elec-
prevalent use in the late stages of total synthesis efforts. In-
terestingly, in these strategies lactonisation is subordinate to
two, often sequential, unproductive deprotection steps[5] for
both the alcohol and the carboxylic acid moieties prior to
the actual lactonisation event, as exemplified by the selected
examples shown in Scheme 1a,b.[6]
trophilic character.[1] Therefore,
it is not surprising that amide
hydrolysis is a difficult transfor-
mation, which typically pro-
ceeds with notoriously long re-
action times (t1/2 of >102 years
at pH 7 and 258C in aqueous
solution).[2] Many elegant strat-
egies have been developed to
prepare so-called “twisted”
amides, for which the afore-
mentioned resonance stabilisa-
tion is greatly reduced by care-
fully designed strain elements[1a]
and which have carbonyl reac-
tivities akin to ketones.[3] On
the other hand, lactones form
structural units within a large
range of naturally occurring
bioACHTUNGTRENNUNGlogically active compounds,
and many methods of lactone
preparation have gained rele-
vance over the past few dec-
ACHTUNGTRENNUNG
ades.[4] Nevertheless, the chemi-
cal synthesis of lactones of all
ring sizes still relies predomi-
nantly on ring-closure tech-
Scheme 1. Classical lactonisation strategies in multi-step synthesis (a,b). Thermal lactonisation of hydroxy-
AHCTUNGTREGUNaNN mides (c) and room-temperature lactonisation of protected amidoethers (d). DDQ=2,3-dichloro-5,6-dicyano-
ACHTUNGTRENNUNGniques starting from w-hydroxy-
1,4-benzoquinone, DMAP=4-(dimethylamino)pyridine, TFBA= 4-(trifluoromethyl)benzoic anhydride, Tf=
trifluoromethanesulfonate, PPTS=pyridinium-p-toluenesulfonate.
carboxylic acids (seco-acids),
esters or their activated deriva-
tives, as evidenced by their
We herein report a new methodology for the formation of
lactones directly from a protected alcohol and an “amide-
[a] V. Valerio, D. Petkova, C. Madelaine, Dr. N. Maulide
Max-Planck-Institut fꢁr Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mꢁlheim an der Ruhr (Germany)
Fax : (+49)2083062999
masked” carboxylic acid (Scheme 1d), which proceeds in a
single step under mild conditions. To the best of our knowl-
edge, lactonisations of this type have not been systematically
explored in the literature and usually require rather harsh
conditions, as exemplified by an early thermally induced g-
lactonisation key-step in Storkꢀs landmark enantioselective
E-mail: maulide@mpi-muelheim.mpg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203906.
2606
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 2606 – 2610

COMMUNICATION
total synthesis of quinine (Scheme 1c).[7] The method de-
scribed in this manuscript, in contrast, proceeds at room
Table 1. Optimisation of conditions for the lactonisation of 4a.
ACHTUNGTRENNUNG
temperature over a few minutes.
Our group has previously reported an unexpected Clai-
ACHTUNGTRENNUNGsen-like rearrangement of keteniminium salts that allows a
Entry
Solvent
Tf2O [equiv]
H2O [equiv]
5d/6[a]
stereoselective entry to challenging substituted a-allyl/allen-
yl/aryl lactones (Scheme 2a).[8]
1
2
3
4
5
6
7
8
9
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
1.05
2.00
3.00
1.05
1.05
1.05
2.00
1.05
2.00
1.05
2.00
2.00
2.00
1.00
–
–
–
–
–
–
–
–
–
–
5
20
20
–
67:33
75:25
64:36
74:26
trace
50:50[b]
34:66
31:69
34:66
SM
pentane
benzene
benzene
toluene
toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
10[c]
11
12
13[d]
14[e]
75:25
90:10
86:14
38:62
[a] Ratio determined by 1H NMR spectroscopy. [b] ꢀ50% unreacted
starting material was detected. [c] With molecular sieves (4 ꢃ). [d] Run
at 0.01m concentration. [e] TfOH was used instead of Tf2O. All reactions
were worked up with saturated aqueous NaHCO3; see the Supporting In-
formation for details. SM=starting material.
Scheme 2. Claisen rearrangement of allyloxyamide 1 (a) and unexpected
dealkylative lactonisation in the absence of base (b).
In the course of our studies on this reaction, we were sur-
prised to observe the exclusive formation of a-unsubstituted,
deallylated g-lactone product 3 by treatment of the starting
w-allyloxyamide 1 with triflic anhydride in the absence of
collidine (Scheme 2b).
This unexpected outcome presaged opportunities for the
development of a new lactonisation strategy, which would
proceed directly from a protected alcohol onto otherwise
inert, stable amides. The attainment of such a goal would ef-
fectively bypass the need for the two sequential deprotec-
tion steps mentioned above, because both the hydroxy and
the carboxy moietiesꢀ protection would actually be a crucial
feature of the process.
Realising that the allyl moiety on substrate 1 might be re-
placed by other protecting groups, we initially turned our at-
tention to the use of secondary alcohols bearing several dif-
ferent protecting groups (such as various silyl derivatives,
acetals, benzyl derivatives and trityl among others).[9] We
observed that all substrates studied were rapidly consumed
in relatively short reaction times (between 5 min and 1 h) at
room temperature to give the lactone along with variable
amounts of an elimination by-product. Substrates bearing a
benzylic secondary ether were found to be the most chal-
lenging in terms of competition between the formation of
these two products. In our initial investigations, higher
yields of lactone were obtained when tert-butyldimethylsilyl
(TBS) was used as protecting group. Thus, we chose com-
pound 4d as a model substrate to gauge how to efficiently
promote lactonisation (Table 1).
tries 12 and 13). Although the concurrent addition of the
electrophilic Tf2O and water appeared paradoxical at this
time, it is important to note that their replacement by TfOH
as lactonisation promoter (entry 14) led to markedly inferior
results, as well as the joint use of various amounts of TfOH
and Tf2O[11] (not shown; see discussion below).
With optimised conditions in hand, we then inspected the
scope of this protocol. As shown in Scheme 3, a broad range
of substrates were tested. For the sake of comparison, the
analogous substrates bearing a naked hydroxyl group were
also subjected to the reaction, and the results are presented
in combined fashion.
As depicted, various lactones of different ring sizes could
be prepared by this direct cyclisation, bearing alkyl, alkenyl
and aryl substituents. The use of a free hydroxyl-bearing
substrate tends to be similarly effective to the use of a TBS-
protected moiety, but as the lactone ring size increases this
trend fades and the silyl ethers prove to be superior. En-
couraging results were also obtained from the application of
this methodology to the preparation of more challenging
ring sizes (compounds 5m and 5n, Scheme 3), with the trityl
protecting group proving to be an interesting alternative to
TBS in one instance. All the lactonisations depicted in
Scheme 3 proceed at room temperature and are generally
complete within minutes (up to 1 h).
To ascertain whether this protocol would be of synthetic
utility, it was of particular importance to test the tolerance
of typical functional groups encountered in multistep syn-
thetic sequences. To probe this, we added equimolar
amounts of substrates decorated with such functional groups
to reaction mixtures in which the lactonisation of protected
hydroxyamide 4b would take place and assessed their recov-
After screening different solvents and concentrations, the
best ratios of 5a/6[10] were obtained by running reactions in
CH2Cl2. The striking negative result obtained upon addition
of molecular sieves (Table 1, entry 10) suggested the possi-
ble beneficial effect of water. Indeed, the deliberate addi-
tion of 20 equivalents of H2O provided the best results (en-
Chem. Eur. J. 2013, 19, 2606 – 2610
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2607

N. Maulide et al.
Scheme 3. Reaction
conditions:
[a] Tf2O
(1.05 equiv),
[b] Tf2O
(2.00 equiv), [c] Tf2O (2.00 equiv), H2O (20.0 equiv), [d] Tf2O
(1.05 equiv), H2O (20.0 equiv); see the Supporting Information for de-
tails.
Scheme 4. Tolerance of typical functional groups to the reaction condi-
tions.
summarised in Scheme 5. We presume that triflic anhydride
activates the amide to form the iminium triflate 11, which is
prone to nucleophilic attack by the pendant oxygen (path-
ery at the end of the reaction. The results are depicted in
Scheme 4.
As can be seen, the mild con-
ditions and short reaction times
under which the lactonisation
operates allow the survival of
most typical functional groups.
These include sulACTHUNRTGENNUGfonACHUTNGTRENNaUGN mides,
benzyl ethers and thioketals.
The chemoselectivity of carbon-
yl activation by Tf2O is reflect-
ed by the fact that most esters
(acetate, benzoate) are oblivi-
ous to this reagent and are thus
tolerated by the procedure. The
limits of the method are en-
countered with acetonides and
tert-butoxycarbonyl (Boc)-pro-
tected amines, which undergo
deprotection under these condi-
tions. Importantly, sulfides and
various electron-rich aromatic
heterocycles, such as indole or
thiophene derivatives, are per-
fectly tolerated by the reaction
conditions.
Our mechanistic hypothesis
for the direct lactonisation is Scheme 5. Working mechanistic hypothesis.
2608
www.chemeurj.org
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 2606 – 2610

Lactonisation of Alcohols and Ethers onto Amides
COMMUNICATION
way a). After ejection of triflate, the iminium ether inter-
mediate 12 is hydrolysed during the post-reaction treatment
with saturated aqueous NaHCO3. Alternatively, the electro-
philic triflic anhydride can react first with the alcohol/ether
moiety to transiently generate a triflate/triflyloxonium leav-
ing group (pathway b). Elimination of the last (presumably
by E1 pathways) or its direct intramolecular displacement by
the amide carbonyl oxygen leads to either the olefin by-
product 16 or an analogous iminium ether intermediate
17.[12,13]
An experimentally appealing way to distinguish between
pathways a and b would be to label one of the oxygen
atoms. Thus, we prepared TBS-protected precursor 19b in
which the silylether oxygen was isotopically labeled as 18O
and subjected it to the reaction conditions.[11] In the event
(Scheme 6), a near complete retention of the 18O label was
termined by NMR experiments), it appears that the benefi-
cial effect of water on these lactonisations could be due to
its selective “mopping” of undesired TBSOTf as it is
formed, thus protecting the most effective reaction pathway.
Finally, we applied this lactonisation method as the key
step in the synthesis of epi-6-hydroxyundecan-4-olide 26,
one of the metabolites produced by the giant white butterfly
Idea leucona.[14] As shown in Scheme 7, our short sequence
begins with the aldol merger of the amide aldehyde 22
(available by Dess–Martin oxidation of the corresponding
alcohol) with the boron enolate of commercially available 2-
heptanone.[15] Following TBS-protection of the aldol product
and syn-diastereoselective reduction employing Super-Hy-
dride,[16] direct lactonisation affords the target compound in
moderate overall yield. This streamlined, unoptimised syn-
thetic route outlines what could be termed as an “amide
strategy” for synthesis: it showcases the advantages of carry-
ing a robust, amide-masked carboxyl through strongly basic
conditions in steps such as an aldol addition or a carbonyl
reduction (which might not have been tolerated by the car-
boxylic acid or ester[17] analogues of 22 and 25), while allow-
ing for its selective activation at a late stage of the synthetic
pathway.[18]
Scheme 6. 18O-labelling experiments. Reaction conditions: Tf2O
(2.00 equiv), CH2Cl2, 10 min. Similar results were obtained upon addition
of H2O (20.0 equiv); see the Supporting Information for details.
Herein, we disclose a new approach to the synthesis of
lactones by room-temperature cyACTHUNRTGENNUGcliACHTUNGTRNEsNUGN ation of alcohols and
ethers onto amides. This allows a direct disconnection of lac-
observed (within experimental
error). This result supports
pathway a being operative in
the case of TBS-protected
ethers. Intriguingly, when the
same experiment was per-
formed on the free 18O-labeled
alcohol 19a, significant erosion
of the 18O label was observed,
suggesting that pathway b be-
comes competitive when the
free alcohols are employed as
substrates.
This mechanistic proposal
further highlights the release of
an equivalent of TBSOTf
during the reaction of silyleth-
ers (and TfOH in the case of
Scheme 7. Four-step synthesis of rac-epi-6-hydroxyundecan-4-olide (26).
free alcohol substrates). In con-
trol experiments, we established
that TBSOTf is also capable of
promoting lactonisation under these reaction conditions, but
with concomitant formation of significant amounts of elimi-
nation and other by-products.[11] Further experiments also
suggest that the hydrolysis of TBSOTf into ultimately
TBSOTBS (which can be detected by GC-MS analysis of re-
action mixtures) proceeds very readily. Since, in our proce-
dure, water is added to the reaction mixture after Tf2O and
as the conversion of the amide starting material into its im-
tone products back to fully protected starting materials, but
without requiring any wasteful deprotection steps.[19] The
notion that the amide functional group can be taken through
demanding synthetic sequences as a robust carboxylate
mask and at the same time be selectively activated under
mild conditions at room temperature is intriguing.[18] The
elaboration of this concept is currently underway in our lab-
oratories.
ACHTUNGTRENNUNGinACHTUNGTRENNUNGium triflate 11 takes place virtually instantaneously (as de-
Chem. Eur. J. 2013, 19, 2606 – 2610
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2609

N. Maulide et al.
laine, N. Maulide, Chem. Eur. J. 2011, 17, 4742; for pioneering work
on keteniminium chemistry, see: c) J.-B. Falmagne, J. Escudero, S.
Taleb-Sahraoui, L. Ghosez, Angew. Chem. 1981, 93, 926; Angew.
Chem. Int. Ed. Engl. 1981, 20, 879; for additional important work on
electrophilic activation of amides, see: d) K.-J. Xiao, Y. Wang, K.-Y.
Ye, P.-Q. Huang, Chem. Eur. J. 2010, 16, 12792; e) K.-J. Xiao, J.-M.
Luo, K.-Y. Ye, Y. Wang, P.-Q. Huang, Angew. Chem. 2010, 122,
3101; Angew. Chem. Int. Ed. 2010, 49, 3037; f) K.-J. Xiao, A.-E.
Wang, P.-Q. Huang, Angew. Chem. 2012, 124, 8439; Angew. Chem.
Int. Ed. 2012, 51, 8314; g) A. B. Charette, M. Grenon, Can. J. Chem.
2001, 79, 1694; h) G. Barbe, A. B. Charette, J. Am. Chem. Soc. 2008,
130, 18; i) G. Pelletier, W. S. Bechara, A. B. Charette, J. Am. Chem.
Soc. 2010, 132, 12817; j) A. B. Charette, M. Grenon, A. Lemire, M.
Pourashraf, J. Martel, J. Am. Chem. Soc. 2001, 123, 11829; k) W. S.
Bechara, G. Pelletier, A. B. Charette, Nat. Chem. 2012, 4, 228.
[9] Additional protecting groups screened, including (but not limited
to) Bn, MOM, SEM and THP, all proved inferior to TBS; see the
Supporting Information for details.
Acknowledgements
We are grateful to the Max-Planck Society and the Max-Planck-Institut
fꢁr Kohlenforschung for generous funding of our research programs. This
work has been supported by the Deutsche Forschungsgemeinschaft
(Grants MA-4861/1-1 and 1-2), the Alexander von Humboldt-Foundation
(Scholarship to C.M.) and the Fonds der Chemischen Industrie (Sachkos-
tenzuschuss to N.M.).
Keywords: amides · electrophilic activation · lactones ·
mechanistic studies · triflic anhydrides
[1] a) M. B. Smith, J. March, Advanced Organic Chemistry, Wiley, New
York, 2001; b) J. Clayden, N. Geeves, S. Warren, Organic Chemistry,
Oxford University Press, 2000; c) H. Ishida, Z. Naturforsch. 2000,
55a, 769.
[2] a) M. Hutchby, C. E. Houlden, M. F. Haddow, S. N. G. Tyler, G. C.
Lloyd-Jones, K. I. Booker-Milburn, Angew. Chem. Int. Ed. 2012, 51,
548; b) A. Radzicka, R. Wolfenden, J. Am. Chem. Soc. 1996, 118,
6105; c) R. M. Smith, D. E. Hansen, J. Am. Chem. Soc. 1998, 120,
8910.
[3] a) M. C. Brçhmer, S. Mundinger, S. Brꢄse, W. Bannwarth, Angew.
Chem. 2011, 123, 6299; Angew. Chem. Int. Ed. 2011, 50, 6175; b) K.
Tani, B. M. Stoltz, Nature 2006, 441, 731; c) A. J. Kirby, I. R. Komar-
ov, J. Chem. Soc. Perkin Trans. 2 2001, 2, 522; d) C. G. Bashore, I. J.
Samardjiev, J. Bordern, J. W. Coe, J. Am. Chem. Soc. 2003, 125,
3268; e) A. Greenberg, D. T. Moore, T. D. Dubois, J. Am. Chem.
Soc. 1996, 118, 8658; f) G. M. Blackburn, C. J. Skaife, I. T. Kay, J.
Chem. Res. Synop. 1980, 294; g) M. Szostak, J. Aubꢅ, Org. Biomol.
Chem. 2011, 9, 27; for an overview, see: h) J. Aubꢅ, Angew. Chem.
2012, 124, 3117; J. Aubꢅ, Angew. Chem. Int. Ed. 2012, 51, 3063; for
earlier amide hydrolysis procedures, see: i) S. Hanessian, Tetrahe-
dron Lett. 1967, 8, 1549; j) G. E. Keck, M. D. McLaws, T. T. Wager,
Tetrahedron 2000, 56, 9875.
[10] No efforts were made to fully characterise compound 6 beyond
NMR spectroscopy; see the Supporting Information for details.
[11] See the Supporting Information for details.
[12] The intermediacy of 12 and 17 prior to hydrolysis has been con-
firmed by online NMR analysis of reaction mixtures; see the Sup-
porting Information for details.
[13] Importantly, the isolated olefin by-product 16 (R1 =Ph) does not un-
dergo conversion to 12 or 17 under either the reaction conditions or
upon addition of TfOH.
[14] a) J. S. Yadav, P. A. N. Reddy, H. Ather, A. S. Kumar, A. R. Prasad,
B. V. S. Reddy, A. Al Khazim, Synthesis 2012, 579; b) H. Nago, M.
Matsumoto, Biosci. Biotechnol. Biochem. 1994, 58, 1267; c) S.
Schulz, Chem. Commun. 1999, 1239; d) K. Stritzke, S. Schulz, R.
Nishida, Eur. J. Org. Chem. 2002, 3884; e) R. Nishida, S. Schulz,
C. S. Kim, H. Fukami, Y. Kuwahara, K. Honda, N. Hayashi, J.
Chem. Ecol. 1996, 22, 949; f) K. Honda, N. Hayashi, F. Abe, T. Ya-
mauchi, J. Chem. Ecol. 1997, 23, 1703.
[15] L. C. Dias, J. Fattori, C. C. Perez, V. M. De Oliveira, A. M. Aguilar,
Tetrahedron 2008, 64, 5891.
[16] Stereochemistry assigned by the Rychnovsky acetonide method;
see: S. D. Rychnovsky, B. Rogers, G. Yang, J. Org. Chem. 1993, 58,
3511; see the Supporting Information for details.
[17] Interestingly, esters are not competent in this lactonisation proce-
dure and lead to substantial amounts of elimination products (pre-
sumably as a consequence of pathway b in Scheme 5 being domi-
nant); see the Supporting Information for details.
[4] a) K. J. Fraunhoffer, N. Prabagaran, L. E. Sirois, M. C. White, J. Am.
Chem. Soc. 2006, 128, 9032; for an authoritative review, see: b) A.
Parenty, X. Moreau, J.-M. Campagne, Chem. Rev. 2006, 106, 911.
[5] These could also be considered as “concession” steps, see: T. Gaich,
P. S. Baran, J. Org. Chem. 2010, 75, 4657.
[6] a) D. J. Critcher, S. Connoly, M. Wills, J. Org. Chem. 1997, 62, 6638;
b) I. Shiina, Y. Fukuda, T. Ishii, H. Fujisawa, T. Mukaiyama, Chem.
Lett. 1998, 831; for additional selected examples of two sequential
deprotection steps preceding the lactonisation key step, see: c) L.
Ratjen, P. Garcia-Garcia, F. Lay, M. E. Beck, B. List, Angew. Chem.
2011, 123, 780; Angew. Chem. Int. Ed. 2011, 50, 754; d) S. L. Schreib-
er, S. E. Kelly, J. A. Porco Jr., T. Sammakia, E. M. Suh, J. Am.
Chem. Soc. 1988, 110, 6210; e) K.-i. Takao, H. Saegusa, G. Wata-
nabe, K.-i. Tadano, Tetrahedron: Asymmetry 2000, 11, 453; f) M. Ko-
bayashi, K. Ishigami, H. Watanabe, Tetrahedron Lett. 2010, 51, 2762.
[7] a) G. Stork, D. Niu, A. Fujimoto, E. R. Koft, J. M. Balkovec, J. R.
Tata, G. R. Dake, J. Am. Chem. Soc. 2001, 123, 3239; for rare exam-
ples of chelation-biased lactonisation onto amides, see: b) A. G.
Schultz, N. J. Green, J. Am. Chem. Soc. 1991, 113, 4931; c) A. G.
Schultz, S. J. Kirincich, J. Org. Chem. 1996, 61, 5626; d) T. Katsuki,
M. Yamaguchi, Tetrahedron Lett. 1987, 28, 651; e) J.-H. Yang, G.-C.
Yang, C.-F. Lu, Z.-X. Chen, Tetrahedron: Asymmetry 2008, 19, 2164;
f) J.-F. Brazeau, A.-A. Guilbault, J. Kochuparampil, P. Mochirian, Y.
Guindon, Org. Lett. 2010, 12, 36; g) K. Mori, Y. Nakazono, Tetrahe-
dron 1986, 42, 6459.
[18] For
a similar synthetic concept applied to the use of alkynes
(“alkyne strategy”), see: a) B. M. Trost, A. H. Weiss, Angew. Chem.
2007, 119, 7808; Angew. Chem. Int. Ed. 2007, 46, 7664; b) B. M.
Trost, J. D. Sieber, W. Qian, R. Dhawan, Z. T. Ball, Angew. Chem.
2009, 121, 5586; B. M. Trost, J. D. Sieber, W. Qian, R. Dhawan, Z. T.
Ball, Angew. Chem. Int. Ed. 2009, 48, 5478.
[19] For selected examples of protecting-group-free total syntheses, see:
a) V. Hickmann, A. Kondoh, B. Gabor, M. Alcarazo, A. Fꢁrstner, J.
Am. Chem. Soc. 2011, 133, 13471; b) P. S. Baran, T. J. Maimone,
J. M. Richter, Nature 2007, 446, 404; c) Q. Zhou, X. Chen, D. Ma,
Angew. Chem. 2010, 122, 3591; Angew. Chem. Int. Ed. 2010, 49,
3513. For recent reviews on protecting group-free total synthesis,
see: d) E. Roulland, Angew. Chem. 2011, 123, 1260; Angew. Chem.
Int. Ed. 2011, 50, 1226; e) I. S. Young, P. S. Baran, Nat. Chem. 2009,
1, 193; f) R. W. Hoffmann, Synthesis 2006, 3531.
[8] a) C. Madelaine, V. Valerio, N. Maulide, Angew. Chem. 2010, 122,
1628; Angew. Chem. Int. Ed. 2010, 49, 1583; b) V. Valerio, C. Made-
Received: October 31, 2012
Published online: January 10, 2013
2610
www.chemeurj.org
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 2606 – 2610