Environmentally friendly and regioselective C3-alkylation of indoles with alcohols through a hydrogen autotransfer strategy

Article abstract of DOI:10.1016/j.tetlet.2013.04.062

The direct alkylation of indoles using KOH and alcohols, as initial source of the electrophile, under solvent-free conditions is a safe and environmentally benign strategy for selective modification of these structures at the C ₃-position, without using hazardous and difficult to handle bromide or iodide derivatives or toxic and expensive transition metal catalysts. The protocol shows a broad scope, including halogenated indoles and secondary alcohols.

Full text of DOI:10.1016/j.tetlet.2013.04.062

Accepted Manuscript
Environmentally Friendly and Regioselective C 3-Alkylation of Indoles with
Alcohols through a Hydrogen Autotransfer Strategy
Rafael Cano, Miguel Yus, Diego J. Ramón
PII:
S0040-4039(13)00657-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.04.062
TETL 42830
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
16 March 2013
12 April 2013
16 April 2013
Please cite this article as: Cano, R., Yus, M., Ramón, D.J., Environmentally Friendly and Regioselective C -
3
Alkylation of Indoles with Alcohols through a Hydrogen Autotransfer Strategy, Tetrahedron Letters (2013), doi:
http://dx.doi.org/10.1016/j.tetlet.2013.04.062
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Environmentally Friendly and Regioselective C3-
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Rafael Cano, Miguel Yus, and Diego J. Ramón*

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Environmentally Friendly and Regioselective C3-Alkylation of Indoles with Alcohols
through a Hydrogen Autotransfer Strategy
Rafael Cano,a Miguel Yus,a and Diego J. Ramóna
Instituto de Síntesis Orgánica (ISO), and Departamento de Química Orgánica, Facultad de Ciencias, Universidad de Alicante, Apdo. 99, E-03080-Alicante, Spain
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
The direct alkylation of indoles using KOH and alcohols, as initial source of the electrophile,
under solvent-free conditions is a safe and environmental benign strategy for selective
modification of these structures at the C3-position, without using hazardous and difficult to
handle bromide or iodide derivatives or toxic and expensive transition metal catalysts. The
protocol shows a broad scope, including halogenated indoles and secondary alcohols.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Alcohols
Alkylation
Hydrogen autotransfer
Indoles
Transition metal-free
1. Introduction
reduction of the formed double bond, there are several examples
in which this hydrogen transfer has been carried out in absence of
metallic catalyst.12 Therefore, we anticipate that, if the
dehydrogenation of alcohols takes place, the most nucleophilic
C3-position of indoles could condensate to form a α,β-unsaturated
imine derivative which could be reduced by the product of
another dehydrogenation process.
The indole scaffold is one of the most industrial, agricultural
and medicinal relevant cores.1 Substituted indoles have been
referred to as privileged structures since they are capable of
binding to many receptors with high affinity. Therefore, their
synthesis and selective modification have been the focus of active
research over many years.2 The simple alkylation of indole
suffers from several fundamental problems, including the
regioselectivity at C-3,3 C-2 and N-14 positions, which have been
partially solved using different strategies, conditions and
catalysts. However, from the environmental, and even human
hazard (due to the use of highly reactive electrophiles), point of
view still there are some problems to be solved.
The standard problem of using hazard electrophilic reagents,
such as alkyl bromides or iodides was solved by the use of
alcohols as the source of electrophile. This alcohol strategy has
been successfully used when allylic metal intermediates were
involved5 and when carbocations were generated by either acid
treatment6 or harsh conditions.7
The hydrogen autotransfer methodology8 (also named
borrowing hydrogen) has been also applied to the selective
alkylation of indole at C3,9 C210 and N111 positions using alcohols
as initial source of electrophile, just by using the appropriate
transition metal catalysts (Scheme 1).
Scheme 1. General scheme for the indole C-3 alkylation trough a
hydrogen autotransfer.
2. Results and discussion
Although the presence of a metallic catalyst seems to be
In our ongoing project on the use of alcohols as alkylating
reagents through a hydrogen autotransfer strategy,13 we report
mandatory for the initial dehydrogenation of the alcohol and the
———
 Corresponding author. Tel.: +34-965903986; fax: +34-965903549; e-mail: djramon@ua.es

2
herein the transition metal-free C3-alkylation of indoles under
solvent-free conditions.
The optimized protocol was applied to other substrates in
order to study the scope of the reaction (Table 2). The reaction
gave practically the same quantitative yields using 4-
functionalised benzylic alcohols independently of the electronic
nature of the functionalisation (entries 1-4). However, the
possible steric hindrance of a group at the two position of
aromatic ring seems to have an important effect, decreasing the
yield (compare entries 1, 5 and 6). Other arylmethanol
derivatives (entries 7 and 8) as well as the related heteroaryl
derivatives (entries 9 and 10) gave similar results, depending on
the presence of steric hindrance at the ortho-position of the
aromatic ring of the alcohol.
The reaction between indole (1a) and benzyl alcohol (2a) was
selected as a model reaction in order to optimize the conditions
(Table 1). The reaction in a mixture of xylenes using KOH (90%
purity) as base at 150 ºC under an air atmosphere gave after 3 h
the expected benzylated compound 3a in an excellent yield (entry
1), as the only regioisomer.
Whereas the increase of base did not have any influence on
the result, the decrease of KOH produced an important and
negative effect on the chemical yield (Table 1, entries 2 and 3).
The use of strict stoichiometric amount of alcohol, one equivalent
to produce the electrophile and another to produce the
hydrogenating source, decrease the yield meanwhile the use of a
large excess did not produce any significant change in the result
(entries 4 and 5). Then, the influence of solvent (entries 6-9),
temperature (entry 10), and nature of base (entries 11-13) was
studied, with the solvent-free conditions presented in the entry 9
of Table 1 showing a quantitative yield of product 3a. The
reaction could be scaled-up to 5 mmols without any problem.
Table 2
C3-Alkylation of indoles using primary alcoholsa
After finding the best reaction conditions, we focused on the
presence of a masked metallic catalyst on the reaction media. We
repeated the reaction with a new source of KOH (99.9% purity;
with less than 200 ppm of other metals) obtaining the same result
(entry 14). To verify this aspect, after repeating the reaction
presented in Table 1, entry 9, the ICP-MS study of the obtained
mixture was performed finding that the amount of copper was
only 5 × 10-3 mol% and that of palladium even lower 3 × 10-4
mol%. Although we believe that the trace impurities are not
responsible for the reaction depicted in this process, it could not
be completely ruled out.
Entry
R1
R2
R3
R4
No Yield (%)b
1
2
H
H
H
H
H
H
H
H
H
H
H
H
Et
H
H
H
Ph
3a
3b
3c
3d
3e
3f
99
88
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
4-MeOC6H4
4-MeC6H4
4-ClC6H4
2-MeOC6H4
3-MeOC6H4
1-C10H8
3
H
99
4
H
98
5
H
76
6
H
96
7
H
3g
92
Table 1
8
H
2,3-(OCH2O)C6H3 3h
71c,d
Optimization of the reaction conditionsa
9
H
2-Piridyl
3i
3j
73
10
11
12
13
14
H
2-Thienyl
90
MeO
F
Ph
Ph
Ph
Ph
3k
3l
98d
63d
76c,d
99d
H
3m
3n
Entry
1
2a (mol%)
Base (mol%) solvent T (ºC) Yield (%)b
H
300
300
300
200
400
300
300
300
300
300
300
300
300
300
a Reaction carried out using compounds 1 (1.0 mmol) and 2 (3.0 mmol).
b
KOH (130) Xylene
KOH (50) Xylene
150
150
150
150
150
150
150
150
150
130
150
150
150
150
97
21
95
68
72
92
0
c
Isolated yield after crystallization.
Isolated yield after column
2
chromatography. d Reaction performed during 18 h.
3
KOH (250) Xylene
KOH (130) Xylene
KOH (130) Xylene
4
The presence of a substituent at the indole position had a
significant influence in the reaction time, needing longer time to
finish the process but keeping, in general, the previous results
(entries 11-14). It should be pointed out that the reactions of
indole (1a) with 1-butanol or with 1-heptanol failed, recovering
the starting indole and the corresponding alcohol.
5
6
KOH (130)
PhMe
7
KOH (130) Dioxane
8
KOH (130)
KOH (130)
KOH (130)
NaOH (130)
tBuOK (130)
K2CO3 (130)
KOHd (130)
H2O
0
9
-
-
-
-
-
-
99 (95)c
The optimized protocol could be applied not only to primary
alcohols but also to secondary ones (Table 3). In this case, the
required reaction times were increased up to one day, and the
presence of several by-products arising from the auto-
condensation of the in situ formed ketones (which diminished the
yield of desired compound 5) was observed. In the case of using
diphenylmethanol the yield was similar to those obtained using
primary alcohols thanks to the impossibility of auto-condensation
of the benzophenone intermediate (entry 2 in Table 3).
10
11
12
13
14
5
74
45
0
99
a Reaction carried out using compound 1a (1.0 mmol). b Isolated yield after
c
d
crystallization. Reaction carried out using compound 1a (5.0 mmol).
KOH used with less than 200 ppm of other metals.

3
Table 3
condensation step followed by a reduction of the corresponding
3-alkylidene-3H-indole intermediate (Scheme 2). The reaction of
1a with benzaldehyde (6) and (4-methoxyphenyl)methanol (2b)
gave a mixture of compounds 3, in which the main product 3a
was that arising from the condensation with the aldehyde
followed by reduction, confirming our mechanistic hypothesis.
C3-Alkylation of indole using secondary alcoholsa
Entry
R1
Ph
Ph
R2
Me
Ph
No
5a
5b
5c
Yield (%)b
1
2
3
57
91
56
-(CH2)5
b
a Reaction carried out using compounds 1a (1.0 mmol) and 4 (3.0 mmol).
Isolated yield after crystallization.
Scheme 2. Competitive experiments between an aldehyde and an
alcohol.
Finally, the possible hydrogen autotransfer mechanism was
studied. The reaction of indole (1a) with benzyl bromide (300
mol%) and KOH in xylene at 150 ºC gave a mixture of
compound 3a (43%), 1-benzyl-1H-indole (35%) and 1,3-
dibenzyl-1H-indole (18%), according to the CG and the MS
spectra. The comparison with the result of entry 1 in Table 1 calls
our attention to the possible difference in the mechanism, with
the one followed in the reaction with benzyl bromide being close
to be mediated by a cationic intermediate. Then, we repeated the
reaction described in the entry 9 in Table 1 but using the
alkylated 3-methyl-1H-indole and after 18 h the starting
unchanged indole was recovered,5f which is another indirect
proof of that the cationic benzyl intermediate is not involved. The
last evidence in this direction was obtained when the reaction
was performed with triphenylmethanol:7a after 24 h of reaction
the staring indole (1a) was recovered unchanged.
3. Conclusion
In conclusion, the direct alkylation of indoles using KOH and
alcohols at 150 ºC under solvent-free conditions is a safe and
environmental benign strategy for selective modification of these
structures, without using hazardous and difficult to handle
bromide or iodide derivatives or toxic and expensive transition
metal catalysts. All these facts permit us to anticipate a good
future for the process shown in this study not only in the
laboratory but also in the industry.
Acknowledgments
After being quite confident that the mechanism did not
involve the formation of a cationic-like intermediate from the
alcohol, the role of the base was studied. The standard reaction
(entry 9 in Table 1) in absence of base failed, as it was expected
from the fact that a softer base than KOH had failed previously
(entry 13 in Table 1). A similar reaction but using 1-methyl-1H-
indole also failed, and only the starting material was recovered.
This means that the base seems to deprotonate, at least partially,
the starting indole 1, which is in concordance with the pKa
values. Following with the possible role of the base, benzylic
alcohol (2a) was heated in the presence of KOH at 150 ºC in a
sealed tube during 18 h. We found the formation of a small
amount of benzaldehyde (6), 15% according to GC-analysis of
the crude mixture using anisol as an internal standard. However,
benzaldehyde was not detected when the alcohol 2a was heated
up in absence of base. From these results, the base seems to play
two different roles, the first one is the deprotonation of the
alcohol, favoring the first dehydrogenation step, and the second
is the deprotonation of indole increasing the nucleophilicity at the
C3-position. To corroborate the last result, indole (1a) and
benzaldehyde (6) were heated at 150 ºC overnight, in the absence
or presence of KOH. In the second case both starting reagents
were recovered unchanged, however, in the first one benzyl
alcohol (2a, 32%), benzoic acid, and the benzylated product 3a
(11%) appeared, together with the starting materials. These
experiments highlight the central role of the base in the
condensation step, as well as in the following reduction of the C-
C double bond.
This work was supported by the current Spanish Ministerio de
Economía y Competitividad (Consolider Ingenio 2010 CSD2007-
00006, CTQ2011-24151) and the Generalitat Valenciana (G.V.;
PROMETEO 2009/03, FEDER). R.C. thanks to G.V. for a
fellowship through the PROMETEO program. We gratefully
acknowledge the polishing of our English by Mrs. Oriana C.
Townley.
Supplementary data
A MOL file for representative products, detailed experimental
procedures, and spectral data for all compounds associated with
this article can be found in the online version, at http://dx.doi.org/
4. References and notes
1. (a) Ramírez, A.; García-Rubio, S. Curr. Med. Chem. 2003, 10, 1891-
1915; (b) Somei, M.; Yamada, F. Nat. Prod. Rep. 2005, 22, 73-103; (c)
Wu, Y.-J. Top. Heterocycl. Chem. 2010, 26, 1-29; (d) Barden, T. C. Top.
Heterocycl. Chem. 2010, 26, 31-46; (e) Palerno, V.; Pieri, L.; Silvestri,
R.; La Regina, G.; Falcone, C.; Mazzoni, C. Cell Cycle 2011, 10, 3208-
3209; (f) Dhani, R.; Avinash, A.; Salenaagina, S. K.; Teja, M. V. S.;
Masthanaiah, P.; Rathnam, P. R.; Silpa, V. C. J. Chem. Pharm. Res.
2011, 3, 519-523; (g) Ahmad, A.; Sakr, W. A.; Rahman, K. M. W.;
Cancers 2011, 3, 2955-2974; (h) Biswal, S.; Sahoo, U.; Sethy, S.;
Kumar, H. K. S.; Banerjee, M. Asian J. Pharm. Clin. Res. 2012, 5, 1-6.
2. (a) Sundberg, R. in Comprehensive Heterocyclic Chemistry II; Katritzky,
A. R.; Rees, C. W.; Scriven, E. F. V.; Bird, C. W., Eds.; Pergamon Press,
Oxford, 1996; Vol. 2, pp 119-206; (b) Gribble, G. W. in Comprehensive
Heterocyclic Chemistry II; Katritzky, A. R.; Rees, C. W.; Scriven, E. F.
Finally, a competitive experiment was performed to rule out
the benzyl cation intermediate and to point out the sequential

4
V.; Bird, C. W., Eds.; Pergamon Press, Oxford, 1996,;Vol. 2, pp 207-
257; (c) Joule, J. A. in Science of Synthesis; Thomas, E. J., Ed.; Georg
Thieme Verlag, Stuttgart, 2000; Vol. 10, pp 361-652; (d) Bandini, M.;
Melloni, A.; Tommasi, S.; Umani-Ronchi, A. Synlett 2005, 1199-1222;
(e) Bandini, M.; Eichholzer, A. Angew. Chem. Int. Ed. 2009, 48, 9608-
9644; (f) Sundberg, R. J. Top. Heterocycl. Chem. 2010, 26, 47-115; (g)
Zeng, M.; You, S.-L. Synlett 2010, 1289-1301; (h) Taber, D. F.;
Tirunahari, P. K. Tetrahedron 2011, 67, 7195-7210; (i) Karchava, A. V.;
Melkonyan, F. S.; Yurovskaya, M. A. Chem. Heterocycl. Compd. 2012,
48, 391-407; (j) Shiri, M. Chem. Rev. 2012, 112, 3508-3549; (k) Platon,
M.; Amardeil, R.; Djakovitch, L.; Hierso, J.-C.; Chem. Soc. Rev. 2012,
41, 3929-3968.
Y.; Gu, Z.-Y.; Meng, H.; Wang, S.-Y.; Ji, S.-J. Tetrahedron 2013, 69,
1166-1174.
7. (a) Hirashita, T.; Kuwahara, S.; Okochi, S.; Tsuji, M.; Araki, S.
Tetrahedron Lett. 2010, 51, 1847-1851; (b) Osyanin, V. A.; Sidorina, N.
E.; Klimochkin, Y. N. Russ. J. Gen. Chem. 2011, 81, 115-121.
8. For reviews, see (a) Guillena, G.; Ramón, D. J.; Yus, M. Angew. Chem.
Int. Ed. 2007, 46, 2358-2364; (b) Hamid, M. H. S. A.; Slatford, P. A.;
Williams, J. M. J. Adv. Synth. Catal. 2007, 349, 1555-1575; (c) Nixon, T.
D.; Whittlesey, M. K.; Williams, J. M. J. Dalton Trans. 2009, 753-762;
(d) Fujita, K.-i.; Yamaguchi, R. in Iridium Complexes in Organic
Synthesis; Oro, L. A.; Claver, C.; Eds.. Wiley-VCH, Weinheim, 2009; pp
107-143; (e) Dobereiner, G. E.; Crabtree, R. H. Chem. Rev. 2010, 110,
681-703; (f) Guillena, G.; Ramón, D. J.; Yus, M. Chem. Rev. 2010, 110,
1611-1641; (g) Yamaguchi, R.; Fujita, K.-i.; Zhu, M. Heterocycles 2010,
81, 1093-1140; (h) Alonso, F.; Foubelo, F.; González-Gómez, J. C.;
Martínez, R.; Ramón, D. J.; Riente, P.; Yus, M. Mol. Divers. 2010, 14,
411-424; (i) Watson, A. J. A.; Williams, J. M. J. Science 2010, 329, 635-
636; (j) Kimura, H. Catal. Rev. Sci. Eng. 2011, 53, 1-90; (k) Bähn, S.;
Imm, S.; Neubert, L.; Zhang, M.; Neumann, H.; Beller, M.
ChemCatChem 2011, 3, 1853-1864.
3. (a) Zhu, X.; Ganesan, A. J. Org. Chem. 2002, 67, 2705-2708; (b)
Sharma, R.; Chouhan, M.; Sood, D.; Nair, V. A. Appl. Organometal.
Chem. 2011, 25, 305-309, and the literature quoted therein.
4. (a) Heaney, H.; Ley, S. V. in Organic Synthesis, Collective Volume;
Noland, W. E., Ed.; John Wiley & Sons, New York, 1988; Vol. 6, pp
104-105; (b) Rubotton, G. M.; Chabala, J. C. in Organic Synthesis,
Collective Volume; Noland, W. E., Ed.; John Wiley & Sons, New York,
1988; Vol. 6, pp 106-107.
5. See for instace: (a) Nishibayashi, Y.; Yoshikawa, M.; Inada, Y.; Hidai,
M.; Uemura, S. J. Am. Chem. Soc. 2002, 124, 11846-11847; (b) Inada,
Y.; Yoshikawa, M.; Milton, M. D.; Nishibayashi, Y.; Uemura, S. Eur. J.
Org. Chem. 2006, 881-890; (c) Matsuzawa, H.; Kanao, K.; Miyake, Y.;
Nishibayashi, Y. Org. Lett. 2007, 9, 5561-5564; (d) Sundararaju, B.;
Achard, M.; Demerseman, B.; Toupet, L.; Sharma, G. V. M.; Bruneau,
C.; Angew. Chem. Int. Ed. 2010, 49, 2782-2785; (e) Detz, R. J.; Abiri, Z.;
le Griel, R.; Hiemstra, H.; van Maarseveen, J. H. Chem. Eur. J. 2011, 17,
5921-5930; (f) Zhu, Y.; Rawal, V. H. J. Am. Chem. Soc. 2011, 134, 111-
114; (g) Thies, N.; Hrib, C. G.; Haak, E. Chem. Eur. J. 2012, 18, 6302-
6308.
6. (a) Sefkow, M.; Buchs, J. Org. Lett. 2003, 5, 193-196; (b) Motokura, K.;
Nakagiri, N.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Org. Chem. 2007,
72, 6006-6015; (c) Yadav, J. S.; Reddy, B. V. S.; Reddy, A. S. J. Mol.
Catal. A: Chem. 2008, 280, 219-223; (d) Robertson, F. J.; Kenimer, B.
D.; Wu, J. Tetrahedron 2011, 67, 4327-4332; (e) Yang, J.; Zhang, J.;
Chen, T. T.; Sun, D. M.; Li, J.; Wu, X. F. Chin. Chem. Lett. 2011, 22,
1391-1394; (f) Zhang, L.; Zhu, X.; Yin, G.; Lu, P.; Wang, Y. J. Org.
Chem. 2012, 77, 9510-9520; (g) Kozhevnikov, I. V.; Nuzhdin, A. L.;
Bukhtiyarova, G. A.; Martyanov, O. N.; Chibiryaev, A. M. J. Supercrit.
Fluids 2012, 69, 82-90; (h) Sato, Y.; Aoyama, T.; Takido, T.; Kodomari,
M.; Tetrahedron 2012, 68, 7077-7081; (i) Gohain, M.; Marais, C.;
Bezuidenoudt, B. C. B. Tetrahedron Lett. 2012, 53, 4704-4707; (j)
Nobuta, T.; Fujiya, A.; Tada, N.; Miura, T.; Itoh, A. Synlett 2012, 2975-
2980; (k) Chung, J. Y. L.; Steinhuebel, D.; Krska, S. W.; Hartner, F. W.;
Cai, C.; Rosen, J.; Mancheño, D. E.; Pei, T.; DiMichele, L.; Ball, R. G.;
Chen, C.-y.; Tan, L.; Alorati, A. D.; Brewer, S. E.; Scott, J. P. Org.
Process Res. Dev. 2012, 16, 1832-1845; (l) Chu, X.-Q.; Jiang, R.; Fang,
9. (a) Pratt, E. F.; Botimer, L. W. J. Am. Chem. Soc. 1957, 79, 5248-5250
(b) Whitney, S.; Grigg, R.; Derrick, A.; Keep, A. Org. Lett. 2007, 9,
3299-3302; (c) Imm, S.; Bähn, S.; Tillack, A.; Mevius, K.; Neubert, L.;
Beller, M. Chem. Eur. J. 2010, 16, 2705-2709.
10. Lee, D.-H.; Kwon, K.-H.; Yi, C. S. Science 2011, 333, 1613-1616.
11. a) Plieninger, H.; Kraemer, H. P.; Roth, C. Chem. Ber. 1975, 108, 1776-
1778; (b) De Angelis, F.; Crasso, M.; Nicoletti, R. Synthesis 1977, 335-
336; (c) Botta, M.; De Angelis, F.; Nicoletti, R. J. Heterocycl. Chem.
1979, 16, 501-504; (d) Bähn, S.; Imn, S.; Mevius, K.; Neubert, L.;
Tillack, A.; Williams, J. M. J.; Beller, M. Chem. Eur. J. 2010, 16, 3590-
3593; (e) Zhang, Y.; Qi, X.; Cui, X.; Shi, F.; Deng, Y. Tetrahedron Lett.
2011, 52, 1334-1338.
12. (a) Ekström, J.; Wettergren, J.; Adolfsson, H. Adv. Synth. Catal. 2007,
349, 1609-1613; (b) Martínez, R.; Ramón, D. J.; Yus, M. J. Org. Chem.
2008, 73, 9778-9780; (c) Vander Mierde, H.; Van Der Voort, P.;
Verpoort, F. Tetrahedron Lett. 2008, 49, 6893-6895; (d) Vander Mierde,
H.; Van Der Voort, P.; Verpoort, F. Tetrahedron Lett. 2009, 50, 201-203;
(e) Xu, Q.; Li, Q.; Zhu, X.; Chen, J. Adv. Synth. Catal. 2013, 335, 73-80.
13. (a) Martínez, R.; Brand, G. J.; Ramón, D. J.; Yus, M. Tetrahedron Lett.
2005, 46, 3683-3686; (b) Martínez, R.; Ramón, D. J.; Yus, M.
Tetrahedron 2006, 62, 8982-8987; (c) Martínez, R.; Ramón, D. J.; Yus,
M.; Tetrahedron 2006, 62, 8988-9001; (d) Martínez, R.; Ramón, D. J.;
Yus, M. Org. Biomol. Chem. 2009, 7, 2176-2181; (e) Martínez-Asencio,
A.; Ramón, D. J.; Yus, M. Tetrahedron Lett. 2010, 51, 325-327; (f)
Martínez-Asencio, A.; Ramón, D. J.; Yus, M.; Tetrahedron 2011, 67,
3140-3149; (g) Cano, R.; Ramón, D. J.; Yus, M. J. Org. Chem. 2011, 76,
5547-5557; (h) Martínez-Asencio, A.; Yus, M.; Ramón, D. J. Synthesis
2011, 3730-3740; (i) Cano, R.; Yus, M.; Ramón, D. J. Chem. Commun.
2012, 48, 7628-7630.