Cobalt-catalyzed oxidative cyclization of gem-disubstituted conjugated alkenols

Article abstract of DOI:10.1016/j.tetlet.2016.06.064

Aryl gem-disubstituted conjugated alkenols underwent oxidative cyclization affording 2,5,5-trisubstituted tetrahydrofurans in reasonable yields and good diastereoselectivities using the reductive termination variation of the Mukaiyama aerobic oxidative reaction. Under oxidative termination, the same alkenols produced diols and ketonic by-products via the double hydration and beta-scission competing pathways. Furthermore, the differences in alkenol reactivity under the reductive and oxidative termination conditions were investigated.

Full text of DOI:10.1016/j.tetlet.2016.06.064

Tetrahedron Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Cobalt-catalyzed oxidative cyclization of gem-disubstituted
conjugated alkenols
⇑
Tânia M. F. Alves, Mateus O. Costa, Beatriz A. D. Bispo, Fabiana L. Pedrosa, Marco A. B. Ferreira
Department of Chemistry, Laboratório de Química Bio-Orgânica, Federal University of São Carlos—UFSCar, Rod. Washington Luis, km 235, C.P. 676 São Carlos, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Aryl gem-disubstituted conjugated alkenols underwent oxidative cyclization affording 2,5,5-trisubstituted
tetrahydrofurans in reasonable yields and good diastereoselectivities using the reductive termination
variation of the Mukaiyama aerobic oxidative reaction. Under oxidative termination, the same alkenols
produced diols and ketonic by-products via the double hydration and beta-scission competing pathways.
Furthermore, the differences in alkenol reactivity under the reductive and oxidative termination
conditions were investigated.
Received 10 May 2016
Revised 11 June 2016
Accepted 14 June 2016
Available online xxxx
Keywords:
Ó 2016 Elsevier Ltd. All rights reserved.
Mukaiyama oxidative cyclization
2,5,5-Trisubstituted tetrahydrofuran
Metal catalysis
Introduction
extremely powerful tool in asymmetric synthesis of natural
products and biologically active compounds.9
Substituted tetrahydrofurans (THFs) are common structural
elements of natural products and other biologically active
molecules.1 Therefore, significant efforts have been dedicated to
the development of new stereoselective methods for the construc-
tion of such moieties.2 In particular, synthetic approaches for the
formation of 2,5-disubstituted-THFs have received considerable
attention.2 However there is a lack of stereoselective methodolo-
gies for 2,5,5-trisubstituted-THFs possessing aromatic motifs in
tertiary carbinol centers, presented in some biologically active
compounds (Fig. 1).3
A powerful strategy to achieve 2,5-trans-THFs in a stereoselective
manner involves the Mukaiyama aerobic oxidative cyclization reac-
tion of bis-homoallylic alcohols under a catalytic amount of Co(II)
complexes in the presence of O2 (Scheme 1).4 The low cost and
non-toxic metal, open reaction conditions, lack of moisture
sensitivity, and use of green solvents make this approach appealing.
Therefore, the reaction mechanism has been investigated by
Hartung, showing evidences of the formation of carbon free radical
intermediates.5 Additionally, with slight variations on reaction
conditions, these radical intermediates can be transformed into syn-
thetically useful functional groups such as alcohols,4–6,9 bromides,6b
alkyls,6b,7 and alkylsulfanyls.8 They are also able to perform a
nucleophilic radical addition to electron-withdrawing conjugated
double/triple bonds.7 This methodology has proven to be an
A critical issue for the application of this methodology has been
the degree of substitution of the double bond. Previous work has
explored tertiary pentenol derivatives forming 2,5,5-trisubsti-
tuted-THFs in moderate yields and selectivities (Scheme 2).3c
Moreover, excellent selectivities and yields for terminal
unsubstituted and gem-alkyl olefins have been achieved.4–9 Alkyl
or aryl substituents attached at the terminal position provide
excellent 2,5-trans diastereoselectivity independent of the double
bond geometry.5a,6b However, the newly formed stereocenter in
the side chain is obtained in a ratio of 1:1 from the radical
symmetric intermediate under the traditional oxidative
termination.5a,6b Harsh reaction conditions were also necessary to
cyclize trialkyl-trisubstituted terminal olefins, affording the
products with low yield and diastereoselectivity.5
We envisioned whether this transformation could be performed
employing aryl gem-disubstituted conjugated double bonds
(Scheme 2). We hypothesized that this reaction would stereoselec-
tively form a 2,5,5-trisubstituted THF, thus creating a tertiary
carbinol center with an aromatic group. Herein we report our
contribution toward extending the scope of this methodology
and in understanding the reactivity of these systems.
Results and discussion
Reductive termination
Initially, we focused on the feasibility of the Mukaiyama
cyclization of simple model substrates 1a, under reductive
⇑
Corresponding author.
E-mail address: marco.ferreira@ufscar.br (M.A.B. Ferreira).
http://dx.doi.org/10.1016/j.tetlet.2016.06.064
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Alves, T. M. F.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.06.064

2
T. M. F. Alves et al. / Tetrahedron Letters xxx (2016) xxx–xxx
by silica gel chromatography of 2,5-trans (8a–h) and 2,5-cis
(9a–h) THFs in moderate to poor yields and good diastereoselectiv-
ities in favor of 8a–h (2,5-trans). The relative stereochemistry was
confirmed by NOE experiments of both 8b/9b diastereoisomers.10
Better diastereoselectivities were obtained at 60 °C, but a reduction
in the yield also occurred. When comparing the reaction involving
the compounds 7a and 7h, erosion of diastereoselectivity was
observed for electron-donating aromatic substituents.
HO
O
N
N
(7S,10S)-boivinianin B
N
F
O
O
O
N
N
N
N
Despite the quantitative consumptions of starting materials the
isolated yields after flash chromatography were lower than
expected. We were not able to assess the yields from NMR using
an internal standard of the crude material due to paramagnetic
character of the cobalt catalyst, which produced broad signals in
most cases. To examine these results further, we collected kinetic
information by monitoring the cyclization of alkenols 7b and 7h
at 60 °C via quantitative GC–MS analysis. The consumption of
starting material and formation of cyclized products were moni-
tored and the changes in concentration are plotted as a function
of time in Figure 3. After approximately 2 and 3 h, respectively,
the starting material was completely consumed for the reactions
of 7b and 7h. The amount of products remains unchanged after this
point, with yields of 49% for 7b and 54% for 7h. The achieved
diastereoselectivity ratios were 5.7:1 for 8b:9b and 1.2:1 for
8h:9h. Additionally, the diastereomeric ratios of the products are
nearly constant with respect to conversion.
While the diastereoselectivities obtained from the kinetic
progress were very similar to those obtained from isolated prod-
ucts after silica gel chromatography, the yields decrease by approx.
30–40%. Pagenkopf obtained similar results for 2,5,5-trisubstituted
THF.3c We hypothesize in our case a product decomposition by a
ring opening due to exposure to silica gel through the formation
of a stable tertiary carbocation.
F
N
SCH 45009
Figure 1. Examples of biologically active compound containing 2,2,5-trisubstituted
THF structural motifs.
termination version for construction of the 2a core (Table 1). In this
initial screening, we used 1,4-cyclohexadiene (1,4-CHD) as a
reducing agent (Table 1). The combination of CoL2 (3, 5, or 6)
(Fig. 2), 1,4-CHD (20 equiv) in toluene and open flask conditions
(atmospheric air), catalyzed the reaction to afford the expected
THFs (2a–c) and complete conversion of alkenol 1a after 4 h
(entries 1–3). The mass balance was a critical issue leading us to
suspect that under these conditions we had decomposition of the
starting material and/or product. A more sterically bulky alkenol
could also be associated with the lower selectivity due to repulsive
interaction in catalyst coordination.3c Aiming to improve the
reaction conditions, we systematically varied the concentration.
At lower and higher concentrations (entries
4
and 5,
respectively), we experienced difficulties in achieving acceptable
yields. It is noteworthy that when only 1,4-CHD is used as
solvent, as described by Hartung,5,7 decomposition of the starting
material was observed (entry 6). The best yield was achieved with
0.06 M employing catalyst 5 at 15 mol % (entry 7). We were not
able to reduce the catalyst loading, as 7.5 mol % of 5 lead to a
reduced reaction yield, producing 2a in only 13% (entry 8). No sub-
strate consumption occurred in the absence of O2 and 1,4-CHD
(entry 9). Finally, the alkenol 1b (entry 10) lead to similar results
to 1a, revealing a minor influence of an electron withdrawing
group on aromatic ring on this reaction. However, it was noted a
higher yield for the alkenol 1c (entry 11) possessing an electron
donating group.
Finally, a control experiment was performed in order to verify if
the presence of the catalyst was critical to induction of selectivity.
The alkenol 7g was cyclized in the presence of p-TSA (1 equiv) and
CH2Cl2 (0.06 M), at room temperature affording
a separable
mixture by silica gel chromatography of 8g:9g in 58% yield with
no diastereoselectivity (dr = 1:1), proving the importance of the
catalyst in this reaction.
Oxidative termination
The alkenol 1a0 possessing a trisubstituted double bond also
generated the cyclic product 2a0 in 31% yield (Scheme 3). However,
attempts to lengthen the alkyl chain proved to be unfruitful as no
products were observed with the alkenol 1a00 even after 24 h, with
partial recovery of the starting material.
With the best reaction condition in hand, we next investigated
the diastereoselectivity of the Mukaiyama cyclization with
alkenols 7a–h (Scheme 4). The reaction gave a separable mixture
In a second stage of this study, we explored the oxidative
termination version of the Mukaiyama cyclization. In this screen-
ing, we used catalysts 3–6 (Fig. 2), t-BuOOH as additive, and i-PrOH
as solvent under O2 (1 atm) atmosphere. Surprisingly, as shown in
Table 2, we were unable to obtain the cyclized product 10 under
oxidative termination for alkenols 1a–c. Depending on the cobalt
OH
OH
OH
CoL2, O2
O
R
radical functionalization
R
R
ref 3c
reductor
60-80 ˚C
oxidative termination
reductive termination
dr 2:1 to 5:1
R = alkyl, aryl, alkynyl
CoL2
O2, H2X
X
OH
R'
O
SR
OH
Br
R
CoL2, O2
O2
R'
R
O
O
O
RS-SR
R
R
R
iPrOH
1,4-CHD
ref 4-9
reductor
60-80 ˚C
O
R
dr > 95:5
R' = alkyl, H
BrCCl3
1,4-CHD
EWG
1,4-CHD
X
OH
R
R'
O
1,4-CHD
CoL2, O2
R'
R
H
this work
H
O
reductor
60-80 ˚C
O
R
R
EWG
R' = Aryl
Scheme 1. Synthetic variations of Mukaiyama oxidative cyclization.
Scheme 2. Scope of the Mukaiyama oxidative cyclization.
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T. M. F. Alves et al. / Tetrahedron Letters xxx (2016) xxx–xxx
3
Table 1
8 (15 mol%), air
1,4-CHD (20 equiv)
HO
7
n-Pr
Ar
(
Ar
O
O
n-Pr
n-Pr
Mukaiyama oxidative cyclization reactions of alkenols 1a–c under reductive termi-
nation conditions
Me
Me
+
Ar
60 ˚C, PhMe, 3-4 h
)-8
2,5-trans
(
)-9
2,5-cis
F
Cl
Br
O
O
O
n-Pr
n-Pr
n-Pr
Entry CoL2
Alkenol Concentration Time Yielda (2)
Me
Me
Me
1
2
3
4
3 (15 mol %)
1a
1a
1a
1a
1a
1a
1a
0.04 M
0.04 M
0.04 M
0.02 M
0.1 M
4 h
4 h
4 h
16 h
3 h
18 h
3 h
18 h
18 h
3 h
30%
25%
31%
<5%
8b, 30% dr=5.4:1
8a, 43% dr=4.7:1
8c, 46% dr=4.7:1
(8b, 53%,dr=5.9:1)*
6 (15 mol %)
5 (15 mol %)
5 (15 mol %)
5 (15 mol %)
5 (15 mol %)
5 (15 mol %)
(8a, 56% dr=2.5:1)*
Me
Me
Me
O
5
6%
O
6b
7
0.6 M
Complex mixture
n-Pr
O
n-Pr
n-Pr
Me
Me
Me
0.06 M
0.06 M
0.06 M
0.06 M
0.06 M
34%
13%
<5%
32%
44%
8
5 (7.5 mol %) 1a
8d, 30% dr=5.3:1
8e, 39%, dr=3.5:1
8f, 17% dr=3.4:1
O
n-Pr
9c
10
11
5 (15 mol %)
5 (15 mol %)
5 (15 mol %)
1a
1b
1c
PhO
MeO
3 h
O
a
b
c
n-Pr
Yield of isolated product 2a–c.
The reaction was conducted without solvent.
The reaction was conducted without 1,4-CHD and under argon atmosphere.
Me
Me
8g, 26% dr=4.2:1
8h, 20% dr=2.5:1
*reaction at 80 ˚C
Scheme 4. Substrate scope for Mukaiyama cyclization under reductive termina-
tion. Yields and diastereoselectivities obtained from isolated products.
O
O
O
MeN
O
O
N
N
t-Bu
t-Bu
Again, the mass balance was a critical issue in all entries leading
us to speculate that under these reaction conditions we had
decomposition of the starting material.
O
O
O
4
3
CoL2
O
O
O
Mechanistic interpretation
F3C
CF3
Based on the experimental results from this study and the liter-
ature, the conjugation of aryl gem-disubstituted double bonds
appears to prevent oxidative cyclization reactions via traditional
oxidative termination, but proceed under reductive termination.
Furthermore, gem-disubstituted non-conjugated double bonds, or
conjugated E/Z-disubstituted olefins normally react under any
activation mode.5a,6b
5
6
CF3
Figure 2. Cobalt catalysts used in this work.
Based on the detailed mechanism study of Hartung and col.,5a it
is expected that after the oxygen activation of cobalt-II (I), a
superoxo-binding mode provides a stronger oxidant, converting
the olefin into radical cation II (Scheme 5a). The lowest-energy
chair-like conformer was proposed based on the experimentally
observed diastereoselectivities. The decrease in bond order and,
consequently, the rotational barrier, can lead to the
interconversion of II to II0 by a C–C rotation. This step is the key
to understand the reactivity differences between oxidative and
reductive termination. The inability of the catalyst to promote
the desired reaction via oxidative termination is because the
redox potential in the aromatic gem-disubstituted double bond
increases substantially compared to vicinal disubstituted or non-
substituted derivatives.12 In addition, a reduction of the oxidative
potential of cobalt complex is expected in polar media.13 A more
sterically bulky alkenol could also be associated with the lower
reactivity due to repulsive interaction in catalyst coordination.3a
The low cyclization reactivity eventually allows the occurrence of
secondary radical reactions (Scheme 5b).5a The double hydration
of olefins in the presence of i-PrOH, O2, and cobalt-II (via cobalt
hydride species) is well-known,14 leading to the formation of
radical intermediate V in our system. Hydrogen abstraction from
O
n=1
8 (15mol%)
OH
air (1 atm)
2a'
( )n
1,4-CHD (20 equiv)
PhMe, 80 ˚C, 3 h
n=2
1a'. n=1
1a''.n=2
31%
Scheme 3. Mukaiyama oxidative cyclization reactions of alkenols 1a0 and 1a00 under
reductive termination conditions.
catalyst and additive (t-BuOOH) used, the formation of
by-products 11a and 13a were observed (Table 2, entries 1, 4,
7–9). Hartung and colleagues have detected similar diols using
trialkyl-trisubstituted double bonds.5a In some cases (entries 2, 3,
5, and 6), we were not able to detect the formation of these
compounds, obtaining only complex mixtures. The use of
molecular sieves and dry conditions (entry 8) were also
considered in order to avoid the supposed hydration by-product
11a. However a similar result was obtained, revealing a possible
radical path. We employed an alternative method involving the
pre-activation of catalyst prior the introduction of the alkenol, as
described by Pagenkopf and coworkers,11 but this was
unsuccessful (entry 9). We investigated the reaction profile using
electron-withdrawing (Cl–) and electron-donating (MeO–) groups
on aromatic rings by using alkenols 1b and 1c, respectively (entries
10 and 11), however the desired products were not obtained.
isopropanol by
V leads to formation of 11. A beta-scission
mechanism can lead to the products 12 (path a) or 13 (path b).
Finally, we have noted that only the less polar solvent media,
corresponding to the reductive termination variation of oxidative
cyclization, converted the olefins into the desired products. Under
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4
T. M. F. Alves et al. / Tetrahedron Letters xxx (2016) xxx–xxx
100
90
80
70
60
50
40
30
20
10
0
100
(a)
(b)
9b
13b
9h
13h
8b
12b
8h
12h
7b
11b
80
60
40
20
0
7h
11h
0
50
100
150
200
250
300
0
50
100
150
200
250
300
350
Time (min)
Time (min)
Figure 3. Reaction progress profile for the reaction in Scheme 4 at 60 °C obtained using quantitative GC–MS. Yield versus time for alkenols 7b (a) and 7h (b).
Table 2
Screening of conjugated alkenols 1a–c for the Mukaiyama aerobic oxidative cycliza-
Oxidative cyclization methanism
tion under oxidative termination
2 H2O +
Ar
HO
R
Ar
not observed
O
R
2,5-trans
OH
L2CoII -- O2
CoL2 (10 mol%)
O2 (1 atm)
HO
O
OH
OH
O
O
+
+
R
+
R
(a)
R
R
additive
i-PrOH, 60 ˚C
24h
L2CoIII -- O2
3[ H ] / O2
R
OH
13a-c
10a-c
11a-c
12c
1a. R = Ph
1b. R = p-Cl-C6H4
H
O
Ar
L2CoIII -- O2H
L2CoII -- O2
1c. R = p-CH3O-C6H4
R
Ar
O
H
O
R
I
Ar
Conva
87%
Complex mixture
Complex mixture
Yieldb
22% (11a)
III
Entry
CoL2
Alkenol
R
II
1
3
3
4
4
5
5
6
3
3
3
3
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
—
—
23% (13a)
C-C rotation
2c
3
L2CoII -- O2
Ar
4c
5
78%
13% (11a)
—
Ar
O
H
O
R
2,5-cis
Complex mixture
Complex mixture
Quant.
89%
79%
79%
6c
7
R
II'
beta-scission mechanism
19% (11a)
—
—
—
—
—
8d
9c,e
10
11
21% (11a)
15% (11a)
21% (11b)
15% (11c)
25% (13a)
19% (13a)
12% (13b)
12% (13c)
(b)
OH
Ar
i-PrOH
HO
11
85%
25% (12c)
O2CoIII
L
O
Ar
O
CoL2, O2
i-PrOH
path a
+
Ar
HO
Ar
a
b
c
Ar
Conversion based on recovery of starting material.
Yield of isolated by-product.
t-BuOOH (10 mol %) was used as additive.
Use of MS 4 Å, and dry i-PrOH.
12
HO
O
IV
V
path b
+
Ar
CH3
d
e
OH
OH
OH
13
Catalyst was pre-activated.
Scheme 5. Proposed mechanism.
this condition, a higher oxidative potential, combined with a
complete inhibition of hydration of olefins due to the absence of
i-PrOH, explains the reactivity difference between reaction
conditions. In regard to diastereoselectivity, formation of a stable
benzylic tertiary carbocation may explain the lower diastereoselec-
tivities of 2,5-trans diastereoisomer where C–C rotation and shift
equilibrium lead to II0. However, the preference for the pseudo-
axial or equatorial orientation of aryl substituent of intermediate
II is not clear. Steric repulsion between the alkenol and the cobalt
ligand may play a major role in this equilibrium.
Concluding, in this study, aryl gem-disubstituted conjugated
alkenols underwent the reductive termination variation of the
Mukaiyama aerobic oxidative cyclization, providing 2,5,5-trisub-
stituted THF with moderate yields and diastereoselectivities.
Although the diastereoselectivity of products can be improved,
the difference in reactivity under these termination conditions
provides further insights into the mechanism of the Mukaiyama
oxidative cyclization and competing reactions. Studies regarding
the structural nature of the cobalt complex and ligand coordination
in solution are currently underway.
Acknowledgments
The authors acknowledge FAPESP (13/02311-3), CNPq
(477944/2013-2) and CAPES for financial support and fellowships.
We also thank Prof. Márcio W. Paixão and Prof. Timothy J.
Brocksom for the careful reading of the manuscript and helpful
suggestions.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.06.
064.
Please cite this article in press as: Alves, T. M. F.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.06.064

T. M. F. Alves et al. / Tetrahedron Letters xxx (2016) xxx–xxx
8. Fries, P.; Müller, M. K.; Hartung, J. Org. Biomol. Chem. 2013, 11, 2630.
5
References and notes
9. (a) Valot, G.; Mailhol, D.; Regens, C. S.; O’Malley, D. P.; Godineau, E.; Takikawa,
H.; Philipps, P.; Fürstner, A. Eur. J. Chem. 2015, 21, 2398; (b) Phillips, G. A.;
Wright, T. B.; Stevens, A. C.; Pagenkopf, B. L. Can. J. Chem. 2015, 93, 196; (c)
Sheikh, N. S. RSC Adv. 2015, 5, 3941; (d) Wu, D.; Forsyth, C. J. Org. Lett. 2013, 15,
1178; (e) Valot, G.; Regens, C. S.; O’Malley, D. P.; Godineau, E.; Takikawa, H.;
Fürstner, A. Angew. Chem., Int. Ed. 2013, 52, 9534; (f) Morra, N. A.; Pagenkopf, B.
L. Tetrahedron 2013, 69, 8632; (g) Dias, L. C.; Farina, L. S.; Ferreira, M. A. B. J.
Braz. Chem. Soc. 2013, 24, 184; (h) Dias, L. C.; Ferreira, M. A. B. J. Org. Chem.
2012, 77, 4046; (i) Chen, Z.; Sinha, S. C. Tetrahedron Lett. 2009, 50, 6398; (j)
Wang, J.; Pagenkopf, B. L. Org. Lett. 2007, 9, 3703; (k) Zhao, H. D.; Gorman, J. S.
T.; Pagenkopf, B. L. Org. Lett. 2006, 8, 4379; (l) Evans, P. A.; Cui, J.; Gharpure, S. J.;
Polosukhin, A.; Zhang, H.-R. J. Am. Chem. Soc. 2003, 125, 14702; (m) Takahashi,
S.; Kubota, A.; Nakata, T. Angew. Chem., Int. Ed. 2002, 41, 4751.
1. (a) Bermejo, A.; Figadere, B.; Zafra-Polo, M.-C.; Barrachina, I.; Estornell, E.;
Cortes, D. Nat. Prod. Rep. 2005, 22, 269; (b) Kobayashi, J. J. Antibiot. 2008, 61,
271; (c) Lorente, A.; Lamariano-Merketegi, J.; Albericio, F.; Álvarez, M. Chem.
Rev. 2013, 113, 4567.
2. (a) Wolfe, J. P.; Hay, M. B. Tetrahedron 2007, 63, 261; (b) Clark, P. Tetrahedron
2011, 67, 4949; (c) Hartung, J.; Greb, M. J. Organomet. Chem. 2002, 661, 67; (d)
Jalce, G.; Franck, X.; Figadère, B. Tetrahedron: Asymmetry 2009, 20, 2537.
3. (a) Faul, M. M.; Huff, B. E. Chem. Rev. 2000, 100, 2407; (b) Abecassis, K.; Gibson,
S. E. Eur. J. Org. Chem. 2010, 2938; (c) Phillips, G. A.; Palmer, C.; Stevens, A. C.;
Piotrowski, M. L.; Dekruyf, D. S. R.; Pagenkopf, B. L. Tetrahedron Lett. 2015, 56,
6052; (d) Lovely, R. G.; Saksena, A. K.; Girijavallabhan, V. M.; Blundell, P.; Guzik,
H.; Loebenberg, D.; Parmegiani, R. M.; Cacciapuoti, A. Bioorg. Med. Chem. Lett.
2002, 12, 1739.
10. See ESI for further details.
11. Morra, N. A.; Pagenkopf, B. L. Eur. J. Org. Chem. 2013, 756.
4. Inoki, S.; Mukaiyama, T. Chem. Lett. 1990, 1, 67.
12. Hamilton, D. S.; Nicewicz, D. A. J. Am. Chem. Soc. 2012, 134, 18577.
13. Araujo, C. M.; Doherty, M. D.; Konezny, S. J.; Luca, O. R.; Usyatinsky, A.; Grade,
H.; Lobkovsky, E.; Soloviechik, G. L.; Crabtree, R. H.; Batista, V. S. Dalton Trans.
2012, 41, 3562.
14. (a) Kato, K.; Yamada, T.; Takai, T.; Inoki, S.; Isayama, S. Bull. Chem. Soc. Jpn. 1990,
63, 179; (b) Mukaiyama, T.; Yamada, T. Bull. Chem. Soc. Jpn. 1995, 68, 17.
5. (a) Peérez, B. M.; Schuch, D.; Hartung, J. Org. Biomol. Chem. 2008, 6, 3532; (b)
Schuch, D.; Fries, P.; Donges, M.; Pérez, B. M.; Hartung, J. J. Am. Chem. Soc. 2009,
131, 12918.
6. (a) Palmer, C.; Morra, N. A.; Stevens, A. C.; Bajtos, B.; Machin, B. P.; Pagenkopf, B.
L. Org. Lett. 2009, 11, 5614; (b) Pérez, B. M.; Hartung, J. Tetrahedron Lett. 2009,
50, 960.
7. Fries, P.; Halter, D.; Kleinschek, A.; Hartung, J. J. Am. Chem. Soc. 2011, 133, 3906.
Please cite this article in press as: Alves, T. M. F.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.06.064