Intramolecular Mannich and Michael Annulation Reactions of Lactam Derivatives Bearing Enals to Afford Bicyclic N-Heterocycles

Article abstract of DOI:10.1021/acs.orglett.9b03210

Acid-catalyzed intramolecular vinylogous Mannich reactions and intramolecular Michael reactions affording pyrrolizinone-fused N-heterocycles from hydroxylactam derivatives bearing enals have been developed. Depending on the substituent on the hydroxylacta

Full text of DOI:10.1021/acs.orglett.9b03210

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Intramolecular Mannich and Michael Annulation Reactions of
Lactam Derivatives Bearing Enals To Afford Bicyclic N‑Heterocycles
Yarkali Krishna, Kola Shilpa, and Fujie Tanaka*
Chemistry and Chemical Bioengineering Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha,
Onna, Okinawa 904-0495, Japan
S
* Supporting Information
ABSTRACT: Acid-catalyzed intramolecular vinylogous Man-
nich reactions and intramolecular Michael reactions affording
pyrrolizinone-fused N-heterocycles from hydroxylactam deriva-
tives bearing enals have been developed. Depending on the
substituent on the hydroxylactam, the enal moiety acted either
as a nucleophile (i.e., as an enol/enolate) or as an electrophile to
react with the N-acyliminium ion or enamide generated from
the hydroxylactam moiety, respectively. The reactions were
demonstrated in the construction of fused N-heterocycles with
5- to 8-membered rings.
icyclic N-heterocycles bearing pyrrolidine rings or
pyrrolidin-2-one moieties, such as pyrrolizines,1 indoli-
Scheme 1. Reaction Paths Leading to Pyrrolidinone-Fused
Bicyclic N-Heterocycles
B
zines,2 indolizinones,2 pyrroloazepines,3 and pyrroloazocine,3
are present in bioactive natural products1−3 (Figure 1).
Figure 1. Bioactive natural products with pyrrolizine and related
bicyclic N-heterocycle cores.
which is generated from N-acyliminium ion B, with the enal
moiety (Scheme 1, path b).
Hydroxylactam derivatives5,6 have been used as starting
materials for the synthesis of fused N-heterocyclic systems; in
these reactions, N-acyliminium ions7 are generated in situ and
are used as electrophiles to react with nucleophiles to lead to
the formation of a new ring.5,6 Nucleophiles and nucleophile
precursors previously used in these reactions include
allylsilanes,5a,d alkenes,5b,e,f,j alkynes,5h,k heteroaryls,5c,i enami-
des,5g acetals,5l−n dithioacetals,6a and enals5o,6c−e (in pre-
viously reported examples, the hydroxylactam enals were used
for Morita−Baylis−Hillman reactions5o,6c−e). In these re-
ported reactions, for each method, the substituents on the
hydroxylactam ring and the ring size that can be synthesized
Methods for the synthesis of these N-heterocycles are of
interest in drug discovery and related research.1−3 Whereas
various methods for the synthesis of the N-heterocycles have
been reported, each can be used only for the synthesis of
certain types of N-heterocycles.1−4 Here we report a strategy
that allows the synthesis of various pyrrolidinone-fused bicyclic
N-heterocycles with different ring sizes (Scheme 1).
In our strategy, hydroxylactam enals A are used as starting
materials. We hypothesized that, depending on the substituents
on the lactam ring of A, the cyclization would occur either
through the intramolecular vinylogous Mannich reaction of the
enal enolate or the enol with the N-acyliminium ion B
generated in situ (Scheme 1, path a) or through the
intramolecular Michael addition reaction of enamide C,
Received: September 10, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03210
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
are limited. Only a few examples of construction of rings larger
than six-membered rings in these reactions have been
reported.5f,g,i,l−n,6e
high loading of TfOH gave the product in a short time, the use
of 0.3 equiv of TfOH relative to 1a at room temperature (25
°C) led to the formation of the product in a high yield as
almost a single diastereomer within a reasonable reaction time
(75%, dr >20:1, after 20 h, entry 5).
In previous reports, vinylogous reactions of enals were often
performed in the presence of amine-based catalysts to form
enamines10a−f as intermediates or in the presence of metal
catalysts to form enolates10g as intermediates. In the reaction
of 1a, the acid catalysis worked for the formation of the enol/
enolate from the enal group as well as the formation of the
iminium ion from the hydroxylactam to afford 2a; the use of
silyl enol ether derivatives12 of the enal was not required to
give the product.
Next, the scope of the intramolecular vinylogous Mannich
reaction was evaluated using various hydroxylactam enals 1, in
which R2 is H or Ph, to afford bicyclic N-heterocycles 3 under
the conditions identified for the formation of 3a, i.e., in the
presence of TfOH (0.3 equiv) in CH3CN (Scheme 2). For the
formation of 3a−e, single enantiomers 1a−e were used as
starting materials. Depending on the methylene chain length of
1, five- and six-membered rings were constructed. Desired
products were obtained from the reactions of hydroxylactam
derivatives bearing benzyloxy, methoxy, or p-methoxybenzy-
loxy groups, dibenxyloxy groups, or fused benzene or
substituted benzene rings. For the reactions constructing
five-membered rings, the products were obtained as single
diastereomers in most cases (dr >20:1 for 3a−c, 3f−i, and 3k;
dr 10:1 for 3d). For the reactions generating six-membered
rings, two diastereomers were formed, and each diastereomer
of 3e and of 3j was isolated as the single diastereomer by usual
purification. The hydroxylactam enal bearing a substituent at
the enal α-position was also converted to the desired product
(formation of 3h). The pyrrolizinone derivative bearing a
tetrasubstituted carbon center was also constructed (formation
of 3i). The hydroxylactam bearing an enone moiety instead of
an enal moiety also afforded the corresponding desired product
(formation of 3k). Whereas five- and six-membered rings were
readily constructed through the intramolecular vinylogous
Mannich reactions, formation of seven-membered rings was
not observed under the same conditions.
We hypothesized that the use of hydroxylactam derivatives
with enolizable substituents at the carbon bearing the hydroxy
group would alter the reaction mode (Scheme 1, path b) and
expand the range of the fused N-heterocycles that can be
synthesized. When enamides would be generated from the N-
acyliminium ions, the enamides5g,8,9 should act as nucleophiles,
and the bond formation positions will differ from those of the
reactions that the N-acyliminium ions act as electrophiles (path
a). With the reactions of the enamides, the construction of
seven- and eight-membered rings should be readily achieved.
We designed an α,β-unsaturated aldehyde (or enal) to serve as
the reacting group with the N-acyliminium ions (path a) and
with enamides (path b), because the α,β-unsaturated aldehyde
moiety can act as a nucleophile10 (as an enol or an enolate)
and as an electrophile11 (as a Michael reaction acceptor).
First, the Mannich reaction of the enal donor with the N-
acyliminium ion generated in situ (Scheme 1, path a) was
examined using hydroxylactam enal 1a to afford hexahydro-
pyrrolizinone derivative 2a, which was directly reduced by
NaBH4 to give 3a (Table 1). Enantiomerically pure
hydroxylactam 1a was synthesized from (R)-(+)-malic acid
(Supporting Information). When TfOH was used as an acid
catalyst in CH3CN, 3a was obtained in high yields with high
diastereoselectivity (entries 3−5). Although conditions with a
Table 1. Screening of Catalysts and Conditions for the
a
Formation of 2a from 1a
time
(h)
yield
b
c
entry
1
catalyst (equiv)
solvent
(%)
dr
d
d
BnNH2 (0.3)−PhCOOH
CH3CN
24
−
−
(0.4)
d
d
2
Pyrrolidine (0.3)−
CH3CN
24
−
−
PhCOOH (0.4)
The stereochemistries of 3a and ( )-2f were determined to
be as shown in Scheme 2 by X-ray crystal structural analysis of
their derivatives (see below and the Supporting Information).
Next, the intramolecular Michael reactions via the formation
of the enamides (Scheme 1, path b) were examined (Scheme
3). When the reaction to form 3l was tested in the presence of
TfOH (0.3 equiv to 1) in CH3CN, which was the same
conditions used for the intramolecular Mannich reactions to
form 3a−k, the reaction was faster than the reactions shown in
Scheme 2. Reducing the loading of TfOH to 0.1 equiv to 1 also
afforded 3l in the same yield. Thus, the intramolecular Michael
reactions of various substrates 1l−u were performed in the
presence of TfOH (0.1 equiv). In all the cases, the
intramolecular Michael reaction step was completed within 1
h. Depending on the chain length between the amide group
nitrogen and the enal group, products with six-, seven-, and
eight-membered rings 3l−u were obtained. Although the
reactions were performed in the presence of TfOH, product 3r,
which has an acetal group, was obtained from 1r. Product 3u,
which has an oxygen-containing eight-membered ring, was also
obtained.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
TfOH (1.0)
TfOH (0.5)
TfOH (0.3)
TfOH (0.2)
TfOH (0.1)
TfOH (0.3)
TfOH (0.3)
TfOH (0.3)
TsOH (0.3)
MeOTf (0.3)
MsOH (0.3)
BF3.OEt3 (0.3)
TMSOTf (0.3)
TfOH (0.3)
TfOH (0.3)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
toluene
acetone
THF
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
8
80
78
75
68
63
<10
50
50
50
63
50
45
73
70
40
>20:1
>20:1
>20:1
>20:1
>20:1
-
>20:1
>20:1
6:1
10:1
10:1
10:1
18:1
12
20
30
40
48
48
48
24
24
36
24
24
72
72
>20:1
>20:1
e
17
a
Reaction conditions (first step): 1a (0.11 mmol), catalyst (equiv),
solvent (1.0 mL) at room temperature (25 °C) for indicated time; see
b
Supporting Information for details. Isolated yield of 3a from 1a.
c
d
Data of 2a before purification, determined by 1H NMR analysis. 2a
e
was not formed (1a remained). Reaction at 0 °C.
B
DOI: 10.1021/acs.orglett.9b03210
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Organic Letters
Letter
Scheme 2. Scope of the Intramolecular Vinylogous Mannich
Reactions
Scheme 3. Scope of the Intramolecular Michael Reactions
a
a
Conditions: 1 (0.1 mmol, 1.0 equiv) and TfOH (0.1 equiv) in
CH3CN (1.0 mL) at rt (25 °C) for the indicated time, then reduction
b
c
using NaBH4. A 1-g scale reaction. TfOH (0.3 equiv).
Scheme 4. Isolation of Enamide Intermediates and the
Reactions of the Enamides
a
Conditions: 1 (0.1 mmol, 1.0 equiv) and TfOH (0.3 equiv) in
CH3CN (1.0 mL) at rt (25 °C) for the indicated time, then reduction
using NaBH4, except where noted. Isolated yield of 3 from 1 is listed.
The dr values of 2 determined by 1H NMR analysis before
b
c
purification are shown. No reduction step was used. Data taken
d
from Table 1, entry 5. A 1 g-scale reaction; 1a (3.6 mmol).
Formation of 2 was performed in THF. Corresponding ketone (not
e
f
aldehyde) was used as the starting material.
Whereas aryl-group-fused hydroxylactam enals afforded the
intramolecular Michael reaction products, product 3v was not
formed from the corresponding starting material under the
same conditions as those used for the formation of 3l−u.
When the reaction of the formation of 1l was stopped at an
early stage of the reaction, enamide 4 and 2l were obtained
(Scheme 4). When enamide 4 was treated under the same
conditions with TfOH, product 2l was formed. These results
indicate that the enamide is the intermediate of the reaction for
the formation of 2l from 1l. Similarly, (E)- and (Z)-isomers of
enamide 5 were isolated, and these were also transformed to
product 2p (Scheme 4).
In previously reported reactions, when five-membered
hydroxylactam derivatives with enolizable substituents at the
carbon bearing the hydroxy group were used for the formation
of fused N-heterocycles, products obtained were only through
the N-acyliminium ions.5c,d,l Thus, the formation of the
C
DOI: 10.1021/acs.orglett.9b03210
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Organic Letters
Letter
enamide intermediates to lead to products 3l−u shown in
Scheme 3 is notable.
To demonstrate the utility of the reactions, products 2 and 3
were transformed to various derivatives 6−11 (Scheme 5; see
Experimental procedures, characterization data of
compounds (PDF)
NMR spectra (PDF, PDF, PDF)
Accession Codes
Scheme 5. Transformations of the Products
CCDC 1950699−1950700 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ftanaka@oist.jp.
ORCID
Fujie Tanaka: 0000-0001-6388-9343
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. Michael Chandro Roy and Dr. Hyung Been
Kang, Research Support Division, Okinawa Institute of Science
and Technology Graduate University for mass analyses and X-
ray crystal structural analysis of compound 10, respectively. We
thank Dr. Mikio Yamasaki, Rigaku for the refinement and the
cif file formation of the crystal structure of 10. This study was
supported by the Okinawa Institute of Science and Technology
Graduate University.
REFERENCES
■
also Supporting Information for additional transformations).
X-ray crystal structural analyses of 8 and 10 (CCDC 1950699
and CCDC 1950700, respectively) were used to deduce the
stereochemistry of 3a and 3f, respectively. Further, direct
transformation after the intramolecular Michael reaction was
also tested; the intramolecular Michael addition reaction of 1l
followed by Wittig reaction afforded 12 in one pot.
In summary, we have developed intramolecular vinylogous
Mannich reactions and intramolecular Michael reactions of
hydroxylactam derivatives bearing enal groups to afford
pyrrolidinone-fused N-heterocycles. With these reactions,
oxy-functionalized products retaining the starting material
chirality and benzene-fused products were synthesized.
Depending on the substituent on the hydroxylactam
derivatives, the C−C bond formation proceeded through
either the enal enolate (or enol) addition to the N-acyliminium
ion or the enamide addition to the enal. With a nonenolizable
substituent, a pyrrolizinone derivative bearing a tetrasubsti-
tuted carbon center was obtained. With enolizable substituents,
construction of rings larger than a six-membered ring was
achieved. Our strategy allowed the construction of five- to
eight-membered rings to lead to various pyrrolidinone-fused
N-heterocycles.
(1) (a) Robertson, J.; Stevens, K. Nat. Prod. Rep. 2017, 34, 62.
(b) Mulzer, J.; Scharp, M. Synthesis 1993, 1993, 615. (c) Hashimura,
K.; Tomita, S.; Hiroya, K.; Ogasawara, K. A. J. Chem. Soc., Chem.
Commun. 1995, 2291.
(2) (a) Michael, J. P. Nat. Prod. Rep. 2007, 24, 191. (b) Suga, H.;
Hashimoto, Y.; Yasumura, S.; Takezawa, R.; Itoh, K.; Kakehi, A. J.
Org. Chem. 2013, 78, 10840. (c) Cutter, A. C.; Miller, I. R.; Keily, J.
F.; Bellingham, R. K.; Light, M. E.; Brown, R. C. D. Org. Lett. 2011,
13, 3988.
(3) (a) Yoritate, M.; Takahashi, Y.; Tajima, H.; Ogihara, C.;
Yokoyama, T.; Soda, Y.; Oishi, T.; Sato, T.; Chida, N. J. Am. Chem.
Soc. 2017, 139, 18386. (b) Hou, Y.; Shi, T.; Yang, Y.; Fan, X.; Chen,
J.; Cao, F.; Wang, Z. Org. Lett. 2019, 21, 2952. (c) Miloserdov, F. M.;
Kirillova, M. S.; Muratore, M. E.; Echavarren, A. M. J. Am. Chem. Soc.
2018, 140, 5393. (d) Taniguchi, T.; Iwasaki, K.; Uchiyama, M.;
Tamura, O.; Ishibashi, H. Org. Lett. 2005, 7, 4389. (e) Yao, T.; Guo,
Z.; Liang, X.; Qi, L. J. Org. Chem. 2018, 83, 13370. (f) Zhu, W.; Tong,
S.; Zhu, J.; Wang, M.-X. J. Org. Chem. 2019, 84, 2870. (g) Petrone, D.
A.; Yen, A.; Zeidan, N.; Lautens, M. Org. Lett. 2015, 17, 4838.
(h) Zhang, X.; Mu, T.; Zhan, F.; Ma, L.; Liang, G. Angew. Chem., Int.
Ed. 2011, 50, 6164. (i) Volvoikar, P. S.; Tilve, S. G. Tetrahedron Lett.
2018, 59, 2567.
(4) (a) Honma, T.; Hashimoto, N.; Hayashi, K.; Kawanishi, N.;
Fukasawa, K.; Takaki, T.; Ikeura, C.; Ikuta, M.; Suzuki-Takahashi, I.;
Hayama, T.; Nishimura, S.; Morishima, H. J. Med. Chem. 2001, 44,
4628. (b) Boulahjar, R.; Ouach, A.; Matteo, C.; Bourg, S.; Ravache,
M.; Guevel, R. L.; Marionneau, S.; Oullier, T.; Lozach, O.; Meijer, L.;
Guguen-Guillouzo, C.; Lazar, S.; Akssira, M.; Troin, Y.; Guillaumet,
G.; Routier, S. J. Med. Chem. 2012, 55, 9589. (c) Chiurato, M.;
Routier, S.; Troin, Y.; Guillaumet, G. Eur. J. Org. Chem. 2009, 2009,
3011. (d) Oukoloff, K.; Buron, F.; Routier, S.; Jean, L.; Renard, P.-Y.
Eur. J. Org. Chem. 2015, 2015, 2450. (e) Saikia, A. K.; Das, M. J. Org.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b03210.
D
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Organic Letters
Letter
Chem. 2018, 83, 6178. (f) Zeidan, N.; Beisel, T.; Ross, R.; Lautens, M.
Org. Lett. 2018, 20, 7332.
(5) (a) Park, Y.; Schindler, C. S.; Jacobsen, E. N. J. Am. Chem. Soc.
2016, 138, 14848. (b) Knowles, R. R.; Lin, S.; Jacobsen, E. N. J. Am.
Chem. Soc. 2010, 132, 5030. (c) Raheem, I.; Thiara, P. S.; Jacobsen, E.
N. Org. Lett. 2008, 10, 1577. (d) Liu, H.; Yu, J.; Li, X.; Yan, R.; Xiao,
J.-C.; Hong, R. Org. Lett. 2015, 17, 4444. (e) Li, X.; Li, C.; Zhang, W.;
Lu, X.; Han, S.; Hong, R. Org. Lett. 2010, 12, 1696. (f) Li, C.; Li, X.;
Hong, R. Org. Lett. 2009, 11, 4036. (g) Andna, L.; Miesch, L. Org.
Lett. 2018, 20, 3430. (h) Gharpure, S.; Shelke, Y. G.; Kumar, D. P.
Org. Lett. 2015, 17, 1926. (i) Tanis, S. P.; Deaton, M. V.; Dixon, L. A.;
McMills, M. C.; Raggon, J. W.; Collins, M. A. J. Org. Chem. 1998, 63,
6914. (j) Indukuri, K.; Unnava, R.; Deka, M. J.; Saikia, A. K. J. Org.
Chem. 2013, 78, 10629. (k) Das, M.; Saikia, A. K. J. Org. Chem. 2018,
83, 6178. (l) Srinivas, K.; Singh, N.; Das, D.; Koley, D. Org. Lett.
2017, 19, 274. (m) Koley, D.; Srinivas, K.; Krishna, Y.; Gupta, A. RSC
Adv. 2014, 4, 3934. (n) Koley, D.; Krishna, Y.; Srinivas, K.; Khan, A.
A.; Kant, R. Angew. Chem., Int. Ed. 2014, 53, 13196. (o) Alonso-
Fernandez, J.; Benaiges, C.; Casas, E.; Alibes, R.; Bayon, P.; Busque,
F.; Alvarez-Larena, A.; Figueredo, M. Tetrahedron 2016, 72, 3500.
(6) Alkoxylactam and acyloxylactam derivatives have also been used
for the synthesis of fused N-heteocycles: (a) Chamberlin, A. R.;
Chung, J. Y. J. Am. Chem. Soc. 1983, 105, 3653. (b) Nilson, M. G.;
Funk, R. L. Org. Lett. 2006, 8, 3833. (c) Myers, E. L.; de Vries, J. G.;
Aggarwal, V. K. Angew. Chem., Int. Ed. 2007, 46, 1893. (d) Kurasaki,
H.; Okamoto, I.; Morita, N.; Tamura, O. Chem. - Eur. J. 2009, 15,
12754. (e) Kurasaki, H.; Okamoto, I.; Morita, N.; Tamura, O. Org.
Lett. 2009, 11, 1179. (f) Hanessian, S.; Tremblay, M.; Marzi, M.; Del
Valle, J. R. J. Org. Chem. 2005, 70, 5070.
(7) Wu, P.; Nielsen, T. E. Chem. Rev. 2017, 117, 7811.
(8) (a) Matsubara, R.; Kobayashi, S. Acc. Chem. Res. 2008, 41, 292.
(b) Wang, M.-X. Chem. Commun. 2015, 51, 6039.
(9) (a) Yang, L.; Wang, D.-X.; Huang, Z.-T.; Wang, M.-X. J. Am.
Chem. Soc. 2009, 131, 10390. (b) Yang, L.; Lei, C.-H.; Huang, Z.-T.;
Wang, M.-X. Org. Lett. 2010, 12, 3918. (c) Tong, S.; Wang, D.-X.;
Zhao, L.; Zhu, J.; Wang, M.-X. Angew. Chem., Int. Ed. 2012, 51, 4417.
(d) He, L.; Zhao, L.; Wang, D.-X.; Wang, M.-X. Org. Lett. 2014, 16,
5972. (e) Xu, X.-M.; Lei, C.-H.; Tong, S.; Zhu, J.; Wang, M.-X. Org.
Chem. Front. 2018, 5, 3138. (f) Lei, C.-H.; Wang, D.-X.; Zhao, L.;
Zhu, J.; Wang, M.-X. J. Am. Chem. Soc. 2013, 135, 4708. (g) Nilson,
M. G.; Funk, R. L. Org. Lett. 2006, 8, 3833. (h) Ryder, G. M.; Wille,
U.; Willis, A. C.; Pyne, S. G. Org. Biomol. Chem. 2019, 17, 7025.
(10) Examples of vinylogous reactions of enals (enals act as
nucleophiles): (a) Enders, D.; Huttl, M. R. M.; Grondal, C.; Raabe, G.
̈
Nature 2006, 441, 861. (b) Bergonzini, G.; Vera, S.; Melchiorre, P.
Angew. Chem., Int. Ed. 2010, 49, 9685. (c) Quintard, A.; Lefranc, A.;
Alexakis, A. Org. Lett. 2011, 13, 1540. (d) Basu, S.; Gupta, V.; Nickel,
J.; Schneider, C. Org. Lett. 2014, 16, 274. (e) Feng, J.; Li, X.; Cheng,
J.-P. J. Org. Chem. 2017, 82, 1412. (f) Kutwal, M. S.; Dev, S.; Appayee,
C. Org. Lett. 2019, 21, 2509. (g) Zhong, F.; Yue, W.-J.; Zhang, H.-J.;
Zhang, C.-Y.; Yin, L. J. Am. Chem. Soc. 2018, 140, 15170.
(11) Examples of Michael addition reactions to enals: (a) Paras, N.
A.; MacMillan, D. W. C. J. Am. Chem. Soc. 2001, 123, 4370.
(b) Austin, J.; MacMillan, D. W. C. J. Am. Chem. Soc. 2002, 124, 1172.
(c) Brown, S. P.; Goodwin, N. C.; MacMillan, D. W. C. J. Am. Chem.
Soc. 2003, 125, 1192. (d) Gotoh, H.; Ishikawa, H.; Hayashi, Y. Org.
Lett. 2007, 9, 5307. (e) Quintard, A.; Lefranc, A.; Alexakis, A. Org.
Lett. 2011, 13, 1540. (f) Dell’Amico, L.; Albrecht, L.; Naicker, T.;
Poulsen, P. H.; Jorgensen, K. A. J. Am. Chem. Soc. 2013, 135, 8063.
(g) Guo, Q.; Fraboni, A. J.; Brenner-Moyer, S. E. Org. Lett. 2016, 18,
2628. (h) Hayashi, Y.; Umekubo, N. Angew. Chem., Int. Ed. 2018, 57,
1958. (i) Hayashi, Y.; Yamada, T.; Sato, M.; Watanabe, S.; Kwon, E.;
Iwasaki, K.; Umemiya, S. Org. Lett. 2019, 21, 5183. (j) Marshall, J. A.;
Herold, M.; Eidam, H. S.; Eidam, P. Org. Lett. 2006, 8, 5505.
(k) Palais, L.; Babel, L.; Quintard, A.; Belot, S.; Alexakis, A. Org. Lett.
2010, 12, 1988.
(12) (a) Gieseler, M. T.; Kalesse, M. Org. Lett. 2011, 13, 2430.
(b) Gonzales, A. S.; Arrayas, R. G.; Rivero, M. R.; Carretero, J. C. Org.
Lett. 2008, 10, 4335.
E
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