Guanidine-catalyzed γ-selective morita-baylis-hillman reactions on α,γ-dialkyl-allenoates: Access to densely substituted heterocycles

Article abstract of DOI:10.1055/s-0033-1339471

N-Methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) was discovered as an excellent catalyst for the Morita-Baylis-?-Hillman reaction for previously hard-to-activate α,γ-dialkyl allenoate substrates. The obtained densely substituted allenic alcohols, which are generally inaccessible with other Lewis base catalysts, could be further converted into 2,5-dihydrofuran and 2H-pyran-2-one heterocyclic structures with challenging substitution patterns. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0033-1339471

CLUSTER
▌2535
cluster
Guanidine-Catalyzed γ-Selective Morita–Baylis–Hillman Reactions on
α,γ-Dialkyl-Allenoates: Access to Densely Substituted Heterocycles
Morita–Baylis–Hillman Reactions on α,γ-Dialkyl Allenoates
Philipp Selig,*a Aleksej Turočkin,a William Ravenb
a
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
Fax +49(241)8092127; E-mail: philipp.selig@rwth-aachen.de
b
Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
Received: 27.06.2013; Accepted: 27.06.2013
umpolung reaction by a proton shift to form vinyl ylides
C with nucleophilic properties at the β- and δ-positions.
Abstract: N-Methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD)
was discovered as an excellent catalyst for the Morita–Baylis–
Hillman reaction for previously hard-to-activate α,γ-dialkyl allenoate
substrates. The obtained densely substituted allenic alcohols, which
are generally inaccessible with other Lewis base catalysts, could be
further converted into 2,5-dihydrofuran and 2H-pyran-2-one het-
erocyclic structures with challenging substitution patterns.
.
CO2R3
CO2R3
R
1 = H
R2 = Me
α
β
Nu
Nu
γ
δ
R1
R2
CO2R3
Nu
Key words: allenes, catalysis, nucleophiles, guanidines, cycliza-
tion
H+ transfer
⇓
B
C
CO2R3
umpolung
CO2R3
Nu
Nu
A
In organic chemistry, ‘superbases’ are most commonly
defined by their Brønsted basic properties.1 While more
thorough definitions do in fact exist,2 a superbase is often
simply regarded as any species of which the correspond-
ing acid can no longer be easily deprotonated by the hy-
droxide ion OH–. Considering only the organic, that is,
nonmetal superbases, guanidines have emerged as espe-
cially versatile reagents and organocatalysts for synthetic
organic chemistry.3 While the employment of the Brøn-
sted basic properties of guanidines is thus quite firmly es-
tablished nowadays, applications of their pronounced
Lewis basic properties are still in their infancy.4 Especial-
ly bicyclic guanidines,5 however, have recently been rec-
ognized as not only strongly Brønsted basic, but also
remarkably Lewis basic and highly nucleophilic re-
agents.6 We herein report a further application of these
‘super’ Lewis basic properties of guanidines for the nu-
cleophilic activation of densely substituted allenoates.
β
β'
γ
R1 = Me
2 = H
B'
R
C'
Scheme 1 Potentially undesirable umpolung reactions after nucleo-
philic activation of α- or γ-methyl allenoates by phosphine catalysts.
Nucleophilic positions are shown in red.
Similarly, an α-methyl allenoate derived dienolate B′
could be transformed into a methylene ylide C′, which is
nucleophilic at the β- and β′-positions. While both umpo-
lung reactions have been exploited synthetically,10,11 they
also represent a major limitation on the use of dienolate
reactivity for substituted allenoate substrates. The use of
nitrogen Lewis bases (e.g. DABCO) could theoretically
prevent unwanted umpolung reactions and allow for di-
enolate reactivity even in alkyl-substituted cases. Unfor-
tunately, however, tertiary amines generally proved
insufficiently reactive for the activation of sterically de-
manding substrates and accordingly, many dienolate-
based transformations, like the prototypical reaction be-
tween allenoates and aromatic aldehydes, could be real-
ized for simple buta-2,3-dienoates only.12 The
introduction of further alkyl groups either effectively shut
down the reaction for N-nucleophiles, or opened compet-
ing reaction pathways by umpolung for P-nucleophiles.
Lewis basic allenoate activations and their applications in
organic synthesis have undergone a staggering develop-
ment since their initial discovery in 1995.7,8 Most com-
monly, tertiary phosphines are employed as the
nucleophilic catalysts due to their high nucleophilicity
and Lewis basicity.9 In general, the initial attack of a nu-
cleophilic catalyst on an allenoate ester A generates zwit-
terionic dienolate intermediates B/B′, which behave as
nucleophiles at the α- and/or γ-positions (Scheme 1).
While this dienolate reactivity is readily exploited in the
case of buta-2,3-dienoates, that is, allenoates without any
further alkyl substituents, the situation becomes much
more complex in the case of α- and/or γ-substituted allen-
oates. γ-Methyl-derived dienolates B can thus undergo an
Following our previous work with the bicyclic guanidine
1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as highly ac-
tive Lewis base catalyst for the umpolung-free nucleo-
philic activation of allenoates,13 we set out to investigate
the reaction of very challenging α,γ-dialkyl-substituted
substrates with aromatic aldehydes. To our delight, a test
reaction employing ethyl 2-methylpenta-2,3-dienoate
(1a), 4-chlorobenzaldehyde (2a), TBD as the catalyst, and
SYNLETT 2013, 24, 2535–2539
Advanced online publication: 26.07.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1339471; Art ID: ST-2013-D0596-C
© Georg Thieme Verlag Stuttgart · New York

2536
P. Selig et al.
CLUSTER
MeCN as the solvent smoothly provided the desired allen- creasing the amount of allenoate to 2.0 equivalents
ic alcohol 3aa as the only discernible product (Table 1, relative to the aldehyde. Even bigger excesses of 3.0
entry 1).
equivalents of allenoate led to no further improvement
(Table 1, entries 3–5). The reaction was most efficient in
highly polar DMF, but yields decreased dramatically in
CH2Cl2, and in THF no reaction was observable at all (Ta-
ble 1, entries 6 and 7). In order to further suppress unwant-
ed side reactions, we also investigated slightly less
reactive Lewis base catalysts. Indeed, yields of 3aa could
be further improved up to 80% when using the amidine
base DBU (Table 1, entries 8–10), and finally, N-methyl
TBD (MTBD) was identified as the optimal catalyst,
which gave the product 3aa in an excellent and reproduc-
ible yield of 88% (Table 1, entry 11).
Table 1 γ-Selective Morita–Baylis–Hillman Reaction of Ethyl
2-Methylpenta-2,3-dienoate (1a) and 4-Chlorobenzaldehyde (2a)a
O
Me
CO2Et
+
Me
CO2Et
cat. (25 mol%)
(solvent)
r.t.
H
Ar
Cl
Me
Me
OH
1a
2a
3aa
(Ar = 4-ClC6H4)
Entry Catalyst
Solvent
1a (equiv) Time
Yield of
3aa (%)b
High yields of >80% could still be achieved even at –20
°C (Table 1, entry 12), or with only 15 mol% of MTBD
catalyst (Table 1, entry 13). Most importantly, common
N-nucleophiles like DABCO or DMAP were entirely in-
effective in catalyzing this reaction (Table 1, entries 14
and 15).
1
TBD
MeCN
DMF
DMF
DMF
DMF
CH2Cl2
THF
1.0
1.0
1.5
2.0
3.0
2.0
2.0
1.0
1.5
2.0
2.0
2.0
2.0
2.0
2.0
30 min
20 min
45 min
30 min
1.5 h
5.25 h
6 h
52
2
TBD
60
3
TBD
65
4
TBD
76
Mechanistically, the reaction most likely follows a classic
MBH pathway originating from the zwitterionic interme-
diate B′′ (Scheme 2). Blocking of the usually more reac-
tive α-position13a with the α-Me substituent as well as the
suppression of any umpolung reactions forces the nucleo-
philic attack on the aldehyde to take place at the γ-posi-
tion. A subsequent proton transfer and elimination finally
liberates both the product 3aa and the catalyst. Side reac-
tions probably originate from zwitterion B′′ as well, as its
attack on a second molecule of allenoate would lead to oli-
go- and later on higher polymeric products.
5
TBD
75
6
TBD
38
7
TBD
n.r.c
62
8
DBU
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
1.5 h
1.5 h
1.5 h
1.5 h
15 h
9
DBU
78
10
11
12d
13e
14
15
DBU
80
MTBD
MTBD
MTBD
DABCO
DMAP
88
81
allenoate
1a
1.5 h
1.5 h
1.5 h
84
1a
n.r.c
traces
CO2Et
side
products
N
MTBD
Me
Me
B''
N
N
γ
a Reactions were run at a 0.3–0.4 mmol scale with c (2a) = 0.1 M,
b Yield of isolated product (dr = ca. 1:1).
c No reaction.
Me
(MTBD)
no
umpolung!
d Reaction run at –20 °C.
CO2Et
e Reaction run with 15 mol% of catalyst.
MTBD
Me
ArCHO
2a
Me
Ar
MBH
product
3aa
Alcohol 3aa, which can be described as the product of a
γ-selective Morita–Baylis–Hillman (MBH) reaction,14
was formed as a mixture of diastereoisomers within very
short reaction times. To the best of our knowledge, this
represents the first example of a MBH reaction on α,γ-di-
substituted allenoates.15 Moreover, even the correspond-
ing transformation of γ-unsubstituted allenic substrates
under DMAP catalysis could previously only be realized
for highly reactive allenic ketones, but not for the synthet-
ically more versatile allenic esters.16 Yields could be
slightly improved by conducting the reaction in DMF (Ta-
ble 1, entry 2). As some undesired TBD-induced polym-
erization reactions of the allenoate substrate were still
encountered as side reactions at room temperature, a fur-
ther improvement of yields to 76% was achieved by in-
O
Scheme 2 Mechanistic proposal for the γ-selective MBH reaction
Having established the optimal reaction conditions of the
γ-selective MBH reaction (15–30 mol% MTBD, 2.0 equiv
allenoate, DMF, r.t.), we investigated the scope of this
novel transformation both with regard to different aromat-
ic aldehydes and – more importantly – differently substi-
tuted allenoates (Table 2). In reactions with allenoate 3a,
moderate to high yields of γ-MBH products could be iso-
lated with both electron-poor and electron-rich aromatic
aldehydes (Table 2, entries 1–6). The ortho- and disubsti-
tuted aldehydes 2d,f were equally suitable reactants. Em-
ploying nonenolizable aliphatic pivaloylaldehyde (2g),
Synlett 2013, 24, 2535–2539
© Georg Thieme Verlag Stuttgart · New York

CLUSTER
Morita–Baylis–Hillman Reactions on α,γ-Dialkyl Allenoates
2537
the expected product 3ag could be isolated, but only in (de ≤16%). Scale-up was unproblematic, and in a large-
low yields (Table 2, entry 7). Excellent yields of >90% scale experiment, 3aa could be isolated in 84% yield giv-
were achieved with α-n-Pr-γ-Me allenoate 1b (Table 2, ing 0.8 gram of product without any further tuning of the
entries 8–10), and even the α-Bn substituted allenoate 1c, reaction conditions.
a substrate which is especially prone to undesired umpol-
ung reactions, was successfully employed to give the de-
Having established a straightforward access to both
densely and diversely substituted γ-MBH products 3, we
sired products in satisfactory yields of 61–87% (Table 2,
next envisioned the employment of these allenic products
entries 11–13). Introduction of a branched i-Pr substituent
for the synthesis of heterocyclic structures. While Bu3P
was entirely unsuitable for the preparation of the γ-MBH
products 3, it retained its activity for the Lewis base in-
to the γ-position in substrate 1d reduced its reactivity sig-
nificantly and therefore required higher catalyst loadings
and longer reaction times. Still, however, the sterically
duced cyclization of these products to give 3,5-dimethyl-
highly encumbered products 3da, 3db, 3dc, and 3df were
6-aryl-2H-pyran-2-ones 4.12c Pyranones 4aa, 4ac, and 4ae
obtained in good yields of up to 83% (Table 2, entries 14–
were isolated in moderate to good yields after stirring with
1.0 equivalent of Bu3P in MeCN for 18 hours (Scheme 3).
17). Interestingly, with aldehyde 2f the reaction proceed-
ed in a slightly diastereoselective fashion (dr of 3df = ca.
2:1), while all other reactions were basically unselective
Table 2 Scope of the MTBD-Catalyzed γ-Selective Morita–Baylis–Hillman Reactiona
N
N
N
R1
R2
CO2Et
+
R1
R2
CO2Et
Me
(MTBD)
O
(DMF), r.t.
R3
H
R3
OH
1 (2.0 equiv)
2
3
Entry
Allenoate
Aldehyde Product R1
R2
R3
MTBD (mol%)
Time (h)
Yield of 3 (%)b
1
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1c
1c
1c
1d
1d
1d
1d
2a
2b
2c
2d
2e
2f
3aa
3ab
3ac
3ad
3ae
3af
Me
Me
Me
Me
Me
Me
Me
n-Pr
n-Pr
n-Pr
Bn
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
4-ClC6H4
Ph
25
20
20
20
20
20
20
15
20
20
20
20
20
15
30
30
30
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
5.0
5.0
5.0
5.0
89
95
74
81
68
85
15
90
96
90
61
87
73
83
54
67
76c
2
3
4-O2NC6H4
2-O2NC6H4
4-MeOC6H4
3,5-(MeO)2C6H3
t-Bu
4
5
6
7
2g
2a
2b
2f
3ag
3ba
3bb
3bf
3ca
3cb
3cf
8
4-ClC6H4
Ph
9
10
11
12
13
14
15
16
17
3,5-(MeO)2C6H3
4-ClC6H4
Ph
2a
2b
2f
Bn
Bn
3,5-(MeO)2C6H3
4-ClC6H4
Ph
2a
2b
2c
2f
3da
3db
3dc
3df
Me
Me
Me
Me
i-Pr
i-Pr
i-Pr
i-Pr
4-O2NC6H4
3,5-(MeO)2C6H3
a Reactions were run at a 0.3–0.4 mmol scale with c (2) = 0.2 M.
b Yield of isolated product (dr = ca. 1:1).
c dr = ca. 2:1.
© Georg Thieme Verlag Stuttgart · New York
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2538
P. Selig et al.
CLUSTER
O
Me
Me
CO2Et
Me
Bu3P (100 mol%)
(MeCN), r.t., 18 h
O
OH
Ar
Ar
Me
3aa
3ac
3ae
4aa 67%
Ar = 4-ClC6H4
Ar = 4-O2NC6H4
Ar = 4-MeOC6H4
83%
56%
4ac
4ae
Scheme 3 Phosphine-catalyzed cyclization of γ-MBH products 3 to
3,5-dimethyl-6-aryl-2H-pyran-2-ones 4
The organocatalytic synthesis of pyranones, which was
previously only reported for α,γ-unsubstituted butadieno-
ates,12c could thus be successfully expanded to α,γ-dialkyl
allenoates. It is also noteworthy that the only two previ-
ously reported syntheses of such 3,5-dialkyl-substituted
pyranones are both dependent on the use of expensive ru-
thenium or rhodium catalysts as well as additional silver
and copper salts and high reaction temperatures of 100–
120 °C.17 Our MTBD/Bu3P catalyzed two-step procedure
to 2H-pyran-2-ones could thus reasonably offer a cheap,
simple, and mild organocatalytic alternative.
Figure 1 X-ray crystal structure of the tosylcarbamate derivative 6
of product 3aa21
As a second example to illustrate the synthetic versatility
of γ-MBH products 3 we chose the popular transition-
metal-catalyzed cycloisomerization of hydroxyallenes
into 2,5-dihydrofurans 5 (Scheme 4).18 While the reaction
did not proceed with Ag(I) salts,19 a cationic gold catalyst
formed in situ from Ph3PAuCl and AgOTf (10 mol%
each)20 enabled the efficient 5-endo-trig cyclization of
α,γ-dimethyl allenic alcohols under mild conditions and
within short reaction times.
In conclusion, we have developed the first γ-selective
MBH reaction of α,γ-dialkyl-substituted allenoate esters 1
and aromatic aldehydes 2. Using MTBD as a tuned-down
variant of the exceedingly active TBD catalyst, a wide va-
riety of allenic alcohols 3 were readily available within
short reaction times and under very mild and simple reac-
tion conditions. The reaction is quite general with regard
to substitution at the allenoate α- and γ-positions as well
as the electronic and steric properties of the aromatic alde-
hydes. This generality is most likely caused by the ab-
sence of any undesired umpolung reactions which
typically complicate selective transformations of substi-
tuted allenoates under phosphine catalysis. The obtained
γ-MBH products 3 were proven to be versatile intermedi-
ates for the synthesis of densely substituted and potential-
ly bioactive22 2H-pyran-2-ones 4 and 2,5-dihydrofurans 5.
Further work is currently directed towards the control of
both diastereo- and enantioselectivity as well as the fur-
ther elaboration of the now easily accessible MBH prod-
ucts into biologically active derivatives.
Me
CO2Et
Me
Ph3PAuCl (10 mol%)
AgOTf (10 mol%)
(CH2Cl2), r.t., 1.5 h
CO2Et
O
OH
Ar
Me
Me
Ar
5aa 86%
5ad
5af
3aa
3ad
3af
Ar = 4-ClC6H4
Ar = 2-O2NC6H4
Ar = 3,5-(MeO)2C6H3
88%
61%
Scheme 4 Gold-catalyzed cycloisomerization of γ-MBH products 3
to 2,5-dihydrofurans 5
Representative Procedure for the γ-Selective MBH Reaction of
α,γ-Dialkyl Allenoates and Aromatic Aldehydes
2,5-Dihydrofurans 5aa, 5ad, and 5af displaying very
dense substituent patterns as well as a quaternary center
on C-2 were thus easily synthesized in yields of up to 88%
from their corresponding alcohol precursors.
Allenoate 1a (106 mg, 756 μmol, 2.0 equiv) and aldehyde 2a (51
mg, 366 μmol, 1.0 equiv) were dissolved in DMF (3.8 mL). MTBD
(13 μL, 14 mg, 91 μmol, 25 mol%) was added all at once, and the
mixture was stirred at r.t. for 1.5 h. The reaction was quenched by
the addition of solid NH4Cl (excess), filtered, and the solids washed
with EtOAc (3 × 10 mL). The filtrate was washed with H2O (30
mL), LiCl (5% aq, 30 mL), dried over MgSO4, filtered, and the sol-
vent evaporated. Flash column chromatography of the residue
(SiO2, 2.0 × 20 cm, pentane–EtOAc = 8:2) gave the γ-MBH prod-
uct 3aa (92 mg, 326 mmol, 89%, dr = 48:52) as a slightly yellow
oil.
In order to facilitate further investigations regarding the
stereoselectivity of this and related functionalizations of
allenoates, the identification of the product diastereoiso-
mers was also deemed desirable. We therefore obtained
the X-ray crystal structure of the tosylcarbamate deriva-
tive 6 of product 3aa which could be crystallized in a dia-
stereomerically pure form (Figure 1).21
Synlett 2013, 24, 2535–2539
© Georg Thieme Verlag Stuttgart · New York

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Morita–Baylis–Hillman Reactions on α,γ-Dialkyl Allenoates
Beutner, G. L. Angew. Chem. Int. Ed. 2008, 47, 1560;
2539
Representative Procedure for the Bu3P-Catalyzed Cyclization
of γ-MBH Products to 2H-Pyran-2-ones
Angew. Chem.; 2008, 120, 1584. (e) Methot, J. L.; Roush,
William. R. Adv. Synth. Catal. 2004, 346, 1035.
γ-MBH product 3ac (64 mg, 220 μmol) was dissolved in MeCN
(1.0 mL). Bu3P (54 μL, 44 mg, 220 μmol, 1.0 equiv) was added, and
the mixture was stirred at r.t. for 18 h. After evaporation of all vol-
atile matter, the residue was purified by flash column chromatogra-
phy (SiO2, 2.0 × 20 cm, pentane–EtOAc = 75:25 to 70:30) to give
the pyrone product 4ac (45 mg, 183 μmol, 83%) as a yellow solid.
(10) For examples regarding umpolung reactions of γ-alkyl
allenoates, see: (a) Xu, S.; Zhou, L.; Zeng, S.; Ma, R.; Wang,
Z.; He, Z. Org. Lett. 2009, 11, 3498. (b) Xu, S.; Zhou, L.;
Ma, R.; Song, H.; He, Z. Chem. Eur. J. 2009, 15, 8698.
(c) Ma, R.; Xu, S.; Tang, X.; Wu, G.; He, Z. Tetrahedron
2011, 67, 1053. (d) Li, E.; Huang, Y.; Liang, L.; Xie, P. Org.
Lett. 2013, 15, 3138.
Representative Procedure for the Au-Catalyzed Cycloisomer-
ization of γ-MBH Products to 2,5-Dihydrofurans
(11) For examples regarding umpolung reactions of α-alkyl
allenoates, see: (a) Xu, S.; Zhou, L.; Ma, R.; Song, H.; He,
Z. Org. Lett. 2010, 12, 544. (b) Xu, S.; Zou, W.; Wu, G.;
Song, H.; He, Z. Org. Lett. 2010, 12, 3556. (c) Wang, T.; Ye,
S. Org. Lett. 2010, 12, 4168. (d) Khong, S. N.; Tran, Y. S.;
Kwon, O. Tetrahedron 2010, 66, 4760. (e) Martin, T. J.;
Vakhshori, V. G.; Tran, Y. S.; Kwon, O. Org. Lett. 2011, 13,
2586.
γ-MBH product 3aa (71 mg, 253 μmol, dr = 48:52) was dissolved
in CH2Cl2 (1.0 mL). Ph3PAuCl (12.5 mg, 25 μmol, 10 mol%) and
AgOTf (6.5 mg, 25 μmol, 10 mol%) were added, and the resulting
suspension was stirred at r.t. for 1.5 h. After evaporation of the sol-
vent, the residue was purified by flash column chromatography
(SiO2, 2.0 × 20 cm, pentane–EtOAc = 10:1) to give the dihydrofu-
ran product 5aa (61 mg, 217 μmol, 86%, dr = 48:52) as a yellow oil.
(12) (a) Tsuboi, S.; Kuroda, H.; Takatsuka, S.; Fukawa, T.; Sakai,
T.; Utaka, M. J. Org. Chem. 1993, 58, 5952. (b) Zhu, X.-F.;
Henry, C. E.; Wang, J.; Dudding, T.; Kwon, O. Org. Lett.
2005, 7, 1387. (c) Zhu, X.-F.; Schaffner, A.-P.; Li, R. C.;
Kwon, O. Org. Lett. 2005, 7, 2977. (d) Creech, G. S.; Kwon,
O. Org. Lett. 2008, 10, 429. (e) Creech, G. S.; Zhu, X.-F.;
Fonovic, B.; Dudding, T.; Kwon, O. Tetrahedron 2008, 64,
6935.
Acknowledgment
We thank the Fonds der Chemischen Industrie (FCI) for financial
support (Liebig Scholarship to P.S., PhD scholarship to A.T.). Spe-
cial thanks go to Prof. Dr. D. Enders, Institute of Organic Chemi-
stry, RWTH Aachen University, for the generous provision of
laboratory space and chemicals.
(13) (a) Selig, P.; Turočkin, A.; Raven, W. Adv. Synth. Catal.
2013, 355, 297. (b) Selig, P.; Turočkin, A.; Raven, W.
Chem. Commun. 2013, 49, 2930.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
m
iotSrat
ungIifoop
r
t
(14) We intentionally avoid the expression ‘vinylogous Morita–
Baylis–Hillman reaction’, as this term is now generally used
to describe the related Rauhut–Currier reaction. For a
review, see: Aroyan, C. E.; Dermenci, A.; Miller, S. J.
Tetrahedron 2009, 65, 4069.
(15) For the synthesis of similar products by Brønsted base
catalysis, see: Xu, B.; Hammond, G. B. Angew. Chem. Int.
Ed. 2008, 47, 689; Angew. Chem. 2008, 120, 701.
(16) Zhao, G.-L.; Shi, M. Org. Biomol. Chem. 2005, 3, 3686.
(17) (a) Mochida, S.; Hirano, K.; Satoh, T.; Miura, M. J. Org.
Chem. 2009, 74, 6295. (b) Chinnagolla, R. K.; Jeganmohan,
M. Chem. Commun. 2012, 48, 2030.
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