A novel tandem Pd-catalyzed intramolecular allylic decarboxylative coupling and “diradical conjugated 1,3-dien-5-yne cycloaromatization”: An unusual ortho-selectivity in the cycloaromatization and the mechanistic implications

Article abstract of DOI:10.1016/j.tetlet.2017.06.079

A domino fashioned allylic rearrangement, intramolecular decarboxylative coupling, propargyl-allene rearrangement and diradical 1,3-dien-5-yne cycloaromatization of aryl propiolates of the Baylis-Hillman alcohols in the presence of catalytic Pd(PPh₃)₄ and DBU leading to the formation 4-benzylnaphthoates has been described. Interestingly, the meta-substituted propiolates (1i–t) provided sterically hindered cyclization products (12–23) via the unusual ortho-selective cycloaromatization; while the meta-alkoxy (1i–m, 1r–t) group provided predominantly the ortho-selective 1,3-dien-5-yne cycloaromatization products, meta-Me group (1?n–q), products formed both via ortho-selective cyclization and para-selective cyclization products in comparable ratio. A novel mechanism involving diradical species has been proposed to explain the unusual ortho-selectivity in 1,3-dien-5-yne cycloaromatization. Preference for sterically hindered ortho-selective cyclization may be attributed to the formation of a transition state radical possessing more stable ortho-methoxy cyclohexadiene radical moiety (driving force).

Full text of DOI:10.1016/j.tetlet.2017.06.079

Accepted Manuscript
A novel tandem Pd-catalyzed intramolecular allylic decarboxylative coupling
and “diradical conjugated 1,3-dien-5-yne cycloaromatization”: an unusual or-
tho-selectivity in the cycloaromatization and the mechanistic implications
Satyanarayana Tummanapalli, Dhanunjaya Naidu Vangapandu, Parthasarathy
Muthuraman, Sanjeeva Kambampati, Gnanakalai Shanmugavel
PII:
S0040-4039(17)30817-1
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.06.079
TETL 49071
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Accepted Date:
21 June 2017
24 June 2017
Please cite this article as: Tummanapalli, S., Vangapandu, D.N., Muthuraman, P., Kambampati, S., Shanmugavel,
G., A novel tandem Pd-catalyzed intramolecular allylic decarboxylative coupling and “diradical conjugated 1,3-
dien-5-yne cycloaromatization”: an unusual ortho-selectivity in the cycloaromatization and the mechanistic
implications, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.2017.06.079
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2
Graphical Abstract
A novel tandem Pd-catalyzed intramolecular
allylic decarboxylative coupling and
“diradical conjugated 1,3-dien-5-yne
cycloaromatization”: an unusual ortho-
selectivity in the cycloaromatization and the
mechanistic implications.
Leave this area blank for abstract info.
Satyanarayana Tummanapalli∗ Dhanunjaya Naidu Vangapandu, Parthasarathy Muthuraman, Sanjeeva Kambampati, Gnanakalai
Shanmugavel
EWG
O
para substitution
R =
R
O
Bn
Ar
R2
EWG
Pd(PPh3
)
4 (15 mol%.)
Exclusive product
DBU (20 mol%)
THF, 70 oC, 2 h
EWG
RO
EWG
meta substitution
(alkoxy)
R1
1j-t
R =
EWG = CO2Me, CO2Et
SO2Ph, COEt
unusual ortho-selective
cyclization
Bn
OR
Bn
Minor product
Major product

1
Tetrahedron Letters
journal homepage: www.els evier.com
A novel tandem Pd-catalyzed intramolecular allylic decarboxylative coupling and
“diradical conjugated 1,3-dien-5-yne cycloaromatization”: an unusual ortho-
selectivity in the cycloaromatization and the mechanistic implications.
a
Satyanarayana Tummanapalli∗ , Dhanunjaya Naidu Vangapandu,b Parthasarathy Muthuraman,b Sanjeeva
Kambampati,b Gnanakalai Shanmugavelb
aAlbany Molecular Research Hyderabad Research Center, Plot#9, MN Park, Turkapally, Shameerpet, Genome Valley, RR District, Hyderabad – 500 078, India
bAlbany Molecular Research Singapore Research Center, 61 Science Park Road, #03-13 The Galen, Singapore Science Park II, Singapore 117525
ARTICLE INFO
ABSTRACT
A domino fashioned allylic rearrangement, intramolecular decarboxylative coupling, propargyl-
allene rearrangement and diradical 1,3-dien-5-yne cycloaromatization of aryl propiolates of the
Baylis-Hillman alcohols in the presence of catalytic Pd(PPh3)4 and DBU leading to the
formation 4-benzylnaphthoates has been described. Interestingly, the meta-substituted
propiolates (1i-t) provided sterically hindered cyclization products (12-23) via the unusual
ortho-selective cycloaromatization; while the meta-alkoxy (1i-m, 1r-t) group provided
predominantly the ortho-selective 1,3-dien-5-yne cycloaromatization products, meta-Me group
(1n-q), products formed both via ortho-selective cyclization and para-selective cyclization
products in comparable ratio. A novel mechanism involving diradical species has been proposed
to explain the unusual ortho-selectivity in 1,3-dien-5-yne cycloaromatization. Preference for
sterically hindered ortho-selective cyclization may be attributed to the formation of a transition
state radical possessing more stable ortho-methoxy cyclohexadiene radical moiety (driving
force).
Article history:
Received
Received in revised form
Accepted
Available online
Keywords:
Intramolecular Decarboxylative Coupling
Cycloaromatization
Baylis-Hillman Chemistry
Diradical
Palladium
2009 Elsevier Ltd. All rights reserved
.

2
Decarboxylative coupling reactions have many advantages over
traditional oxidative coupling reactions. Decarboxylative cross
coupling reactions employ more stable and inexpensive
carboxylic acids as key reaction partners while the counterpart
traditional coupling reactions have typically involved expensive
and usually air-sensitive organometallic species as the reaction
partners.1-2 Alkynyl carboxylic acids have been lately introduced
into decarboxylative coupling for providing the aryl alkyne
derivatives.3 Tunge and co-workers independently reported the
first intramolecular Pd-catalyzed decarboxylative allylic
rearrangement of allyl alkynoates in 2005.4 In 2008, Lee and
coworkers reported the first decarboxylative coupling of alkynyl
carboxylic acids with aryl halides. We recently reported a highly
regio- and stereo-selective Pd-catalyzed tandem intramolecular
decarboxylative coupling via the allylic rearrangement of aryl
propiolates derived from the Baylis-Hillman adducts.5
Subsequently, we reported Pd-catalyzed decarboxylative allylic
coupling of acetates of the Baylis-Hillman alcohols with the
propiolic acids leading to the formation of important class of 1,5-
diarylpent-1-en-4-ynes in a highly regio- and stereoselective
manner.6
Fascinated by the importance of polysubstituted naphthalenes,
and as part of our continued interest in the application of
decarboxylative coupling of the Baylis-Hillman adducts for
various synthetic organic transformations,5,6,9 we have undertaken
a study to develop an efficient methodology for accessing
polysubstituted naphthalenes. We envisioned that palladium-
catalyzed allylic intramolecular decarboxylative coupling of
propiolates of Baylis-Hillman alcohols in the presence of a base
would provide naphthalene derivatives in a domino fashioned
reaction sequence; first an intramolecular decarboxylative
coupling by extrusion CO2 (step-1), followed by propargyl-allene
rearrangement (step-2), and finally a cycloaddition (step-3) as
described in Scheme 1.21,22 Accordingly, we subjected the freshly
prepared23 phenyl propiolate 1a to the decarboxylative coupling
reaction conditions5,6 in the presence of various bases (Cs2CO3,
DBU, DBN, t-BuOK and KHMDS). Though the desired product
was observed using all bases in varying yields, better conversion
was observed when the phenyl propiolate 1a was heated at 70 °C
in THF in the presence of 10 mol% Pd(PPh3)4 and 25 mol% DBU
for 16 hours. Silica gel column chromatography of crude product
The Baylis–Hillman reaction is an important atom-economical
carbon–carbon bond forming reaction providing densely
functionalized molecules whose applications in many organic
transformations have been well documented.7 Baylis–Hillman
adducts have also been used in the synthesis of naphthalene
derivatives via various organic transformations.8 In continuation
of our ongoing research on the application of decarboxylative
coupling of the Baylis-Hillman adducts in various synthetic
organic transformations5,6,9 we report herein the "Pd-catalyzed
intramolecular allylic decarboxylative coupling, alkyne-allene
rearrangement and 1,3-dien-5-yne cycloaddition of aryl propiolates
of the Baylis-Hillman alcohols leading directly to polysubstituted
naphthalenes".
provided methyl 4-benzyl-2-naphthoate
4
in 78% yield.24
The naphthalene scaffold is an important building block present
in many pharmaceutical and biologically active natural
products.10-12 Naphthalene is also an important structural moiety
present in organic electronic materials.13 Therefore, construction
of polysubstituted naphthalene molecules continues to be an
interesting area in organic synthesis.10 Although a variety of
methods involving various strategies have been developed for the
preparation polysubstituted naphthalene molecules, most of them
involve a multistep synthesis.10 Transition metal-catalyzed
[2+2+2] cyclotrimerization of alkynes is a well-known method
for the construction of polyaromatic rings.14 Annulations via the
Encouraged by this result and to explore the generality of the
reaction, we extended the study to other examples and observed
interesting results. Consistent with the first example 1a, aryl
propiolates derived from Baylis-Hillman adducts of
benzaldehyde (1b), para-tolualdehyde (1c, 1d), 3,5-dimethoxy
benzaldehyde (1e) and 2-methyl benzaldehyde (1f) provided the
corresponding 4-benzyl-2-naphthoates (5-9) in good yields under
standard tandem cycloaddition conditions via the Pd-catalyzed
intramolecular allylic decarboxylative coupling.24 We have also
studied two examples by varying the EWG (electron withdrawing
groups) to ethyl ketone. Accordingly, we subjected the phenyl
propiolates 1g and 1h [derived from Baylis-Hillman adducts of
benzaldehyde (1b) and ethyl vinyl ketone] to the tandem Pd-
catalyzed intramolecular decarboxylative coupling/cycloaddition
using the standard reaction conditions24 and isolated the
corresponding 1-(4-benzylnaphthalen-2-yl)propan-1-ones (10 and
11) in good yields.
palladium-catalyzed
cyclization
of
alkynes,15,16
[3+3]
benzannulation of benzenoids,17 [4+2] benzannulation between
enynes-dienynes,18 or between enynes-enynes19 have been
developed recently for the preparation of naphthalene molecules.
Allene-mediated electrocyclization reactions is yet another
important annulation strategy for accessing naphthalene
derivatives.20
However, an unexpected result was observed when the aryl
propiolate 1i derived from the Baylis-Hillman adduct of meta-
anisaldehyde was used (Scheme 2). m-Methoxy group on the
benzene ring has two directing places for cyclization, i) at the
sterically hindered C2-carbon of the benzene ring providing 12a
or ii) at sterically favoured C6-carbon of benzene ring providing
12b (Scheme 2). Earlier work on similar cyclization involving
Scheme 1: Pd-catalyzed tandem intramolecular allylic
decarboxylative coupling, propargyl-allene rearrangement and
cycloaddition of propiolate 1a providing 4-benzyl naphthaoate
4.24
Table 1. Pd-catalyzed tandem intramolecular allylic
decarboxylative coupling, propargyl-allene rearrangement and
cycloaddition of propiolates 1b-1h providing directly 4-benzyl
naphthaoate 5-11.24

3
O
MeO
CO2Me
CO2Me
O
Pd(PPh3)4 (10 mol%.)
CO2Me
OMe
DBU (20 mol%)
THF, 70 oC, 2 h
Me
12a/12b = 82:18
OMe
Me
Me
unusual ortho-selective
cyclization
1i
12b
12a
Observed minor product
(18%)
Observed major product
(82%)
-CO2
Pd(0)
expected major
product based on
sterics in the
expected minor
product based on
sterics in the
diene-allene
cycloaddition
diene-allene
cycloaddition
transition states
transition states
CO2Me
MeO
CO2Me
CO2Me
DBU
OMe
O
Me
TS-1a
2a
TS-1b
Me
Me
more favoured based
on steric factors
less favoured based
on steric factors
3
two possible transition states of intermediate f or
a concerted [2+2+2] pericyclic cycloaddition
(6 electrocyclization)
The structures were further ascertained by 2D NOESY. A strong
nOe cross peak between the CH2 (benzylic methylene proton)
and OCH3 (ortho-methoxy proton of the benzene ring) was
clearly observed in 12a confirming OMe group on C5-carbon of
naphthalene ring (12a). There was no nOe correlation between
the CH2 (allylic methylene) and OCH3 (para-methoxy proton of
the benzene ring) in 12b again confirming methyl 7-methoxy-4-
(4-methylbenzyl)-2-naphthoate for 12b. This result clearly
suggested that cycloaddition favoured unusual sterically hindered
ortho-selective cyclization. Intrigued by the result and to further
understand the generality of the unusual ortho-selectivity in the
tandem Pd-catalyzed intramolecular decarboxylative coupling-
cycloaddition reaction, we subjected various aryl propiolates 1j-
1t possessing the meta-substitution on the benzene ring to
standard reaction condition24 and isolated the corresponding 4-
benzylnaphthoates (13-23) in good yields (Table 2).
Interestingly, all the meta-substituted propiolates (1j-1q)
provided products via the unusual ortho-selective cyclization.
While the meta-OMe group (1i-1l) provided predominantly 4-
benzylnaphthoates 12-15 via ortho-selective cycloaddition (Table
2), 3,4-dimethoxyphenyl propiolate (1m) provided exclusively
ortho-cyclized product 16a. In the case of a meta-Me group (1n-
1q, Scheme 2 and Table 2) too, ortho-selective cyclization
products (17a-20b) were observed in addition to para-cyclization
products (17b-20b) in comparable ratios (Table 2). To
understand the effect of other EWG (electron withdrawing
groups), we also subjected the phenyl propiolates 1r (derived
from Baylis-Hillman adducts of benzaldehyde phenyl vinyl
sulfone) and 1s and 1t (derived from Baylis-Hillman adducts of
3-methoxybenzaldehyde
and
3-isopropoxybenzaldehyde
respectively with ethyl vinyl ketone) to tandem Pd-catalyzed
intramolecular decarboxylative coupling and cycloaddition under
the standard reaction conditions.24 Isolated products 21a-23a
(major products) and 21b-23b (minor products) again ascertained
that cycloaddition favoured ortho-selective cyclization.
a diene-allene intermediate (similar to 3) was thought to proceed
via 6π electrocyclization,22 and as 6π electrocyclization is a
known pericyclic reaction involving a concerted mechanism,25
we expected that aryl propiolate 1i would provide methyl 7-
methoxy-4-(4-methylbenzyl)-2-naphthoate 12b as the major
product with para-cyclization at C6-carbon via the sterically
more favoured transition state TS-1b (Scheme 2). But contrarily,
aryl propiolate 1i, under the standard reaction conditions,24
provided methyl 5-methoxy-4-(4-methylbenzyl)-2-naphthoate
(12a) as the predominant product with ortho-cyclization
occurring at the sterically hindered C2-carbon (TS-1a, Scheme
2). Isomeric structures of 12a and 12b were clear from 1H NMR
spectra with distinct proton splitting patterns of ortho-substituted
(12a) and para-substituted (12b) benzene rings of 12a and 12b.25
The results described in Scheme 2 and Table 2 clearly
demonstrate that meta-alkoxy propiolates (1i-1m, 1r-t), under
standard tandem intramolecular decarboxylative coupling and
cycloaddition conditions,24 favoured the formation of sterically
hindered 5-alkoxy-2-naphthoates (12a-16a, 21a-23a) via ortho-
selective cyclization. This evidently suggests that [2+2+2]
Table 2. Examples describing the ortho-selective cyclization in
the Pd-catalyzed tandem intramolecular allylic decarboxylative
coupling and diene-allene cycloaromatization.24
Scheme 2: Ortho-selective cyclization in Pd-catalyzed tandem
intramolecular allylic decarboxylative coupling and cycloaddition
of propiolate 1i.24

4
It is noteworthy to mention here that diradical intermediates TS-
6a and TS-6b contain “2-methoxycyclohexa-1,3-diene radicle
moiety A” and “1-methoxycyclohexa-1,3-diene radicle moiety
B” respectively as shown in Scheme 4. McMillan and Houk,28-29
earlier showed, using the density functional theory calculation,
that similar 2-methoxycyclohexa-1,3-diene radicle is actually
more stable than the isomeric 1-methoxycyclohexa-1,3-diene
radicle by ~3.6 kcal/mole.28-29 Therefore, putting together our
ortho-selective cyclization results described in Scheme 2 and
Table 2 and support from McMillan and Houk’s density
functional theory calculations results which suggested the greater
stability of 2-methoxycyclohexa-1,3-diene radicle A, we strongly
believe that our Pd-catalyzed tandem intramolecular allylic
decarboxylative coupling-cycloaddition involve
cycloaromatization mechanism as described in Scheme 3 giving
a diradical
rise to ortho-selectivity.
Scheme 4: Structures depicting the more stable 2-
methoxycyclohexa-1,3-diene radical moiety A in TS-4a and less
stable 1-methoxycyclohexa-1,3-diene radical moiety B in TS-
4b.29
Conclusions
cycloaddition in this case (Scheme 2, Table 2) was influenced
more by the electronic factors rather than the steric factors. This
led us to presume that the cycloaddition step in our tandem
cyclization reaction would proceed via Hopf type cyloaddition.26
First allenic intermediate 3a (formed by decarboxylative coupling
in presence of DBU/[Pd]) rearranges to a Hopf type 1,3-dien-5-
yne intermediate 3b. Then 3b undergoes Hopf type cycloaddition
leading to the formation of diradical27 intermediates TS-6a and
TS-b via hydrogen shift.26b
In conclusion, we have developed a novel domino fashioned Pd-
catalyzed allylic intramolecular decarboxylative coupling,
isomerization to conjugated 1,3-dien-5-yne from diene-allene
intermediate followed by cycloaromatization of aryl propiolates
of the Baylis-Hillman alcohols in the presence of catalytic
amount DBU leading to the formation 4-benzylnaphtho-2-ates.
We found unusual ortho-selective cyclization in the case of meta-
alkoxy propiolates. A rational mechanism with step-wise diene-
allene isomerization to a conjugated 1,3-dien-5-yne followed by
the cycloaromatization involving a Hopf type diradical species
has been proposed. Thus, the unusual ortho-selectivity observed
in this work can be attributed to the greater stability of 2-
Scheme 3: Plausible mechanism for the tandem Pd-catalyzed
coupling
intramolecular
cycloaromatization involving diradical species.
allylic
decarboxylative
and

5
8
For the application of Baylis-Hillman addicts in the synthesis of
methoxycyclohexa-1,3-diene radical species that would form in
the proposed diradical mechanism.
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Acknowledgments
We sincerely appreciate the financial support from Albany
Molecular Research Inc. The authors also thank Dr. Anjan
Chakrabarti and Dr. Valluri Muralikrishna for their support
during the preparation of this manuscript.
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16 Saito, S.; Yamamoto, Y. Chem. Rev. 2000, 100, 2901-2915.
17 Katrizky, A. R.; Li, J.; Xie, L. Tetrahedron. 1999, 55, 8263-8293.
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3
For selected papers on Pd-catalyzed decarboxylative coupling alkynyl
carboxylic acids, see: (a) Moon, J.; Jeong, M.; Nam, H.; Ju, J.; Moon, J.
H.; Jung, H. M.; Lee, S. Org. Lett. 2008, 10, 945-948. (b) Moon, J.; Jang,
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Zhang, W. -W.; Zhang, X. -G.; Li, J. -H. J. Org. Chem. 2010, 75, 5259-
5264.
19 (a) Saito, S.; Salter, M. M.; Gevorgyan, V.; Tsuboya, N.; Tando, K.;
Yamamoto, Y. J. Am. Chem. Soc. 1996, 118, 3970-3971. (b) Gevorgyan,
V.; Yamamoto, Y. J. Organomet. Chem. 1999, 576, 232-247. (c)
Kawasaki, T.; Saito, S.; Yamamoto, Y. J. Org. Chem. 2002, 67, 2653-
2658.
4
5
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Rayabarapu, D. K.; Tunge, J. A. J. Am.Chem. Soc. 2005, 127, 13510-
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Satyanarayana, T.; Dhanunjaya Naidu, V.; Parthasarathy, M.;
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20 For the allene-mediated electrocyclization reactions to naphthalene, see
(a) Zhou, H.; Xing, Y.; Yao, J.; Lu, Y. J. Org. Chem. 2011, 76, 4582-
4590. (b) Zhou, H.; Xing, Y.; Yao, J.; Chen, J. Org. Lett. 2010, 12, 3674-
3677. (c) Souto, J. A.; Perez, M.; Lopez, C. S.; Alvarez, R.; Torrado, A.;
De Lera, A. R. J. Org. Chem. 2010, 75, 4453-4462. (c) Knobloch, K.;
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7
For reviews on Baylis-Hillman Chemistry and the application of the
Baylis-Hillman adducts, see (a) Basavaiah, D.; Sahu, B. C. Chimia.,
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21 Earlier, similar polysubstituted naphthalenes were prepared following the
different synthetic protocols using other Baylis-Hillman adducts as
described in Scheme 2. Though

6
22. Lim, J.W.; Kim, K. H.; Kim, S. H.; Kim, J. N. Tetrahedron Lett. 2012,
53, 5449-5454.
23. To a stirred solution of methyl 2-(hydroxy(phenyl)methyl)acrylate 1 (100
mg, 0.52 mmol), 3-phenylpropiolic acid (96 mg, 0.67 mmol) in
dichloromethane (5 mL), was added drop wise a solution of N,N'-
Dicyclohexylcarbodiimide
(160 mg, 0.78 mmol) and 4-
Dimethylaminopyridine (6 mg, 0.05 mmol) in dichloromethane (5 mL) at
-20 °C. Upon reaction completion as judged by TLC, the resulting
mixture was diluted with dichloromethane (10 mL), washed with sat.
NaHCO3 solution and dried over Na2SO4. The organic layer was
concentrated under reduced pressure and purified by column
chromatography
phenylpropioloyl)oxy)methyl)acrylate 2 (150 mg, 0.46 mmol, 90%).
24. To stirred solution of methyl 2-(phenyl((3-phenylpropioloyl)-
oxy)methyl)acrylate 2 (100 mg, 0.31 mmol) in dry THF (15 mL), argon
gas was purged for mins, then
to
afford
methyl
2-(phenyl((3-
a
5
Tetrakis(triphenylphosphine)palladium(0) (36 mg, 0.03 mmol) was
added and followed by 1,8-Diazabicyclo[5.4.0]undec-7-ene (14 mg, 0.09
mmol) was added and the mixture was heated to 70 °C for 16 h. Upon
reaction completion as judged by TLC, the resulting mixture was diluted
with ethyl acetate (10 mL), washed with water (5 mL), brine (5 mL) and
dried over Na2SO4. The organic layer was concentrated under reduced
pressure and purified by column chromatography to afford methyl 4-
benzyl-2-naphthoate 3 (65 mg, 0.23 mmol, 75%).
25. (a) Houk, K. N.; Lin, T-Y.; Brown, F. K. J. Am. Chem. Soc. 1986, 108,
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81-90.(c) Houk, K. N.; Li, Y.; Evanseck, J. D. Angew. Chem., Int. Ed..
1992, 31, 682-708.
26. (a) Hopf, H.; Musso, H. Angew. Chem. Int. Ed. Engl. 1969, 8, 680. (b)
Hopf, H.; Kruger, A.; Schreiner, P. R. Chem. Eur.J. 2001, 7, 4386.
27. For reviews on cycloaromatization reactions that involve biradicle
species, please see: (a) Mohamed, R. K.; Peterson, P. W.; Alabugin, I. V.
Chem. Rev. 2013, 113, 7089-7129. (r) Mondal, S.; Mitra, T.; Mukherjee,
R.; Addy, P. S.; Basak, A. Synlett. 2012, 23, 2582-2602.
28. McMillan during their pioneering work on the development of organo-
SOMO catalysis,30 they noticed similar ortho-selective cyclization in
intramolecular α-arylation of aldehydes. McMillan and his group has
established thoroughly that organo-SOMO catalysis proceeds via radical
cation species. They attributed ortho-selectivity of cyclization to greater
stability of 1-methoxycyclohexa-1,3-diene radicle.30 In fact McMillan
and Houk30b showed that 1-methoxycyclohexa-1,3-diene radicle A is 3.6
kcal/mole more stable 2-methoxycyclohexa-1,3-diene radicle B. 1,3-
diene radicle using the density functional theory calculation.
29. (a) Conrad, J. C.; Kong, J.; Laforteza, B. N.; MacMillan, D. W. C.
J. Am. Chem. Soc
Gutierrez, O.; Schoenebeck, F.; Houk, K.N.; MacMillan,
C . J. Am. Chem. Soc 2010 132, 6001–6005.
.
2009
,
131
,
11640–11641. (b) Um, J. M.;
D. W.
.
,
Supplementary Material
Experimental procedures, 1H NMR, 2D NOESY, 13C NMR and
MS data of all the products are available free of charge via the
Internet at http://www.sciencedirect.com.