Facile one-pot synthesis of (benzoxazol-2'-yl)bicyclo[2.2.2]octen-2-one derivatives

Article abstract of DOI:10.1016/j.tet.2015.06.073

Abstract A simple and efficient one-pot synthesis of 5-(benzoxazol-2'-yl)bicyclo[2.2.2]octen-2-one derivatives from readily accessible starting materials has been demonstrated. The current protocol involves diacetoxyiodobenzene mediated domino oxidative c

Full text of DOI:10.1016/j.tet.2015.06.073

Tetrahedron 71 (2015) 6245e6253
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Facile one-pot synthesis of (benzoxazol-2ʹ-yl)bicyclo[2.2.2]octen-2-
one derivatives
*
Ram Tilak Naganaboina, Rama Krishna Peddinti
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple and efficient one-pot synthesis of 5-(benzoxazol-2ʹ-yl)bicyclo[2.2.2]octen-2-one derivatives
from readily accessible starting materials has been demonstrated. The current protocol involves diac-
etoxyiodobenzene mediated domino oxidative cyclizationeacetalizationeDielseAlder reaction.
Ó 2015 Elsevier Ltd. All rights reserved.
Received 1 January 2015
Received in revised form 17 June 2015
Accepted 20 June 2015
Available online 25 June 2015
Keywords:
Aldimines
Hypervalent iodine reagent
Cyclohexadienones
Masked o-benzoquinones
Cycloaddition
1. Introduction
Lewis acid are shown to be good acceptors in the reaction with
electron-rich arenes to generate highly oxygenated unsymmetrical
Organic compounds containing the benzoxazole scaffold belong
to a significant class of heterocyclic compounds that are encoun-
tered in a number of natural products.1 For example, benzoxazole
core containing bicyclic systems calcimycin, cezomycin and rou-
tiennocin are isolated from the various strains of Streptomyces. and
are found to act as good ionophore antibiotics.2 Benzoxazole de-
rivatives are found to have many applications in the field of me-
dicinal chemistry. They act as estrogen receptor agonists,3
melatonin receptor agonists,4 anti tumor agents,5 exhibit antimi-
crobial activities6 and FAAH inhibitors.7 These compounds are also
used as herbicides like fenoxaprop and found to have applications
in electronic devices,8 photoluminescent dyes9 and as sensors for
metals,10 due to their fluorescent properties. The wide spectrum of
applications of benzoxazoles has fascinated the chemists to de-
velop novel benzoxazole derivatives for various applications.11,12
biaryls.17 These cyclic dienones underwent conjugative addition
with thiols under catalyst-free conditions to furnish unsymmetrical
alkyl aryl/diaryl sulfides.18 We have recently synthesized bicy-
clooctenone derivatives by the DielseAlder reaction of relatively
stable dienic 4-halo MOBs with various 2p
-components.19 In further
investigation we envisaged the introduction of biologically signifi-
cant benzoxazole core on the bicyclooctenone system by domino
reaction. The strategy involves the sequential oxidative cycliza-
tion20 of an aldimine 1 into the benzoxazol-2ʹ-yl guaiacol 2, oxida-
tive dearomatization of 2 into MOB 3 and the DielseAlder
cycloaddition of 3 to provide the anticipated benzoxazolyl bicyclo
OH
N
DIB (1 eq)
N
OH
OMe
R
OMe
O
R
MeOH
2
OH
Masked o-benzoquinones (MOBs),
a
class of 2,4-
1
cyclohexadienones that can be transiently generated by the oxida-
tive dearomatization of guaiacol derivatives13,14 are useful synthons
in organic synthesis. The bicyclic systems derived from the Diel-
seAlder reaction of these intermediates with various dienophiles
are used as potential precursors in the synthesis of various complex
natural products.13,15,16 The MOB intermediates in the presence of
DIB (2 eq)
MeOH
"one-pot"
DIB (1 eq)
MeOH
X
Y
X
X
Y
N
N
Y
R
O
R
O
O
OMe
O
OMe
MeO OMe
3
* Corresponding author. E-mail addresses: ramakpeddinti@gmail.com,
rkpedfcy@iitr.ac.in (R.K. Peddinti).
Scheme 1. Domino oxidative cyclizationeacetalizationeDielseAlder strategy.
http://dx.doi.org/10.1016/j.tet.2015.06.073
0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.

6246
R.T. Naganaboina, R.K. Peddinti / Tetrahedron 71 (2015) 6245e6253
[2.2.2]octenone derivatives (Scheme 1). Herein we report the one-
pot synthesis of 5-(benzoxazol-2ʹ-yl)bicyclo[2.2.2]octenone de-
rivatives from easily accessible starting materials.
6 (Table 1). We extended our investigations with electron-deficient
dienophiles like methyl acrylate and methyl methacrylate. For this
purpose, we performed the reaction of 1a (0.5 mmol) with
2.2 equiv of DIB at rt in methanol in the presence of methyl acrylate
(20 equiv). The reaction was found to be completed in 40 min and
after purification by the silica gel column chromatography the
corresponding bicycloadduct 7a was obtained in 62% yield (Table
1). The generality of the cycloaddition of electron-deficient dien-
ophiles in current strategy was also tested with MOBs derived from
other aldimines. The bicyclic adducts 7 and 8 were obtained in
relatively lower yields in comparison to the products derived from
electron-rich ethyl vinyl ether (Table 1). Interestingly, the reaction
of aldimine 1e (R1¼Cl, R2¼H) derived from 2-amino-4-
chlorophenol, with methyl acrylate and methyl methacrylate did
not give any isolable products (Table 1). It appears that the MOB 3e
bearing chlorobenzoxazole moiety is not sufficiently electron-rich
owing to the electron pulling nature of chlorine atom to drive the
reaction with electron-deficient dienophiles used in this study.
The cyclic dienophilesedihydrofuran and furandwere also used
in the present protocol. The reaction of 1a gave optimum results
with 5 equiv of dihydrofuran and 20 equiv of furan. The reaction of
aldimines 1aee with these dienophiles produced the correspond-
ing adducts 9 and 10 in very high yields (Table 2).
2. Results and discussions
Our studies beganwith the synthesis of aldimines from the readily
available vanillin and o-aminophenol derivatives. Initially the aldi-
mine 1a was dissolved in methanol, and subjected to oxidation with
2.2 equiv of diacetoxyiodobenzene (DIB)21 at room temperature. The
in situ generated MOB has undergone self-dimerization in 4 h in the
absence of external reactant to afford the corresponding DielseAlder
dimer 4a in 79% yield. Of the eight possible isomers from the di-
merization of substituted MOB 3a provided single isomer 4a. The
assigned structure of this dimer is unambiguously confirmed by its
single crystal X-ray analysis (Fig. 1).22
O
MeO
MeO
N
O
N
O
O
OMe
MeO
4a
Each cycloadduct was obtained as racemic mixture of a single
diastereomer. The structures of these DielseAlder adducts were
assigned based on the 1H (500 MHz) and 13C (125 MHz) NMR, DEPT
and HRMS spectral analysis. In order to identify the chemical shifts of
protons and carbon atoms, we have carried out 1He1H decoupling
experiments and 2D experiments such as 1He1H COSY and HMQC.
The protons Ha and Hd resonate in the range of
d 3.5e3.8 and
Fig. 1. Single crystal X-ray structure of 4a.22
4.1e4.7 ppm, respectively. The coupling constants of HdeHe or HdeHf
are observed predominantly in the range of J¼2.5e3.5 Hz, which are
in agreement with the assigned ortho-regiochemistry (Table 3).
We were delighted with this event and proceeded further to
carry out the reaction of aldimine 1a with external dienophiles.
Thus the reaction of 1a with ethyl vinyl ether (5 equiv) in 5 mL of
methanol in the presence of 2.2 equiv of DIB underwent smoothly
to furnish the desired bicycloadduct 6a in 82% yield via the Diel-
seAlder reaction of in situ generated MOB 3a (R¼H).
The proton Hf resonates in the range of
d 1.5e2.1 ppm and He
resonate at downfield in the range of 2.4e2.7 ppm. The higher
d
chemical shift of He may be attributed predominantly to the mag-
netic anisotropic effect of exo-methoxy group of ketal function,
which is lying in its proximity. The coupling constants
(J¼6.0e7.0 Hz) between HfeHg, and those (J¼8.5e10.5 Hz) between
HeeHg reveals the cis relationship of the protons He and Hg, which
confirms the assigned endo-stereochemistry.
Though the result obtained was encouraging, we were curious
to know whether the first step is oxidative cyclization or oxidative
dearomatization. To understand the reaction sequence, we carried
out the reaction with 1.1 equiv of oxidizing agent. The product
formed in 10 min was isolated in 80% yield and its 1H and 13C NMR
analysis revealed that the product was benzoxazole derivative 2a.
The product 2a obtained in the initial step was oxidized further
with DIB (1.1 equiv) in the presence of ethyl vinyl ether and
methanol afforded the corresponding DielseAlder adduct 6a in 72%
yield. As the overall chemical yield (58%) of 6a obtained from the
two individual steps is less than one-pot reaction (82%), we pro-
ceeded with one-pot synthesis of benzoxazolyl bicyclo[2.2.2]octe-
none derivatives. To further optimize the reaction conditions,
several bases such as NaHCO3, K2CO3, NEt3 and KHCO3 were tested.
However, in all the cases only traces of benzoxazol-2ʹ-yl guaiacol 2a
was observed from the 1H NMR. At this juncture, we turned our
focus on the quantity of dienophile. The reaction of 1a was per-
formed with 10 equiv of ethyl vinyl ether; and no appreciable
change was noticed on the chemical yield. At higher amount
(20 equiv) of styrene, the corresponding product 5a was isolated in
about 10% higher yield than that carried out with 10 equiv.
In the 13C NMR of the DielseAlder adducts, the ring carbonyl
carbon appears at around 200 ppm and the ketal quaternary carbon
appears at around 93 ppm. Among the bridge-head carbons Ca
(d
55e57 ppm) and Cd ( 39e40 ppm), the former, which is positioned
d
next to the ring carbonyl resonates downfield. The cycloadducts
exhibited IR absorptions at 1735e1745 cmꢀ1, a characteristic ab-
sorption of carbonyl function of bicyclo[2.2.2]octenones derived
from MOBs. The selected 1H and 13C chemical shifts of some of the
cycloadducts are shown in Fig. 2. The assigned regio- and stereo-
selectivities are further established by the single crystal X-ray anal-
ysis of the product 5e (Fig. 3).23 The regio-, stereo- and site-selectivity
of the dimerization is in harmony with the literature precedents.24
In the initial step, the attack of lone pair of electrons of the imino
nitrogen on the Lewis acidic trivalent iodine centre makes the ad-
jacent carbon more electrophilic, making more susceptible for the
attack of nucleophilic oxygen atom leading to the benzoxazoline
intermediate A with concomitant elimination of acetic acid. The
subsequent aromatization of dihydro oxazole ring takes place along
with the elimination of acetic acid and iodobenzene to give ben-
zoxazole derivative 2a. Now, the attack of nucleophilic oxygen of
methoxyphenol 2a at the electrophilic iodine centre of DIB triggers
acetalization by knocking acetic acid out and the addition of
methanol either by associative pathway or by dissociative pathway
leads to MOB 3a. The nucleofugality of phenyliodanyl group helps
the oxidation by two-electron reduction of iodine(III) centre to
With the optimization conditions in hand, we advanced for
uncovering the scope of this domino reaction. Several aldimines
1be1e bearing different substitutions on the aminophenol moiety
were subjected to hypervalent iodine-mediated domino oxidative
cyclizationeacetalizationeDielseAlder protocol in methanol in the
presence of dienophiles ethyl vinyl ether and styrene. The reaction
proceeded smoothly to provide the bicyclic octenone adducts 5 and

R.T. Naganaboina, R.K. Peddinti / Tetrahedron 71 (2015) 6245e6253
6247
Table 1
Scope of substratesa
OH
X
R1
DIB (2.2 eq)
MeOH
N
R1
R2
N
Y
OMe
X
Y
O
R2
O
OMe
OH
MeO
5-8
Ph
Ph
Ph
Ph
Ph
N
O
N
O
N
O
N
O
Cl
N
O
O
O
O
O
O
MeO
MeO
MeO
MeO
OMe
OMe
OMe
OMe
MeO
OMe
5a, 64%
5c, 64%
5d, 52%
5b, 67%
5e, 69%
EtO
EtO
EtO
EtO
EtO
Cl
N
O
N
N
N
O
N
O
O
O
O
O
O
O
O
MeO
MeO
MeO
MeO
MeO
OMe
OMe
OMe
OMe
OMe
6e, 82%
6a, 82%
6d, 62%
6b, 80%
6c, 82%
O
O
O
O
MeO
MeO
MeO
MeO
MeO
MeO
N
O
N
O
N
N
O
O
O
OMe
O
OMe
O
O
OMe
MeO
MeO
OMe
7d, 57%
7c, 56%
7a, 62%
7b, 59%
O
O
O
O
MeO
MeO
MeO
MeO
N
O
N
O
N
O
N
O
O
O
OMe
O
OMe
O
OMe
MeO
MeO
MeO
MeO
OMe
8c, 55%
8d, 53%
8b, 55%
8a, 66%
aReaction was carried out with aldimine 1 (0.5 mmol), a dienophile and 2.2 equiv. of DIB in 5 mL of methanol at rt for 20 min (40 min
in case of electron-deficient dienophiles).
energetically preferable monovalent iodide i.e., iodobenzene. The
dienone 3a undergoes the [4þ2] cycloaddition with the dienophile
to deliver the desired cycloadduct (Scheme 2).
wavenumbers (cmꢀ1). NMR spectra were recorded in CDCl3 using
TMS as an internal standard. Chemical shifts of 1H NMR spectra are
given in parts per million with respect to TMS, and coupling con-
stants (J) are reported in hertz. The signals from solvent CDCl3 at
7.26 and 77.0 ppm were set as the reference peaks in 1H NMR and
13C NMR spectra, respectively. The following abbreviations are used
to explain the multiplicities: s¼singlet, d¼doublet, dd¼doublet of
doublets, dt¼doublet of triplets, t¼triplet, q¼quartet, m¼multiplet,
3. Conclusions
In summary, an efficient hypervalent iodine mediated domino
oxidative cyclizationeacetalizationeDielseAlder protocol for the
title compounds bearing diverse functionalities obtained from this
rapid synthesis shall be seen as overall yields of three steps. We are
currently pursuing the application of domino reactions of benzo-
quinone monoketals in our laboratory.
br¼broad. HRMS was performed on
a micrOTOF-Q II mass
spectrometer.
4.2. General procedure
4. Experimental section
4.1. General information
To a solution of aldimine 1 (0.5 mmol) and a dienophile [ethyl
vinyl ether/dihydrofuran (5 equiv) or styrene/furan/methyl acry-
late/methyl methacrylate (20 equiv)] in MeOH (5 mL), DIB (177 mg,
0.55 mmol) was added portion-wise at rt over a period of 5 min,
then allowed to stir for 5 min. After complete disappearance of
starting material as shown by TLC, another portion of DIB (177 mg,
0.55 mmol) was added over a period of 5 min and allowed to stir for
Unless otherwise noted, chemicals were purchased at the
highest purity grade available and were used without further pu-
rification. IR spectra of the compounds are expressed as

6248
R.T. Naganaboina, R.K. Peddinti / Tetrahedron 71 (2015) 6245e6253
Table 2
Scope of substratesa
OH
X
R1
DIB (2.2 eq)
Y
N
O
R1
R2
N
MeOH
OMe
X
R2
O
OH
MeO
OMe
9,10
Y
O
O
O
O
O
N
O
N
O
N
O
Cl
N
O
N
O
O
OMe
O
OMe
O
OMe
O
OMe
O
OMe
MeO
MeO
MeO
MeO
MeO
9a, 73%
9b, 71%
9c, 69%
9d, 54%
9e, 62%
O
O
O
O
N
O
N
O
N
O
Cl
N
O
N
O
O
O
OMe
O
OMe
O
OMe
O
OMe
O
OMe
MeO
MeO
MeO
MeO
MeO
10e, 71%
10a, 74%
10b, 72%
10c, 73%
10d, 61%
aReaction was carried out with aldimine 1 (0.5 mmol), a dienophile (2.5 mmol of dihydrofuran or 10 mmol of furan) and 2.2 equiv. of
DIB in 5 mL of methanol at rt for 20 min.
Table 3
Selected coupling constants (J in Hz) of the adducts 5e10
additional 5 min (or 25 min in case of an electron-deficient dien-
ophile). After completion of the reaction as indicated by TLC,
methanol was evaporated and the crude reaction mixture was
purified by silica gel column chromatography with ethyl acetate/
hexanes (20e30%) as eluting system.
O
Ph
Hf
O
Hg
He
Hg
He
Hg
He
MeO
Hf
Hf
O
O
O
OMe
OMe
OMe
OMe
R
R
R
OMe
OMe
5
6
7
4.2.1. 4-(Benzo[d]oxazol-2ʹ-yl)-2-methoxyphenol (2a). Yield: 97 mg
(80%) as white solid. Mp: 164e165 ꢁC. IR (KBr): nmax 2924, 1601,
O
1500, 1457, 1302, 1255, 1180 cmꢀ1. 1H NMR (500 MHz, CDCl3):
d 7.81
O
Hg
He
Hg
He
MeO
Hf
He
(dd, J¼2.0, 8.5 Hz, 1H), 7.77 (d, J¼2.0 Hz, 1H), 7.74e7.72 (m, 1H),
O
O
O
O
7.56e7.54 (m, 1H), 7.34e7.31 (m, 2H), 7.05 (d, J¼8.0 Hz, 1H), 6.20 (br
OMe
OMe
OMe
OMe
OMe
s, 1H), 4.01 (s, 3H) ppm. 13C NMR (125 MHz, CDCl3):
d 163.2, 150.7,
R
R
R
OMe
8
9
10
149.1, 146.9, 142.2, 124.7, 124.5, 121.9, 119.6, 119.3, 114.8, 110.4, 109.8,
56.2 ppm. HRMS (ESþ): m/z calcd for [C14H12NO3þ1]þ: 242.0817,
found: 242.0811.
S.No.
Adduct
J (Hz)
HeeHg
HfeHg
HdeHe
HdeHf
1
2
3
4
5
6
7
8
5a
5b
5c
5d
5e
6a
6b
6c
6d
6e
7a
7b
7c
7d
8a
8b
8c
8d
9a
9b
9c
8.5
9.5
9.5
9.5
10.0
8.0
8.5
8.5
8.0
8.5
10
6.5
7.0
7.0
6.5
7.0
d
3.0
3.0
3.0
3.0
3.0
2.5
3.0
2.5
2.5
2.5
3.0
2.5
d
2.5
2.5
3.0
2.5
3.0
3.0
3.5
3.5
3.5
3.5
3.0
3.0
2.5
3.0
2.0
2.5
2.5
2.5
d
4.2.2. (1S*,2S*,7R*,8R*)-7,11-(bis-Benzoxazol-2ʹ-yl)-3,3,10,1-
tetramethoxytricyclo[6.2.2.02,7]dodeca-5,11-diene-4,9-dione
(4a). Yield: 106 mg (79%) as yellow solid. Mp: 200e201 ꢁC. IR
(KBr): nmax 2947, 2830, 1745, 1708, 1590, 1533, 1449, 1237,
1115 cmꢀ1 1H NMR (500 MHz, CDCl3):
. d 7.78e7.75 (m, 2H),
d
7.56e7.54 (m, 1H), 7.50e7.48 (m, 1H), 7.40e7.36 (m, 2H), 7.35e7.30
(m, 2H), 7.02 (dd, J¼2.0, 7.0 Hz, 1H), 6.98 (d, J¼10.0 Hz, 1H), 6.17 (d,
J¼10.0 Hz, 1H), 4.46 (s, 1H), 4.35 (t, J¼2.0 Hz, 1H), 3.76 (d, J¼7.0, Hz,
1H), 3.59 (s, 3H), 3.54 (s, 3H), 3.38 (s, 3H), 3.00 (s, 3H) ppm. 13C NMR
d
9
d
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
d
6.0
6.0
6.0
6.0
d
10
d
(125 MHz, CDCl3):
d 197.8, 192.0, 164.9, 159.0, 150.8, 150.5, 143.8,
141.5, 140.6, 132.7, 130.7, 128.7, 125.7, 125.3, 124.6, 124.3, 120.8,
120.3, 110.5, 110.4, 97.5, 94.1, 58.9, 51.1, 50.0, 49.9, 49.5, 48.8, 43.5,
41.2 ppm. HRMS (ESþ): m/z calcd for [C30H26N2O8þNa]þ: 565.1581,
found: 565.1587.
10.5
d
3.0
3.5
3.5
3.5
3.5
3.0
3.0
3.0
d
d
d
d
d
d
d
8.0
8.0
8.0
d
d
d
d
4.2.3. (1S*,4R*,7S*)-5-(Benzoxazol-2ʹ-yl)-3,3-dimethoxy-7-
phenylbicyclo[2.2.2]oct-5-en-2-one (5a). Yield: 121 mg (64%) as
yellow solid. Mp: 99e100 ꢁC. IR (KBr): nmax 2949, 2830, 1737, 1633,
d
d
9d
9e
d
d
d
d
d
d
1529, 1447, 1132 cmꢀ1. 1H NMR (500 MHz, CDCl3):
d 7.77e7.75 (m,
10a
10b
10c
10d
10e
9.5
9.5
9.5
9.5
9.5
d
4.0
4.0
4.0
4.0
4.0
d
1H), 7.52e7.49 (m, 1H), 7.34e7.30 (m, 2H), 7.25e7.22 (m, 2H),
7.20e7.16 (m, 2H), 7.14e7.12 (m, 2H), 4.28 (q, J¼2.5 Hz, 1H),
3.60e3.55 (m, 1H), 3.54 (dd, J¼1.5, 6.5 Hz, 1H), 3.49 (s, 3H), 3.41 (s,
3H), 2.74 (ddd, J¼3.0, 8.5, 13.0 Hz, 1H), 1.78 (ddd, J¼2.5, 6.5, 13.5 Hz,
d
d
d
d
d
d
d
d

R.T. Naganaboina, R.K. Peddinti / Tetrahedron 71 (2015) 6245e6253
6249
O
O
3.32
3.65
MeO
3.58
Hg
3.65
CH3
Ph
MeO
2.74
He
Ha
H
e 2.74
1.78
2.07
H
7.18
b
Hf
Hf
1.88
Hf
Ha
O
O
7.17
Hb
H
3.61
3.54
OMe
7.15
b
OMe
N
N
N
Hd
4.11
Hd
4.28
Hd
4.18
3.49, 3.41
3.42, 3.32
OMe
OMe
O
O
O
7a
8a
5a
O
O
25.5
CH3
47.1
Ph
MeO
32.0 57.1
39.5
50.6
40.4
55.7
199.7
OMe
OMe
MeO
24.7
29.7
129.5
O
O
O
129.4
131.5
200.7
199.4
OMe
OMe
N
N
N
39.6
40.0
93.2
39.3
93.4
OMe
93.7
OMe
50.6, 49.8
50.1, 49.6
50.3, 49.9
O
O
O
7a
8a
5a
Fig. 2. Selected 1H and 13C chemical shifts (in ppm) of the cycloadducts 5a, 7a, and 8a.
1H) ppm. 13C NMR (125 MHz, CDCl3):
d 199.7, 160.0, 150.5, 143.1,
Ph
O
141.5, 134.4, 129.5, 128.4, 127.1, 126.7, 125.4, 124.3, 120.0, 110.3, 93.2,
55.7, 50.3, 49.9, 40.4, 40.0, 29.7 ppm. HRMS (ESþ): m/z calcd for
[C23H21NO4þNa]þ: 398.1363, found: 398.1363.
MeO
O
MeO
N
5e
4.2.4. (1S*,4R*,7S*)-3,3-Dimethoxy-5-(5ʹ-methylbenzoxazol-2ʹ-yl)-7-
phenylbicyclo[2.2.2]oct-5-en-2-one (5b). Yield: 131 mg (67%) as
yellow solid. Mp: 141e142 ꢁC. IR (KBr): nmax 2942, 2839, 1744, 1647,
Cl
1534,1452,1139 cmꢀ1. 1H NMR (500 MHz, CDCl3):
d 7.52 (s,1H), 7.39
(d, J¼8.0 Hz,1H), 7.24 (d, J¼3.0 Hz,1H), 7.23 (s,1H), 7.20 (d, J¼7.0 Hz,
1H), 7.16 (d, J¼9.5 Hz, 1H), 7.14e7.11 (m, 3H), 4.23 (q, J¼2.5 Hz, 1H),
3.56e3.53 (m, 1H), 3.51 (dd, J¼1.5, 6.5 Hz, 1H), 3.47 (s, 3H), 3.37 (s,
3H), 2.70 (ddd, J¼3.0, 9.5, 13.0 Hz, 1H), 2.46 (s, 3H), 1.76 (ddd, J¼2.5,
7.0, 13.5 Hz, 1H) ppm. 13C NMR (125 MHz, CDCl3):
d 200.1, 160.3,
148.9, 143.3, 141.8, 134.7, 134.4, 129.2, 128.6, 127.3, 126.9, 126.8,
Fig. 3. Single crystal X-ray structure of 5e.23
O
O
O
O
O
O
O
O
I
Ph
Ph
O
I
I
O
O
PhI
AcOH
OH
N
N
O
I
N
O
AcOH
OH
OMe
OH
OH
OMe
N
O
O
"cyclization"
H
OMe
H
OMe
A
2a
OH
OAc
AcOH
O
O
O
I
Ph
N
O
N
O
PhI
O
"oxidative
acetalization"
O
Me
O
Me
MeOH
(Dissociative pathway)
OAc
AcOH
O
X
X
O
I
O
PhI
AcOH
N
O
Ph
N
O
N
O
O
"oxidative
acetalization"
O
"Diels-Alder"
OMe
OMe
OMe
MeO
O
MeOH
Me
3a
cycloadduct
(Associative pathway)
Scheme 2. Plausible mechanism.

6250
R.T. Naganaboina, R.K. Peddinti / Tetrahedron 71 (2015) 6245e6253
120.1, 109.8, 93.4, 55.8, 50.5, 50.2, 40.6, 40.2, 30.0, 21.4 ppm. HRMS
141.8, 134.3, 133.8, 128.4, 126.8, 120.1, 109.8, 93.4, 75.0, 64.5, 54.6,
50.7, 49.9, 38.8, 30.1, 21.5, 15.2 ppm. HRMS (ESþ): m/z calcd for
[C20H23NO5þNa]þ: 380.1474, found 380.1462.
(ESþ): m/z calcd for [C24H23NO4þNa]þ: 412.1525, found: 412.1522.
4.2.5. (1S*,4R*,7S*)-3,3-Dimethoxy-5-(6ʹ-methylbenzoxazol-2ʹ-yl)-7-
phenylbicyclo[2.2.2]oct-5-en-2-one (5c). Yield: 123 mg (64%) as pale
yellow solid. Mp: 99e100 ꢁC. IR (KBr): nmax 2937, 2834, 1742, 1631,
4.2.10. (1S*,4R*,7S*)-3,3-Dimethoxy-7-methoxy-5-(6ʹ-methylbenzox-
azol-2ʹ-yl)bicyclo[2.2.2]oct-5-en-2-one (6c). Yield: 146 mg (82%) as
thick brown liquid. IR (KBr): nmax 2934, 2865,1741,1638,1533,1449,
1530, 1449, 1245, 1128 cmꢀ1. 1H NMR (500 MHz, CDCl3):
d 7.62 (d,
J¼8.0, 1H), 7.32 (s, 1H), 7.26e7.22 (m, 2H), 7.20e7.11 (m, 5H), 4.25
(q, J¼3.0 Hz, 1H), 3.58e3.54 (m, 1H), 3.52 (dd, J¼1.5, 6.5 Hz, 1H),
3.49 (s, 3H), 3.40 (s, 3H), 2.72 (ddd, J¼3.0, 9.5, 13.0 Hz, 1H), 2.48 (s,
3H), 1.77 (ddd, J¼3.0, 7.0, 13.5 Hz, 1H) ppm. 13C NMR (125 MHz,
1244, 1102 cmꢀ1. 1H NMR (500 MHz, CDCl3):
d
7.58 (d, J¼8.0 Hz, 1H),
7.29 (s, 1H), 7.12 (d, J¼8.0 Hz, 1H), 7.04 (d, J¼6.0 Hz, 1H), 4.12e4.08
(m, 2H), 3.81 (dd, J¼2.5, 6.0 Hz, 1H), 3.56e3.50 (m, 1H), 3.46e3.40
(m, 1H), 3.38 (s, 3H), 3.32 (s, 3H), 2.59 (ddd, J¼2.5, 8.5, 14.0 Hz, 1H),
2.47 (s, 3H), 1.50 (ddd, J¼3.5, 3.5, 14.0 Hz, 1H), 1.48 (t, J¼6.5 Hz, 3H)
CDCl3):
d 200.0, 159.7, 151.0, 143.3, 139.5, 136.2, 134.7, 128.9, 128.6,
127.3, 126.8, 125.8, 119.6,110.6, 93.4, 55.8, 50.5, 50.1, 40.6, 40.2, 29.9,
21.7 ppm. HRMS (ESþ): m/z calcd for [C24H23NO4þNa]þ: 412.1519,
found: 412.1513.
ppm. 13C NMR (125 MHz, CDCl3):
d 200.0, 159.8, 151.0, 139.4, 136.2,
133.8, 128.1, 125.8, 119.5, 110.6, 93.4, 75.0, 64.4, 54.7, 50.7, 49.9, 38.9,
30.1, 21.8, 15.2 ppm. HRMS (ESþ): m/z calcd for [C20H23NO5þNa]þ:
380.1474, found: 380.1465.
4.2.6. (1S*,4R*,7S*)-5-(5ʹ-tert-Butylbenzoxazol-2ʹ-yl)-3,3-
dimethoxy-7-phenylbicyclo[2.2.2]oct-5-en-2-one (5d). Yield: 113 mg
(52%) as pale yellow liquid. IR (KBr): nmax 2958, 2934, 1740, 1639,
4.2.11. (1S*,4R*,7S*)-5-(5ʹ-tert-Butylbenzoxazol-2ʹ-yl)-3,3-
dimethoxy-7-ethoxybicyclo[2.2.2]oct-5-en-2-one
(6d). Yield:
1532, 1475, 1268, 1197 cmꢀ1. 1H NMR (500 MHz, CDCl3):
d
7.80e7.79
124 mg (62%) as pale yellow liquid. IR (KBr): nmax 2965, 2865, 1741,
(m, 1H), 7.45e7.44 (m, 2H), 7.27e7.24 (m, 2H), 7.22e7.18 (m, 1H),
7.15e7.12 (m, 3H), 4.26 (q, J¼2.5 Hz, 1H), 3.58e3.55 (m, 1H), 3.52
(dd, J¼1.5, 6.5 Hz,1H), 3.49 (s, 3H), 3.40 (s, 3H), 2.73 (ddd, J¼3.0, 9.5,
1639, 1534, 1475, 1360, 1271, 1103 cmꢀ1. 1H NMR (500 MHz, CDCl3):
d
7.75 (s, 1H), 7.41e7.39 (m, 2H), 7.05 (dd, J¼1.5, 6.5 Hz, 1H),
4.12e4.09 (m, 2H), 3.81 (dd, J¼2.5, 6.5 Hz, 1H), 3.56e3.50 (m, 1H),
3.45e3.40 (m, 1H), 3.38 (s, 3H), 3.31 (s, 3H), 2.59 (ddd, J¼2.5, 8.0,
13.5 Hz, 1H), 1.49 (ddd, J¼3.5, 3.5, 14.0 Hz, 1H), 1.36 (s, 9H), 1.15 (t,
13.0 Hz, 1H), 1.78 (ddd, J¼2.5, 6.5, 13.5 Hz, 1H), 1.39 (s, 9H) ppm. 13
C
NMR (125 MHz, CDCl3):
d 200.3, 160.5, 148.8, 148.3, 143.4, 141.6,
134.8, 129.2, 128.7, 127.4, 127.0, 123.5, 116.8, 109.7, 93.5, 56.0, 50.6,
50.2, 40.8, 40.3, 34.9, 31.7, 30.0 ppm. HRMS (ESþ): m/z calcd for
[C27H29NO4þNa]þ: 454.1989, found: 454.1976.
J¼5.5 Hz, 3H) ppm. 13C NMR (125 MHz, CDCl3):
d 200.0, 160.4, 148.7,
148.0, 141.5, 133.8, 128.3, 123.4, 116.7, 109.6, 93.4, 75.0, 64.4, 54.6,
50.7, 49.9, 38.9, 34.8, 31.7, 30.1, 15.2 ppm. HRMS (ESþ): m/z calcd for
[C23H29NO5þNa]þ: 422.1938, found: 422.1941.
4.2.7. (1S*,4R*,7S*)-5-(50-Chlorobenzoxazol-2ʹ-yl)-3,3-dimethoxy-7-
phenylbicyclo[2.2.2]oct-5-en-2-one (5e). Yield: 142 mg (69%) as
orange solid. Mp: 173e174 ꢁC. IR (KBr): nmax 2943, 2836, 1736,
4.2.12. (1S*,4R*,7S*)-5-(50-Chlorobenzoxazol-2ʹ-yl)-3,3-dimethoxy-
7-ethoxybicyclo[2.2.2]oct-5-en-2-one (6e). Yield: 154 mg (82%) as
orange solid. Mp: 80 ꢁC. IR (KBr): nmax 2978, 2839, 1740, 1637, 1529,
1626, 1527, 1453, 1127 cmꢀ1. 1H NMR (500 MHz, CDCl3):
d 7.73 (d,
J¼2.0 Hz, 1H), 7.45 (d, J¼8.5 Hz, 1H), 7.32 (dd, J¼2.0, 8.5 Hz, 1H),
7.28e7.25 (m, 2H), 7.22e7.19 (m, 2H), 7.14e7.12 (m, 2H), 4.23 (q,
J¼3.0 Hz, 1H), 3.60e3.58 (m, 1H), 3.56 (dd, J¼1.5, 6.5 Hz, 1H), 3.49
(s, 3H), 3.39 (s, 3H), 2.74 (ddd, J¼3.0, 10.0, 13.5 Hz, 1H), 1.78 (ddd,
1448, 1126 cmꢀ1. 1H NMR (500 MHz, CDCl3):
d 7.70e7.69 (m, 1H),
7.42 (d, J¼8.5 Hz, 1H), 7.33e7.30 (m, 1H), 7.11 (d, J¼7.5 Hz, 1H),
4.14e4.11 (m, 1H), 4.09e4.07 (m, 1H), 3.84 (dd, J¼2.5, 6.0 Hz, 1H),
3.57e3.51 (m, 1H), 3.46e3.40 (m, 1H), 3.38 (s, 3H), 3.31 (s, 3H), 2.60
(ddd, J¼2.5, 8.5, 14.0 Hz, 1H), 1.50 (ddd, J¼3.5, 3.5, 14.0 Hz, 1H), 1.15
J¼3.0, 7.0, 13.5 Hz, 1H) ppm. 13C NMR (125 MHz, CDCl3):
d 199.9,
161.4, 149.3, 143.2, 142.8, 134.3, 130.7, 130.0, 128.7, 127.3, 127.0,
125.9, 120.2, 111.2, 93.3, 56.0, 50.6, 50.2, 40.6, 40.2, 29.9 ppm.
HRMS (ESþ): m/z calcd for [C23H20ClNO4þNa]þ: 432.0973, found:
432.0991.
(t, J¼7.0 Hz, 3H) ppm. 13C NMR (125 MHz, CDCl3):
d 199.8, 161.4,
149.2, 142.7, 133.3, 129.9, 129.8, 125.8, 120.1, 111.2, 93.2, 74.9, 64.5,
54.7, 50.7, 49.8, 38.8, 30.0, 15.2 ppm. HRMS (ESþ): m/z calcd for
[C19H20ClNO5þNa]þ: 400.0928, found: 400.0917.
4.2.8. (1S*,4R*,7S*)-5-(Benzoxazol-2ʹ-yl)-3,3-dimethoxy-7-
ethoxybicyclo[2.2.2]oct-5-en-2-one (6a). Yield: 142 mg (82%) as
yellow solid. Mp: 101e102 ꢁC. IR (KBr): nmax 2939, 2842, 1738, 1643,
4.2.13. (1S*,4R*,7S*)-5-(Benzoxazol-2ʹ-yl)-3,3-dimethoxy-7-
methoxycarbonylbicyclo[2.2.2]oct-5-en-2-one (7a). Yield: 111 mg
(62%) as thick yellow liquid. IR (KBr): nmax 2952, 2839, 1737, 1636,
1531, 1447, 1191 cmꢀ1. 1H NMR (500 MHz, CDCl3):
d
7.68e7.67 (m,
1525, 1443, 1332,1165 cmꢀ1. 1H NMR (500 MHz, CDCl3):
d 7.73e7.69
1H), 7.46e7.43 (m, 1H), 7.29e7.24 (m, 2H), 7.04 (d, J¼6.0 Hz, 1H),
4.07e4.06 (m, 2H), 3.78 (dd, J¼2.0, 6.0 Hz, 1H), 3.52e3.45 (m, 1H),
3.41e3.35 (m, 1H), 3.34 (s, 3H), 3.28 (s, 3H), 2.55 (ddd, J¼2.5, 8.0,
13.5 Hz, 1H), 1.46 (ddd, J¼3.0, 3.0, 13.5 Hz, 1H), 1.10 (t, J¼7.0 Hz, 3H)
(m, 1H), 7.49e7.46 (m, 1H), 7.34e7.29 (m, 2H), 7.15e7.13 (m, 1H),
4.18 (q, J¼2.5 Hz, 1H), 3.76 (dd, J¼2.0, 7.0 Hz, 1H), 3.66 (s, 3H), 3.40
(s, 3H), 3.33 (s, 3H), 3.19 (ddd, J¼1.5, 6.0, 10.0 Hz, 1H), 2.49 (ddd,
J¼3.0, 10.0, 13.5 Hz, 1H), 1.88 (ddd, J¼3.0, 6.0, 13.5 Hz, 1H) ppm. 13
C
ppm. 13C NMR (125 MHz, CDCl3):
d
199.9, 160.2, 150.6, 141.6, 133.6,
NMR (125 MHz, CDCl3): d 199.4, 172.8, 159.9, 150.6, 141.5, 134.4,
128.8,125.5,124.4,120.1,110.4, 93.3, 74.9, 64.4, 54.7, 50.6, 49.8, 38.8,
30.0, 15.2 ppm. HRMS (ESþ): m/z calcd for [C19H21NO5þNa]þ:
366.1312, found: 366.1329.
129.4,125.7,124.5,120.2,110.4, 93.4, 52.4, 50.6, 50.6, 50.1, 39.5, 39.3,
24.7 ppm. HRMS (ESþ): m/z calcd for [C19H19NO6þNa]þ: 380.1105,
found: 380.1102.
4.2.9. (1S*,4R*,7S*)-3,3-Dimethoxy-7-ethoxy-5-(5ʹ-methylbenzox-
azol-2ʹ-yl)bicyclo[2.2.2]oct-5-en-2-one (6b). Yield: 142 mg (80%) as
yellow solid. Mp: 109e110 ꢁC. IR (KBr): nmax 2972, 2843, 1738, 1643,
4.2.14. (1S*,4R*,7S*)-3,3-Dimethoxy-7-methoxycarbonyl-5-(5ʹ-meth-
ylbenzoxazol-2ʹ-yl)bicyclo[2.2.2]oct-5-en-2-one (7b). Yield: 109 mg
(59%) as pale yellow solid. Mp: 118e119 ꢁC. IR (KBr): nmax 2946,
2838, 1738, 1633, 1527, 1440, 1323, 1143 cmꢀ1. 1H NMR (500 MHz,
1534, 1447, 1128 cmꢀ1. 1H NMR (500 MHz, CDCl3):
d 7.50 (s, 1H), 7.37
(d, J¼8.5 Hz, 1H), 7.15 (dd, J¼1.5, 8.5 Hz, 1H), 7.06 (d, J¼6.5 Hz, 1H),
4.14e4.09 (m, 2H), 3.82 (dd, J¼2.5, 6.5 Hz, 1H), 3.57e3.51 (m, 1H),
3.46e3.41 (m, 1H), 3.38 (s, 3H), 3.32 (s, 3H), 2.59 (ddd, J¼3.0, 8.5,
14.0 Hz, 1H), 2.46 (s, 3H), 1.50 (ddd, J¼3.5, 3.5, 14.0 Hz, 1H), 1.15 (t,
CDCl3):
d
7.49 (s, 1H), 7.34 (d, J¼8.0 Hz, 1H), 7.12 (dt, J¼1.5, 9.0 Hz,
2H), 4.17 (q, J¼2.5 Hz, 1H), 3.75 (dd, J¼2.0, 6.5 Hz, 1H), 3.66 (s, 3H),
3.40 (s, 3H), 3.34 (s, 3H), 3.19 (ddd, J¼1.5, 6.0, 10.0 Hz, 1H), 2.49
(ddd, J¼2.5, 10.0, 13.0 Hz, 1H), 2.44 (s, 3H), 1.88 (ddd, J¼3.0, 6.0,
J¼7.0 Hz, 3H) ppm. 13C NMR (125 MHz, CDCl3):
d
200.1, 160.3, 148.9,
13.5 Hz, 1H) ppm. 13C NMR (125 MHz, CDCl3):
d 199.4, 172.8, 160.0,

R.T. Naganaboina, R.K. Peddinti / Tetrahedron 71 (2015) 6245e6253
6251
148.9, 141.7, 134.6, 134.4, 129.0, 126.9, 120.1, 109.8, 93.4, 52.4, 50.6,
50.5, 50.1, 39.6, 39.4, 24.8, 21.4 ppm. HRMS (ESþ): m/z calcd for
[C20H21NO6þNa]þ: 394.1261, found: 394.1253.
25.4, 21.7 ppm. HRMS (ESþ): m/z calcd for [C21H23NO6þNa]þ:
408.1418, found: 408.1402.
4.2.20. (1S*,4R*,7S*)-5-(5ʹ-tert-Butylbenzoxazol-2ʹ-yl)-3,3-
dimethoxy-7-methoxycarbonyl-7-methylbicyclo[2.2.2]oct-5-en-2-
one (8d). Yield: 114 mg (53%) as yellow solid. Mp: 90 ꢁC. IR (KBr):
4.2.15. (1S*,4R*,7S*)-3,3-Dimethoxy-7-methoxycarbonyl-5-(6ʹ-meth-
ylbenzoxazol-2ʹ-yl)bicyclo[2.2.2]oct-5-en-2-one (7c). Yield: 103 mg
(56%) as pale yellow solid. Mp: 95e96 ꢁC. IR (KBr): nmax 2952,
nmax 2961, 2878,1734,1634,1534,1468,1382,1264, 1208,1114 cmꢀ1
.
2839, 1740, 1626, 1528, 1444, 1327, 1184, 1136 cmꢀ1
.
1H NMR
1H NMR (500 MHz, CDCl3):
d
7.74 (s, 1H), 7.39 (s, 2H), 7.14 (dd, J¼2.0,
(500 MHz, CDCl3):
d
7.57 (d, J¼8.0 Hz, 1H), 7.28 (s, 1H), 7.13e7.09
6.5 Hz, 1H), 4.11e4.10 (m, 1H), 3.65 (s, 3H), 3.61 (d, J¼6.5 Hz, 1H),
3.42 (s, 3H), 3.22 (s, 3H), 2.39 (dd, J¼3.5, 14.0 Hz, 1H), 2.07 (dd,
J¼2.5,14.0 Hz,1H), 1.38 (s, 3H),1.35 (s, 9H) ppm. 13C NMR (125 MHz,
(m, 2H), 4.17e4.16 (m, 1H), 3.75 (d, J¼6.5 Hz, 1H), 3.67 (s, 3H),
3.40 (s, 3H), 3.34 (s, 3H), 3.21e3.17 (m, 1H), 2.51e2.48 (m, 1H),
2.46 (s, 3H), 1.88 (ddd, J¼2.5, 6.0, 13.0 Hz, 1H) ppm. 13C NMR
CDCl3):
d 200.7, 175.7, 160.3, 148.7, 148.1, 141.5, 133.9, 131.0, 123.5,
(125 MHz, CDCl3):
d
199.5, 172.8, 159.5, 151.0, 139.4, 136.3, 134.6,
116.7, 109.6, 93.7, 57.1, 52.6, 50.5, 49.8, 47.2, 39.7, 34.8, 32.0, 31.7,
25.5 ppm. HRMS (ESþ): m/z calcd for [C24H29NO6þNa]þ: 450.1893,
found: 450.1893.
128.7, 125.8, 119.6, 110.6, 93.5, 52.4, 50.6, 50.1, 39.6, 39.4, 24.8,
21.8 ppm. HRMS (ESþ): m/z calcd for [C20H21NO6þNa]þ: 394.1261,
found: 394.1251.
4.2.21. (1R*,2R*,6R*,7S*)-11-(Benzoxazol-2ʹ-yl)-8,8-dimethoxy-3-
oxatricyclo[5.2.2.02,6]undec-10-en-9-one (9a). Yield: 124 mg (73%)
as green colour solid. Mp: 107e108 ꢁC. IR (KBr): nmax 2939, 2843,
1737, 1643, 1539, 1448, 1230, 1146 cmꢀ1. 1H NMR (500 MHz, CDCl3):
4.2.16. (1S*,4R*,7S*)-5-(5ʹ-tert-Butylbenzoxazol-2ʹ-yl)-3,3-
dimethoxy-7-methoxycarbonylbicyclo[2.2.2]-oct-5-en-2-one
(7d). Yield: 118 mg (57%) as yellow solid. Mp: 94e95 ꢁC. IR (KBr):
nmax 2952, 2839, 1740, 1626, 1526, 1444, 1327, 1184 cmꢀ1. 1H NMR
d
7.76e7.73 (m, 1H), 7.52e7.50 (m, 1H), 7.38e7.31 (m, 2H), 7.14e7.11
(500 MHz, CDCl3):
d
7.74 (s, 1H), 7.40 (s, 2H), 7.11 (dd, J¼2.0, 6.5 Hz,
(m, 1H), 4.47 (dd, J¼3.0, 8.0 Hz, 1H), 4.21 (t, J¼2.5 Hz, 1H), 3.84 (dd,
J¼3.0, 6.0 Hz, 1H), 3.80e3.78 (m, 1H), 3.57e3.52 (m, 1H), 3.42 (s,
3H), 3.31 (s, 3H), 3.12 (ddd, J¼3.0, 8.0, 17.5 Hz, 1H), 2.18e2.11 (m,
1H), 4.18 (q, J¼2.5 Hz, 1H), 3.76 (dd, J¼2.0, 6.5 Hz, 1H), 3.66 (s, 3H),
3.40 (s, 3H), 3.34 (s, 3H), 3.20 (ddd, J¼1.5, 6.0, 10.0 Hz, 1H), 2.49
(ddd, 3.0, 10.5, 13.5 Hz, 1H), 1.88 (ddd, 3.0, 6.0, 13.5, 1H), 1.35 (s, 9H)
1H), 1.57e1.50 (m, 1H) ppm. 13C NMR (125 MHz, CDCl3):
d 199.7,
ppm. 13C NMR (125 MHz, CDCl3):
d
199.5, 172.8, 160.1, 148.7, 148.1,
161.1, 150.7, 141.6, 131.8, 130.5, 125.8, 124.6, 120.3, 110.6, 93.1, 79.2,
69.1, 55.7, 50.7, 50.0, 43.3, 38.3, 30.2 ppm. HRMS (ESþ): m/z calcd for
[C19H19NO5þNa]þ: 364.1155, found: 364.1143.
141.4, 134.6, 128.9, 123.5, 116.7, 109.6, 93.5, 52.4, 50.6, 50.6, 50.1,
39.6, 39.4, 34.8, 31.6, 24.7 ppm. HRMS (ESþ): m/z calcd for
[C23H27NO6þNa]þ: 436.1736, found: 436.1736.
4.2.22. (1R*,2R*,6R*,7S*)-8,8-Dimethoxy-11-(5ʹ-methylbenzoxazol-
4.2.17. (1S*,4R*,7S*)-5-(Benzoxazol-2ʹ-yl)-3,3-dimethoxy-7-
methoxycarbonyl-7-methylbicyclo[2.2.2]oct-5-en-2-one (8a). Yield:
121 mg (66%) as yellow solid. Mp: 101e102 ꢁC. IR (KBr): nmax 2949,
2843, 1739, 1634, 1531, 1447, 1204, 1149 cmꢀ1. 1H NMR (500 MHz,
2ʹ-yl)-3-oxatricyclo[5.2.2.02,6]undec-10-en-9-one
(9b). Yield:
126 mg (71%) as green solid. Mp: 144e145 ꢁC. IR (KBr): nmax 2939,
2878, 1737, 1636, 1526, 1451, 1262, 1146 cmꢀ1. 1H NMR (500 MHz,
CDCl3):
d
7.51 (s, 1H), 7.37 (d, J¼8.0 Hz, 1H), 7.15 (dd, J¼1.0, 8.5 Hz,
CDCl3):
d
7.71 (d, J¼7.5 Hz, 1H), 7.47 (d, J¼7.5 Hz, 1H), 7.34e7.28 (m,
1H), 7.08 (d, J¼6.5 Hz, 1H), 4.45 (dd, J¼3.0, 8.5 Hz, 1H), 4.19 (t,
J¼2.5 Hz, 1H), 3.82 (dd, J¼3.0, 6.5 Hz, 1H), 3.79 (dd, J¼3.0, 8.5 Hz,
1H), 3.56e3.50 (m, 1H), 3.40 (s, 3H), 3.30 (s, 3H), 3.11 (ddd, J¼3.0,
8.0, 18.0 Hz, 1H), 2.45 (s, 3H), 2.16e2.10 (m, 1H), 1.55e1.47 (m, 1H)
2H), 7.18 (d, J¼6.5 Hz, 1H), 4.12e4.11 (m, 1H), 3.65 (s, 3H), 3.61 (d,
J¼6.5 Hz, 1H), 3.42 (s, 3H), 3.32 (s, 3H), 2.40 (dd, J¼3.5, 14.0 Hz, 1H),
2.07 (dd, J¼2.0, 14.0 Hz, 1H), 1.39 (s, 3H) ppm. 13C NMR (125 MHz,
CDCl3):
d
200.7, 175.7, 160.1, 150.6, 141.6, 133.8, 131.5, 125.7, 124.5,
ppm. 13C NMR (125 MHz, CDCl3):
d 199.7, 161.2, 148.9, 141.8, 134.4,
120.2, 110.5, 93.7, 57.1, 52.6, 50.6, 49.8, 47.1, 39.6, 32.0, 25.5 ppm.
HRMS (ESþ): m/z calcd for [C20H21NO6þNa]þ: 394.1261, found:
394.1247.
131.9, 130.0, 126.8, 120.1, 109.9, 93.0, 79.1, 69.1, 55.6, 50.6, 50.0, 43.2,
38.2, 30.2, 21.4 ppm. HRMS (ESþ): m/z calcd for [C20H21NO5þNa]þ:
378.1312, found: 378.1312.
4.2.18. (1S*,4R*,7S*)-3,3-Dimethoxy-7-methoxycarbonyl-7-methyl-
5-(5ʹ-methylbenzoxazol-2ʹ-yl)bicyclo[2.2.2]oct-5-en-2-one
(8b). Yield: 106 mg (55%) as brown solid. Mp: 98 ꢁC. IR (KBr): nmax
2949, 2839, 1738, 1634, 1530, 1452, 1382, 1256, 1113 cmꢀ1. 1H NMR
4.2.23. (1R*,2R*,6R*,7S*)-8,8-Dimethoxy-11-(6ʹ-methylbenzoxazol-
2ʹ-yl)-3-oxatricyclo[5.2.2.02,6]undec-10-en-9-one
(9c). Yield:
121 mg (69%) as brown solid. Mp: 119e120 ꢁC. IR (KBr): nmax 2939,
2863, 1735, 1630, 1530, 1449, 1232, 1141 cmꢀ1. 1H NMR (500 MHz,
(500 MHz, CDCl3):
d
7.48 (s, 1H), 7.34 (d, J¼8.5 Hz, 1H), 7.16e7.11 (m,
CDCl3):
d
7.61 (d, J¼8.0 Hz, 1H), 7.33 (s, 1H), 7.16 (d, J¼8.5 Hz, 1H),
2H), 4.11e4.09 (m, 1H), 3.65 (s, 3H), 3.60 (d, J¼6.5 Hz, 1H), 3.42 (s,
3H), 3.32 (s, 3H), 2.43 (s, 3H), 2.39 (dd, J¼3.5, 14.0 Hz, 1H), 2.07 (dd,
J¼2.5, 14.0 Hz, 1H), 1.38 (s, 3H) ppm. 13C NMR (125 MHz, CDCl3):
7.10e7.08 (m, 1H), 4.48 (dd, J¼3.0, 8.5 Hz, 1H), 4.22e4.21 (m, 1H),
3.84 (dd, J¼3.0, 6.5 Hz, 1H), 3.81 (dd, J¼3.0, 8.5 Hz, 1H), 3.58e3.53
(m, 1H), 3.42 (s, 3H), 3.32 (s, 3H), 3.12 (ddd, J¼3.0, 8.0, 17.5 Hz, 1H),
2.51 (s, 3H), 2.19e2.13 (m, 1H), 1.57e1.49 (m, 1H) ppm. 13C NMR
d
200.7, 175.7, 160.2, 148.9, 141.7, 134.3, 133.9, 131.1, 126.8, 120.1,
109.8, 93.6, 57.1, 52.6, 50.5, 49.8, 47.1, 39.6, 32.0, 25.5, 21.4 ppm.
HRMS (ESþ): m/z calcd for [C21H23NO6þNa]þ: 408.1418, found:
408.1401.
(125 MHz, CDCl3): d 199.7, 160.6, 150.8, 139.2, 136.3, 131.7, 129.7,
125.7, 119.5, 110.6, 93.0, 79.0, 69.0, 55.5, 50.6, 49.9, 43.1, 38.1, 30.1,
21.7 ppm. HRMS (ESþ): m/z calcd for [C20H21NO5þNa]þ: 378.1312,
found: 378.1320.
4.2.19. (1S*,4R*,7S*)-3,3-Dimethoxy-7-methoxycarbonyl-7-methyl-
5-(6ʹ-methylbenzoxazol-2ʹ-yl)bicyclo[2.2.2]oct-5-en-2-one
(8c). Yield: 106 mg (55%) as white solid. Mp: 109e110 ꢁC. IR (KBr):
nmax 2951, 2837, 1733, 1625, 1530, 1446, 1326, 1211, 1144 cmꢀ1. 1H
4.2.24. (1R*,2R*,6R*,7S*)-11-(5ʹ-tert-Butylbenzoxazol-2ʹ-yl)-8,8-
dimethoxy-3-oxatricyclo[5.2.2.02,6]undec-10-en-9-one (9d). Yield:
106 mg (54%) as green colour solid. Mp: 158e159 ꢁC. IR (KBr): nmax
2952, 2894, 1743, 1639, 1532, 1269, 1141 cmꢀ1. 1H NMR (500 MHz,
NMR (500 MHz, CDCl3):
d
7.55 (d, J¼8.5 Hz, 1H), 7.25 (s, 1H),
7.13e7.09 (m, 2H), 4.08e4.07 (m, 1H), 3.64 (s, 3H), 3.59 (d, J¼6.0 Hz,
1H), 3.41 (s, 3H), 3.31 (s, 3H), 2.44 (s, 3H), 2.38 (dd, J¼3.5, 14.0 Hz,
1H), 2.05 (dd, J¼2.5, 14.0 Hz, 1H), 1.37 (s, 3H) ppm. 13C NMR
CDCl3): d 7.80e7.77 (m, 1H), 7.43e7.42 (m, 2H), 7.10e7.08 (m, 1H),
4.46 (dd, J¼3.0, 8.0 Hz, 1H), 4.21e4.20 (m, 1H), 3.82 (dd, J¼3.0,
6.5 Hz, 1H), 3.79 (dt, J¼3.0, 8.5 Hz, 1H), 3.56e3.51 (m, 1H), 3.41 (s,
3H), 3.31 (s, 3H), 3.14e3.08 (m,1H), 2.17e2.11 (m,1H),1.57e1.48 (m,
(125 MHz, CDCl3):
d 200.6, 175.7, 159.7, 150.9, 139.3, 136.2, 133.9,
130.7, 125.7, 119.5, 110.5, 93.6, 57.0, 52.5, 50.5, 49.8, 47.1, 39.6, 31.9,
1H), 1.37 (s, 9H) ppm. 13C NMR (125 MHz, CDCl3):
d 199.8, 161.3,

6252
R.T. Naganaboina, R.K. Peddinti / Tetrahedron 71 (2015) 6245e6253
148.7, 148.2, 141.5, 131.9, 130.0, 123.6, 116.8, 109.7, 93.1, 79.2, 69.1,
55.7, 50.7, 50.0, 43.3, 38.2, 34.9, 31.7, 30.2 ppm. HRMS (ESþ): m/z
calcd for [C23H27NO5þNa]þ: 420.1781, found: 420.1781.
160.8, 148.5, 148.2, 148.0, 141.5, 130.4, 129.8, 123.3, 116.9, 109.5,
100.0, 93.4, 79.1, 52.8, 50.4, 45.4, 44.3, 34.8, 31.6, 28.5 ppm. HRMS
(ESþ): m/z calcd for [C23H25NO5þNa]þ: 418.1625, found: 418.1649.
4.2.25. (1R*,2R*,6R*,7S*)-11-(50-Chlorobenzoxazol-2ʹ-yl)-8,8-
dimethoxy-3-oxatricyclo[5.2.2.02,6]undec-10-en-9-one (9e). Yield:
116 mg (62%) as yellow solid. Mp: 127e128 ꢁC. IR (KBr): nmax 2940,
4.2.30. (1R*,2R*,6R*,7S*)-9-(50-Chlorobenzoxazol-2ʹ-yl)-10,10-
dimethoxy-3-oxatricyclo[5.2.2.02,6]undeca-4,8-dien-11-one
(10e). Yield: 132 mg (71%) as yellow solid. Mp: 149e150 ꢁC. IR
(KBr): nmax 2943, 2839, 1739, 1634, 1530, 1454, 1138 cmꢀ1. 1H NMR
2882, 1740, 1589, 1499, 1450, 1275, 1218, 1130 cmꢀ1 1H NMR
.
(500 MHz, CDCl3):
d
7.71 (d, J¼2.0 Hz, 1H), 7.44 (d, J¼8.0 Hz, 1H),
(500 MHz, CDCl3):
d
7.71 (d, J¼2.0 Hz, 1H), 7.42 (d, J¼9.0 Hz, 1H),
7.33 (d, J¼2.0, 9.0 Hz, 1H), 7.15e7.14 (m, 1H), 4.47 (dd, J¼3.0, 8.0 Hz,
1H), 4.18e4.17 (m, 1H), 3.85 (dd, J¼3.0, 6.5 Hz, 1H), 3.81 (dt, J¼3.0,
13.0 Hz, 1H), 3.57e3.52 (m, 1H), 3.42 (s, 3H), 3.31 (s, 3H), 3.15e3.09
(m, 1H), 2.19e2.14 (m, 1H), 1.54e1.46 (m, 1H) ppm. 13C NMR
7.31 (dd, J¼2.5, 9.0 Hz, 1H), 7.17 (dd, J¼2.5, 7.0 Hz, 1H), 6.18e6.17 (m,
1H), 5.29 (dd, J¼4.0, 9.5 Hz, 1H), 4.80 (t, J¼2.5 Hz, 1H), 4.68 (dd,
J¼2.0, 3.5 Hz, 1H), 3.59 (dd, J¼1.5, 9.5 Hz, 1H), 3.49 (dd, J¼2.0,
6.5 Hz, 1H), 3.45 (s, 3H), 3.38 (s, 3H) ppm. 13C NMR (125 MHz,
(125 MHz, CDCl3):
d
199.6, 162.3, 149.3, 142.8, 131.6, 131.5, 130.1,
CDCl3): d 199.3, 161.9, 149.2, 148.4, 142.9, 132.0, 130.0, 129.5, 125.9,
126.0, 120.3, 111.3, 93.0, 79.2, 69.1, 55.8, 50.8, 50.0, 43.3, 38.2,
30.3 ppm. HRMS (ESþ): m/z calcd for [C19H18ClNO5þNa]þ:
398.0766, found: 398.0766.
120.4, 111.2, 100.2, 93.4, 79.2, 53.0, 50.5, 45.5, 44.4 ppm. HRMS
(ESþ): m/z calcd for [C19H16ClNO5þNa]þ: 396.0615, found:
396.0610.
4.2.26. (1R*,2R*,6R*,7S*)-9-(Benzoxazol-2ʹ-yl)-10,10-dimethoxy-3-
oxatricyclo[5.2.2.02,6]undeca-4,8-dien-11-one (10a). Yield: 126 mg
(74%) as pale yellow solid. Mp: 129e130 ꢁC. IR (KBr): nmax 2953,
2839, 1743, 1621, 1534, 1456, 1135 cmꢀ1. 1H NMR (500 MHz, CDCl3):
Acknowledgements
The authors thank Science & Engineering Research Board (New
Delhi, research grant No. SR/S1/OC-38/2011) for financial support,
DST-FIST program for providing HRMS facility and Prof. V. R. Ped-
ireddi, IIT Bhubaneswar for X-ray crystallographic assistance. RTN
thanks CSIR for a research fellowship.
d
7.75e7.73 (m, 1H), 7.50e7.48 (m, 1H), 7.36e7.31 (m, 2H), 7.16e7.13
(m, 1H), 6.17e6.16 (m, 1H), 5.28 (dd, J¼4.0, 9.5 Hz, 1H), 4.78 (t,
J¼2.5 Hz, 1H), 4.72e4.70 (m, 1H), 3.58 (dd, J¼1.5, 9.5 Hz, 1H), 3.47
(dd, J¼2.5, 7.0 Hz, 1H), 3.44 (s, 3H), 3.39 (s, 3H) ppm. 13C NMR
(125 MHz, CDCl3):
d 199.5, 160.7, 150.6, 148.4, 141.7, 130.9, 129.8,
125.6, 124.5, 120.5, 110.4, 100.0, 93.4, 79.2, 52.9, 50.5, 45.4,
44.4 ppm. HRMS (ESþ): m/z calcd for [C19H17NO5þNa]þ: 362.0998,
found: 362.0993.
References and notes
1. (a) Priestap, H. A.; de los Santos, C.; Quirke, J. M. E. J. Nat. Prod. 2010, 73, 1979;
(b) Rodríguez, I. I.; Rodríguez, A. D. J. Nat. Prod. 2003, 66, 855; (c) Rodríguez, A.
D.; Ramírez, C. J. Nat. Prod. 2001, 64, 100.
2. (a) Dias, L. C.; Correia, V. G.; Finelli, F. G. Tetrahedron Lett. 2007, 48, 7683; (b)
Boeckman, R. K., Jr.; Charette, A. B.; Asberom, T.; Johnston, B. H. J. Am. Chem. Soc.
1991, 113, 5337; (c) Boeckman, R. K., Jr.; Charette, A. B.; Asberom, T.; Johnston,
B. H. J. Am. Chem. Soc. 1987, 109, 7553.
3. (a) Manas, E. S.; Unwalla, R. J.; Xu, Z. B.; Malamas, M. S.; Miller, C. P.; Harris, H.
A.; Hsiao, C.; Akopian, T.; Hum, W.-T.; Malakian, K.; Wolfrom, S.; Bapat, A.; Bhat,
R. A.; Stahl, M. L.; Somers, W. S.; Alvarezr, J. C. J. Am. Chem. Soc. 2004, 126, 15106;
(b) Malamas, M. S.; Manas, E. S.; Mc Devitt, R. E.; Gunawan, I.; Xu, Z. B.; Collini,
M. C.; Miller, C. P.; Dinh, T.; Henderson, R. A.; Keith, J. C., Jr.; Harris, H. A. J. Med.
Chem. 2004, 47, 5021.
4. (a) Sun, L.-Q.; Chen, J.; Takaki, K.; Johnson, G.; Iben, L.; Mahle, C. D.; Xu, C. Bi-
oorg. Med. Chem. Lett. 2004, 14, 1197; (b) Sun, L.-Q.; Chen, J.; Bruce, M.; Deskus,
J. A.; Epperson, J. R.; Takaki, K.; Johnson, G.; Iben, L.; Mahle, C. D.; Ryan, E.; Xu, C.
Bioorg. Med. Chem. Lett. 2004, 14, 3799.
5. Aiello, S.; Wells, G.; Stone, E. L.; Kadri, H.; Bazzi, R.; Bell, D. R.; Stevens, M. F. G.;
Matthews, C. S.; Bradshaw, T. D.; Westwell, A. D. J. Med. Chem. 2008, 51, 5135.
6. Mc Culloch, M. W. B.; Berrue, F.; Haltli, B.; Kerr, R. G. J. Nat. Prod. 2011, 74, 2250.
7. Estiarte, M. A.; Johnson, R. J.; Kaub, C. J.; Gowlugari, S.; O’Mahony, D. J. R.;
Nguyen, M. T.; Emerling, D. E.; Kelly, M. G.; Kincaid, J.; Vincent, F.; Duncton, M.
A. J. Med. Chem. Commun. 2012, 3, 611.
8. Ko, C.-W.; Tao, Y.-T.; Danel, A.; Krzeminska, L.; Tomasik, P. Chem. Mater. 2001, 13,
2441.
9. Kocher, C.; Smith, P.; Weder, C. J. Mater. Chem. 2002, 12, 2620.
10. Tian, Y.; Chen, C.-Y.; Yang, C.-C.; Young, A. C.; Jang, S.-H.; Chen, W.-C.; Jen, A. K.-
Y. Chem. Mater. 2008, 20, 1977.
11. For general synthetic methods of benzoxazoles, see: Schnurch, M.; Hammerle,
J.; Stanetty, P. Chapter 13: Benzoxazoles and Other Annulated Oxazoles. Science of
Synthesis: Houben-Weyl Methods of Molecular Transformations, 2001, Vol. 11.
12. For recent synthetic methodologies, see: (a) Yu, P.; Zhang, G.; Chen, F.; Cheng, J.
Tetrahedron Lett. 2012, 53, 4588; (b) Kalkhambkar, R. G.; Laali, K. K. Tetrahedron
Lett. 2012, 53, 4212; (c) Wu, M.; Hu, X.; Liu, J.; Liao, Y.; Deng, G.-J. Org. Lett. 2012,
14, 2722; (d) Leardini, R.; McNab, H.; Nanni, D.; Tenan, A. G.; Thomson, A. Org.
Biomol. Chem. 2012, 10, 623; (e) Chen, F.; Shen, C.; Yang, D. Tetrahedron Lett.
2011, 52, 2128; (f) Qiao, J. X.; Wang, T. C.; Hu, C.; Li, J.; Wexler, R. R.; Lam, P. Y. S.
Org. Lett. 2011, 13, 1804; (g) Guru, M. M.; Ali, Md A.; Punniyamurthy, T. J. Org.
Chem. 2011, 76, 5295; (h) Zong, J. P. C.; Ye, M.; Chen, T.; Gao, D.; Wang, Y.; Chen,
C. Org. Biomol. Chem. 2011, 9, 1225; (i) Saha, P.; Ali, Md A.; Ghosh, P.; Pun-
niyamurthy, T. Org. Biomol. Chem. 2010, 8, 5692.
4.2.27. (1R*,2R*,6R*,7S*)-10,10-Dimethoxy-9-(5ʹ-methylbenzoxazol-
2ʹ-yl)-3-oxatricyclo[5.2.2.02,6]undeca-4,8-dien-11-one (10b). Yield:
127 mg (72%) as pale orange solid. Mp: 138e139 ꢁC. IR (KBr): nmax
2952, 2847, 1741, 1613, 1526, 1454, 1135 cmꢀ1. 1H NMR (500 MHz,
CDCl3):
d
7.52 (s, 1H), 7.35 (d, J¼8.5 Hz, 1H), 7.14 (d, J¼8.0 Hz, 1H),
7.11 (dd, J¼2.0, 7.0 Hz, 1H), 6.16 (t, J¼2.5 Hz, 1H), 5.28 (dd, J¼4.0,
9.5 Hz, 1H), 4.77 (t, J¼2.5 Hz, 1H), 4.70 (dd, J¼2.0, 4.0 Hz, 1H), 3,57
(dd, J¼2.0, 9.5 Hz, 1H), 3.45 (dd, J¼2.0, 7.0 Hz, 1H), 3.44 (s, 3H), 3.38
(s, 3H), 2.45 (s, 3H) ppm. 13C NMR (125 MHz, CDCl3):
d 199.5, 160.7,
148.8,148.3,141.9, 134.3,130.5,129.8, 126.7, 120.3,109.7,100.0, 93.4,
79.2, 52.8, 50.5, 50.4, 45.4, 44.3, 21.4 ppm. HRMS (ESþ): m/z calcd
for [C20H19NO5þNa]þ: 376.1155, found: 376.1153.
4.2.28. (1R*,2R*,6R*,7S*)-10,10-Dimethoxy-9-(6ʹ-methylbenzoxazol-
2ʹ-yl)-3-oxatricyclo[5.2.2.02,6]undeca-4,8-dien-11-one (10c). Yield:
126 mg (73%) as white solid. Mp: 108e109 ꢁC. IR (KBr): nmax 2940,
2834, 1738, 1621, 1534, 1456, 1234, 1139 cmꢀ1. 1H NMR (500 MHz,
ꢀ
CDCl3):
d
7.58 (d, J¼11.5 Hz, 1H), 7.28 (s, 1H), 7.11 (dd, J¼1.0, 8.0 Hz,
1H), 7.08 (dd, J¼1.5, 5.5 Hz,1H), 6.15 (dd, J¼2.0, 2.5 Hz,1H), 5.26 (dd,
J¼4.0, 9.5 Hz, 1H), 4.76 (t, J¼2.5 Hz, 1H), 4.69 (dd, J¼2.0, 3.5 Hz, 1H),
3.57e3.53 (m, 1H), 3.44 (dd, J¼2.5, 5.0 Hz, 1H), 3.43 (s, 3H), 3.37 (s,
€
€
3H), 2.45 (s, 3H) ppm. 13C NMR (125 MHz, CDCl3):
d 199.4, 160.2,
150.8, 148.2, 139.4, 136.1, 130.2, 129.7, 125.6, 119.7, 110.5, 100.0, 93.4,
79.1, 52.8, 50.4, 45.3, 44.3, 21.7 ppm. HRMS (ESþ): m/z calcd for
[C20H19NO5þNa]þ: 376.1155, found: 376.1142.
4.2.29. (1R*,2R*,6R*,7S*)-9-(5ʹ-tert-Butylbenzoxazol-2ʹ-yl)-10,10-
dimethoxy-3-oxatricyclo[5.2.2.02,6]undeca-4,8-dien-11-one
(10d). Yield: 121 mg (61%) as yellow solid. Mp: 116e118 ꢁC. IR
(KBr): nmax 2960, 2846, 1739, 1626, 1537, 1472, 1267, 1139 cmꢀ1. 1H
ꢀ
13. For reviews on MOB chemistry, see: (a) Pouysegu, L.; Deffieux, D.; Quideau, S.
NMR (500 MHz, CDCl3): d 7.79e7.77 (m, 1H), 7.41e7.40 (m, 2H), 7.11
Tetrahedron 2010, 66, 2235; (b) Liao, C.-C. Pure Appl. Chem. 2005, 77, 1221; (c)
Magdziak, D.; Meek, S. J.; Pettus, T. R. R. Chem. Rev. 2004, 104, 1383; (d) Liao, C.-
C.; Peddinti, R. K. Acc. Chem. Res. 2002, 35, 856; (e) Singh, V. Acc. Chem. Res.
1999, 32, 324; For reviews on asymmetric dearomatizations, see: (f) Harned, A.
M. Tetrahedron Lett. 2014, 55, 4681; (g) Zhuo, C.-X.; Zhang, W.; You, S.-L. Angew.
Chem., Int. Ed. 2012, 51, 12662.
(ddd, J¼1.0, 2.5, 7.0 Hz, 1H), 6.14 (dd, J¼1.5, 2.5 Hz, 1H), 5.27 (dd,
J¼4.0, 9.5 Hz, 1H), 4.78e4.76 (m, 1H), 4.71 (dd, J¼2.0, 3.5 Hz, 1H),
3.56 (dd, J¼1.5, 9.5 Hz, 1H), 3.45 (dd, J¼2.0, 6.5 Hz, 1H), 3.44 (s, 3H),
3.38 (s, 3H), 1.36 (s, 9H) ppm. 13C NMR (125 MHz, CDCl3):
d 199.5,

R.T. Naganaboina, R.K. Peddinti / Tetrahedron 71 (2015) 6245e6253
6253
14. For recent papers on MOB chemistry, see: (a) Hirokane, T.; Hirata, Y.; Ishimoto,
T.; Nishii, K.; Yamada, H. Nat. Commun. 2014, 5, 4478; (b) Niu, G.-H.; Hou, C.-S.;
Chuang, G. J.; Wu, C.-P.; Liao, C.-C. Eur. J. Org. Chem. 2014, 3794; (c) Chittimalla,
S. K.; Bandi, C.; Putturu, S.; Kuppusamy, R.; Boellaard, K. C.; Tan, D. C. A.; Lum,
D. M. J. Eur. J. Org. Chem. 2014, 2565; (d) Chittimalla, S. K.; Kuppusamy, R.;
Thiyagarajan, K.; Bandi, C. Eur. J. Org. Chem. 2013, 2715; (e) Chittimalla, S. K.;
Bandi, C. RSC Adv. 2013, 3, 13663; (f) Gao, J.; Simon, J. O.; Rodrigo, R.; Assoud, A.
19. (a) Surasani, S. R.; Parumala, S. K. R.; Peddinti, R. K. Org. Biomol. Chem. 2014, 12,
5656; (b) Surasani, S. R.; Peddinti, R. K. Tetrahedron Lett. 2011, 52, 4615; (c)
Surasani, S. R.; Rajora, V. S.; Bodipati, N.; Peddinti, R. K. Tetrahedron Lett. 2009,
50, 773.
20. (a) Zhang, J.-Z.; Zhu, Q.; Huang, X. Synth. Commun. 2002, 32, 2175; (b) Varma, R.
S.; Saini, R. K.; Prakash, O. Tetrahedron Lett. 1997, 38, 2621.
21. For DIB mediated generation of benzoquinone monoimine derivatives, see: (a)
Bodipati, N.; Peddinti, R. K. Org. Biomol. Chem. 2012, 10, 4549; (b) Bodipati, N.;
Peddinti, R. K. Org. Biomol. Chem. 2012, 10, 1958.
ꢀ
J. Org. Chem. 2013, 78, 48; (g) Quideau, S.; Pouysegu, L.; Deffieux, D.; Ozanne, A.;
Gagnepain, J.; Fabre, I.; Oxoby, M. Arkivoc 2003, 6, 106.
15. Roche, S. P.; Porco, J. A., Jr. Angew. Chem., Int. Ed. 2011, 50, 4068.
16. For recent papers on total synthesis using MOB Diels-Alder strategy, see: (a)
Nishiyama, Y.; Ya, Y. H.; Yokoshima, S.; Fukuyama, T. J. Am. Chem. Soc. 2014, 136,
6598; (b) Sacher, J. R.; Weinreb, S. M. Org. Lett. 2012, 14, 2172; (c) Lu, P.-H.; Yang,
C.-S.; Devendar, B.; Liao, C.-C. Org. Lett. 2010, 12, 2642; (d) Snyder, S. A.; Kontes,
F. J. Am. Chem. Soc. 2009, 131, 1745; (e) Shiao, H.-Y.; Hsieh, H.-P.; Liao, C.-C. Org.
Lett. 2008, 10, 449.
22. 4a CCDC No. 972848.
23. 5e CCDC No. 910161.
ꢀ
ꢀ
24. (a) Gagnepain, J.; Mereau, R.; Dejugnac, D.; Leger, J.-M.; Castet, F.; Deffieux, D.;
ꢀ
Pouysegu, L.; Quideau, S. Tetrahedron 2007, 63, 6493; (b) Deffieux, D.; Fabre, I.;
ꢀ
Titz, A.; Leger, J.-M.; Quideau, S. J. Org. Chem. 2004, 69, 8731; (c) Quideau, S.;
ꢀ
Looney, M. A.; Pouysegu, L.; Ham, S.; Birney, D. M. Tetrahedron Lett. 1999, 40,
615; (d) Liao, C.-C.; Chu, C.-S.; Lee, T.-H.; Rao, P. D.; Ko, S.; Song, L. D.; Shiao, H.-
17. Parumala, S. K. R.; Peddinti, R. K. Org. Lett. 2013, 15, 3546.
C. J. Org. Chem. 1999, 64, 4102.
18. Parumala, S. K. R.; Surasani, S. R.; Peddinti, R. K. New J. Chem. 2014, 38, 5268.