Tandem ring-contraction/decarbonylation of 2,4-diphenyl-3H-1-benzazepine to 2,4-diphenylquinoline

Article abstract of DOI:10.1016/j.tet.2012.10.052

Attempted free-radical bromination of 2,4-diphenyl-3H-1-benzazepine (3) with NBS led to an unusual ring-contraction reaction, giving rise to 2,4-diphenylquinoline (5) in high yield. This is a convenient path for the synthesis of a quinoline in one step from the easily accessible 1-benzazepine. We have elucidated the mechanism of ring-contraction reaction using ¹³C-labeled and deuterated benzazepines, and DFT calculations. There is strong experimental evidence that the departing carbon atom is initially part of a reactive intermediate dibromomethyl cation, which leads to methyl formate upon reaction with methanol. In the absence of methanol, water can complete the ring-contraction. In this case, the departing carbon atom is in the form of carbon monoxide.

Full text of DOI:10.1016/j.tet.2012.10.052

Tetrahedron 69 (2013) 147e151
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Tandem ring-contraction/decarbonylation of 2,4-diphenyl-3H-1-
benzazepine to 2,4-diphenylquinoline
,
b
b
b
c
Sasan Karimi a , Keith Ramig , Edyta M. Greer , David J. Szalda , William F. Berkowitz ,
*
Prakash Prasad c, Gopal Subramaniam c
a Department of Chemistry, Queensborough Community College of the City University of New York, 222-05 56th Ave., Bayside, NY 11364, USA
b Department of Natural Sciences, Baruch College of the City University of New York, 17 Lexington Ave., New York, NY 10010, USA
c Department of Chemistry and Biochemistry, Queens College of the City University of New York, 65-30 Kissena Blvd., Flushing, NY 11367, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Attempted free-radical bromination of 2,4-diphenyl-3H-1-benzazepine (3) with NBS led to an unusual
ring-contraction reaction, giving rise to 2,4-diphenylquinoline (5) in high yield. This is a convenient path
for the synthesis of a quinoline in one step from the easily accessible 1-benzazepine. We have elucidated
the mechanism of ring-contraction reaction using 13C-labeled and deuterated benzazepines, and DFT
calculations. There is strong experimental evidence that the departing carbon atom is initially part of
a reactive intermediate dibromomethyl cation, which leads to methyl formate upon reaction with
methanol. In the absence of methanol, water can complete the ring-contraction. In this case, the de-
parting carbon atom is in the form of carbon monoxide.
Received 14 September 2012
Received in revised form 15 October 2012
Accepted 17 October 2012
Available online 24 October 2012
Keywords:
Benzazepine
NBS
Ring-contraction
Quinoline
Ó 2012 Elsevier Ltd. All rights reserved.
Carbon monoxide
1. Introduction
fate of the missing carbon atom was not ascertained, thus the
intimate mechanistic details were not soundly established. This
We have reported a synthesis of 2-aryl-3H-1-benzazepines such
as 1, in one step from 2-haloanilines and acetophenone derivatives
(Scheme 1).1 The impetus for that study was the possibility of new
and efficient preparations of biologically active 1-benzazepines,2
which have the same framework as 1, but have a higher level of
saturation. In an effort to functionalize C-3, benzazepine 1 was
treated with 1 equiv of NBS and a small amount of the free-radical
initiator dibenzoyl peroxide (DBP), in refluxing chloroform. Instead
of the expected bromobenzazepine 2, quinoline 4 was isolated
(Scheme 2). This ring-contraction with loss of a carbon atom may
be related to conversion of 3H-azepines to pyridines3 and
1H-benzazepines to isoquinolines.4 However, in those studies the
paper describes the results of our investigation on the fate of the
missing carbon and the theoretical calculations to support the
proposed mechanistic path for the ring-contraction reaction.
2. Result and discussion
When the benzazepine 1 was treated with 2 equiv of NBS with
a
catalytic amount of DBP, ring-contraction proceeded very
welldquinolines 4,5 6,6 and 8 were isolated for a total of 92% yield,
and the structures were confirmed by NMR spectra and HRMS. The
structure of 6 was also confirmed by an X-ray crystal structure. We
rationalize the formation of 6 and 8 by reaction of 4 with NBS, as
Ph
Ph
Ph
N
5
N
5
NH2
F
O
NBS
X
DBP
TsOH
3
3
+
Br
Ph
Ph
1
2
Scheme 1. Previously published synthesis of benzazepine 1 and attempted bromination.
bromination of quinoline derivatives with NBS is a known re-
action.7 It has also been suggested8 that the quinolineebromine
* Corresponding author. E-mail address: skarimi@qcc.cuny.edu (S. Karimi).
complex is probably the source of the electrophile for ring
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.10.052

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S. Karimi et al. / Tetrahedron 69 (2013) 147e151
Br
calculations show that the 3-bromobenzazepine 2 can isomerize to
its 5-bromo isomer 11, which ultimately results in quinoline 5 (see
Scheme S1 and its accompanying discussion in Supplementary
data).
Ph
nC
N
Ph
N
Ph
N
2 NBS
N
Ph
+
+
nC
nC
nC
DBP
nC
Br
CHCl3
Br
Ph
Ph
Ph
Ph
1 (n=12)
3 (n=13)
4 (n=12), 72%
5 (n=13)
6 (n=12), 12%
7 (n=13)
8 (n=12), 8%
9 (n=13)
Since we find that use of 2 equiv of NBS maximizes the yield, we
first considered that a second bromination at C-5 could have oc-
curred, followed by electrocyclization. In this case, expulsion of
dibromocarbene would rationalize formation of quinoline 5.
However, we observed no trapping of the putative carbene with
trans-stilbene, and did not detect any carbene dimerization prod-
ucts. As an alternate method to trap the carbene, excess methanol
was added to the reaction containing the labeled benzazepine 3
and NBS, under scrupulously dry conditions, with the expectation
of finding the trapping product dibromomethoxymethane.11 Sur-
prisingly, aside from labeled quinoline 5, we detected methyl for-
mate, which contained the label in the carbonyl group. The
presence of the labeled methyl formate was initially deduced by the
13C NMR data (161.6 ppm for the C]O) of the reaction crude in an
NMR tube.
Because of the difficulties associated with detection and iso-
lation of methyl formate and methyl bromide, the putative by-
product we propose for the very last step producing methyl for-
mate, we ran the reaction in the presence of EtOH. The result was
production of 13C-labeled ethyl formate and ethyl bromide, both of
which were isolated and identified by their NMR spectra.
Although formation of the 13C-labeled formate ester can support
the presence of a carbene, the failure to trap the carbene with trans-
stilbene makes this route implausible. Thus, we considered that the
production of quinoline and methyl formate did proceed through
monobromide 11, and could have proceeded through a geminally
dibrominated derivative, but not with expulsion of a carbene.
Evidence to be given later will rule out the intermediacy of a gem-
inally dibrominated benzazepine.
We rationalize the formation of methyl formate in Scheme 3.
Monobromide 11 undergoes electrocyclization, giving cyclopro-
pane 12. Assuming that the free-radical bromination giving 11 did
not finish completely at this stage, there will be some bromine
available to rupture the cyclopropane ring in either way, giving
cation 13a or 13b. Loss of dibromomethyl cation completes the
ring-contraction, giving quinoline 5. The cation is trapped by
methanol, and subsequent reaction with more methanol gives
bromodimethoxymethane. Loss of a bromide ion followed by
nucleophilic attack on the oxonium ion provides methyl formate.
A similar nucleophilic attack on an oxonium ion was also noted by
Grob and Freiberg.12
Scheme 2. Ring-contraction of unlabeled and labeled benzazepines, giving quinoline
derivatives.
bromination of an uncomplexed quinoline like 4, which leads to 6.
Similarly, dibromination leading to 8 would be facilitated by the
quinolineebromine complex of
quinoline 6.
6
with the uncomplexed
Bromination of 1 with Br2 on the other hand, produced a com-
plex mixture, affording quinolines 4, 6, and 8, in poor yields: 10, 3.6,
and 2.3%, respectively.
We then performed the important control experiment of leaving
out the DBP initiator, which resulted in production of quinoline 4 in
less than 10% yield. This establishes that at least part of the
mechanism must be of the free-radical type. In order to find
a product containing the missing carbon atom, we prepared doubly
labeled benzazepine 3 from 2-fluoroaniline and acetophenone-
b-
13C. The assumption here is that C-5 could be the carbon atom
that is lost, and having it labeled would make it easier to find.
Consistent with the mechanism we reported earlier,1 the labels
ended up at C-3 and C-5 in the benzazepine. Treatment of labeled
benzazepine 3 with NBS led to the ejection of one of the labeled
carbon atoms (presumably C-5), while the other labeled carbon
atom remained to give products 5, 7, and 9. We eventually found
the ‘missing’ carbon atom to be the carbon atom in carbon mon-
oxide. The series of experiments leading to this will be described
below.
A mechanism, which fits all the data given above, and that to be
given, is applied to doubly labeled benzazepine 3 in Scheme 3
(labels are indicated by asterisks). Resonance-stabilized radicals
10a and 10b are produced as intermediates in free-radical bromi-
nation with part of the first equivalent of NBS. Reaction of 10b with
bromine, which is produced from the conversion of NBS by HBr, and
is the actual species that brominates,9 yields monobromide 11 and
the chain-carrying bromine radical.
The cation 13a or 13b, which produces dibromomethyl cation
and 2,4-diphenylquinoline does not necessarily have to arise from
bromine-induced rupture of cyclopropane 12, as shown in Scheme
3. The data we have presented so far are also consistent with initial
dibromination giving 14 and acid-catalyzed conversion of a dibro-
minated cyclopropane intermediate, 15 (Scheme 4). Substitution of
CD3OD for CH3OH would allow the processes of Schemes 3 and 4 to
be differentiated. If production of methyl formate were to occur as
in Scheme 3, then presence of CD3OD should not result in
incorporation of deuterium at the carbon atom of the carbonyl
Ph
Ph
Ph
Ph
Ph
N
N
N
Scheme 3. Mechanism for ring-contraction of benzazepine 3, and the fate of the di-
bromomethyl cation in the presence of either MeOH or water.
NBS
HBr
Br-
NBS
all-12C-11
1
+
Br2HC
all-12C-13
Br
Ph
Br
15
Br
14
Br
We note that bromination of C-3 could actually be the first step
of the mechanism. The Mulliken spin densities using the Gaussian
09 program (B3LYP method and 6-31G(d) basis set10) for the radical
10a/b revealed the values of the spin density of 0.055, 0.980, and
0.036 at N-1, C-3, and C-5, respectively. This implies that bromi-
nation occurs mainly at C-3 to yield 2. However, additional
+
CH3OH
HCO2CH3
HBr (feeds back into 15
4
+
CHBr2
13)
Scheme 4. Hypothetical ring-contraction reaction mechanism, if dibromination had
first occurred.

S. Karimi et al. / Tetrahedron 69 (2013) 147e151
149
group. On the other hand, if the dibromocyclopropane 15 was an
intermediate, then presence of CD3OD should result in a significant
level of deuteration at the carbonyl group of methyl formate. This is
because most of the acid would be in the form of DBr, split off in the
last steps.
In the event, benzazepine 3 was treated with 2 equiv of NBS,
a small amount of DBP, and excess CD3OD, in chloroform as before.
The result was production of methyl formate, which had in-
corporated no detectable deuterium at the carbonyl group carbon
atom (Fig. 1, bottom trace). This appears to rule out dibromination
of benzazepine 1 as a possibility. In order to verify this in the ‘re-
verse’ sense, we synthesized benzazepine 1, selectively deuterated
at C-3 and C-5 (65% deuteration as analyzed by 1H NMR), by
is the loss of a gaseous molecule carrying the missing carbon atom.
It has been reported that photochemical dehalogenation of bro-
moform with water produces carbon monoxide and HBr via the
CHBr2(OH) intermediate.13 Since the CHBr2 cation depicted in
Scheme 3 may form the same intermediate by reaction with water,
it is conceivable that carbon monoxide is produced during the ring-
contraction. To prove this, we set up the reaction with an added CO
detector tube. We observed a strong positive test (color change
from yellow to gray) to indicate that indeed CO was being gener-
ated (Fig. 2). Because the ring-contraction was not carried out under
scrupulously dry conditions, generation of CO, at first slow, was
observed in the detector. As the reaction continued for several more
hours, the darkening of the color in the detector tube became more
visible. After 18 h of reflux, water was added through a syringe and
the tube became completely dark as more CO was generated. The
darkening of the detector tube (described in the vendor’s manual)
is a result of reduction of the palladium complex Na2Pd(SO3)2 by CO
to metallic Pd, CO2, SO2, and Na2SO3. Thus, in the absence of CH3OH,
adventitious water is required for completion of the ring-
contraction and results in formation of carbon monoxide.
treating
acetophenone-(methyl-d3)
with
deuterated
2-
fluoroaniline (ND2). The ring-contraction of this deuterated ben-
zazepine in the presence of excess CH3OH produced DCO2CH3 as
evidenced by the splitting of the signal due to the carbonyl carbon
atom (Fig. 1, top trace). The peak marked DCO is a triplet with
a coupling constant of 34.6 Hz and a 0.20 ppm upfield shift from the
HCO singlet. This result, consistent with the observed ring-
contraction of 13C-labeled benzazepine 3 in the presence of
CD3OD, also supports formation of the monobromocyclopropane
intermediate 12. In short, the hydrogen atom attached to the car-
bon atom of the carbonyl group in methyl formate has to come from
the intermediate itself and not from the alcohol additive.
Fig. 2. The effect of released gases on the CO sensor during the ring-contraction re-
action. Tubes A and B indicate reference colors for the unexposed and CO-exposed
sensors. Tube C shows slow discoloration to gray (due to CO) after 18 h of reflux.
Tube D shows substantial release of CO after addition of water. Tube E captures the
effect of refluxing a mixture of HBr, NBS, benzoyl peroxide, and bromine, without
benzazepine, for 3 h to rule out the possibility of a false positive (absence of a gray
color). The reddening of tube E is due to Br2.
Fig. 1. 13C NMR spectrum of the crude ring-contraction reaction mixture. Top trace:
using deuterated 1 and CH3OH. Bottom trace: using 3 and CD3OD.
The mechanism of Scheme 3 has two components, which utilize
the bromine produced from 2 equiv of NBS in different ways. The
first component is of the free-radical type, giving monobromide 11
and succinimide by-product. Before this process is complete, the
bromine begins to convert cyclopropane 12 into cation 13a or 13b.
This is the start of the second component, an ionic chain reaction.
Methyl formate is eventually produced, with loss of HBr along the
way. Some of this HBr will react with more NBS, giving more bro-
mine (and more succinimide by-product), which then converts
more cyclopropane 12. Once the whole reaction is complete, there
will be 2 equiv of succinimide present, which we have detected by
its NMR spectrum, 1 equiv of CH3Br, and a little less (allowing for
the bromine atoms in bromoquinolines 7 and 9) than 1 equiv of
HBr. We infer the presence of HBr by a drop in the pH of the reaction
mixture we observe as time passes.
With the mechanism established for the process containing
methanol, the question remained: how does ring-contraction occur
in its absence? First, we found that the reaction does not proceed
well under scrupulously anhydrous conditions in the absence of
CH3OH. The reaction was carried out in a sealed dry NMR tube in
dry CDCl3, and a complex mixture of products resulted, without
formation of quinoline derivatives. However, if the reaction is run in
glassware that has not been flame-dried, and no special pains are
taken to exclude atmospheric moisture, then the reaction proceeds
well. We had originally assumed that, in the absence of MeOH, if
adventitious water was to complete the ring-contraction process,
we would find the departing carbon atom in the carbonyl group of
formic acid. Nonetheless, after the work-up, we never detected
formic acid either in the organic or the aqueous layer. An alternative

150
S. Karimi et al. / Tetrahedron 69 (2013) 147e151
3. Conclusion
7.72 (1H, dd, J¼9.04, 2.18 Hz), 7.52e7.38 (8H, m); 13C NMR (CDCl3,
100 MHz)
d 155.5(q), 146.7 (q), 145.7 (q), 137.5 (q), 136.0 (q), 133,
In summary, reaction of benzazepine 1 with NBS gave the ex-
pected product of allylic bromination only as an intermediate in
a complex series of transformations. The result was formation of
2,4-diphenylquinoline by ring-contraction with loss of one of the
carbon atoms and two of the hydrogen atoms. When methanol was
added to the reaction mixture, the carbon atom was found to be in
the carbonyl group of methyl formate. Several possible mechanisms
were ruled out by using either doubly 13C-labeled or deuterated
benzazepines, in the presence of either methanol-d4 or methanol,
respectively. These experiments strongly suggested the in-
termediacy of the dibromomethyl cation, leading eventually to
methyl formate upon reaction with methanol. In the absence of
methanol, adventitious water reacts with the ejected dibromo-
methyl cation to give carbon monoxide. This ring-contraction
provides a convenient method to synthesize substituted quinolines.
131.9, 129.6, 129.5 (2C), 128.9 (2C), 128.9 (2C), 128.7, 127.8, 127.5
(2C), 125.3 (q), 118.7 (q), 118.4. HRMS calcd for C21H14NBrþH m/z
360.0388, found 360.0401; mp 151e153 ꢁC.
4.2.2. Compound 8. 1H NMR (CDCl3, 400 MHz)
J¼6.85, 1.58 Hz), 8.10 (1H, d, J¼2.11 Hz), 7.90 (1H, d, J¼2.11 Hz), 7.83
(1H, s), 7.53e7.42 (8H, m); 13C NMR (CDCl3, 100 MHz)
157.2 (q),
d 8.25 (2H, dt,
d
149.1 (q), 144.5 (q), 138.6 (q), 137.4 (q), 136.0, 130.1, 129.5 (2C), 129.0
(2C), 128.9 (2C), 127.8 (q), 127.7 (2C), 127.0 (q), 127.6 (2C), 120.3,
119.6 (q). HRMS calcd for C21H13NBr2þH m/z 439.9474, found
439.9475; mp 154e155 ꢁC.
4.3. Synthesis of 13C-labeled benzazepine 3, and quinolines 5,
7, and 9
13C-labeled benzazepine at positions 3 and 5 (3) was obtained
by treating methyl-13C-labeled acetophenone and 2-fluoroaniline
according to the reported procedure.1 Compound 3 was con-
verted to 5, 7, and 9 using the same procedure described earlier for
unlabeled benzazepine 1.
4. Experimental section
4.1. General
All 1H and 13C NMR spectra were referenced to TMS and recor-
ded on a 400 MHz spectrometer. To observe the CeD coupling, the
experimental parameters are adjusted to a delay time of 10 s.
Gastec Detector Tube, no.1La Carbon Monoxide 25e500 ppm (scale
range) was used to detect CO. HRMS data were obtained using ESI-
TOF method.
4.3.1. Compound 3 (labeled at C-3 and C-5). C-3 and C-5 are ob-
served at 33.88 and 126.86 ppm, respectively. The methylene pro-
tons at C-3 are split as broad doublets centered at 3.33 ppm with
JCeH¼132.9 Hz. The proton attached to C-5 appears as a doublet of
doublets centered at 8.02 ppm. It is coupled to C3 and C5 with JCeH
values of 156.2 and 7.79 Hz. HRMS calcd for C22H17NþH m/z
298.1506, found 298.1495.
Collection and reduction of X-ray data for (6).
A crystal
(0.30ꢀ0.07ꢀ0.07 mm) of C21H14BrN (6) was cut from a large cluster
of crystals and mounted on a glass fiber. It was transferred to
a Bruker Kappa Apex II diffractometer for the collection of diffrac-
tion data. Diffraction data for (6) indicated: orthorhombic sym-
metry and systematic absences consistent with space group Pca21
and Pbcm. Pca21 was used for the solution and refinement of the
structure based on E statistics. The Flack parameter refined to
0.003(12). The supplementary crystallographic data for this com-
pound can be obtained free of charge from The Cambridge Crys-
tallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif
with the access number 901542.
Computational methods. Optimizations were performed with
Gaussian 0914 using the B3LYP method and a 6-31G(d) basis set. The
uB3LYP/6-31G(d) was employed to study radical 10. This level of
DFT calculation is in agreement with the experimentally observed
product 4. We also calculated harmonic vibrational frequencies to
confirm whether the optimized structures were minima or transi-
tion states. The intrinsic reaction coordinate (IRC) calculations were
conducted to verify the reaction pathways involving TS-I and TS-II
(Scheme S1 of Supplementary data).
4.3.2. Compound 5. Labeled C-3 is observed at 119.41 ppm. The
proton attached to C-3 appears as a doublet centered at 7.75 ppm
with JCeH¼161.3 Hz. HRMS calcd for C21H15NþH m/z 283.1316,
found 283.1308.
4.3.3. Compound 7. Labeled C-3 is observed at 120.07 ppm. The
proton attached to C-3 appears as a doublet centered at 7.76 ppm
with JCeH¼160.8 Hz. An important feature is the singlet at 7.97 ppm
for the proton attached to C-5. The coupling pattern observed in
this ring is important for assigning the compound as the 6-bromo
and not the 5-bromo. The 5-bromo adduct would have made all
protons in this ring appear as multiplets. HRMS calcd for
C21H14NBrþH m/z 361.0421, found 361.0412.
4.3.4. Compound 9. Labeled C-3 is observed at 120.35 ppm. The
proton attached to C-3 appears as a doublet centered at 7.84 ppm
with JCeH¼162.0 Hz. An important feature is a small meta coupling
in the proton spectrum due to protons attached to C-5 and C-7.
HRMS calcd for C21H13NBr2þH m/z 440.9507, found 440.9514.
4.2. Synthesis of 2,4-diphenylquinoline (4), 6-bromo-2,4-
diphenylquinoline (6), 6,8-dibromo-2,4-diphenylquinoline (8)
4.4. Preparation of deuterated benzazepine and variation of
the reaction with added MeOH/CD3OD
A solution of benzazepine 1 (100 mg, 0.34 mmol), NBS (120 mg,
0.67 mmol, recrystallized from CHCl3), and a catalytic amount of
dibenzoyl peroxide (w10 mg) in 8 mL of distilled CHCl3 was heated
at reflux for 18 h under N2. After cooling, the yellow mixture was
extracted with 10 mL of dilute aqueous Na2S2O3, 10 mL of saturated
NaCl, and dried over MgSO4. Rotary evaporation gave a crude
product (0.250 g), which was purified by radial chromatography
(silica gel, hexane) to give, in order of elution, 12.0 mg (8%) of
dibromide 8, 15.2 mg (12%) of monobromide 6,6 and 68.8 mg (72%)
of quinoline 4.5 Compounds 4 and 6 have very close Rf values.
Deuterated 2-fluoroaniline (ND2) was synthesized by adding
D2O to 2-fluoroaniline and heating the mixture to 60 ꢁC for 24 h.
The enrichment was at best 97% and did not increase on longer
heating. Deuterated benzazepine (65% enriched at positions 3 and
5) was synthesized by treating deuterated 2-fluoroaniline (ND2)
with methyl-deuterated acetophenone. To the labeled benzazepine
3 (50 mg, 0.17 mmol) was sequentially added, a catalytic amount of
dibenzoyl peroxide (w5 mg), CDCl3 (3 mL), MeOH (0.4 mL), and
NBS (60 mg, 0.33 mmol, recrystallized from CHCl3), and the re-
action mixture was heated in an NMR tube at 70 ꢁC for 18 h under
N2. The work-up was performed as before. The reaction was
4.2.1. Compound 6. 1H NMR (CDCl3, 400 MHz)
1.57 Hz), 8.03 (1H, d, J¼9.04 Hz), 7.96 (1H, d, J¼2.18 Hz), 7.76 (1H, s),
d
8.11 (2H, dt, J¼6.94,

S. Karimi et al. / Tetrahedron 69 (2013) 147e151
151
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replacing CH3OH with CD3OD.
Acknowledgements
We thank Dr. E. Fujita of Brookhaven National Laboratory for the
use of the Bruker Kappa Apex II diffractometer for X-ray data col-
lection, Dr. Matthew Donahue for helpful suggestions on some of the
labeling experiments, Mr. Pedro Irigoyen for locating the source of
CO detector tube, Baruch College Technology Center and the Com-
putational Facility at the College of Staten Island for computational
support, and the Professional Staff Congress of the City University of
New York is acknowledged for financial support of this work.
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Supplementary data
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X-ray crystal structure for 6 (Fig. S1), Mulliken atomic spin
densities for 10, chemical shifts for compounds 4, 6, and 8 (Table
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the CIF file for 6, calculated energies for the ring-contraction
pathway (Scheme S1) and a discussion of Scheme S1. Supplemen-
tary data associated with this article can be found in the online
version, at http://dx.doi.org/10.1016/j.tet.2012.10.052.
3323e3
332.
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References and notes
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