Synthesis of electron-poor 4-halo-2-azabuta-1,3-dienes by Rh(II)-catalyzed diazo ester-azirine coupling. 2-Azabuta-1,3-diene-2,3-dihydroazete valence isomerism

Article abstract of DOI:10.1016/j.tetlet.2012.08.063

The Rh₂(OAc)₄-catalyzed reaction of alkyl 2-acyl-2-diazoacetates and dimethyl diazomalonate with methyl 2-bromo- and 2-chloro-3-phenyl-2H-azirine-2-carboxylates gives rise to electron-poor 4-halo-substituted (3E)-2-azabuta-1,3-dienes

Full text of DOI:10.1016/j.tetlet.2012.08.063

Tetrahedron Letters 53 (2012) 5777–5780
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Tetrahedron Letters
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Synthesis of electron-poor 4-halo-2-azabuta-1,3-dienes by Rh(II)-catalyzed
diazo ester–azirine coupling. 2-Azabuta-1,3-diene-2,3-dihydroazete
valence isomerism
Mikhail S. Novikov a, , Ilia A. Smetanin a, Alexander F. Khlebnikov a, Nikolai V. Rostovskii a, Dmitry S. Yufit b
⇑
a Department of Chemistry, Saint Petersburg State University, Universitetskii pr. 26, 198504 St. Petersburg, Russia
b Department of Chemistry, Durham University, Durham, South Rd., DH1 3LE, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The Rh2(OAc)4-catalyzed reaction of alkyl 2-acyl-2-diazoacetates and dimethyl diazomalonate with
methyl 2-bromo- and 2-chloro-3-phenyl-2H-azirine-2-carboxylates gives rise to electron-poor 4-halo-
substituted (3E)-2-azabuta-1,3-dienes. Their formation proceeds with complete stereoselectivity via
ring-opening of the intermediate azirinium ylide. 2-Azabuta-1,3-dienes with electron-withdrawing sub-
stituents at the 1,1,4-positions are stable compounds at room temperature, but are in equilibrium with
cyclic valence isomers, 2,3-dihydroazetes, at elevated temperatures.
Received 3 July 2012
Revised 30 July 2012
Accepted 15 August 2012
Available online 23 August 2012
Keywords:
Azirines
Ó 2012 Elsevier Ltd. All rights reserved.
Diazo compounds
Carbenoids
Ylides
2-Azabuta-1,3-dienes
2,3-Dihydroazetes
Electrocyclic reactions
Due to their availability and high reactivity, 2H-azirines are
widely used as building blocks for the synthesis of various nitro-
gen-containing compounds. They can act as electrophiles, nucleo-
philes, dienophiles, dipolarophiles, and serve as sources of
nitrogen ylides.1 One of the novel synthetic applications of these
strained compounds is the Rh(II)-catalyzed azirine–diazo ester
coupling to form functionalized 2-azabuta-1,3-dienes A,2 (which
are useful intermediates in heterocyclic synthesis3) and 2H-1,4-
oxazines B possessing photochromic activity4 (Scheme 1). 2-Azab-
uta-1,3-dienes derived from the diazo esters, methyl 2-phenyl-2-
diazoacetate or dimethyl diazomalonate, are stable compounds.
The reaction of aryl-substituted 2H-azirines with diazo keto esters
furnishes unstable 1-acyl-2-azabuta-1,3-dienes which rapidly un-
dergo 1,6-electrocyclization into 2H-1,4-oxazines.
phenyl-2H-azirine-2-carboxylates by the Rh2(OAc)4-catalyzed
reaction with alkyl 2-acyl-2-diazoacetates and dimethyl diazomal-
onate is described. The involvement of 4-halo-2-azabuta-1,3-
dienes in the equilibrium valence isomerization into 2,3-
dihydroazetes is demonstrated.
It was found that mono-, di-, and triaryl-substituted 2H-azirines
reacted with ethyl diazoacetoacetate (2a) in the presence of a cat-
alytic amount of Rh2(OAc)4 to form substituted 2H-1,4-oxazine B,
as the sole product or as a mixture with functionalized 2-azabu-
ta-1,3-diene A.4 In contrast, the reaction of diazo compound 2a
R
CO2Me
N2
R
2-Halo-substituted 2H-azirines, and particularly the most read-
ily available alkyl 2-bromo- and 2-chloro-2H-azirine-2-carboxyl-
ates,5 seemed to be attractive starting materials for the
construction of halogenated 2-azabuta-1,3-dienes and 2H-1,4-
oxazines.
Rh2(OAc)4
R = Ph, CO2Me
N
CO2Me
Ar
A
Ar
O
N
CO2Et
R
O
R
N2
In the present work the first synthesis of electron-poor 4-bro-
mo- and 4-chloro-2-azabuta-1,3-dienes from methyl 2-halo-3-
Rh2(OAc)4
R = Me, Ph, CF3
Ar
N
B
CO2Et
⇑
Corresponding author. Tel.: +7 812 428 4021; fax: +7 812 428 6939.
E-mail address: m.s.novikov@chem.spbu.ru (M.S. Novikov).
Scheme 1. Rh(II)-catalyzed reactions of aryl-substituted 2H-azirines with diazo
esters and diazo keto esters.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.08.063

5778
M. S. Novikov et al. / Tetrahedron Letters 53 (2012) 5777–5780
R1
CO2R2
ethyl 2-benzoyl-2-diazoacetate (2b). The oxetanone 6a is a known
CO2Me
X
Ph
product of intramolecular insertion of a rhodium carbenoid (gener-
ated from ethyl benzoyldiazoacetate) into a C–H bond of the meth-
ylene group.8,9 The undesirable product of carbenoid CH-insertion,
3-benzoyloxetan-2-one (6b), was also formed, but in substantially
less amounts, when the methyl ester of benzoyldiazoacetic acid 2c
in the presence of Rh2(OAc)4 was reacted with azirine 1a (azadi-
ene/oxetanone ratio = 3:1). For that reason the dimethyl ester 4c
was obtained in higher yield than the methyl, ethyl ester 4b. The
formation of azadienes in these reactions proceeds with complete
stereoselectivity to give only (3E)-2-azabuta-1,3-dienes.
Changing from bromine to chlorine in the starting azirines led
to a decrease in the yield of dihydroazetes, but did not affect the
yields of azadienes or the selectivity of the reaction: complete E-
stereoselectivity of chloroazadiene 4d,e formation and the non-
stereoselective formation of chloroazetine 5d were observed. No
traces of dihydroazetes 5b,c,e were detected in the reactions of
benzoyldiazoacetates 2b,c with azirines 1a,b.
CO2Me
X
2a−d
N2
Ph
N
_
Rh2(OAc)4
N
R1
CO2R2
DCE, 84 °C
1a,b
3a−g
O
CO2R2
R1
CO2Me
N
MeO2C
O
R1
CO2R2
Ph
X
X
+
+
N
O
Ph
R3
Ph
5a,d,f,g
4a−g
6a (R3 = Me)
6b (R3 = H)
Scheme 2. The reaction of azirines 1a,b with diazo compounds 2a–d (4–6 equiv) in
the presence of Rh2(OAc)4) (5 mol %) over 20–90 min.
Table 1
The formation of significant amounts of dihydroazetes was ob-
served in the reaction of azirines 1a,b with dimethyl diazomalo-
nate (2d) under the same reaction conditions (Table 1). It was
found that the product yields depended dramatically on the reac-
tion time. Thus, the addition of diazo compound 2d to a solution
of azirine 1a and Rh2(OAc)4 in DCE over 30 min under reflux pro-
vided azadiene 4f and dihydroazete 5f in 32% and 25% yields,
respectively, while after 80 min of reaction the yields were 14%
and 32%.
Rh2(OAc)4-catalyzed reactions of azirines 1a,b with diazo compounds 2a–d
Azirine
X
Diazo
R1
R2
Yield of 4
Yield of 5
compound
(%)
(%)
1a
1a
1a
1b
1b
1a
1b
Br 2a
Br 2b
Br 2c
Cl 2a
Cl 2b
Br 2d
Cl 2d
Ac
Bz
Bz
Ac
Bz
Et
Et
43 (4a)
22 (5a)a
0 (5b)
35 (4b)b
Me 52 (4c)c
0 (5c)
Et
Et
56 (4d)
10 (5d)a
0 (5e)
30 (4e)b
CO2Me Me 32 (4f)d
CO2Me Me 60 (4g)
25 (5f)d
17 (5g)
Compounds 4a–g and 5a,d,f,g were fully characterized using
standard spectral methods.10 The structure of dihydroazete 5f
and the E configuration of the C@C bond in azadiene 4b were con-
firmed by X-ray analysis (Fig. 1).11,12 The C@N bond of compound
4b in the crystal form had Z configuration, whereas most probably
in solution it exists as a mixture of Z- and E-isomers, which are un-
der rapid equilibrium. This conclusion follows from the magnetic
equivalence of methoxy protons and carbonyl carbons in the 1H
and 13C NMR spectra of compounds 4f,g, as well as from the low
values of the activation barriers for N-inversion for the related
C@N-containing systems.13 The Z-configuration of molecule 4b in
the crystal may be caused by Br. . .O3 halogen bonds (Br. . .O
3.052(1) Å) with the oxygen atom of the carboxylate group in the
Z-position. These pairs of interactions link molecules 4b as cen-
tro-symmetrical dimers in the crystal state.
a
Diastereomeric ratio = 1:1.
3-Benzoyl-4-methyloxetan-2-one (6a) was formed as a by-product (4b,e/6a
b
ratio as measured by 1H NMR spectroscopy is 1:2).
c
3-Benzoyloxetan-2-one (6b) was formed as a by-product (4c/6b ratio as mea-
sured by 1H NMR spectroscopy is 3:1).
d
The reaction time is 30 min.
with methyl 2-bromo-3-phenyl-2H-azirine-2-carboxylate (1a) in
the presence of 5 mol % of Rh2(OAc)4 in 1,2-dichloroethane (DCE)
under reflux gave three products: 2-azabuta-1,3-diene 4a and a
1:1 mixture of two diastereomeric 2,3-dihydroazetes6 5a
(Scheme 2, Table 1). No traces of the corresponding 2-bromo-2H-
1,4-oxazine were found in the reaction mixture. Azadiene 4a was
separated from the non-separable mixture of cis, trans-5a by col-
umn chromatography.7
The reaction mechanism (Scheme 2) involving formation of
ylide intermediate 3 is in good agreement with the observed stere-
oselectivity of the ring-opening of the three-membered ring.
A 1:2 ratio of 2-azadiene 4b and trans-3-benzoyl-4-methyloxe-
tan-2-one (6a) was obtained from the reaction of azirine 1a with
Figure 1. X-ray structures of compounds 4b and 5f.

M. S. Novikov et al. / Tetrahedron Letters 53 (2012) 5777–5780
5779
a solution of the originally pure diastereomeric mixture 5a,d at
room temperature.
CO2Me
Br
Ph
O
In contrast to compounds 4a,d, azadienes 4f,g, derived from di-
methyl diazomalonate, smoothly undergo reversible 1,4-cycliza-
tion to give dihydroazetes 5f,g, as the only products (Scheme 4).
The equilibrium was attained in 25 min when a solution of the bro-
mo-substituted azadiene 4f in C6D6 was heated in a sealed tube at
100 °C giving a mixture 4f/5f in a 1:1 ratio. A change from bromine
to chlorine in azadiene 4 led to stabilization of the open-chain va-
lence isomer 4g, which is present in the equilibrium mixture in 2.6
times higher quantity than the corresponding cyclic isomer 5g.
In summary, we have reported the first example of the reaction
of 2-halo-substituted 2H-azirines with a rhodium carbenoid, gen-
erated from 2-acyl-2-diazoacetates and dimethyl diazomalonate,
leading to electron-poor 4-bromo- and 4-chloro-2-azabuta-1,3-
dienes. The complete stereoselectivity of the reaction is in accor-
dance with a mechanism involving the formation of an intermedi-
ate azirinium ylide and predicts the preferential formation of an
azadiene with E configuration of the C@C bond. It was established
that 2-azabuta-1,3-dienes with electron-withdrawing substituents
at the 1,1,4-positions were stable compounds at room tempera-
ture, but existed in solution at elevated temperatures in equilib-
rium with cyclic valence isomers, 2,3-dihydroazetes.
N
CO2Me O
ΔG≠ = 9.7 (from E-3a)
ΔG≠ = 9.8 (from Z-3a)
Me
O
Br
Ph
OMe
N
Me OMe
E-3a
O
E-4a
CO2Me
Br
ΔG≠ = 17.2 (from E-3a)
ΔG≠ = 18.9 (from Z-3a)
Br
O
Ph
O
Me
MeO2C
Ph
N
OMe
N
O
O
Z-4a
OMe Me
Z-3a
Scheme 3. Activation free energies for the ring-opening of isomeric azirinium
ylides E/Z-3a to give 2-azabuta-1,3-diene isomers 4a (kcal/mol).
Quantum-chemical calculations (DFT B3LYP/6-31G(d)/LANL2DZ)14
for the isomerization of E-3a and Z-3a ylides to azadiene 4a predict
a big difference for the activation barriers (DDG– = 7.5–9.1 kcal/
mol) for the three-membered ring-opening of ylides 3 leading to
(3E)- and (3Z)-2-azabuta-1,3-dienes 4 in favor of the former
(Scheme 3). In fact, according to 1H NMR spectroscopy and TLC
data, no traces of the (3Z)-isomers of 2-azabuta-1,3-dienes 4a–e
were observed in the reaction mixtures.
Acknowledgements
We gratefully acknowledge the financial support of the Russian
Foundation for Basic Research (Grant No. 11-03-00186), the Fed-
eral Grant-in-Aid Program «Human Capital for Science and Educa-
tion in Innovative Russia» (Governmental Contract no.
16.740.11.0442), and Saint Petersburg State University (Grant No.
12.38.78.2012).
The only example of carbenoid-mediated azirine–2,3-dihydro-
azete transformation was observed in the Rh2(OAc)4-catalyzed
reaction of 2,3-diphenyl-2H-azirine with dimethyl diazomalo-
nate.2b No immediate precursor of the 2,3-dihydroazete was ob-
served in this reaction and the mechanism of its formation had
remained unclear. The most reasonable sequence leading to the
2,3-dihydroazete system is a domino reaction involving consecu-
tive formation of a rhodium carbenoid, an azirinium ylide 3, and
a 2-azabuta-1,3-diene (Scheme 2) followed by 1,4-cyclization of
the latter, although to the best of our knowledge, this reaction is
unknown in the literature. The reaction of azirine 1a with diazo
compound 2a was shown to be the first to provide both 2,3-dihyd-
roazete 5a and its probable precursor, 2-azabuta-1,3-diene 4a. It
was found, that heating a benzene solution of azadiene 4a or 4d
in a vial with a screw cap at 100 °C for 20 min led to a complex
mixture of products including two diastereomeric dihydroazetes
5a/5d. The latter were formed in the same 1:1 ratio as in the reac-
tion of 1a,b with 2a (Scheme 4). They were isolated by chromatog-
raphy in 19–20% overall yield. The 1,4-electrocyclization of 4a,d
into 5a,d was found to be reversible: an azadiene 4a,d impurity
was detected by 1H NMR spectroscopy after prolonged storage of
Supplementary data
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.tet-
let.2012.08.063. These data include MOL files and InChiKeys of
the most important compounds described in this article.
References and notes
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2414; (f) Lemos, A. Molecules 2009, 14, 4098–4119.
2. (a) Khlebnikov, A. F.; Novikov, M. S.; Amer, A. A. Tetrahedron Lett. 2004, 45,
6003–6006; (b) Khlebnikov, A. F.; Novikov, M. S.; Amer, A. A.; Kostikov, R. R.;
Magull, J. Russ. J. Org. Chem. 2006, 42, 533–544.
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4. Khlebnikov, V. A.; Novikov, M. S.; Khlebnikov, A. F.; Rostovskii, N. V.
Tetrahedron Lett. 2009, 50, 6509–6511.
CO Me
MeO2C
X
CO2Et
COMe
O
MeO2C
X
COMe
CO2Et
2
benzene
100 °C
Me
X
5. Pinho e Melo, T. M. V. D.; Lopes, C. S. J.; Cardoso, A. L.; Rocha Gonsalves, A. M.
d’A. Tetrahedron 2001, 57, 6203–6208.
+
N
N
N
CO2Et
Ph
Ph
Ph
6. For the synthesis and reactions of 2,3-dihydroazetes see: (a) Hemming, K.;
O’Gorman, P. A.; Page, M. I. Tetrahedron Lett. 2006, 47, 425–428; (b) Petrov, V.
A.; Davidson, F.; Marshall, W. J. Fluorine Chem. 2004, 125, 1621–1628; (c)
Ambhaikar, N. B.; Snyder, J. P.; Liotta, D. C. J. Am. Chem. Soc. 2003, 125, 3690–
3691; (d) Wolfe, S.; Ro, S.; Shi, Z. Can. J. Chem. 2001, 79, 1259–1271; (e)
Hemming, K.; Luheshi, A. N.; Redhouse, A. D.; Smalley, R. K.; Thompson, J. R.
Tetrahedron 1993, 49, 4383–4408; (f) Stierli, F.; Prewo, R.; Bieri, J. H.;
Heimgartner, H. Helv. Chim. Acta 1984, 67, 927–933; (g) Hassner, A.; Currie, J.
O., Jr.; Steinfeld, A. S.; Atkinson, R. F. J. Am. Chem Soc. 1973, 95, 2982–2987; (h)
Levy, A. B.; Hassner, A. J. Am. Chem. Soc. 1971, 93, 2051–2053.
7. General procedure for the preparation of azadienes 4a–g and dihydroazetes
5a,d,f,g. A solution of diazo compound 2a (2.36 mmol) in anhydrous DCE
(1.5 mL) or 2b,c (3.54 mmol) in anhydrous DCE (3 mL), or 2d (1.18 mmol) in
anhydrous DCE (1 ml) was added dropwise to a stirred solution of azirine 1a,b
(0.59 mmol) and Rh2(OAc)4 (15 mg) in anhydrous DCE (1 mL) at reflux under
4a,d
(2RS,3RS)-5a,d
(2RS,3SR)-5a,d
5а, X = Br; 20%; dr = 1 : 1
5d, X = Cl; 19%; dr = 1 : 1
CO2Me
N
CO2Me
CO2Me
MeO2C
X
C6D6
X
CO2Me
CO2Me
100 °C
25 min
N
Ph
Ph
4f,g
5f,g
X = Br, 4f : 5f = 1 : 1
X = Cl, 4g : 5g = 2.6 : 1
Scheme 4. Valence isomerism of 2-azadienes 4a,d,f,g.

5780
M. S. Novikov et al. / Tetrahedron Letters 53 (2012) 5777–5780
an argon atmosphere. The solvent was evaporated in vacuum and the residue
was purified by column chromatography on silica gel (eluent: benzene/EtOAc
from 300:1 to 100:1) followed by crystallization of the resulting solid, to give
compounds 4a–g and 5a,d,f,g.
C
15H14BrNO6, M = 384.18, triclinic, space group P-1, a = 8.3126(2),
b = 9.9057(3), c = 10.3924(3) Å,
a
= 88.56(1), b = 67.44(1),
c
= 86.20(1)°,
U = 788.49(4) Å3, F(000) = 388, Z = 2, Dc = 1.618 mg mÀ3 = 2.637 mmÀ1
, l
(Mo–Ka, k = 0.71073 Å), T = 120(1) K.
8. Lall, M. S.; Ramtohul, Y. K.; James, M. N. G.; Vederas, J. C. J. Org. Chem. 2002, 67,
1536–1547.
11. Crystallographic data for the structure 4b have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publication no.
CCDC 889607. Copies of these data can be obtained on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (email: deposit@ccdc.cam.ac.uk).
12. Crystallographic data for the structure 5f have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publication no.
CCDC 889608. Copies of these data can be obtained on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (email: deposit@ccdc.cam.ac.uk).
13. Galvez, J.; Guirado, A. J. Comput. Chem. 2009, 31, 520–531.
14. The calculations were performed using the Gaussian 03 (Frisch, M. J.; Trucks, G.
W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J.
A. Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi,
J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; A. Petersson, G.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida,
M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.;
Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann,
R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P.
Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez,
C.; Pople, J. A.; Revision B.05, Gaussian, Inc., Pittsburgh PA, 2003) package of
programs at a hybrid density functional theory (DFT) level with basis set 6–
31G(d) for atoms C, N, O, H and LANL2DZ for Br.
9. Balaji, B. S.; Chanda, B. M. Tetrahedron Lett. 1998, 39, 6381–6382.
10. Methyl (2E)-2-bromo-3-{[1-(ethoxycarbonyl)-2-oxo-2-phenyl-ethylidene]amino}-
3-phenylprop-2-enoate (4b), yellow solid, mp 84–86 °C (from hexane to Et2O).
1H NMR (300 MHz, CDCl3): d 1.30 (3H, t, J = 7.3 Hz, CH3), 3.84 (3H, CH3O), 4.34
(2H, q, J = 7.3 Hz, CH2O), 7.37–7.40 (3H, m, Ph), 7.47–7.52 (4H, m, Ph), 7.62–
7.67 (1H, m, Ph), 8.07–7.13 (2H, m, Ph). 13C NMR (75 MHz, CDCl3): d 13.9 (CH3),
53.0 (CH3), 62.8 (CH2), 92.2 (@CBr), 128.1, 128.6, 128.8, 129.7, 130.3, 133.7,
134.4, 136.4 (Ph), 154.2, 157.3, 159.3, 163.5, 189.2 (C@N, C@O). HRMS-ESI
calcd for
21H18NO5Br, M = 444.27, triclinic, space group P-1, a = 8.9820(2),
b = 9.7558(2), c = 11.4619(3) Å, = 95.76(1), b = 96.53(1), = 103.63(1)°,
U = 963.81(4) Å3, F(000) = 452, Z = 2, Dc = 1.531 mg mÀ3 = 2.165 mmÀ1
(Mo–K , k = 0.71073 Å), T = 120(1) K. Dimethyl 2-[(E)-2-bromo-3-methoxy-3-
C
21H18NO5Br [M+H]+ 444.0441, found 444.0437. Crystal data:
C
a
c
,
l
a
oxo-1-phenylprop-1-enylimino]malonate (4f), yellow oil. 1H NMR (300 MHz,
CDCl3): d 3.79 (3H, s, CH3O), 3.90 (6H, s, 2CH3O), 7.41–7.43 (3H, m, Ph), 7.54–
7.58 (2H, m, Ph). 13C NMR (75 MHz, CDCl3): d 53.0 (2CH3O), 53.3 (CH3O), 92.7
(@C–Br), 128.2, 128.6, 129.8, 135.9 (Ph), 148.8 (C@N), 156.9 (@CN), 163.2
(C@O). HRMS-ESI calcd for C15H14BrNO6 [M+Na]+ 405.9897, found 405.9897.
Trimethyl 3-bromo-4-phenyl-2,3-dihydroazete-2,2,3-tricarboxylate (5f), mp
103–104 °C (from hexane to CH2Cl2). 1H NMR (300 MHz, CDCl3): d 3.85 (3H, s,
CH3O), 3.86 (3H, s, CH3O), 3.94 (3H, s, CH3O), 7.47–7.52 (2H, m, Ph), 7.57–7.62
(1H, m, Ph), 7.94–7.97 (2H, m, Ph). 13C NMR (75 MHz, CDCl3): d 53.4 (CH3O),
53.7 (CH3O), 53.9 (3-C), 54.0 (CH3), 79.3 (2-C), 127.6, 127.7, 128.7, 133.4 (Ph),
164.8, 165.1, 165.2 (C@O), 185.0 (4-C). Elem. Anal. Found: C, 46.92; H, 3.42; N,
3.73. Calcd for
C15H14BrNO6: C, 46.89; H, 3.67; N, 3.65. Crystal data: