Palladium-catalyzed four-membered ring annelation reactions at dibenzobarrelene

Article abstract of DOI:10.1055/s-0028-1087375

A palladium-catalyzed coupling process of dibenzobarrelene with annelated aryl halides leads to special benzocyclobutenes, resembling Diels-Alder adducts of antiaromatic benzocyclobutadienes with anthracene. Moderate steric pressure in intermediary five-m

Full text of DOI:10.1055/s-0028-1087375

CLUSTER
133
Palladium-Catalyzed Four-Membered Ring Annelation Reactions at Dibenzo-
barrelene
R
ingAnnelation
R
eact
a
ions at Dib
r
enzobarr
c
e
lene us Pillekamp, Alexandra Aniol, Johannes Heppekausen, Saskia Neukirchen, Stephanie Seel, Iris M. Oppel,
Gerald Dyker*
Fakultät für Chemie, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany
Fax +49(234)3214353; E-mail: Gerald.Dyker@rub.de
Received 7 August 2008
Dedicated to Prof. Dr. Maximilian Zander on the occasion of his 80th birthday
Abstract: A palladium-catalyzed coupling process of dibenzobar-
relene with annelated aryl halides leads to special benzocy-
clobutenes, resembling Diels–Alder adducts of antiaromatic
benzocyclobutadienes with anthracene. Moderate steric pressure in
intermediary five-membered palladacycles seems to be decisive for
1
2
3
4
this type of transformation.
Key words: benzocyclobutenes, C–H activation, domino process-
es, palladium catalysis, steric pressure
Figure 1 Olefin components for palladium-catalyzed coupling
reactions
and its structure was confirmed by analysis of two-dimen-
sional NMR spectra including HMBC, HMQC and
NOESY; somewhat surprisingly deviating from substitu-
tion patterns observed before for annelation reactions us-
ing norbornene.13 Anthracene was found as by-product,
obviously resulting from the anticipated retro-Diels–Al-
der step. In the case of 2-bromo- and 2-iodo-p-xylene14
(7a and 7b) the formation of the phenanthrene moiety sur-
Norbornene (1),1 norbornadiene (2)2 and acenaphthylene
(3)3 have been extensively studied as strained, highly re-
active alkene components in palladium-catalyzed domino
coupling processes, which deviate from the classical Heck
reaction, since the ‘normal’ b-H-elimination step is pro-
hibited because of stereochemical requirements.4 Nor-
bornene (1), in particular, turned out to be very useful as
catalytic template in the Catellani reaction, which is es-
sentially based on the reversibility of the carbopalladation
step.5
Table 1 Coupling Reactions at Dibenzobarrelene (4)a
We became interested in palladium-catalyzed annelation
reactions of the structurally related dibenzobarrelene (4;
Figure 1)6 in order to overcome a preparative obstacle:
while disubstituted alkynes have been successfully ap-
plied for the synthesis of polycyclic ring systems via pal-
ladium-catalyzed annelation processes with aryl halides7
and vinyl halides8, terminal alkynes, and of course the
parent acetylene,9 bearing acidic alkynyl hydrogens prefer
to react in a Sonogashira-type fashion.10 We envisioned,
that dibenzobarrelene (4) could serve as an ‘acetylene an-
nelation synthon’ when applied in a palladium-catalyzed
annelation and retro-Diels–Alder sequence,2a,3c and wish
to report on our initial results with 4 as coupling compo-
nent, focusing on the efficient formation of annelated ben-
zocyclobutenes.11
Entry
Aryl halideb Molar ratio of T (°C) Product (yield)
halide/4
1
2
5a
7a
4.9:1
3:1
3:1
1:3
3:1
1:3
3:1
1:5
3:1
3:1
2.3:1
1:3
100
100
140
140
100
140
100
100
100
140
100
100
6 (63%)
8 (8%)
3
7a
8 (59%)
8 (80%)
8 (36%)
8 (80%)
10 (40%)
10 (36%)
10 (44%)
12 (37%)
14 (21%)
14 (40%)
4
7a
5
7b
6
7b
7
9a
8
9b
9
9b
With the 4-bromobenzoic acid ester 5a, the initial plan in-
deed worked out under Jeffery conditions:12 the function-
alized phenanthrene 6 was isolated with acceptable yield,
10
11
12
11a
13a
13b
a Reaction conditions: DMF, Pd(OAc)2 (5 mol%), K2CO3, n-Bu4NBr,
3 d, 100 °C or 140 °C.
b For a, X = Br; for b, X = I.
SYNLETT 2009, No. 1, pp 0133–0137
x
x.
x
x.
2
0
0
8
Advanced online publication: 12.12.2008
DOI: 10.1055/s-0028-1087375; Art ID: S07008ST
© Georg Thieme Verlag Stuttgart · New York

134
M. Pillekamp et al.
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EtO2C
X
Pd
Pd
CO2Et
EtO2C
EtO2C
5
6
15
16
Figure 3 Intermediary palladacycles 15 and 16
X
8
7
+
X
18 (48%)
19 (28%)
4
Pd cat.
10
9
Br
X
17a
1
3
Pd cat.
Pd cat.
12
11
X
20 (81%)
21 (84%)
Scheme 1 Products from palladium-catalyzed domino processes of
9-bromoanthracene (17a) with olefins 1, 3 and 4
14
13
Figure 2 Aryl halides (a: X = Br, b: X = I) and resulting ring
systems from palladium-catalyzed domino processes with dibenzo-
barrelene (4)
retro-Diels–Alder reaction would deliver an energetically
disfavored antiaromatic benzocyclobutadiene.
This methodology could also be applied to other polycy-
clic aromatic hydrocarbons 9, 11, and 13, bromo- or iodo-
substituted in peri position, to afford the desired benzocy-
clobutenes 10, 12, and 14 (Figure 2). These findings are
somewhat surprising, since the peri-annelated coupling
components offer the opportunity to form the regioiso-
meric five-membered ring products in principle, as ob-
served before with acenaphthylene (3) as alkene
component.3a However, for the mechanistic interpreta-
tion, one can conclude that the formation of the five-mem-
bered palladacyclic intermediates16 (such as 15, Figure 3)
is favored compared to the six-membered analogues (such
as 16, Figure 3). This conclusion is in accord with the typ-
ical selectivity of an electrophilic aromatic substitution: a
prisingly failed. Instead, the highly strained annelated
benzocyclobutene 8 was the result of the palladium-cata-
lyzed domino process in up to 80% yield. For the iodide
7b a reaction temperature of 100 °C was sufficient
(Table 1, entry 5); in the case of the less reactive bromide
7a, 140 °C turned out to be superior (entries 3 and 4). Ap-
plying dibenzobarrelene (4) in excess offers the advantage
of minimizing by-products; however, using an excess of
the readily accessible aryl halides14 should be more feasi-
ble.
Presumably the four-membered ring formation is favored
by steric influence15 of the ortho-methyl group. The ther-
mal stability of this product is explained by the fact that a
Synlett 2009, No. 1, 133–137 © Thieme Stuttgart · New York

CLUSTER
Ring Annelation Reactions at Dibenzobarrelene
135
The mechanistic coherence of products 18 and 19 is ana-
lyzed in Scheme 2: a C–H activation step in the peri posi-
tion in carbopalladation product 22 leads to the six-
membered palladium(IV) palladacycle 23,17 which either
is deprotonated and consecutively transformed to 18, or
reacts in a reductive elimination step to palladium(II) in-
termediate 25 in the sense of a palladium migration.18 The
second equivalent of barrelene 4 is then inserted again by
carbopalladation. In the cyclopalladated intermediate 26,
steric pressure15 activates the reductive elimination to
four-membered carbocycles 19. In comparison, the cou-
pling reaction of 9-bromoanthracene (17a) with nor-
bornene gave the analogous 1:2 domino products 20 (81%
yield), whereas with acenaphthylene (3) the formally
‘simple’ Heck product 21 was obtained (84% yield), pre-
sumably by a different mechanism circumventing the ste-
reochemical requirements as explained previously.2a
Br
Pd
PdBr
H
23
22
– HBr
Pd
BrPd
For the benzocyclobutenes 10, 12, 14 and 19 single crys-
tals were obtained by diffusion of methanol into a dichlo-
romethane solution and their constitution was proved by
X-ray crystal structure analyses (Figure 4).19 The bond
lengths of the C–C bonds connecting the cyclobutene
moiety with the 9,10-positions of the dihydrophenan-
threne unit are minimally elongated in the range of 153.5–
157.4 pm.
25
24
18
4
– HBr
– Pd0
In conclusion, annelated benzocyclobutenes are readily
formed by a palladium-catalyzed coupling process of
dibenzobarrelene (4) based on the steric influence of ortho
substituents in aryl halides as coupling components.20 In
ongoing studies, we will test these coupling products as a
source for antiaromatic benzocyclobutadienes21 under the
conditions of flash vacuum pyrolysis. The successful syn-
thesis of the functionalized phenanthrene 6 via a domino
coupling retro-Diels–Alder sequence starting from a
para-substituted aryl halide motivates for further studying
scope and limitations of this type of domino process.
19
Pd
– Pd0
26
Scheme 2 Mechanistic rationale for the formation of 18 and 19 by
the palladium-catalyzed reaction of 9-bromoanthracene (17a) with di-
benzobarrelene (4)
Acknowledgment
1-alkyl-substituted naphthalene is more reactive in the 2-
position than in the 8-position in terms of a SEAr reaction.
We thank Degussa AG for a generous donation of palladium ace-
tate.
The five-membered palladacycles undergo reductive
elimination to four-membered carbocycles, rather than
oxidatively adding a second equivalent of a somewhat
sterically demanding aryl halide: the steric influence of an
annelated benzene ring in ortho position or an ortho-
methyl group seems to be sufficient, both, for disfavoring
a second oxidative addition step and for supporting the re-
ductive elimination step by steric pressure.15
References and Notes
(1) (a) Catellani, M. Synlett 2003, 298. (b) Catellani, M.; Motti,
E. New J. Chem. 1998, 22, 759.
(2) (a) Hulcoop, D. G.; Lautens, M. Org. Lett. 2007, 9, 1761.
(b) Bhuvaneswari, S.; Jeganmohan, M.; Cheng, C.-H. Org.
Lett. 2006, 8, 5581. (c) Catellani, M.; Ferioli, L. Synthesis
1996, 769. (d) Dyker, G. J. Org. Chem. 1993, 58, 234.
(3) (a) Dyker, G.; Merz, K.; Oppel, I. M.; Muth, E. Synlett 2007,
897. (b) Dyker, G. Tetrahedron Lett. 1991, 32, 7241.
(c) Dyker, G.; Kerl, T.; Bubenitschek, P.; Jones, P. G.
J. Chem. Soc., Perkin Trans. 1 1997, 1163.
(4) (a) Terpko, M. O.; Heck, R. F. J. Am. Chem. Soc. 1979, 101,
5281. (b) de Meijere, A.; Meyer, F. E. Angew. Chem., Int.
Ed. Engl. 1994, 33, 2379. (c) Dyker, G. Handbook of
Organopalladium Chemistry for Organic Synthesis, Vol. 1;
Negishi, E.-i.; de Meijere, A., Eds.; Wiley: New York, 2002,
1255–1282.
9-Bromoanthracene (17a) as coupling component is an
especially interesting case (Scheme 1) since the formation
of a six-membered palladacycle 24 (Scheme 2) as inter-
mediate is structurally predetermined: the resulting anne-
lated five-membered carbocycle 18 was indeed isolated as
the main product with 48% yield. Surprisingly, a diaste-
reoisomeric mixture of annelated benzocyclobutenes 19,
obviously 1:2 domino products, was found as by-product
in a substantial amount (28% yield).
Synlett 2009, No. 1, 133–137 © Thieme Stuttgart · New York

136
M. Pillekamp et al.
CLUSTER
(5) (a) Catellani, M.; Frignani, F.; Rangoni, A. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 119. (b) Lautens, M.; Alberico, D.;
Bressy, C.; Fang, Y.-Q.; Mariampillai, B.; Wilhelm, T. Pure
Appl. Chem. 2006, 78, 351.
(6) Mühlebach, M.; Neuenschwader, M. Helv. Chim. Acta 1993,
76, 2089.
(7) (a) Dyker, G.; Kellner, A. Tetrahedron Lett. 1994, 35, 7633.
(b) Dyker, G.; Borowski, S.; Henkel, G.; Kellner, A.; Dix, I.;
Jones, P. G. Tetrahedron Lett. 2000, 41, 8259. (c) Liu, Z.;
Larock, R. C. Angew. Chem. Int. Ed. 2007, 46, 2535.
(d) Yao, T.; Campo, M. A.; Larock, R. C. Org. Lett. 2004, 6,
2677. (e) Kadnikov, D. V.; Larock, R. C. J. Org. Chem.
2004, 69, 6772. (f) Larock, R. C.; Doty, M. J.; Tian, Q.;
Zenner, J. M. J. Org. Chem. 1997, 62, 7536. (g) Tian, Q.;
Larock, R. C. Org. Lett. 2000, 2, 3329.
(8) Dyker, G.; Siemsen, P.; Sostmann, S.; Wiegand, A.; Dix, I.;
Jones, P. G. Chem. Ber. 1997, 130, 261.
(9) (a) Cassar, L. J. Organomet. Chem. 1975, 93, 253. (b) Li,
C.-J.; Chen, D.-L.; Costello, C. W. Org. Process Res. Dev.
1997, 325.
(10) (a) Siemsen, P.; Livingston, R. C.; Diederich, F. Angew.
Chem. Int. Ed. 2000, 39, 2632. (b) Thorand, S.; Krause, N.
J. Org. Chem. 1998, 63, 8551.
(11) (a) Catellani, M.; Motti, E.; Ghelli, S. Chem. Commun. 2000,
2003. (b) Deledda, S.; Motti, E.; Catellani, M. Can. J. Chem.
2005, 83, 741.
(12) Jeffery conditions: (a) Jeffery, T. Tetrahedron Lett. 1985,
26, 2667. (b) Jeffery, T. Tetrahedron Lett. 1999, 40, 1673.
(13) Catellani, M.; Motti, E.; Paterlini, L.; Bocelli, G.; Righi, L.
J. Organomet. Chem. 1999, 580, 191.
(14) Iodination of arenes was performed in analogy to: Zweig, A.;
Huffman, K. R.; Nachtigall, G. W. J. Org. Chem. 1977, 42,
4049.
(15) Dyker, G. Angew. Chem., Int. Ed. Engl. 1994, 33, 103.
(16) (a) Dyker, G.; Körning, J.; Nerenz, F.; Siemsen, P.;
Sostmann, S.; Wiegand, A.; Jones, P. G.; Bubenitschek, P.
Pure Appl. Chem. 1996, 68, 323. (b) Dyker, G. Chem. Ber./
Recl. 1997, 130, 1567.
(17) Markies, B. A.; Canty, A. J.; Boersma, J.; van Koten, G.
Organometallics 1994, 13, 2053.
(18) Zhao, J.; Yue, D.; Campo, M. A.; Larock, R. C. J. Am. Chem.
Soc. 2007, 129, 5288.
(19) CCDC 697535 (for 10), CCDC 697536 (for 12), CCDC
697537 (for 14), and CCDC 697538 (for 19) contain the
supplementary crystallographic data. These data can be
obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
(20) General Procedure for Palladium-Catalyzed Coupling
Reactions of Aryl Halides with Dibenzobarrelene (4): A
mixture of aryl halide (3.00 mmol), dibenzobarrelene (4; 202
mg, 1.00 mmol; or in the ratio outlined in Table 1), K2CO3
(1.1 g, 8.0 mmol), n-Bu4NBr (645 mg, 2.00 mmol),
Pd(OAc)2 (12 mg, 0.05 mmol) and DMF (10 mL) in a sealed
tube (for convenience) was stirred under argon at 100 °C for
3 d. After diluting with H2O (50 mL), the reaction mixture
was extracted three times with diethyl ether or methyl tert-
butyl ether (50 mL). The Et2O extract was filtered through
silica and concentrated. The crude product was purified by
flash chromatography (hexanes or hexanes–dichloro-
methane on silica gel).
Selected Analytical Data: Compound 6: colorless solid; mp
42 °C. 1H NMR (400 MHz, CDCl3): d = 1.44–1.55 (m, 9 H),
4.44–4.58 (m, 6 H), 7.61 (d, J = 6.6 Hz, 2 H), 7.81 (d, J = 9.2
Hz, 1 H), 7.88 (d, J = 9.2 Hz, 1 H), 8.19–8.29 (m, 3 H), 8.36
(dd, J = 1.7, 8.7 Hz, 1 H), 8.64 (d, J = 1.6 Hz, 1 H), 8.90 (d,
J = 8.8 Hz, 1 H), 9.51 (s, 1 H). 13C NMR (101 MHz, CDCl3):
Figure 4 Structures of benzocyclobutenes 10, 12, 14 and 19 (top to
bottom, respectively) in the crystal form19
Synlett 2009, No. 1, 133–137 © Thieme Stuttgart · New York

CLUSTER
d = 14.53 (2 × q), 14.58 (q), 61.27 (t), 61.41 (t), 61.61 (t),
Ring Annelation Reactions at Dibenzobarrelene
137
J = 8.3 Hz, 1 H), 7.45–7.57 (m, 4 H), 7.60 (‘t’, J = 6.6 Hz, 1
H), 7.99 (d, J = 8.3 Hz, 1 H), 8.01 (s, 1 H), 8.42 (dd, J = 0.8,
8.6 Hz, 1 H). 13C NMR (101 MHz, CDCl3): d = 49.13 (d),
50.17 (d), 51.35 (d), 51.45 (d), 117.37 (d), 122.44 (d), 122.75
(d), 124.15 (d), 124.23 (d), 124.27 (d), 124.37 (d), 124.92
(d), 124.94 (d), 125.01 (d), 125.31 (d), 125.38 (d), 126.25
(d), 126.33 (d), 127.07 (s), 127.49 (d), 129.31 (s), 130.06 (d),
133.78 (s), 139.04 (s), 139.91 (s), 140.17 (s), 141.19 (s),
143.78 (s), 144.20 (s), 146.72 (s). MS (EI): m/z (%) = 380
(5.7) [M+], 202 (87.1), 178 (100), 108 (6.4), 59 (19.3), 43
(16.4), 29 (6.4).
123.54 (d), 124.72 (d), 125.51 (d), 127.39 (d), 128.11 (s),
128.50 (d), 129.27 (s), 129.86 (d, phenylene), 129.98 (d),
130.09 (s), 130.28 (d, phenylene), 130.90 (d), 131.33 (s),
133.40 (s), 133.65 (s), 140.44 (s), 144.75 (s), 166.49 (2 × s),
166.57 (s), one singlet was superimposed.
Compound 8: colorless solid; mp 126 °C. 1H NMR (400
MHz, CDCl3): d = 2.18 (s, 6 H), 3.66 (dd, J = 1.4, 2.6 Hz, 2
H), 4.58 (dd, 2 H), 6.67 (s, 2 H), 6.92 (dd, J = 3.2, 5.4 Hz, 2
H), 7.09 (dd, J = 3.2, 5.3 Hz, 2 H), 7.21 (dd, J = 3.2, 5.4 Hz,
2 H), 7.40 (dd, J = 3.2, 5.3 Hz, 2 H). 13C NMR (101 MHz,
CDCl3): d = 16.63 (q), 46.02 (d), 47.67 (d), 124.05 (d),
124.54 (d), 125.81 (d), 126.00 (d), 128.22 (d), 129.37 (s),
140.54 (s), 143.14 (s), 143.44 (s). MS (EI, 70 eV): m/z (%) =
308.7 (5) [M+], 307.7 (19) [M+ – 1], 178.8 (15), 177.8 (100),
130.9 (7), 129.9 (58).
Compound 19: slightly yellow fluorescent solid;
decomposition started at 160 °C. MS (EI): m/z (%) = 406
(15) [M+ – Manthracene], 228 (29), 202 (6), 178 (100)
[M+anthracene], 152 (10), 89 (7), 76 (6), 45 (8).
Compound 20: Diastereoisomeric mixture; yellow solid; mp
168 °C; six times fractionated crystallization with i-PrOH
gave one pure diastereoisomer. 1H NMR (400 MHz, CDCl3):
d = 0.91–1.01 (m, 2 H), 1.29–1.35 (m, 2 H), 1.47–1.59 (m, 3
H), 1.63–1.89 (m, 6 H), 1.94–2.02 (m, 1 H), 2.32–2.38 (m, 1
H), 2.45–2.60 (m, 4 H), 2.67 (s, 1 H), 3.20 (d, J = 3.5 Hz, 1
H), 3.73 (d, J = 3.5 Hz, 1 H), 4.43 (t, J = 8.3, 8.6 Hz, 1 H),
7.17 (d, J = 8.3 Hz, 1 H), 7.35–7.44 (m, 2 H), 7.89 (d, J = 8.3
Hz, 1 H), 7.94–7.99 (m, 1 H), 8.32 (s, 1 H), 8.62–8.69 (m, 1
H). 13C NMR (101 MHz, CDCl3): d = 28.12 (t), 28.24 (t),
29.18 (t), 31.53 (t), 32.78 (t), 36.30 (d), 37.10 (d), 37.57 (d),
39.63 (t), 39.72 (t), 44.30 (d), 44.42 (d), 49.21 (d), 54.14 (d),
120.43 (d), 124.01 (d), 124.31 (d), 126.67 (d), 128.78 (d),
129.64 (s), 130.01 (d), 130.26 (s), 130.51 (d), 131.79 (s),
132.34 (s), 137.31 (s), 140.46 (s), 144.15 (s). MS (EI): m/z
(%) = 364 (100) [M+], 323 (8), 283 (9), 270 (16), 253 (36),
239 (25), 229 (17), 202 (11), 67 (9), 49 (11), 41 (8).
Compound 21: orange-red solid; mp 222 °C. 1H NMR (200
MHz, CDCl3): d = 7.17 (d, J = 6.8 Hz, 1 H), 7.24 (s, 1 H),
7.28–7.55 (m, 5 H), 7.67 (dd, J = 6.8, 8.1 Hz, 1 H), 7.79–7.97
(m, 3 H), 8.09 (d, J = 9.9 Hz, 4 H), 8.55 (s, 1 H). 13C NMR
(50 MHz, CDCl3): d = 124.30 (d), 124.45 (d), 125.38 (d),
125.42 (d), 127.07 (d), 127.31 (d), 127.36 (d), 127.72 (d),
127.93 (d), 128.14 (d), 128.47 (s), 128.52 (d), 128.73 (s),
130.72 (s), 130.95 (d), 131.03 (s), 131.62 (s), 139.63 (s),
140.68 (s), 141.69 (s). MS (EI): m/z (%) = 328 (100) [M+],
163 (20), 149 (5.4), 43 (5.7), 27 (8.6).
Compound 10: colorless solid; mp 156 °C. 1H NMR (400
MHz, CDCl3): d = 3.81 (t, J = 4.3 Hz, 1 H), 3.94 (t, J = 4.3
Hz, 1 H), 4.63 (d, J = 3.8 Hz, 1 H), 4.76 (d, J = 4.0 Hz, 1 H),
6.65 (t, J = 8.0 Hz, 1 H), 6.67 (t, J = 8.0 Hz, 1 H), 6.88 (d,
J = 7.3 Hz, 1 H), 7.02 (d, J = 7.3 Hz, 1 H), 7.08 (d, J = 8.1
Hz, 1 H), 7.16–7.21 (m, 2 H), 7.34 (t, J = 8.2 Hz, 1 H), 7.41–
7.50 (m, 4 H), 7.68 (d, J = 8.1 Hz, 1 H), 7.77 (d, J = 8.1 Hz,
1 H). MS (EI, 70 eV): m/z (%) = 331 (3), 330 (13), 203 (2),
178 (13), 163 (2), 153 (13), 152 (100), 151 (6).
Compound 12: colorless solid; mp 265 °C. 1H NMR (200
MHz, CDCl3): d = 3.97 (d, J = 4.0 Hz, 2 H), 4.80 (d, J = 4.0
Hz, 2 H), 6.56–6.64 (m, 2 H), 6.87–6.95 (m, 2 H), 7.19–7.24
(m, 2 H), 7.45–7.64 (m, 6 H), 7.83 (d, J = 7.1 Hz, 2 H), 8.54
(d, J = 7.8 Hz, 2 H). MS (EI, 70 eV): m/z (%) = 380 (15), 202
(100), 187 (2), 178 (4), 152 (1).
Compound 14: yellow solid; mp 287 °C. 1H NMR (400
MHz, CDCl3): d = 4.10 (t, J = 4.8 Hz, 1 H), 4.23 (t, J = 4.8
Hz, 1 H), 4.77 (d, J = 4.0 Hz, 1 H), 4.89 (d, J = 4.0 Hz, 1 H),
6.50 (t, J = 7.3 Hz, 1 H), 6.61 (t, J = 7.3 Hz, 1 H), 6.88 (d,
J = 7.3 Hz, 1 H), 6.98 (d, J = 7.3 Hz, 1 H), 7.19–7.24 (m, 2
H), 7.45–7.51 (m, 2 H), 7.72 (s, 1 H), 7.89–7.94 (m, 3 H),
8.02–8.14 (m, 4 H). MS (FAB): m/z (%) = 405 (35), 404
(71), 226 (21).
Compound 18: yellow solid; mp 265 °C. 1H NMR (400
MHz, CDCl3): d = 4.31 (dd, J = 3.5, 7.3 Hz, 1 H), 4.63 (dd,
J = 2.8, 7.3 Hz, 1 H), 4.76 (d, J = 3.5 Hz, 1 H), 5.16 (d, J =
3.0 Hz, 1 H), 6.44 (‘t’, J = 7.3 Hz, 1 H), 6.50 (dd, J = 1.3, 7.3
Hz, 1 H), 6.61 (‘t’, J = 7.3 Hz, 1 H), 6.86 (d, J = 7.3 Hz, 1
H), 7.21–7.26 (m, 2 H), 7.30 (d, J = 6.6 Hz, 1 H), 7.37 (‘t’,
(21) (a) Cava, M. P. Bull. Soc. Chim. Fr. 1959, 1744.
(b) Stanger, A. Chemistry of Cyclobutanes; Rappoport, Z.;
Liebman, J. F., Eds.; Wiley: New York, 2005, 617–654.
Synlett 2009, No. 1, 133–137 © Thieme Stuttgart · New York

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