Benzocycloheptynedicobalt complexes by intramolecular Nicholas reactions

Article abstract of DOI:10.1055/s-2004-837215

Lewis acid mediated intramolecular Nicholas reactions of aryl (Z)-enyne propargyl acetate-Co₂(CO)₆ complexes 1 afford benzocycloheptenyne-Co₂(CO)₆ complexes 2 and their heterocyclic analogues.

Full text of DOI:10.1055/s-2004-837215

LETTER
271
Benzocycloheptynedicobalt Complexes by Intramolecular Nicholas Reactions
B
enzocycloheptyn
u
e
dicobalt Complex
D
es ing, James R. Green*
Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario, N9B 3P4, Canada
Fax +1(519)9737098; E-mail: jgreen@uwindsor.ca
Received 2 November 2004
The substrates for these cyclization reactions were de-
Abstract: Lewis acid mediated intramolecular Nicholas reactions
of aryl (Z)-enyne propargyl acetate-Co2(CO)6 complexes 1 afford
benzocycloheptenyne-Co2(CO)6 complexes 2 and their heterocyclic
analogues.
rived from the appropriate arylaldehydes 3 (Scheme 1).
Carbon tetrabromide-PPh3 mediated conversion of 3 to
the corresponding dibromoalkenes 4 occurred in high
yield.10 Critical for access to Z-alkenes of high stereo-
chemical purity was the stereoselective Pd-catalyzed
reduction of the trans carbon-bromine bond of 4, with
subsequent Sonogashira coupling of the remaining cis
bromide and the appropriate propargyl alcohol without
intermediate isolation, according to the method of
Uenishi.11 The Z-enyne-propargyl alcohols 5 were ob-
tained in fair to good yields,12 and were subjected to
acetylation and complexation under conventional condi-
tions to afford 1 (Table 1).13
Key words: Nicholas reactions, alkynes, arenes, complexes, transi-
tion metals
The use of acyclic alkynedicobalt complexes and
Nicholas reactions1 have proven themselves to be highly
successful in the rapid preparation of cycloheptyne-
Co2(CO)6 complexes.2–4 Only a limited number of ring
fused versions of these cycloheptyne complexes are
known; of these, our interest has been drawn to the benzo-
cycloheptyne complexes. Just one report of the synthesis
of benzocycloheptynedicobalt complexes has been pub-
lished, involving an impressive carbonylative Heck reac-
tion of an enyne complex.5 Nevertheless, the success of
this process required replacement of two CO ligands by a
dppm ligand, and furthermore employed diphenylacetyl-
ene-Co2(CO)6 as the optimal carbonylation source. A
variety of naturally occurring compounds containing
benzocycloheptane units are known, particularly the
icetexanes6 or other diterpenes7 and the colchicines,8 or
their heterocyclic analogues, particularly the furanocyclo-
heptanes.9 Therefore, we considered further synthetic
effort towards the ready synthesis of benzocyclo-
heptynedicobalt systems to be of importance. Given the
known ability of propargyldicobalt cations to enter into
Nicholas reactions with electron rich arenes, it was our
belief that the ionization of 1 would provide access to
benzocycloheptynedicobalt complexes 2 by way of an
intramolecular Nicholas reaction (Equation 1).
Scheme 1 Reagents and conditions: (a) CBr4, PPh3, CH2Cl2;
(b) Bu3SnH, Pd(PPh3)4, CH2Cl2; then HNi-Pr2, CuI, propargyl
alcohol; (c) Ac2O, pyridine; (d) Co2(CO)8, CH2Cl2.
With the requisite substrates in hand, attention was turned
to cyclization reactions. BF3·OEt2 was chosen as the pre-
ferred Lewis acid due to its minimized tendency to induce
decomposition in related cobalt complexes.14 Although
unactivated arenes are generally insufficiently reactive to
participate in Nicholas reactions, unsubstituted arene 1a
did undergo gradual reaction at 0 °C. After 3.5 hours, a
small amount of starting material remained, but benzocy-
cloheptyne 2a could be isolated in 49% yield (58% yield
based on recovered starting material). Longer reaction
periods resulted in increased amounts of unproductive
decomposition.
With electron donating substituents on the benzene ring,
the reactions were noticeably more rapid. m-Methyl-sub-
stituted 1b and m-methoxy-substituted 1c underwent
complete starting material consumption in 90 minutes and
30 minutes, respectively. Compound 2b was obtained in
67% yield as a mixture of products of substitution p- and
o- to the methyl group (2b:2b¢ = 3.5:1, inseparable),
whereas methoxy compound 2c was obtained in 53%
yield as a 4.9:1 separable mixture of 2c and 2c¢ (Figure 1).
Substitution at the propargylic site did not interfere with
the cyclization process. Despite the potential for a
competitive elimination process, methyl-substituted 1d
Equation 1 Intramolecular Nicholas reaction for benzocyclohep-
tenyne-Co2(CO)6 preparation.
SYNLETT 2005, No. 2, pp 0271–0274
0
1.
0
2.
2
0
0
5
Advanced online publication: 17.12.2004
DOI: 10.1055/s-2004-837215; Art ID: S10504ST
© Georg Thieme Verlag Stuttgart · New York

272
Y. Ding, J. R. Green
Table 1 Preparation of Propargyl Acetate 1: Yields of Intermediates
ArCHO 3 Yield 4 Yield 5 Yield 1
LETTER
(%)
(%)
(%)
a
94
71
80
b R¢ = Me
c R¢ = OMe
d R¢ = OMe
R = Me
92
100
–
58
49
90
86
76
70
e R¢ = OMe
R = Ph
–
70
80
f X = O
g X = S
94
90
54
51
70
81
h
77
62
72
i
93
65
70
j
100
45a
85b
k
91
68
80
Figure 1 Benzocycloheptenynedicobalt cyclization products 2.
a Yield of isolated acetate.
b Yield based on acetate.
product mixture apparently containing a benzocyclohep-
tenyne complex, but it was both in low yield and not readi-
ly purified. Trimethoxyarene 1j also cyclized only in low
yield (22%) to give 2j under conventional conditions, but
high dilution conditions (10–3 M) allowed formation of 2j
in 51% yield. As a result of these difficulties, our choice
of a p-disubstituted substrate for investigation was p-tri-
methylsilyl arene 1k, by virtue of the minimally o-/p-acti-
vating nature of the TMS group and the facility with
which arylsilanes are converted into other arenes.16 In the
event, reaction of 1k was somewhat slow, and it was again
prudent to stop the reaction before complete starting ma-
terial consumption, but 2k could be obtained in 58% yield
(70% based on recovered starting material).17
reacted readily, giving 2d in good yield (77%,
2d:2d¢ = 7:1, separable) and with no elimination side-
product. Phenyl-substituted 2e also reacted promptly,
affording 2e in excellent yield (90%, 2e:2e¢ = 12.5:1,
separable).
Heteroaryl based systems were also capable of forming
the corresponding fused cycloheptyne complexes. 3-Sub-
stituted furyl system 1f afforded 2f quickly and in good
yield (78%). The corresponding thienyl system also cy-
clized rapidly, but gave a small amount of C-4 substitution
product 2g¢ in addition to the expected 2g (83% yield,
2g:2g¢ = 5:1, inseparable).15 The C-3-substituted indole
substrate 1h gave cycloheptyne complex 2h in 38% yield,
but the major product was actually cyclooctyne complex
2h¢ (47% yield), the result of competitive reaction through
C-4.
We wished to demonstrate the possibility of removal of
the metal complex in one of these cases. Therefore, sub-
strate 2c was chosen for reductive decomplexation under
the Bu3SnH protocol of Isobe.18 Under the literature con-
ditions (Bu3SnH, benzene, 65 °C) modest yields of a mix-
ture of benzocycloheptadiene 6 were obtained along with
its double bond migration isomer 6¢ (40% yield,
6:6¢ = 2:1). Reducing the reaction temperature to 52 °C
allowed improvement in both the yield and isomeric ratio
(58% yield, 6:6¢ = 3:1, Equation 2).
Accomplishing cyclization at sites disfavored for electro-
philic aromatic substitution by the existing substituents or
by the ring system itself proved to be more problematic,
but possible in some cases. 2-Substituted furyl system 1i
gave only dimeric-oligomeric products under convention-
al conditions. Higher dilution conditions did result in a
Synlett 2005, No. 2, 271–274 © Thieme Stuttgart · New York

LETTER
Benzocycloheptynedicobalt Complexes
273
References
Table 2 Cycloheptenyne Complexes 2 from Propargyl Acetates 1
(1) For recent reviews, see: (a) Green, J. R. Curr. Org. Chem.
2001, 5, 809. (b) Teobald, B. J. Tetrahedron 2002, 58, 4133.
(2) (a) Lu, Y.; Green, J. R. Synlett 2001, 243. (b) Patel, M. M.;
Green, J. R. Chem. Commun. 1999, 509. (c) Green, J. R.
Chem. Commun. 1998, 1751.
(3) (a) Tanino, K.; Onuki, K.; Asano, K.; Miyashita, M.;
Nakamura, T.; Takahashi, Y.; Kuwajima, I. J. Am. Chem.
Soc. 2003, 125, 1498. (b) Tanino, K.; Kondo, F.; Shimizu,
T.; Miyashita, M. Org. Lett. 2002, 4, 2217. (c) Tanino, K.;
Shimizu, T.; Miyama, M.; Kuwajima, I. J. Am. Chem. Soc.
2000, 122, 6116. (d) Schreiber, S. L.; Sammakia, T.; Crowe,
W. E. J. Am. Chem. Soc. 1986, 108, 3128.
(4) (a) Young, D. G. J.; Burlison, J. A.; Peters, U. J. Org. Chem.
2003, 68, 3494. (b) Green, J. R. Synlett 2001, 353.
(c) Iwasawa, N.; Sakurada, F.; Iwamoto, M. Org. Lett. 2000,
2, 871.
(5) Iwasawa, N.; Satoh, H. J. Am. Chem. Soc. 1999, 121, 7951.
(6) For examples, see: (a) Uchiyama, N.; Kiuchi, F.; Ito, M.;
Honda, G.; Takeda, Y.; Khodzhimatov, O. K.; Ashurmetov,
O. A. J. Nat. Prod. 2003, 66, 128. (b) Ulubelen, A.; Topcu,
G. J. Nat. Prod. 2000, 63, 879.
Starting 1
Reaction time (h) Product 2
Yielda,b
49 [58]
1a
1b
1c
1d
1e
1f
3.5
1.5
0.5
0.5
0.5
0.5
0.5
2
2a
2b/2b¢
2c/2c¢
2d/2d¢
2e/2e¢
2f
67 (3.5:1)
53 (4.9:1)
77 (7.0:1)
90 (12.5:1)
78
1g
1h
1j
2g/2g¢
2h/2h¢
2j
83 (5.0:1)
85 (0.81:1)
51c
0.5
4
1k
2k
58 [70]
a Yields in square brackets are based on recovered starting material.
b Numbers in parentheses are 2:2¢ ratios.
(7) For examples, see: (a) Ulubelen, A.; Topcu, G.; Tan, N.;
Lin, L.-J.; Cordell, G. A. Phytochemistry 1992, 31, 2419.
(b) Purushothaman, K. K.; Chandrasekharan, S.; Cameron,
A. F.; Connolly, J. D.; Labbe, C.; Maltz, A.; Rycroft, D. S.
Tetrahedron Lett. 1979, 979. (c) Lee, J.; Hong, J. J. Org.
Chem. 2004, 69, 6433.
c Conducted under high dilution (10–3 M) conditions.
In summary, a series of benzocycloheptenynedicobalt
complexes may be obtained by intramolecular Nicholas
reactions of arenes or their heterocyclic analogues. Both
electron neutral and electron rich arenes will participate in
the process, although difficulties may be encountered
where the existing substitutents direct strongly away from
the intended site of intramolecular attack. The hexacarbo-
nyldicobalt unit may be removed with concomitant reduc-
tion of the alkyne function. The use of blocking groups to
address regiochemical issues, and application of this
chemistry to dibenzocycloheptynedicobalt complexes, are
under study and will be reported in due course.
(8) Bentley, K. W. Nat. Prod. Rep. 2004, 21, 395; and
references therein.
(9) For examples, see: (a) Ho, T.-L.; Lin, Y.-J. J. Chem. Soc.,
Perkin Trans. 1 1999, 1207. (b) Inoue, M.; Carson, M. W.;
Frontier, A. J.; Danishefsky, S. J. J. Am. Chem. Soc. 2001,
123, 1878. (c) Asakawa, Y.; Toyota, M.; Tsunematsu, T.;
Kubo, I.; Nakanishi, K. Phytochemistry 1980, 19, 2147.
(10) Ramirez, F.; Desai, N. B.; McKelvie, N. J. Am. Chem. Soc.
1962, 84, 1745.
(11) Uenishi, J.; Kawahama, R.; Yonemitsu, O.; Tsuji, J. J. Org.
Chem. 1996, 63, 8965.
(12) Selected compounds:
Compound 5b: IR (neat, KBr): nmax = 3345, 3020, 2920,
2193 cm–1. 1H NMR: d = 1.77 (1 H, br), 2.38 (3 H, s), 4.50
(1 H, d, J = 2.0 Hz), 5.71 (1 H, dt, J = 11.9, 2.0 Hz), 6.64 (1
H, d, J = 11.9 Hz), 7.11 (1 H, d, J = 7.5 Hz), 7.27 (1 H, dd,
J = 7.7, 7.5 Hz), 7.63 (1 H, s), 7.68 (1 H, d, J = 7.7 Hz). 13
C
NMR: d = 139.4, 137.8, 136.1, 129.4, 128.2, 125.6, 106.5,
93.6, 84.3, 51.8, 21.4. MS (EI): m/z = 172. HRMS (EI): m/z
calcd for C12H12O: 172.0888; found: 172.0888.
Compound 5g: IR (neat, KBr): nmax = 3358, 3097, 2918,
2191, 1699, 1602 cm–1. 1H NMR: d = 2.39 (1 H, s), 4.51 (2
H, d, J = 2.0 Hz), 5.62 (1 H, dt, J = 11.7, 2.3 Hz), 6.70 (1 H,
d, J = 11.7 Hz), 7.29 (1 H, dd, J = 4.9, 2.6 Hz), 7.62 (1 H, d,
J = 5.0 Hz), 7.76 (1 H, d, J = 2.1 Hz). 13C NMR: d = 138.3,
133.0, 127.7, 125.6, 125.2, 105.2, 93.8, 84.4, 51.6. MS (EI):
m/z = 164. HRMS (EI): m/z calcd for C9H8OS: 164.0296;
found: 164.0292.
Equation 2 Reductive decomplexation of a benzocycloheptenyne
complex.
Acknowledgment
We are grateful to NSERC (Canada), the Canada Foundation for
Innovation (CFI), and the Ontario Innovation Trust (OIT) for
support of this research.
(13) Selected compounds:
Compound 1b: IR (neat, KBr): nmax = 3014, 2927, 2092,
2024, 1748 cm–1. 1H NMR: d = 2.03 (3 H, s), 2.39 (3 H, s),
4.52 (2 H, s), 6.61 (1 H, d, J = 11.0 Hz), 6.80 (1 H, d,
J = 11.0 Hz), 7.02 (2 H, d, J = 8.2 Hz), 7.14 (1 H, d, J = 7.6
Hz), 7.29 (1 H, d, J = 7.5 Hz). 13C NMR: d = 199.1 (br),
170.5, 138.2, 137.6, 132.5, 129.0, 128.5, 128.4, 127.5,
125.4, 91.4, 83.5, 64.9, 21.3, 20.3. MS (EI): m/z = 472 [M –
CO]+, 444 [M – 2 CO]+, 416 [M –3 CO]+, 388 [M – 4 CO]+,
360 [M – 5 CO]+, 332 [M – 6CO]+. HRMS (EI): m/z calcd
for C20H14 Co2O8: 443.9454 [M – 2 CO]+; found: 443.9457.
Synlett 2005, No. 2, 271–274 © Thieme Stuttgart · New York

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Y. Ding, J. R. Green
LETTER
Compound 1g: IR (neat, KBr): nmax = 3013, 2956, 2092,
2022, 1746 cm–1. 1H NMR: d = 2.07 (3 H, s), 4.73 (2 H, s),
6.62 (1 H, d, J = 11.0 Hz), 6.67 (1 H, d, J = 11.0 Hz), 6.98 (1
H, d, J = 4.3 Hz), 7.13 (1 H, s), 7.38 (1 H, s). 13C NMR: d =
199.3 (br), 170.5, 138.2, 128.3, 128.2, 126.8, 126.2, 123.4,
91.4, 83.3, 64.5, 20.3. MS (EI): m/z = 464 [M – CO]+, 436
[M – 2 CO]+, 408 [M – 3 CO]+, 380 [M – 4 CO]+, 352 [M –
5 CO]+, 324 [M – 6 CO]+. HRMS (EI): m/z calcd for
MS (EI): m/z = 456 [M]+, 428 [M – CO]+, 400 [M – 2 CO]+,
372 [M – 3 CO]+, 344 [M – 4 CO]+, 316 [M – 5 CO]+, 288
[M – 6 CO]+. HRMS (EI): m/z calcd for C18H10Co2O7:
455.9090; found: 455.9051.
Compound 2e: IR (neat, KBr): nmax = 3026, 2935, 2090,
2052, 2022, 1602, 1556 cm–1. 1H NMR: d = 3.88 (3 H, s),
5.35 (1 H, s), 6.77 (1 H, dd, J = 8.6, 2.6 Hz), 6.82 (1 H, s),
6.83 (1 H, m), 7.01 (1 H, d, J = 10.1 Hz), 7.09 (1 H, d, J = 8.5
Hz), 7.23 (1 H, m), 7.29 (2 H, m), 7.35 (2 H, m). 13C NMR:
d = 199.4 (br), 158.3, 144.4, 138.3, 133.5, 133.1, 131.3,
129.6, 128.6, 127.0, 118.4, 114.2, 107.1, 84.8, 55.4, 55.3.
MS (EI): m/z = 504 [M – CO]+, 476 [M – 2 CO]+, 448 [M –
3 CO]+, 420 [ M – 4 CO]+, 392 [M – 5 CO]+, 364 [M – 6
CO]+. HRMS (EI): m/z calcd for C24H14Co2O7: 503.9454
[M – CO]+; found: 503.9437.
Compound 2f: IR (neat, KBr): nmax = 3025, 2928, 2092,
2051, 2020. 1H NMR: d = 4.47 (2 H, s), 6.31 (1 H, s), 6.39
(1 H, d, J = 9.6 Hz), 6.57 (1 H, d, J = 9.6 Hz), 7.30 (1 H, s).
13C NMR: d = 199.3 (br), 148.8, 140.8, 125.1, 124.1, 120.5,
113.3, 92.2, 86.6, 34.0. MS (EI): m/z = 416 [M]+, 388 [M –
CO]+, 360 [M – 2 CO]+, 332 [M – 3 CO]+, 304 [M – 4 CO]+,
276 [M – 5 CO]+, 248 [M – 6 CO]+. HRMS (EI): m/z calcd
for C15H6Co2O7: 415.8777 [M]+; found: 415.8752.
Compound 2g: IR (neat, KBr): nmax = 2926, 2856, 2092,
2055, 2024, 1699, 1650, 1540 cm–1. 1H NMR: d = 4.45 (2 H,
s), 6.68 (1 H, AB quartet, J = 16.0 Hz), 6.74 (1 H, AB
quartet, J = 16.1 Hz), 6.88 (1 H, d, J = 8.7 Hz), 7.08 (1 H, d,
J = 8.8 Hz); resonances for minor regioisomer (2g¢) could be
observed (in CD3CN) at d = 4.34 (2 H, s), 6.66 (1 H, d,
J = 9.9 Hz), 6.80 (1 H, d, J = 9.7 Hz), 7.20 (1 H, s), 7.38 (1
H, s). 13C NMR: d = 199.1 (br), 136.8, 136.7, 132.0, 126.4,
126.2, 122.0, 97.6, 86.8, 34.6; resonances for the minor
regioisomer could be observed at d = 128.7, 126.9, 124.8,
35.5. MS (EI) m/z = 432 [M]+, 404 [M – CO]+, 376 [M – 2
CO]+, 348 [M – 3 CO]+, 320 [M – 4 CO]+, 292 [M – 5 CO]+,
264 [M – 6 CO]+. HRMS (EI): m/z calcd for C15H6Co2O6S:
403.8575 [M – CO]+; found: 403.8554.
Compound 2j: IR (neat, KBr): nmax = 3055, 2090, 2051,
2021, 1594 cm–1. 1H NMR: d = 3.78 (3 H, s), 3.83 (3 H, s),
3.91 (3 H, s), 4.05 (2 H, s), 6.62 (1 H, s), 6.93 (1 H, d,
J = 10.4 Hz), 7.12 (1 H, d, J = 10.4 Hz). 13C NMR: d =
199.4, 153.3, 152.8, 141.4, 134.3, 127.7, 125.9, 124.7,
108.4, 102.9, 87.7, 61.1, 61.0, 56.0, 41.1. MS (EI): m/z = 516
[M]+, 460 [M – 2 CO]+, 404 [M – 4 CO]+. HRMS: m/z calcd
for C20H14Co2O9: 403.9505 [M – 4 CO]+; found: 403.9492.
Compound 2k: IR (neat, KBr): nmax = 3014, 2957, 2360,
2091, 2052, 2022 cm–1. 1H NMR: d = 0.28 (9 H, s), 4.20 (2
H, s), 6.76 (1 H, d, J = 10.1 Hz), 6.94 (1 H, d, J = 10.1 Hz),
7.16 (1 H, d, J = 7.5 Hz) 7.36 (1 H, s), 7.37 (1 H, d, J = 6.2
Hz). 13C NMR: d = 199.4 (br), 142.0, 137.7, 136.3, 134.2,
133.2, 131.9, 131.7, 129.0, 102.4, 86.6, 40.9, –1.2. MS (EI)
m/z = 498 [M]+, 470 [M – CO]+, 442 [M – 2 CO]+, 414 [M –
3 CO]+, 386 [M – 4 CO]+, 358 [M – 5 CO]+, 330 [M – 6
CO]+. HRMS (EI): m/z calcd for C20H16Co2O6Si: 469.9431
[M – CO]+; found: 469.9434.
C17H10Co2O8S: 435.8862 [M – 2 CO]+; found: 435.8853.
(14) Experimental Procedure:
To a stirred ice cold solution of the propargyl acetate
complex (0.2 mmol) in CH2Cl2 (20 mL) was added BF3·OEt2
(85.2 mg, 0.60 mmol) in CH2Cl2 (4 mL) over 10 min. After
stirring at 0 °C for the time indicated in Table 2, sat.
NaHCO3 solution was added, and the mixture was subjected
to a conventional work up. Purification by flash chroma-
tography afforded sequentially the benzocycloheptenyne
complex and any recovered starting material.
(15) This reflects a tendency for a slightly lower preference for C-
2 reactivity in thiophenes relative to furans and perhaps the
known tendency for electron withdrawing C-3 groups to
deactivate C-2 to a greater degree than C-4. See: Taylor, R.
In The Chemistry of Heterocyclic Compounds, Part 2, Vol.
44; Gronowitz, S., Ed.; Wiley: New York, 1986, Chap. 1, 16.
(16) Brook, M. A. Silicon in Organic, Organometallic, and
Polymer Chemistry; Wiley and Sons: New York, 2000.
(17) Compound 2a: IR (neat, KBr): nmax = 2929, 2091, 2021,
1574 cm–1. 1H NMR: d = 4.18 (2 H, s), 6.77 (1 H, d, J = 10.1
Hz), 6.93 (1 H, d, J = 10.1 Hz), 7.22 (4 H, m). 13C NMR:
d = 199.3 (br), 137.5, 137.3, 133.1, 132.5, 129.4, 128.9,
128.8, 126.9, 102.3, 86.5, 40.8. MS (EI): m/z = 426 [M]+,
398 [M – CO]+, 370 [M – 2 CO]+, 342 [M – 3 CO]+, 314
[M – 4 CO]+, 286 [M – 5 CO]+, 258 [M – 6 CO]+. HRMS
(EI): m/z calcd for C17H8Co2O6: 426.9063 [M + H]+; found:
426.9069.
Compound 2b: IR (neat, KBr): nmax = 3014, 2858, 2090,
2053, 2021, 1557 cm–1. 1H NMR: d = 2.31 (3 H, s), 4.14 (2
H, s), 6.72 (1 H, d, J = 10.1 Hz), 6.90 (1 H, d, J = 10.1 Hz),
7.00 (1 H, s), 7.07 (1 H, d, J = 7.9 Hz), 7.15 (1 H, d, J = 7.6
Hz); resonances for minor regioisomer (2b¢) could be
observed at d = 2.51 (3 H, s), 4.10 (2 H, s), 6.82 (1 H, d,
J = 10.0 Hz), 6.93 (1 H, d, J = 10.0 Hz), 7.03 (1 H, m), 7.10
(1 H, m), 7.17 (1 H, m). 13C NMR: d = 199.4 (br), 137.3,
136.4, 134.5, 133.2, 129.6, 129.4, 128.5, 102.5, 86.9, 40.4,
20.7; resonances for the minor regioisomer could be
observed at d = 133.8, 130.7, 130.4, 128.8, 126.3, 34.2, 21.1.
MS (EI): m/z = 440 [M]+, 412 [M – CO]+, 384 [M – 2 CO]+,
356 [M+ – 3 CO], 328 [M – 4 CO]+, 300 [M – 5 CO]+, 272
[M – 6 CO]+. HRMS (EI): m/z calcd for C18H10Co2O6:
439.9141; found: 439.9137.
Compound 2c: IR (neat, KBr): nmax = 3014, 2956, 2837,
2090, 2050, 2020, 1604 cm–1. 1H NMR: d = 3.77 (3 H, s),
4.10 (2 H, s), 6.68 (1 H, d, J = 10.1 Hz), 6.71 (1 H, s), 6.79
(1 H, d, J = 8.3 Hz), 6.92 (1 H, d, J = 10.1 Hz), 7.16 (1 H, d,
J = 8.3 Hz). 13C NMR: d = 199.3 (br), 158.3, 138.6, 132.8,
130.5, 130.0, 129.3, 117.7, 113.9, 102.9, 86.6, 55.3, 39.9.
(18) Hosokawa, S.; Isobe, M. Tetrahedron Lett. 1998, 39, 2609.
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