A new rearrangement of cyclic allenes via 1,2-dehydro[10]annulenes: Formation of benzo[c]fluorenones

Article abstract of DOI:10.1016/S0040-4039(02)00368-4

A new rearrangement of cyclic allenes giving benzo[c]fluorenones was observed when studying intramolecular dehydro Diels-Alder reactions of silyl-substituted alkynes with arenynes (aryldiacetylenes).

Full text of DOI:10.1016/S0040-4039(02)00368-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 2717–2720
A new rearrangement of cyclic allenes via
1,2-dehydro[10]annulenes: formation of benzo[c]fluorenones
David Rodr´ıguez, Armando Navarro-Va´zquez, Luis Castedo, Domingo Dom´ınguez* and
Carlos Saa´*
Departamento de Qu´ımica Orga´nica y Unidad Asociada al CSIC, Facultad de Qu´ımica, Universidad de Santiago de Compostela,
15782 Santiago de Compostela, Spain
Received 24 January 2001; revised 18 February 2002; accepted 22 February 2002
Abstract—A new rearrangement of cyclic allenes giving benzo[c]fluorenones was observed when studying intramolecular dehydro
Diels–Alder reactions of silyl-substituted alkynes with arenynes (aryldiacetylenes). © 2002 Elsevier Science Ltd. All rights
reserved.
We1,2 have previously shown that the intramolecular
dehydro Diels–Alder reaction3 of alkynes with arenynes
(aryldiacetylenes) provides an interesting new route to
the tetracyclic nucleus of the benzo[b]fluorene anti-
biotics.4 Its key step is the [4+2] cycloaddition of an
alkyne unit linked to an arylacetylene, which involves
the formation of a strained cyclic allene intermediate.2
dehydro[10]annulene to cyclic allene 4, which finally
rearranges to 6.6 Here we report the results of an
investigation of whether the intramolecular dehydro
Diels–Alder reactions of the closely related aryldi-
acetylenes 8,7 which in our earlier work2 had given
benzo[b]fluorenones 9 almost exclusively, would also
give isomers 10 if reacted under appropriate conditions.
In our previous work,2 thermolysis of 8a in commercial
grade toluene gave benzo[b]fluorenone 9a in 85% yield
together with tiny amounts of an unidentified product.
When we heated a solution of 8a in dry, freshly distilled
benzene at 150°C for 12 h, an inseparable mixture of
Since then it has been found in work using starting
aryldiacetylenes 15 and 26 that the expected aromatic
product 5 may be accompanied by variable amounts of
its isomer 6 (Scheme 1), possibly because the initial
strained cyclic allene intermediate 3 isomerizes via 1,2-
Scheme 1.
Keywords: dehydro Diels–Alder reaction; cyclic allenes; 1,2-dehydro[10]annulenes; benzo[b]fluorenones; benzo[c]fluorenones.
* Corresponding author. Fax: +34-981-595012; e-mail: qocsaa@usc.es
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00368-4

2718
D. Rodr´ıguez et al. / Tetrahedron Letters 43 (2002) 2717–2720
two silylated compounds in 2.2:1 ratio was obtained.
Upon removal of the TMS group by reaction with
tetrabutylammonium fluoride solution in THF at room
temperature for a short time,10 only one of the isomers
reacted, allowing isolation of the desilylated ben-
zo[b]fluorenone 9e2 and the benzo[c]fluorenone 10a11,12
in 61% and 28% yields, respectively (Scheme 2). The
structure of 10a was unambiguously established by an
X-ray analysis.13
little 10b but also cyclized very slowly (suggesting that
R had to be a silyl group); the TES derivative 8c gave
more or less the same 2:1 ratio of 9 to 10 as 8a; and the
sterically more compromised TIPS derivative 8d failed
to cyclize.15
Next we investigated the role of the solvent in this type
of reaction, an aspect to which little attention had
hitherto been paid. First we found that the use of
toluene instead of benzene caused little difference in the
results (Table 1, entries 1 and 2), showing that the
presence of possible radical hydrogen donors plays at
most a minor role in the final aromatization step. Then
we used a variety of solvents capable of interfering with
the allene intermediates. With triethylamine, the ben-
zo[b]fluorenone was isolated as the only product in
quantitative yield and extremely high purity (entry 3);
this is attributed to its acting as a catalytic base for the
aromatization step 11“9, thus precluding both the
opening of 11 to the annulene 12 and any side reactions
of the allene.
When the rather unstable aryldiacetylene 8e (R=H),
obtained by desilylation of 8a (aqueous borax, MeOH,
rt),2 was heated under the same conditions as 8a (ben-
zene, 150°C, 12 h), it gave only the expected benzo-
[b]fluorenone 9e in 70% yield (Table 1, entry 10),
suggesting that the presence of a bulky terminal sub-
stituent on the alkyne was necessary for rearrangement
of the cyclic allene intermediate 11 to the ‘‘angular’’
cyclic allene 14 via 1,2-dehydro[10]annulenes 12 and 13
(Scheme 3);14 the dependence of this process on the
R-group of 8 would be in keeping with our previous
results with silyl-substituted phenylpropiolamides 2.6 In
an attempt to increase the yield of 10, we therefore
carried out experiments with 8b–d,7 which have differ-
ent bulky groups on the alkyne terminus (Table 1).
However, the t-butyl derivative 8b not only gave very
With the protic solvent iPrOH there was again a good
yield of 9 but no 10 (entry 4); probably because it
allows aromatization to 9 by proton transfer.2 When
Scheme 2.
Table 1. Thermolysis of aryldiacetylenes 8a
Entry
Starting material
Solvent
Benzo[b]fluorenone (% yield)
Benzo[c]fluorenone (% yield)
Ratio 9:10
1
2
3
4
5
6
7
8
9
8a
8a
8a
8a
8a
8a
8b
8c
8d
8e
Benzene
Toluene
Et3N
iPrOH
CH3CN
DMF
Benzene
Benzene
Benzene
Benzene
9e (61)b
9e (59)b
9a (100)
9a (73)
9a (15), 9e (31)
9a (8), 9e (15)
9bc
10a (28)b
2.2:1
2.6:1
1:0
10a (23)b
–
–
1:0
10a (1)
15:1
1:0
–
10bc
10ce
–
16:1d
2:1d
–
9ce
–
10
9e (70)
–
1:0
a All experiments were carried out using freshly distilled solvents and heating at 150°C in a sealed tube overnight.
b Yields after desilylation of the crude mixture (9a+10a).
c Inseparable mixture 9b+10b, yield 88%.
d By integration of the 1H NMR signals of the inseparable mixture of regioisomers.
e Inseparable mixture 9c+10c, yield 86%.

D. Rodr´ıguez et al. / Tetrahedron Letters 43 (2002) 2717–2720
2719
Scheme 3.
polar aprotic solvents were used, yields were lower
(entries 5 and 6) because the starting material became
thermally unstable due to partial desilylation. We con-
clude that aromatic solvents are the most appropriate
for obtaining significant yields of 10.16
2. Rodr´ıguez, D.; Navarro, A.; Castedo, L.; Dom´ınguez,
D.; Saa´, C. Org. Lett. 2000, 2, 1497–1500.
3. (a) Danheiser, R. L.; Gould, A. E.; Ferna´ndez de la
Pradilla, R.; Helgason, A. L. J. Org. Chem. 1994, 59,
5514–5515; (b) Burrell, R. C.; Daoust, K. J.; Bradley, A.
Z.; DiRico, K. J.; Johnson, R. P. J. Am. Chem. Soc.
1996, 118, 4218–4219; (c) Gonza´lez, J. J.; Francesch, A.;
Ca´rdenas, D. J.; Echavarren, A. M. J. Org. Chem. 1998,
63, 2854–2857.
4. Schmittel, M.; Strittmatter, M.; Schenk, W. A.; Hagel,
M. Z. Naturforsch. 1998, 53b, 1015–1020.
5. Atienza, C.; Mateo, C.; de Frutos, O.; Echavarren, A. M.
Summing up, a study of the intramolecular [4+2]
cycloaddition of aryldiacetylenes 8 in various solvents
shows that in aromatic solvents the major benzo-
[b]fluorenone products of the silyl derivatives 8a and 8c
are accompanied by significant yields of the corre-
sponding benzo[c]fluorenones. This is probably due to
the silyl substituted cyclic allene intermediate 11 (R=
Org. Lett. 2001, 3, 153–155.
TMS or TES) isomerizing via
a
1,2-dehydro-
6. Rodr´ıguez, D.; Navarro-Va´zquez, A.; Castedo, L.;
Dom´ınguez, D.; Saa´, C. J. Am. Chem. Soc. 2001, 123,
9178–9179.
7. Compounds 8 were prepared from o-phenylethynylbenz-
aldehyde (7) (obtained in 96% yield by Sonogashira
coupling of o-bromobenzaldehyde and phenylacetylene)8
by treatment with the corresponding substituted
acetylides followed by oxidation with activated man-
ganese oxide.9
[10]annulene to the ‘‘angular’’ cyclic allene 14, which
then undergoes aromatization. Unsubstituted and non-
silylated derivatives (8e and 8b) gave negligible amounts
of rearranged products 10.
Acknowledgements
8. Sonogashira, K. In Comprehensive Organic Synthesis;
Trost, B. M.; Fleming, I., Eds.; Pergamon: Oxford, 1991;
Vol. 3, p. 521.
9. All new compounds were fully characterized spectroscop-
ically and had satisfactory elemental analyses or HRMS
data.
We thank the Direccio´n General de Ensen˜anza Supe-
rior (Spain) for financial support under Project PB-
980606, and the Ministerio de Educacio´n y Ciencia for
a predoctoral grant to D. Rodr´ıguez.
10. Toyota, M.; Terashima, S. Tetrahedron Lett. 1989, 30,
829–832.
References
11. Benzo[c]fluorene has recently been reported to be car-
cinogenic: Koganti, A.; Singh, R.; Rozett, K.; Modi, N.;
Goldstein, L. S.; Roy, T. A.; Zhang, F.-J.; Harvey, R. G.;
Weyand, E. H. Carcinogenesis 2000, 21, 1601–1609.
1. Rodr´ıguez, D.; Castedo, L.; Dom´ınguez, D.; Saa´, C.
Tetrahedron Lett. 1999, 40, 7701–7704.

2720
D. Rodr´ıguez et al. / Tetrahedron Letters 43 (2002) 2717–2720
12. Benzo[c]fluorenone 10a: mp 171–172°C (EtOAc–hexane);
1H NMR (CDCl3, 250 MHz) l: 8.52 (d, J=9.0 Hz, 1H),
8.05 (d, J=7.5 Hz, 1H), 7.95 (s, 1H), 7.90 (dd, J=7.5, 2.3
Hz, 1H), 7.50–7.68 (m, 4H), 7.31 (t, J=7.5 Hz, 1H), 0.45
(s, 9H). 13C NMR+DEPT (CDCl3, 62.8 MHz) l: 195.8
(CO), 145.0 (C), 143.0 (C), 137.0 (CH), 136.7 (C), 135.8 (C),
134.9 (C), 134.8 (CH), 134.0 (C), 129.7 (CH), 129.1 (C),
128.5 (CH), 128.2 (CH), 128.0 (CH), 124.4 (CH), 123.6
(CH), 123.2 (CH), −0.9 (3×CH3). MS m/z (rel. intensity%):
302 (M+, 30), 288 (63), 287 (100), 257 (76); Anal. calcd (%)
for C20H18OSi: C, 79.42; H, 6.00; found: C, 79.03; H, 6.03.
13. Details of the structure of 10a and its X-ray crystallo-
graphic determination have been deposited at the Cam-
14. Rearrangements of arylcarbenes via cyclic allenes have
been observed. Xie, Y.; Schreiner, P. R.; Schleyer, P. v. R.;
Schaefer, H. F., III J. Am. Chem. Soc. 1997, 119, 1370–
1377. See also: Bonvallet, P. A.; McMahon, R. J. J. Am.
Chem. Soc. 2000, 122, 9332–9333.
1
15. Diagnostic H NMR signals to distinguish between ben-
zo[b]fluorenones 9 and benzo[c]fluorenones 10 are one
doublet at 8.4–8.5 ppm (d, J:8.0 Hz, H-1) for ben-
zo[b]fluoren-11-ones 9 and two doublets at 8.0–8.1 ppm (d,
J:7.5 Hz, H-1) and 8.5–8.6 ppm (d, J:8 Hz, H-8) for
benzo[c]fluoren-7-ones 10. See numbering on figures of
Scheme 3.
16. In supplementary experiments, various concentrations of
8a ranging from 20 to 100 mM gave virtually the same ratio
of 9 to 10.
bridge
Crystallographic
Data
Centre
as
no.
CCDC–178032.