Intramolecular dehydro Diels-Alder reactions of diarylacetylenes: Switching between benzo[b]- and benzo[c]fluorenones as products by controlling the rearrangement of cyclic allene intermediates

Article abstract of DOI:10.1021/jo0498213

Thermal cyclization of 1-[2-(arylethynyl)phenyl] -3-trimethylsilylpropynones affords a mixture of benzo[b]fluorenones and benzo[c]fluorenones. The ratio of the two isomers can be efficiently varied between 100:0 and 0:100 by introducing substituents with

Full text of DOI:10.1021/jo0498213

In tr a m olecu la r Deh yd r o Diels-Ald er Rea ction s of
Dia r yla cetylen es: Sw itch in g betw een Ben zo[b]- a n d
Ben zo[c]flu or en on es a s P r od u cts by Con tr ollin g th e
Rea r r a n gem en t of Cyclic Allen e In ter m ed ia tes
David Rodr´ıguez, Mar´ıa Fernanda Mart´ınez-Espero´n, Armando Navarro-Va´zquez,
Luis Castedo, Domingo Dom´ınguez, and Carlos Saa´*
Departamento de Quı´mica Orga´nica y Unidad Asociada al CSIC, Facultad de Qu´ımica,
Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain
qocsaa@usc.es
Received J anuary 30, 2004
Thermal cyclization of 1-[2-(arylethynyl)phenyl]-3-trimethylsilylpropynones affords a mixture of
benzo[b]fluorenones and benzo[c]fluorenones. The ratio of the two isomers can be efficiently varied
between 100:0 and 0:100 by introducing substituents with appropriate electronic and steric
properties on the aryl rings and using an appropriate solvent.
In tr od u ction
cyclization of 5a to the new cyclic allene 6a , and final
aromatizing isomerization of this allene (Scheme 1). With
1a , or analogues in which the terminal TMS is replaced
with other SiR3 groups,4,5b,7 the rearrangement product
7 is usually the minor product, but results with other
arylpropynones4 suggested to us that the yield of 7 might
increase with an electron-rich substituent in the aryl-
ethynyl moiety. In this paper, we report the synthesis of
several benzo[b]-, benzo[c]-, and dibenzo[b,g]fluorenones
in the course of a systematic study of how steric and
electronic effects of arylethynyl substitution influence the
2:6 ratio in the thermal cyclization of 1-[2-(arylethynyl)-
phenyl]-3-trimethylsilylpropynones 1b-o.8
Intramolecular dehydro Diels-Alder reactions between
alkynes and arenynes are well-known processes,1 some
having first been reported in the 19th century. The
intermediates in these reactions are 1,2,4-cyclohexatrienes,
strained cyclic allenes2 that normally evolve to aromatic
products by isomerization.3 Echavarren’s group4 and
ours5 have recently reported that isonaphthalenes (a class
of cyclic allenes) can undergo electrocyclic rearrangement
to isomeric isonaphthalenes via 1,2-dehydro[10]annulenes.
As a result, thermal cyclization of propynone 1a (R1 )
R2 ) R3 ) H), for example, followed by selective desily-
lation, affords a mixture of benzo[b]fluorenone 3a (R )
H), in which a 2-phenylnaphthalene skeleton arises from
direct aromatizing isomerization of the initial [4 + 2]
cyclization product 2a and benzo[c]fluorenone 7a ,6 in
which a 1-phenylnaphthalene skeleton arises through a
six-electron electrocyclic opening of 2a to (5Z,7Z)-1,2-
dehydro[10]annulene 4a , isomerization of the latter to
(5E,7E)-1,2-dehydro[10]annulene 5a by simultaneous
rotation of the C5-C6 and C7-C8 bonds, [1,6]-electro-
Resu lts a n d Discu ssion
Trimethylsilylpropynones 1b-e, in which the terminal
aryl ring has a single substituent in the para position,
were synthesized in two steps by Sonogashira coupling
of alkyne 89 or 910 with the appropriate aryl iodide (10b-
e)11 followed by oxidation of the propargylic alcohols with
activated MnO2 (Scheme 2).
Thermal cyclization, carried out by heating solutions
of 1a -e in freshly distilled toluene at 150 °C in an argon-
filled sealed tube, afforded inseparable mixtures of sily-
lated benzofluorenones 3′a -e (Scheme 1) and 7a -e,12 but
selective removal of the TMS group of 3′ by reaction with
(1) (a) Michael, A.; Bucher, J . E. Chem. Ber. 1895, 28, 2511-2512.
(b) Danheiser, R. L.; Gould, A.; Ferna´ndez de la Pradilla, R.; Helgason,
A. L. J . Org. Chem. 1994, 59, 5514-5515. (c) Burrell, R. C.; Daoust,
K. J .; Bradley, A. Z.; DiRico, K. J .; J ohnson, R. P. J . Am. Chem. Soc.
1996, 118, 4218-4219. (d) Schmittel, M.; Strittmatter, M.; Schemk,
W. A.; Hagel, M. Z. Naturforsch. 1998, 53b, 1015-1020.
(2) For a review of the reactivities and structures of cyclic allenes,
see: J ohnson, R. P. Chem. Rev. 1989, 89, 1111-1124.
(3) (a) Rodr´ıguez, D.; Navarro, A.; Castedo, L.; Dom´ınguez, D.; Saa´,
C. Org. Lett. 2000, 2, 1497-1500. (b) Rodr´ıguez, D.; Navarro-Va´zquez,
A.; Castedo, L.; Dom´ınguez, D.; Saa´, C. J . Org. Chem. 2003, 68, 1938-
1946.
(7) Tertiary alcohols gave only 3, and tert-butyl gave 3 and 7 in a
16:1 ratio.
(8) On electronic control in the Schmittel and Bergman cyclizations,
see: (a) Schmittel, M.; Maywald, M. Chem. Commun. 2001, 155-156.
(b) J ones, G. B.; Warner, P. M. J . Am. Chem. Soc. 2001, 123, 2134-
2145. (c) Choy, N.; Kim, C.-S.; Ballestero, C.; Artigas, L.; Diez, C.;
Lichtenberger, F.; Shapiro, J .; Russell, K. C. Tetrahedron Lett. 2000,
41, 6955-6958. Schmittel, M.; Kiau, S. Chem. Lett. 1995, 953-954.
(9) Bru¨ckner, R. Synlett 1994, 51-53; Liebigs Ann. 1996, C41, 447-
456 and 457-471.
(10) Chang, J .; Baik, J .; Lee, C.; Han, M.; Hong, S.-K. J . Am. Chem.
Soc. 1997, 119, 3197-3198.
(11) For synthesis of desilylated 10d , see: Rucker, M.; Bru¨ckner,
R. Tetrahedron Lett. 1997, 38, 7353-7356.
(4) Atienza, C.; Mateo, C.; de Frutos, OÄ .; Echavarren, A. M. Org.
Lett. 2001, 3, 153-155.
(5) (a) Rodr´ıguez, D.; Navarro-Va´zquez, A.; Castedo, L.; Dom´ınguez,
D.; Saa´, C. J . Am. Chem. Soc. 2001, 123, 9178-9179. (b) Rodr´ıguez,
D.; Navarro-Va´zquez, A.; Castedo, L.; Dom´ınguez, D.; Saa´, C. Tetra-
hedron Lett. 2002, 43, 2717-2720.
(6) Benzo[c]fluorene derivatives have been postulated as carcino-
genic metabolites: Wang, J .-Q.; Weyand, E. H.; Harvey, R. G. J . Org.
Chem. 2002, 67, 6216-6219.
10.1021/jo0498213 CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/24/2004
3842
J . Org. Chem. 2004, 69, 3842-3848

Intramolecular Dehydro Diels-Alder Reactions of Diarylacetylenes
SCHEME 1. In tr a m olecu la r Deh yd r o Diels-Ald er of P r op yn on es 1
SCHEME 2. Gen er a l Con d ition s for th e Syn th esis a n d Th er m olysis of P r op yn on es 1a -e
tetrabutylammonium fluoride solution in THF5b allowed
the isolation of silylated benzo[c]fluorenones 7a -e and
desilylated benzo[b]fluorenones 3a -e (Scheme 2 and
Table 1). Propynone 1a (R1 ) H) gave a 23% yield of 7a
and a 59% yield of 3a (entry 1).5b The 7:3 ratio was higher
with the electron-donating substituent Me of 1b (al-
though the total yield decreased; entry 2) and was further
increased by the methoxy group of 1c, confirming that it
tends to increase with the electron-donating capacity of
R1 (entry 3). Although the even more powerful electron
donor of 1d afforded only the linear benzo[b]fluorenone
3d (entry 4), this is thought likely to have been due to
competitive intermolecular protonation of position 5 of
the initial cyclic allene 2d by the acidic phenolic
proton.5a,13,14 Compound 1e, with its electron-withdrawing
nitro group, was smoothly converted to benzo[b]fluo-
(13) Relatively low yield obtained (30%) might be due to the
moderate thermal instability of 1d .
(14) Lithium, potassium, and sodium phenoxides of 1d were pre-
pared by treatment with LDA, K2CO3, and NaH, respectively, but
attempts at their cyclization all failed because they were thermally
unstable and decomposed.
(12) In the case of 1d (R1 ) OH) and 1e (R1 ) NO2), silylated
benzofluorenones 3′d (30%) and 3′e (80%) were the only products
obtained. Attempts to desilylate these compounds led to decomposition.
J . Org. Chem, Vol. 69, No. 11, 2004 3843

Rodr´ıguez et al.
SCHEME 3. Th er m a l Cycliza tion of P r op yn on es 1f-h
TABLE 1. Th er m a l Cycliza tion of Silylp r op yn on es 1a -l
benzo[b]- benzo[c]-
aryl ring fluorenone 3 fluorenone 7 ratio
entry propynone substituent
(%)
(%)a
of 3:7
1
2
3
4
5
6
7
8
9
10
11
12
13
1a
1b
1c
1d
1e
1f
1g
1h
1i
59
33
23
22
30
2.6:1
1.5:1
1:1
1:0
1:0
7.4:1
1:0
1:0
1:1.3
2.3:1
0:1
p-Me
p-OMe
p-OH
p-NO2
m-Me
m-OMe
m-OH
o-Me
31
30b
80b
52
F IGURE 1. Steric interactions in allenes 2i-l.
7
these latter being derived from cycloaddition at the more
crowded position 2 of the aromatic ring (Scheme 3). In
the case of 1h , the failure to obtain 7h is again probably
due to the acidic phenolic proton favoring aromatization
of the initial cyclic allene 2h . Phenoxide derivatives of
1h were thermally unstable and could not be cyclized.
As before, the deprotonation-aromatization of the initial
allene 2 is the most favored pathway irrespective of the
nature of the electron-releasing meta substituent in 1.
59c
44b,c
27
34
17
56
1j
o-OMe
39
1k
1k d
1l
o-tBu
o-tBu
52e
1:0
0:1
o-TMS
36
a
b
Isolated yields after desilylation. Yields of 3′d , 3′e, and 3′h ,
respectively. c 3g′ and 3h ′ were obtained in 8 and 6% yields,
respectively. Reaction carried out in Et3N as a solvent. e Yield
d
We then prepared the 1-[2-(arylethynyl)phenyl]-3-
trimethylsilylpropynones 1i-l to evaluate what were
expected to be the mainly steric effects of ortho substitu-
tion. The terminal trimethylsilyl group of compounds 1
had previously been reported to increase the amount of
rearranged product due to steric repulsion in allene 2
between the TMS and the peri hydrogen, and it was
hoped that ortho substitution on the arylethynyl ring
would further favor rearrangement due to repulsion
between the ortho group and the electron pair of the
central carbon of the allene (Figure 1).
Propynones 1i-l were synthesized following the same
procedure as described above (Scheme 4).16 Gratifyingly,
cyclization of the o-methyl derivative 1i in toluene at 150
°C, followed by selective desilylation, afforded a higher
yield of the rearranged benzo[c]fluorenone 7i than of the
benzo[b]fluorenone 3i, 34 vs 27%. However, in the case
of a better electron-releasing substituent, the o-methoxy
derivative 1j, the benzo[b]fluorenone was once again the
major product (39 vs 17% of the benzo[c]fluorenone 7j).
Unfortunately, analogues of 1 with more electron-rich or
electron-poor ortho susbstituents (R3 ) OH or NO2,
respectively) could not be cyclized due to thermal insta-
bility, but the steric origin of the tendency of 2i to
rearrange appeared to be confirmed when the tert-butyl
and trimethylsilyl analogues 1k and 1l afforded only the
rearranged benzo[c]fluorenones 7k and 7l18 (in 56 and
36% yields, respectively); this is the first time that only
the rearranged products have been obtained due, most
probably, to the steric hindrance for protonation of the
corresponding allenes 2 (Scheme 4). Remarkably, how-
of 3′k .
renone 3e without any production of 7e. From the results,
it seems evident that electronic effects in the reacting
aromatic ring affect largely the regioselectivity of the
reaction since the fate of the first-formed allenic cycload-
duct 2 depends on the nature of the substituent at the
terminal phenyl ring as well as on the nature of the
reaction medium.15 Specifically, the proton at the junction
of rings C and D of 2 is somewhat acidic (the C-H bond
is conjugated with the carbonyl group and with a dienic
system). Electron-withdrawing substituents would favor
a fast deprotonation and the consequent aromatization
of rings C and D to give 3. Electron-releasing substituents
would retard the deprotonation process, making competi-
tive the thermal electrocyclic opening of the C and D
rings to 4 and its isomerization to 5 (this is favored also
by a substantial decrease of strain). Finally, the latter
electrocyclizes to give the more stable allenic adduct 6
from which 7 is formed by the deprotonation-aromati-
zation process.
We next prepared substrates 1f-h , which have meta-
substituted terminal rings.16 The major isolated products
of their cyclization were benzo[b]fluorenones 3f,g, which
were obtained in 52 and 59%, respectively, and 3′h ,17
obtained in 44% yield (Scheme 3). The minor products
were the rearranged benzo[c]fluorenone 7f (7%) and the
benzo[b]fluorenones 3g′ and 3h ′ (8 and 6%, respectively),
(15) Thermolysis of 1a in the presence of triethylamine gives 3a
quantitatively; see ref 5b.
(16) Compounds 1f-o were all prepared in good yields by procedures
similar to the one depicted in Scheme 2. See Supporting Information
for details.
(18) Campo, M. A.; Larock, R. C. J . Org. Chem. 2002, 67, 5616-
5620.
(17) No desilylation step was necessary in this case.
3844 J . Org. Chem., Vol. 69, No. 11, 2004

Intramolecular Dehydro Diels-Alder Reactions of Diarylacetylenes
SCHEME 4. Neu tr a l a n d Ba sic Th er m a l Cycliza tion s of P r op yn on es 1i-l
SCHEME 5. Neu tr a l a n d Ba sic Th er m a l Cycliza tion of P r op yn on e 1m
ever, when the thermolysis of 1k was carried out in
triethylamine as a solvent instead of toluene,5b the only
product isolated was benzo[b]fluorenone 3′k (52%), doubt-
less due to the acidic nature of the proton at the junction
of rings C and D of 2 and the basic nature of triethy-
lamine. Thus, endowing the reacting phenyl group with
bulky ortho substituents subject to steric interaction with
the lone pair of the cyclic allene intermediate seems to
allow either the benzo[b]- or the benzo[c]fluorenone
skeleton to be obtained selectively by just using the
appropriate solvent.
To test the effects of two substituents on the reacting
aryl ring, we first synthesized the diarylacetylene 1m
(Scheme 5) in the hope that the combined effects of the
sterically demanding ortho susbstituent and the electron-
donating p-methoxy group might shift the 3:7 ratio
toward 7 more than the o-Me group by itself. However,
the reaction gave a relatively low global yield of 7 and 3
in 1:1.5 ratio, indicating that the expected combination
of effects had not occurred. Thermolysis of diarylacetylene
1n , in which the terminal aryl ring has o- and p-methoxy
susbstituents, gave similar results (no desilylation is
needed for separation of both isomers, Scheme 6). Dia-
rylacetylene 1o, in which the substituents are two
methyls, afforded an excellent yield, but the 7:3 ratio,
1.4:1, hardly improved on that obtained with the single
methyl of 1i (entry 8 in Table 1). Thermolysis of diary-
lacetylenes 1n -o in Et3N gave the expected benzo[b]-
fluorenones 3′n -o in moderate to excellent yields.3
1,1′-Binaphthalene derivatives such as (S)-BINOL 1119
occupy a prominent position among C2-symmetric chiral
auxiliaries and ligands for asymmetric synthesis.20 In an
J . Org. Chem, Vol. 69, No. 11, 2004 3845

Rodr´ıguez et al.
SCHEME 6. Neu tr a l a n d Ba sic Th er m a l Cycliza tion s of P r op yn on es 1n -o
SCHEME 7. Hyp oth etica l Mech a n istic Cou r se for th e Th er m olysis of Na p h th yl P h en yla cetylen e 12
SCHEME 8. Th er m a l Cycliza tion of P r op yn on es 12a -c
attempt to apply the methods described above to the
synthesis of 1,1′-binaphthalene 15, we subjected naphthyl
phenylacetylene 12 to thermolysis under neutral condi-
tions, hoping that rearrangement of the initial allene 13
to 14 would lead to the desired product (Scheme 7).
Somewhat surprisingly, heating a solution of 12a in
toluene at 150 °C gave exclusively the silylated dibenzo-
[b,g]fluoren-7-one 16′a in an excellent 92% yield (Scheme
8), being a novel route to access this interesting aromatic
nucleus21 (desilylation of 16′a then gave the parent
(19) For a review of BINOL derivatives, see: Chen, Y.; Yekta, S.;
Yudin, A. K. Chem. Rev. 2003, 103, 3155-3211.
(20) For a review of C2 symmetry in asymmetric synthesis, see:
Whitesell, J . K. Chem. Rev. 1989, 89, 1581.
(21) For old methods based on cyclodehydration of â-aryl ketones,
see: (a) Colonge, J .; Weinstein, G. Bull. Soc. Chim. Fr. 1951, 961-
965. (b) Lambert, P.; Martin, R. H. Bull. Soc. Chim. Belg. 1952, 61,
132-138. (c) Martin, R. H. Helv. Chim. Acta 1947, 30, 620-627.
3846 J . Org. Chem., Vol. 69, No. 11, 2004

Intramolecular Dehydro Diels-Alder Reactions of Diarylacetylenes
SCHEME 9. Retr osyn th etic An a lysis of Ca r cin ogen ic Ben zo[c]flu or en e Diep oxid es 18 a n d 19 fr om P h en ol
22
SCHEME 10. Attem p ted Rou te to Ca r cin ogen ic Ben zo[c]flu or en e Meta bolites fr om Ben zo[c]flu or en ic
P h en ol 24
member of the series, 16a , in 95% yield).21c Allene 13
must therefore aromatize much more readily than it
rearranges to the cyclic allene 14. Attempts to invert this
preference by the same means as had proved successful
for allene 2, i.e., by furnishing the terminal aryl ring of
the starting compound with substituents with appropri-
ate electronic and steric properties, were in this case
unsuccessful: 12b gave only a 35% yield of 16′b, and 12c
underwent slow decomposition. Thermolysis of naphth-
ylphenylpropynone 17 under the same conditions gave
only unaltered starting material (Scheme 8).
Harvey et al. have recently described the synthesis of
the diepoxides 18 and 19, which are suspected of being
the ultimate carcinogenic metabolites of 7H-benzo[c]-
fluorene (BcF), a coal tar component implicated in the
causation of lung tumors.6 These metabolites can be
obtained by epoxidation of diol 20, which is in turn
derived from phenol 22 in two steps via the o-quinone
intermediate 21 (Scheme 9).
We wondered whether the previously synthesized
benzo[c]fluorenone 7c might also be convertible into the
o-quinone intermediate 21. However, although desilyla-
tion of 7c followed by hydrogenation over a palladium-
charcoal catalyst gave 23 in quantitative yield, and 23
was smoothly converted to phenol 24 by heating with 48%
HBr in acetic acid (Scheme 10), oxidation of 24 with
Fremy’s salt [(SO3K)2NO] gave only a complex mixture
of products, probably due to the position para to the -OH
group being substitutable.22
In summary, a theoretical and experimental study of
the cyclization of compounds 1 shows that there is a
relationship between the electron-richness of the reacting
aryl group (determined by its para substituent) and the
extent of rearrangement of allene 2 to allene 6. Meta
substituents, whatever their electronic nature, favor the
aromatization of 2 to benzo[b]fluorenones 3. Substitution
in the ortho position allows the choice of solvent to
determine whether rearranged or unrearranged products
are obtained. Dibenzo[b,g]fluoren-7-ones can be obtained
efficiently by thermolysis of naphthylarylacetylenes in
neutral conditions.
Exp er im en ta l Section
Th er m a l Cycliza tion s: Gen er a l P r oced u r e. A toluene
or toluene/Et3N (8:0.5) solution of the substrate (c ) 20-65
mM) was placed in a sealed tube and heated overnight at 150
°C in a silicon oil bath. After evaporation of the solvent, the
crude material was purified by column chromatography on
silica gel using a mixture of hexanes/EtOAc (typically, 6:1) as
the eluent.
8-Meth ylben zo[b]flu or en -11-on e 3b: yellow prisms; mp
144-146 °C; 1H NMR (CDCl3) δ 8.08 (s, 1H, ArH), 7.82 (s, 1H,
ArH), 7.76-7.73 (m, 4H, ArH), 7.55 (td, J ) 7.5, 1.3 Hz, 1H,
ArH), 7.38 (dd, J ) 8.3, 1.6 Hz, 1H, ArH), 7.32 (td, J ) 7.6,
1.0 Hz, 1H, ArH), 2.50 (s, 3H, CH3); 13C NMR/DEPT (CDCl3)
δ 193.2 (CO), 144.9 (C), 137.5 (C), 136.8 (C), 136.0 (C), 134.9
(C), 134.9 (CH), 133.7 (C), 132.7 (C), 131.0 (CH), 129.9 (CH),
128.8 (CH), 128.5 (CH), 125.0 (CH), 124.3 (CH), 120.7 (CH),
118.8 (CH), 21.5 (CH3); MS (70 eV) m/z (%) 244 (M+, 100), 215
(41).
7-Meth ylben zo[b]flu or en -11-on e 3f: yellow prisms; mp
150-152 °C; 1H NMR (CDCl3) δ 8.11 (s, 1H, ArH), 7.88-7.65
(m, 4H, ArH), 7.58 (s, 1H, ArH), 7.54 (td, J ) 7.5, 0.9 Hz, 1H,
ArH), 7.36-7.26 (m, 2H, ArH), 2.51 (s, 3H, CH3); 13C NMR/
DEPT (CDCl3) δ 193.0 (CO), 144.7 (C), 139.2 (C), 138.3 (C),
137.0 (C), 136.1 (C), 134.7 (CH), 131.9 (C), 131.6 (C), 130.4
(CH), 128.9 (2xCH), 128.0 (CH), 125.3 (CH), 124.2 (CH), 120.8
(CH), 118.3 (CH), 21.8 (CH3); MS (70 eV) m/z (%) 244 (M+,
100), 215 (56).
6-Meth ylben zo[b]flu or en -11-on e 3i: yellow prisms; mp
154-155 °C; 1H NMR (CDCl3) δ 8.10 (s, 1H, ArH), 7.95 (s, 1H,
ArH), 7.75-7.66 (m, 3H, ArH), 7.52 (t, J ) 7.3 Hz, 1H, ArH),
7.39-7.22 (m, 3H, ArH), 2.67 (s, 3H, CH3); 13C NMR/DEPT
(CDCl3) δ 193.2 (CO), 145.0 (C), 138.2 (C), 136.1 (C), 135.9 (C),
135.2 (C), 134.9 (CH), 133.7 (C), 132.2 (C), 130.0 (CH), 129.2
(CH), 129.1 (CH), 126.6 (CH), 126.1 (CH), 124.4 (CH), 120.8
(22) Martin Owton, W. J . Chem. Soc., Perkin Trans. 1999, 2409-
2420.
J . Org. Chem, Vol. 69, No. 11, 2004 3847

Rodr´ıguez et al.
(CH), 115.3 (CH), 19.6 (CH3); MS (70 eV) m/z (%) 244 (M+,
100), 243 (25), 215 (46).
(t, J ) 7.6 Hz, 1H, ArH), 7.36 (d, J ) 6.7 Hz, 1H, ArH), 7.25
(t, J ) 7.4 Hz, 1H, ArH), 2.55 (s, 3H, CH3), 0.44 (s, 9H, Si-
(CH3)3); 13C NMR/DEPT (CDCl3) δ 195.9 (CO), 145.1 (C), 142.3
(C), 137.9 (C), 136.8 (CH), 135.9 (C), 135.1 (C), 134.2 (CH),
134.1 (C), 133.8 (C), 130.5 (CH), 129.5 (CH), 129.3 (C), 128.3
(CH), 123.5 (CH), 123.4 (CH), 123.1 (CH), 22.2 (CH3), -0.8 (Si-
(CH3)3); MS (70 eV) m/z (%) 316 (M+, 7), 302 (30), 301 (100).
4-Met h yl-6-(t r im et h ylsilyl)b en zo[c]flu or en -7-on e 7b :
orange prisms; mp 165-167 °C; 1H NMR (CDCl3) δ 8.40 (d, J
) 8.1 Hz, 1H, ArH), 8.16 (s, 1H, ArH), 8.05 (d, J ) 7.6 Hz,
1H, ArH), 7.68-7.63 (m, 1H, ArH), 7.56-7.40 (m, 3H, ArH),
7.30 (t, J ) 7.4 Hz, 1H, ArH), 2.74 (s, 3H, CH3), 0.45 (s, 9H,
Si(CH3)3); 13C NMR/DEPT (CDCl3) δ 195.9 (CO), 145.2 (C),
143.4 (C), 136.0 (C), 135.8 (C), 135.6 (C), 134.6 (C), 134.3 (CH),
134.2 (C), 132.8 (CH), 129.4 (C), 129.2 (CH), 128.5 (CH), 127.7
(CH), 123.7 (CH), 123.4 (CH), 122.8 (CH), 20.1 (CH3), -0.9 (Si-
(CH3)3); MS (70 eV) m/z (%) 316 (M+, 10), 302 (33), 301 (100).
3-Meth yl-6-(tr im eth ylsilyl)ben zo[c]flu or en -7-on e 7f: or-
ange prisms; mp 134-136 °C; 1H NMR (CDCl3) δ 8.39 (d, J )
8.5 Hz, 1H, ArH), 8.01 (d, J ) 7.9 Hz, 1H, ArH), 7.85 (s, 1H,
ArH), 7.69-7.62 (m, 2H, ArH), 7.55-7.42 (m, 2H, ArH), 7.34-
7.25 (m, 1H, ArH), 2.54 (s, 3H, CH3), 0.43 (s, 9H, Si(CH3)3);
13C NMR/DEPT (CDCl3) δ 195.8 (CO), 145.0 (C), 143.1 (C),
138.5 (C), 137.1 (C), 136.3 (CH), 135.2 (C), 135.1 (C), 134.3
(C), 134.2 (C), 130.3 (CH), 128.7 (CH), 128.5 (CH), 127.3 (CH),
124.3 (CH), 123.6 (CH), 123.1 (CH), 21.8 (CH3), -0.9 (Si(CH3)3);
MS (70 eV) m/z (%) 316 (M+, 7), 301 (100).
Ack n ow led gm en t. We thank the Ministerio de
Ciencia y Tecnolog´ıa (Spain) and the European Regional
Development Fund for financial support under Project
BQU2002-02135. D. Rodr´ıguez and F. Mart´ınez-Espero´n
thank the Universidad de Santiago de Compostela
(Spain) for postdoctoral and predoctoral grants, respec-
tively.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and characterization of all new compounds, copies of
1H NMR and 13C NMR for all new compounds, computational
details and geometries for all computed structures, a table of
absolute energies, <S2> expectation values, and a figure with
the structures of 2a and 6a superimposed. This material is
available free of charge via the Internet at http://pubs.acs.org.
2-Meth yl-6-(tr im eth ylsilyl)ben zo[c]flu or en -7-on e 7i: or-
ange prisms; mp 172-174 °C; 1H NMR (CDCl3) δ 8.16 (s, 1H,
ArH), 7.97 (d, J ) 7.4 Hz, 1H, ArH), 7.85 (s, 1H, ArH), 7.73
(d, J ) 7.7 Hz, 1H, ArH), 7.60 (d, J ) 7.2 Hz, 1H, ArH), 7.47
J O0498213
3848 J . Org. Chem., Vol. 69, No. 11, 2004