Generation and trapping of a highly strained bicyclic alkyne: Tricyclo[6.3.1.02,7]dodeca-2,4,6-trien-9-yne

Article abstract of DOI:10.1021/jo0017537

High-temperature bromination of 10-bromotricyclo[6.3.1.0^2,7] dodeca-2,4,6,9-tetraene (8) resulted mainly in the formation of two isomeric tribromides (16 and 17) whose dehydrobromination with DBU afforded the corresponding 9,10-dibromotricyclo [6.3.1.0^2,7] dodeca-2,4,6,9-tetraene (14). Treatment of 14 with tert-butyllithium in THF at -78 °C produces the strained bicyclic alkyne (10) which is trapped by 1,3-diphenylisobenzofuran to give two isomeric cycloadducts 12a and 12b. The adducts 12a and 12b were found to readily rearrange to the isomeric ketones 19a and 19b upon chromatography. Upon treatment with potassium tert-butoxide, the adducts 12a and 12b undergo base-catalyzed double bond isomerization to give four products 20-23. Furthermore, reaction of 14 with one and two mole of potassium tert-butoxide has been studied and the mechanism of the product formation is discussed with regard to either an allene or alkyne as a possible intermediate.

Full text of DOI:10.1021/jo0017537

3806
J . Org. Chem. 2001, 66, 3806-3810
Gen er a tion a n d Tr a p p in g of a High ly Str a in ed Bicyclic Alk yn e:
Tr icyclo[6.3.1.02,7]d od eca -2,4,6-tr ien -9-yn e
Ferhan Tu¨mer,† Yavuz Taskesenligil,‡ and Metin Balci*,§
Department of Chemistry, Atatu¨rk University, 25240 Erzurum, Turkey, Department of Chemistry,
K. K. Faculty of Education, Atatu¨rk University, 25240 Erzurum, Turkey, and Department of Chemistry,
Middle East Technical University, 06531 Ankara, Turkey
mbalci@metu.edu.tr
Received December 18, 2000
High-temperature bromination of 10-bromotricyclo[6.3.1.02,7]dodeca-2,4,6,9-tetraene (8) resulted
mainly in the formation of two isomeric tribromides (16 and 17) whose dehydrobromination with
DBU afforded the corresponding 9,10-dibromotricyclo [6.3.1.02,7] dodeca-2,4,6,9-tetraene (14).
Treatment of 14 with tert-butyllithium in THF at -78 °C produces the strained bicyclic alkyne
(10) which is trapped by 1,3-diphenylisobenzofuran to give two isomeric cycloadducts 12a and 12b.
The adducts 12a and 12b were found to readily rearrange to the isomeric ketones 19a and 19b
upon chromatography. Upon treatment with potassium tert-butoxide, the adducts 12a and 12b
undergo base-catalyzed double bond isomerization to give four products 20-23. Furthermore,
reaction of 14 with one and two mole of potassium tert-butoxide has been studied and the mechanism
of the product formation is discussed with regard to either an allene or alkyne as a possible
intermediate.
Sch em e 1
In tr od u ction
An intriguing part of cyclic hydrocarbon chemistry is
the synthesis and study of highly strained ring systems.
Small-ring cyclic alkynes are of considerable interest in
chemistry because of their high strain and reactivity.1
While cyclononyne and its larger ring homologues are
stable and isolable compounds, cycloheptyne, cyclohex-
yne, and cyclopentyne are unstable and exist only as
short-lived intermediates. Intermediacy of these strained
ring systems is based exclusively on the observation of
trapping products. However, strained bicyclic alkynes
have also been reported. Bicyclo[2.2.1]hept-2-yne (1)2 and
bicyclo[2.2.2]oct-2-yne (2)3 were generated from the cor-
responding 2,3-dibromo compounds as reactive inter-
mediates and trapped with appropriate dienes. Quite
recently, a highly strained bicyclic alkyne, bicyclo[2.2.1]-
hept-2-en-5-yne (3), was generated by reaction of a new
hypervalent iodine precursor and trapped with 1,3-
diphenylisobenzofuran (DPIBF).4
Some years ago, Mohanakrishnan et al.5 reported that
the base-induced (potassium tert-butoxide in DMSO)
dehydrobromination of 46 gives the highly strained
bicyclic allene 5, which was trapped as either enol ether
6 or as its [2 + 2] cycloaddition product 7 (Scheme 1).
An alternate mechanism for the formation of these
products was not discussed.
In a previous paper,7 we initially also proposed the
highly strained bicyclic allene 9 as an intermediate in
the base-induced elimination of HBr from 8, which gives
allene-like cycloadducts 11 in the presence of 1,3-diphen-
ylisobenzofuran (DPIBF) as a trapping agent. However,
as noticed in the same paper, these results also were in
agreement with an alternative mechanism for the forma-
tion of cycloadducts 11. According to this mechanism, the
† Atatu¨rk University, Faculty of Art and Sciences.
‡ Atatu¨rk University, K. K. Faculty of Education.
§ Middle East Technical University.
(3) Komatsu, K.; Aonuma, S.; J inbu, Y.; Tsuji, R.; Hirosawa, C.;
Takeuchi, K. J . Org. Chem. 1991, 56, 195-203.
(4) Kitamura, T.; Kotani, M.; Yokoyama, T.; Fujiwara, Y.; Hori, K.
J . Org. Chem. 1999, 64, 680-681.
(5) Mohanakrishnan, P.; Tayal, S. R.; Vaidyanathhaswamy, R.;
Devaprabhakara, D. Tetrahedron Lett. 1972, 2871-2874.
(6) LeBel, N. A.; Philips, A. G.; Liesemer, R. N. J . Am. Chem. Soc.
1964, 86, 1877.
(7) (a) Balci, M.; Harmandar, M. Tetrahedron Lett. 1984, 25, 237-
240. (b) For a recent review on cyclic strained allenes, see: Balci, M.;
Taskesenligil, Y. In Advances in Strained and Interesting Organic
Molecules; Halton, B., Ed.; J AI: Greenwich, 2000, Vol. 8, pp 43-81.
(1) (a) Hoffmann, R. W. Dehydrobenzene and Cycloalkynes; Academic
Press: New York, 1967. (b) Krebs, A. In Chemistry of Acetylenes; Viehe,
H. G., Ed.; Marcel Dekker: New York, 1969; pp 987-1062. (c)
Nakagava, M. In The Chemistry of the Carbon-Carbon Triple Bond;
Patai, S., Ed.; Wiley: Chichester, 1978; pp 635-712. (d) Greenberg,
A.; Liebman, J . F. Strained Organic Molecules; Academic Press: New
York, 1978; pp 133-138. (e) Gleiter, R.; Merger, R. In Modern Acetylene
Chemistry; Stang, P. J ., Diederich, F., Eds.; VCH: Weinheim, 1995;
pp 285-319.
(2) Gassman, P. G.; Gennick, I. J . Am. Chem. Soc. 1980, 102, 6863-
6864.
10.1021/jo0017537 CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/10/2001

Tricyclo[6.3.1.02,7]dodeca-2,4,6-trien-9-yne
J . Org. Chem., Vol. 66, No. 11, 2001 3807
Sch em e 2
Sch em e 5
mination reaction. In a previous work, we have examined
the addition of bromine to 8 in CHCl3 at -50 and 0 °C
and obtained completely different product distribution.9
From the reaction at -50 °C we have obtained only the
Wagner-Meerwein rearranged product 15 in quantita-
tive yield, which presumably forms via migration of the
aryl group in long-lived exo-bromonium ion intermediate.
In contrast, bromination at 0 °C resulted partly in the
formation of rearranged and nonrearranged tribromides
(Scheme 5). We noticed that the reaction temperature has
a dramatic influence on product distribution. Bromina-
tion at room and lower temperatures gives rearranged
products via Wagner-Meerwein rearrangement with
accompanying alkyl and aryl migration. However, the
bromination of these hydrocarbons at higher tempera-
tures (80-150 °C) resulted partly or completely in the
formation of nonrearranged products.10 High-tempera-
ture bromination prevents skeletal rearrangement.
Vinyl bromide 8, the starting material for the synthesis
of 14, was prepared by a published method11 and sub-
jected to bromination in CCl4 at reflux temperature.
Three products (16-18) were isolated after repeated
column chromatography in yields of 56%, 10%, and 11%,
respectively (Scheme 5). The rearranged tribromide 15
was not detected among the products. The structures of
Sch em e 3
Sch em e 4
dehydrobromination of 8 yields the bicyclic alkyne 10,
which undergoes cycloaddition with DPIBF to give 12.
The base-promoted isomerization of the double bond in
12 would give the observed products 11 (Scheme 2).
To distinguish between these two possible mechanisms,
we recently investigated the generation and trapping of
the alkyne 10 by alternative procedures.8 The alkyne 10
was generated by the base-induced rearrangement of the
bromomethylidene compound 13 and trapped with DP-
BIF to give the alkyne-like cycloadducts 12, which then
isomerize completely to the allene-like products 11 in the
presence of excess base (Scheme 3).
Even with these results, allene formation cannot be
excluded in the base-promoted reaction of 8. At this stage,
the question “What is the real intermediate in the base-
promoted reaction of 8?” still remained open. To reveal
the nature of the intermediate we have undertaken
another independent generation of alkyne 10 where the
formation of allene 9 was excluded. We now report on
the synthesis of 9,10-dibromotricyclo[6.3.1.02,7]dodeca-
2,4,6,9-tetraene (14) including its conversion to 10 and
the subsequent trapping with DPIBF (Scheme 4).
1
16, 17, and 18 were determined principally by H and
13C NMR spectral comparison with literature data.9
Our initial attempt to obtain dibromide 14 from the
dehydrobromination of exo-tribromide 16 using potas-
sium tert-butoxide in THF failed probably due to base-
catalyzed isomerization of the initially formed product
14 (see also below). The exo-dibromide 18 was obtained
as the sole product9 (Scheme 6). In contrast, treatment
of exo-tribromide 16 with 1,8-diazabicyclo[5.4.0]undec-
7-ene (DBU) in refluxing benzene afforded the desired
dibromide 14 (65% isolated yield) together with a small
amount of the parent bromide 8 in a ratio of 78:22. The
latter compound arises from the debromination of 16
under the reaction conditions. Reaction of endo isomer
17 with DBU gave also the dibromide 14 as the only
product in high yield (77%) (Scheme 6).
When 14 was treated with tert-butyllithium in THF
at -78 °C in the presence of DPIBF, two isomeric ketones
(19a and 19b) were obtained after chromatographic
separation (Scheme 7). The same products (19a , 19b)
(9) (a) Harmandar, M.; Balci, M. Tetrahedron Lett. 1985, 26, 5465-
5468. (b) Balci, M.; Harmandar, M. Tetrahedron 1988, 44, 3645-3652.
(10) (a) Dastan, A.; Demir, U¨ .; Balci, M. J . Org. Chem. 1994, 59,
6534-6538. (b) Dastan, A.; Balci, M.; Ho¨kelek, T.; U¨ lku¨, D.; Bu¨yu¨k-
gu¨ngo¨r, O. Tetrahedron 1994, 50, 10555-10578. (c) Tutar, A.; Taske-
senligil, Y.; Cakmak, O.; Abbasoglu, R.; Balci, M. J . Org. Chem. 1996,
61, 8297-8300. (d) Dastan, A.; Taskesenligil, Y.; Tu¨mer, F.; Balci, M.
Tetrahedron 1996, 52, 14005-14020.
Resu lts a n d Discu ssion
Our synthesis plan toward the dibromide 14 has
engaged the techniques of high-temperature bromination,
which prevents skeletal rearrangement during the bro-
(8) Taskesenligil, Y.; Kashyap, R. P.; Watson, W. H.; Balci, M. J .
Org. Chem. 1993, 58, 3216-3218.
(11) Kitahonoki, K.; Takano, Y.; Matsuura, A.; Kotera, K. Tetrahe-
dron 1969, 25, 335-353.

3808 J . Org. Chem., Vol. 66, No. 11, 2001
Tu¨mer et al.
Sch em e 6
Sch em e 8
Sch em e 7
the presence of DPBIF, and (iii) BuLi reaction of 14 in
the presence of DPBIF followed by potassium tert-
butoxide isomerization, strongly suggests that the inter-
mediates have the same structure. Since the allene
intermediate cannot be generated from the BuLi reaction
of 14, we conclude once again that the intermediate is
the alkyne 10.
were obtained previously8 when 13 was reacted with 1
mol of potassium tert-butoxide in the presence of DPIBF.
We suggest that the isolation of 19a and 19b from the
debromination of 14 provides strong evidence for the
intermediacy of the highly strained bicyclic alkyne 10.
This reactive intermediate initially reacts with DPIBF
to give the diastereomeric adducts 12a and 12b. These
trapping products have been observed by 1H NMR
spectroscopy. Attempted isolation of these products 12a
and 12b failed. This mixture isomerized on the chro-
matographic column (Al2O3 or SiO2) to give the corre-
sponding ketones 19a and 19b.
Reaction of dibromide 14 with tert-butyllithium in the
presence of DPIBF was repeated under identical condi-
tions, and after workup, the crude product was heated
for 6 h with potassium tert-butoxide in THF at reflux
temperature. Four products (20-23) were isolated after
repeated column chromatography in yields of 10%, 12%,
6%, and 8% (isolated yields), respectively (Scheme 8).
This experiment indicates clearly that DPIBF trapping
products (12) undergo base-promoted double-bond isomer-
ization when treated with base. These products (20-23)
were also obtained by the reaction of 138 and 87a with
excess potassium tert-butoxide.
Last, we examined the reaction of 14 with potassium
tert-butoxide to force the system to generate an allene,
since the olefinic proton in 8 is substituted by a bromine.
When 14 was treated with 1 mol of potassium tert-
butoxide in refluxing THF, the ether 25 was obtained as
the exclusive product (Scheme 9). No allene dimerization
products were detected. This shows that the bromo allene
24 is not formed in this reaction as the intermediate.
Independently, we have shown that the ether 25 is also
formed by reaction of the exo-dibromide 18 with 1 mol of
potassium tert-butoxide, which indicates that the dibro-
mide 14 initially undergoes base-promoted double bond
isomerization to give exo-dibromide 18. To support the
formation of 18 as the intermediate, we have interrupted
the reaction of 14 with potassium tert-butoxide after 30
min and analyzed the reaction mixture. It has been
shown that 18 was formed as the sole product. Nucleo-
philic displacement of the bromine in 18 (SN2′-type12) by
tert-butoxide ion yields the ether 25 (Scheme 9). The exo
configuration of the t-BuO group was determined by
measuring the vicinal bridgehead-oxymethine coupling
The identical product distribution from the three
different reactions, (i) base-promoted reaction of 8 in the
presence of DPBIF, (ii) base-promoted reaction of 13 in
(12) Cakmak, O.; Taskesenligil, Y.; Balci, M. J . Org. Chem. 1991,
56, 3442-3445.

Tricyclo[6.3.1.02,7]dodeca-2,4,6-trien-9-yne
J . Org. Chem., Vol. 66, No. 11, 2001 3809
gave an oil whose 1H NMR spectrum indicated a mixture of
16, 17, and 18 in a ratio of 72:14:14, respectively. The crude
product (0.95 g) was purified as described in the literature9b
to give 470 mg (56%) of exo-tribromide 16, 75 mg (11%) of exo-
dibromide 18, and 82 mg (10%) of endo-tribromide 17. Com-
parison of the spectral data of these compounds (16, 17, and
18) with those reported in the literature9b were in full
agreement.
Sch em e 9
Rea ction of exo-Tr ibr om id e 16 w ith DBU: 9,10-Dibr o-
m otr icyclo[6.3.1.02,7] d od eca -2,4,6,9-tetr a en e (14). To a
stirred solution of exo-tribromide 16 (1.25 g, 3.98 mmol) in 40
mL of absolute benzene was added 0.96 g (6.31 mmol) of DBU.
The resulting reaction mixture was heated at reflux temper-
ature for 2 h. After the reaction mixture was cooled to room
temperature, the insoluble materials were separated by filtra-
tion. The filtrate was evaporated, and the residue was treated
with water and extracted with ether. The organic phase was
washed with saturated NaHCO3 and water and dried over
MgSO4. Evaporation of the solvent gave a viscous oil (0.77 g)
1
whose H NMR spectrum indicated the presence of a mixture
of 14 and 8 in a ratio of 78:22. The oily viscous residue was
crystallized from ethanol to give 14 (0.65 g, 65%): colorless
1
crystals; mp 68-69 °C; H NMR (200 MHz, CDCl3) δ 7.34-
constant (J ) 2.0 Hz), which indicates clearly the exo
configuration of the tert-butoxide group.13,14
7.12 (m, 4H), 3.77 (d, J ) 4.0 Hz, 1H), 3.39 (t, J ) 4.4 Hz,
1H), 3.06 (dd, A part of AB system, J ) 17.2, 4.9 Hz, 1H), 2.48
(bd, B part of AB system, J ) 17.2 Hz, 1H), 2.32 (dt, A part of
AB system, J ) 10.7, 4.4 Hz, 1H), 2.24 (d, B part of AB system,
J ) 10.7 Hz, 1H); 13C NMR (50 MHz, CDCl3) δ 148.4, 145.4,
127.8, 127.2 (2x), 123.9, 121.6, 119.4, 52.8, 44.2, 42.6, 41.8;
MS (70 eV) m/z 312/314/316 (M+, 32), 233/235 (M+ - Br, 30),
154 (M+-2Br, 100), 76 (51); IR (NaCl, film, cm-1) 3085-2817,
1617, 1410, 1338, 1162, 1086, 960, 852, 744. Anal. Calcd for
On the other hand, treatment of 14 with 2 mol of
potassium tert-butoxide afforded the enol ether 27, which
was also formed upon reaction of 25 with 1 mol of
potassium tert-butoxide (Scheme 9). On standing at room
temperature or hydrolysis with dilute HCl, 27 converts
to the corresponding exo-ketone 28. The structures of 27
and 28 have been elucidated on the basis of spectral data.
The exo configuration of the t-BuO group in 27 again was
determined by measuring the vicinal bridgehead-oxy-
methine coupling constant (J ) 2.3 Hz) as in 25.
We assume that the alkyne 26 is the reactive inter-
mediate in the dehydrobromination of 25, which then
adds t-BuOH to give the enol ether 27. Because of the
unsymmetrical structure of the intermediate alkyne 26,
the formation of two isomeric enol ethers was expected;
however, theoretical calculations15 on analogue alkyne
10 indicated that the unsymmetric triple bond is polar-
ized and the positive end is the site of tert-butoxide
attack, which explains the formation of only one isomer
27 in the dehydrobromination of 25.
C
12H10Br2: C, 45.90; H, 3.20. Found: C, 46.29; H, 3.17.
Rea ction of en d o-9,10,10-Tr ibr om otr icyclo[6.3.1.02,7]-
d od eca -2,4,6-tr ien e 17 w ith DBU. The reaction was carried
out as described above by using 180 mg (0.45 mmol) of endo-
tribromide 17 and 130 mg (0.85 mmol) of DBU. Dibromide 14
(110 mg, 77%) was obtained as sole product.
Rea ction 14 w ith t-Bu Li in th e P r esen ce of DP IBF . A
solution of 1.7 M t-BuLi in pentane (2.1 mL, 3.57 mmol) was
added dropwise to a stirred solution of 14 (1.0 g, 3.2 mmol)
and diphenylisobenzofuran (0.86 g, 3.2 mmol) in dry THF (20
mL) over 5 min at -78 °C under nitrogen. The reaction
mixture was allowed to warm to room temperature and then
quenched with wet THF (3 mL). Stirring was continued at
room temperature for 2 h. After a major part of the THF was
removed, the mixture was treated with water (30 mL). The
aqueous layer was extracted with ether (4 × 30 mL) and CH2-
Cl2 (2 × 25 mL). The combined organic layers were washed
with water, dried over MgSO4, and filtered. The filtrate was
treated with maleic anhydride until the yellow color disap-
peared to remove the excess DPBIF. After removal of the
solvent, the oily residue was chromatographed over alumina
(150 g, grade IV, neutral). Elution with petroleum ether-
toluene (8:2) gave 117 mg (9%) of exo-ketone 19a and 104 mg
(8%) of endo isomer 19b. The 1H NMR, 13C NMR, and IR
spectral data for 19a and 19b are identical with those reported
in the literature.8
Rea ction of 14 w ith t-Bu Li in th e P r esen ce of DP IBF
a n d F ollow ed by P ota ssiu m ter t-Bu toxid e. The first step
of the reaction was carried out as described above by using
2.0 g (6.37 mmol) of dibromide 14, 1.72 g (6.37 mmol) of
diphenylisobenzofuran, and 4.12 mL of t-BuLi in pentane (1.7
M, 7.0 mmol). After the usual workup, the residue (3.5 g) was
dissolved in 20 mL of dry and freshly distilled THF and
potassium tert-butoxide (0.71 g, 6.34 mmol) added. The result-
ing solution was heated at reflux temperature for 6 h under
nitrogen with magnetic stirring and worked up in the same
way as described above. The crude product was chromato-
graphed over alumina (300 g, grade IV, neutral). As the first
fraction we isolated the reduction product 8 (28.5 mg, 22%).
The trapping products (20-23)8 were isolated after repeated
column chromatography with petroleum ether-toluene (8:2,
7:3) in yields of 10%, 12%, 6%, and 8% (isolated yields),
respectively. Total yield of the trapping products, determined
Exp er im en ta l Section
Gen er a l Meth od s. Melting points were determined on a
Bu¨chi model 530 apparatus and are uncorrected. Infrared
spectra were recorded on
a Mattson model 1000 FT-IR
spectrometer. H and 13C NMR spectra were recorded on 200
(50)-MHz spectrometers. Mass spectra (electron impact) were
recorded at 70 eV. Column chromatography was performed
on silica gel (60-200 mesh) and activated alumina (70-230
mesh) from Merck Co. TLC was carried out on Merck 0.2 mm
silica gel 60 F254 analytical aluminum plates.
1
Br om in a t ion of 10-Br om ot r icyclo[6.3.1.02,7]d od eca -
2,4,6,9-tetr a en e 8 a t 77 °C. Vinyl bromide 811 (0.5 g, 2.13
mmol) was dissolved in 5 mL of CCl4 in a 25 mL flask that
was equipped with a reflux condenser. The solution was heated
until carbon tetrachloride started to reflux with magnetic
stirring. To the refluxing solution was added dropwise a hot
solution of bromine (0.36 g, 2.25 mmol) in 0.5 mL of CCl4
during 30 s. The color of bromine disappeared immediately.
The reaction flask was removed from the oil bath and cooled
to room temperature in an ice bath. Evaporation of the solvent
(13) Tu¨mer, F.; Taskesenligil, Y.; Dastan, A.; Balci, M. Aust. J .
Chem. 1996, 49, 599-603.
(14) Wege, D. J . Org. Chem. 1990, 55, 1667-1670.
(15) Balci, M.; Krawiec, M.; Taskesenligil, Y.; Watson, W. H. J .
Chem. Crystallogr. 1996, 26, 413-418.

3810 J . Org. Chem., Vol. 66, No. 11, 2001
Tu¨mer et al.
by 1H NMR integration, was aprroximately 62%. The spectral
data of these compounds (20-23) with those reported in the
literature8 were in full agreement.
2.28 (d, A part of AB system, J ) 9.9 Hz, 1H), 2.07 (dt, B part
of AB system, J ) 9.9, 4.4 Hz, 1H), 1.31 (s, 9H), 1.15 (s, 9H);
13C NMR (50 MHz, CDCl3) δ 156.4, 150.0, 145.5, 128.1, 127.4,
126.3, 122.7, 119.2, 79.7, 76.2, 74.4, 51.0, 41.9, 40.5, 30.9 (3x),
30.8 (3x); IR (NaCl, film, cm-1) 3080-2876, 1651, 1472, 1395,
1242, 1217, 1191, 757.
Reaction of 14 with P otassiu m ter t-Bu toxide (1 Equ iv).
To a stirred solution of dibromide 14 (300 mg, 0.95 mmol) in
20 mL of dry THF was added potassium tert-butoxide (120 mg,
1.07 mmol, 10% excess). The reaction mixture was refluxed
for 6 h and then cooled to room temperature. The mixture was
diluted with water, and the aqueous solution was extracted
with ether, washed with water, and dried over MgSO4. After
removal of the solvent, the residue was chromatographed on
silica gel with hexane-chloroform (9:1) yielding 130 mg (43%)
of ether 25 as a colorless liquid: 1H NMR (200 MHz, CDCl3) δ
7.35 (m, 1H), 7.13 (m, 3H), 6.70 (d, J ) 7.3 Hz, 1H), 3.95 (d, J
) 2.0 Hz, 1H), 3.40 (m, 2H), 2.36 (d, A part of AB system, J )
10.6 Hz, 1H), 2.19 (dt, B part of AB system, J ) 10.6, 4.4 Hz,
1H), 1.39 (s, 9H); 13C NMR (50 MHz, CDCl3) δ 154.1, 145.0,
142.2, 128.5, 128.3, 126.7, 124.0, 123.6, 77.2, 76.4, 51.4, 45.4,
39.9, 30.9 (3x); IR (NaCl, film, cm-1) 2978, 2876, 1625, 1479,
1421, 1395, 1344, 1242, 1217, 1038.
Rea ction of 14 w ith P ota ssiu m ter t-Bu toxid e (2
Equ iv): F or m a tion of exo-10,11-Di(ter t-bu toxy)tr icyclo-
[6.3.1.02,7]d od eca -2,4,6,9-tetr a en e (27). To a stirred solution
of dibromide 14 (500 mg, 1.95 mmol) in 20 mL of dry THF
was added potassium tert-butoxide (400 mg, 3.57 mmol). The
reaction mixture was refluxed for 10 h, cooled to room
temperature, and worked up in the same way as described
above. The crude product was purified on silica gel with
hexane-chloroform (9:1) to yield 270 mg (56%) of the butoxy
enol ether 27 as a colorless liquid: 1H NMR (200 MHz, CDCl3)
δ 7.29 (m, 1H), 7.06 (m, 3H), 5.65 (d, J ) 7.1 Hz, 1H), 3.62 (d,
J ) 2.3 Hz, 1H), 3.33 (dd, J ) 7.1, 4.3 Hz, 1H), 3.25 (m, 1H),
exo-9-(ter t-Bu toxy)tr icyclo[6.3.1.02,7]d od eca -2,4,6-tr ien -
10-on e (28). To a solution of 27 (160 mg, 0.53 mmol) in 5 mL
of CH2Cl2 was added dropwise a solution of 10% HCl (0.5 mL).
The reaction mixture was stirred at room temperature for 30
min and then quenched with NH3 solution. The aqueous
solution was extracted with CH2Cl2, washed with water, and
dried over MgSO4. After removal of the solvent, the residue
was chromatographed on silica gel with petroleum ether-ether
(49:1) to give 75 mg (57%) of ketone 28 as a colorless liquid:
1H NMR (200 MHz, CDCl3) δ 7.24-7.12 (m, 4H), 3.79 (d, J )
3.9, 1H), 3.38-3.28 (m, 2H), 2.94 (dd, A part of AB system, J
) 15.1, 3.3 Hz, 1H), 2.64 (d, A part of AB system, J ) 11.3
Hz, 1H), 2.36 (bd, B part of AB system, J ) 15.1 Hz, 1H), 2.18
(m, B part of AB system, 1H), 1.23 (s, 9H); 13C NMR (50 MHz,
CDCl3) δ 211.1, 149.5, 143.4, 129.8, 129.3, 126.4, 125.5, 79.3,
77.7, 50.3, 49.2, 42.5, 38.8, 30.4 (3×); IR (NaCl, film, cm-1
3055-2902, 1727, 1497, 1370, 1217, 1038, 782.
)
Ack n ow led gm en t. We are indebted to the Depart-
ment of Chemistry and Atatu¨rk University for financial
support of this work and the State Planning Organiza-
tion of Turkey (DPT) for purchasing a 200 MHz NMR
spectrometer.
J O0017537