Benzocyclobutadienes via electrocyclizations of (Z,Z)-3,5-octadiene-1,7-diynes leading to dimers with unusual polycyclic structures

Article abstract of DOI:10.1021/ja960506a

(Z,Z)-3,5-Octadiene-1,7-diynes 15a-e were synthesized by condensation of enynyl aldehydes 12a-e with allenylborane 11 to furnish enynyl alcohols 14a-e followed by the elimination step of the Peterson olefination reaction. Desilylation of 15a with tetrabut

Full text of DOI:10.1021/ja960506a

6860
J. Am. Chem. Soc. 1996, 118, 6860-6867
Benzocyclobutadienes via Electrocyclizations of
(Z,Z)-3,5-Octadiene-1,7-diynes Leading to Dimers with Unusual
Polycyclic Structures
Kung K. Wang,* Bin Liu, and Jeffrey L. Petersen†
Contribution from the Department of Chemistry, West Virginia UniVersity,
Morgantown, West Virginia 26506
ReceiVed February 16, 1996X
Abstract: (Z,Z)-3,5-Octadiene-1,7-diynes 15a-e were synthesized by condensation of enynyl aldehydes 12a-e with
allenylborane 11 to furnish enynyl alcohols 14a-e followed by the elimination step of the Peterson olefination
reaction. Desilylation of 15a with tetrabutylammonium fluoride (TBAF) followed by two consecutive electrocy-
clizations resulted in the formation of the corresponding benzocyclobutadiene 17a, leading to the four angular dimers
18a-d. The use of cyclopentadiene to capture 17a afforded the Diels-Alder adduct 19. Treatment of 15b with
TBAF produced only one angular dimer 21. On the other hand, the presence of a phenyl substituent on the four-
membered ring directed the dimerization of benzocyclobutadiene 17c toward the linear dimer 23, which then underwent
a facile thermal rearrangement to dibenzocyclooctadiene 24. Interestingly, benzocyclobutadienes 17d and 17e having
an alkenyl substituent on the four-membered ring dimerized via a formal [4 + 4] cycloaddition to the
1,5-cyclooctadienes 26 and 32, respectively.
Introduction
Scheme 1
The high reactivity of benzocyclobutadienes has been well
documented.1 The parent benzocyclobutadiene (1a)2 and the
1-alkyl-substituted derivatives 1b-d3 dimerize readily in a
Diels-Alder manner via 2 to afford the corresponding angular
dimers 3 (Scheme 1). Interestingly, the presence of a phenyl
substituent on the four-membered ring in 4 causes the formation
of the linear dimers, dibenzocyclooctadienes 6, presumably via
an initial formation of the syn dimer 5 (Scheme 2).4
The early demonstration of the existence of benzocyclobuta-
dienes as transient reaction intermediates was achieved by
trapping them with reactive dienes, such as cyclopentadiene and
1,3-diphenylisobenzofuran, to form the corresponding Diels-
Alder adducts.5 Direct observation of the infrared and the
ultraviolet spectra of the very reactive 1a was accomplished by
matrix isolation in argon at 8 K.6 The photoelectron spectrum
of 1a was also reported and provided insight into its electronic
structure.7 Because of the high reactivity, it was only until
Scheme 2
† To whom correspondence concerning the X-ray structure should be
addressed.
recently that the 1H NMR spectrum of 1a was obtained by using
the technique of flow NMR.8 In contrast, the sterically hindered
1,2-bis(trimethylsilyl)benzocyclobutadiene is thermally stable
even at 150 °C, and the 1H NMR chemical shifts of the protons
directly attached to the six-membered ring at δ 5.63 and 6.20
(in CCl4) gave the first indication of strong paramagnetic ring
current contributions to the induced ring current.9 Similarly,
1,2-di-tert-butyl-3,4,5,6-tetramethylbenzocyclobutadiene can be
isolated and its X-ray structure permitted the first experimental
determination of the molecular geometry of a free benzocy-
clobutadiene.10
X Abstract published in AdVance ACS Abstracts, July 15, 1996.
(1) For reviews of benzocyclobutadiene, see: (a) Toda, F.; Garratt, P.
Chem. ReV. 1992, 92, 1685-1707. (b) Vollhardt, K. P. C. Top. Curr. Chem.
1975, 59, 113-136. (c) Cava, M. P.; Mitchell, M. J. Cyclobutadienes and
Related Compounds; Academic Press: New York, 1967.
(2) (a) Cava, M. P.; Napier, D. R. J. Am. Chem. Soc. 1956, 78, 500. (b)
Cava, M. P.; Napier, D. R. J. Am. Chem. Soc. 1957, 79, 1701-1705. (c)
Cava, M. P.; Napier, D. R. J. Am. Chem. Soc. 1958, 80, 2255-2257.
(3) (a) Filip, P.; Stefan, N.; Chiraleu, F.; Dinulescu, I. G.; Avram, M.
ReV. Roum. Chim. 1984, 29, 549-555. (b) Avram, M.; Constantinescu,
D.; Dinulescu, I. G.; Nenitzescu, C. D. Tetrahedron Lett. 1969, 5215-
5218. (c) Muller, E.; Fettel, H.; Sauerbier, M. Synthesis 1970, 82-83.
(4) (a) Stiles, M.; Burckhardt, U.; Haag, A. J. Org. Chem. 1962, 27,
4715-4716. (b) Blomquist, A. T.; Bottomley, C. G. J. Am. Chem. Soc.
1965, 87, 86-93.
Benzocyclobutadienes have been generated by the dehalo-
genation of 1,2-dihalobenzocyclobutenes and the dehydroha-
(5) (a) Cava, M. P.; Mitchell, M. J. J. Am. Chem. Soc. 1959, 81, 5409-
5413. (b) Nenitzescu, C. D.; Avram, M.; Dinu, D. Chem. Ber. 1957, 90,
2541-2544. (c) Cava, M. P.; Pohlke, R. J. Org. Chem. 1962, 27, 1564-
1567.
(6) Chapman, O. L.; Chang, C. C.; Rosenquist, N. R. J. Am. Chem. Soc.
1976, 98, 261-262.
(7) Koenig, T.; Imre, D.; Hoobler, J. A. J. Am. Chem. Soc. 1979, 101,
6446-6447.
(8) Trahanovsky, W. S.; Fischer, D. R. J. Am. Chem. Soc. 1990, 112,
4971-4972.
(9) Vollhardt, K. P. C.; Yee, L. S. J. Am. Chem. Soc. 1977, 99, 2010-
2012.
(10) Winter, W.; Straub, H. Angew. Chem., Int. Ed. Engl. 1978, 17, 127-
128.
S0002-7863(96)00506-9 CCC: $12.00 © 1996 American Chemical Society

Electrocyclizations of (Z,Z)-3,5-Octadiene-1,7-diynes
J. Am. Chem. Soc., Vol. 118, No. 29, 1996 6861
Scheme 3
Scheme 4
logenation of 1-halobenzocyclobutenes. These methods have
been successfully adopted for the synthesis of 1a-d2,3 and 4b.4b
Treatment of benzyne generated in situ with phenylacetylene
and 1-phenyl-1-propyne produced 4a and 4b, respectively.4a
Thermolysis of trans-3,4-diethynylcyclobutenes at 100 to 190
°C has also been utilized for the synthesis of benzocyclobuta-
dienes with a permethylated benzene ring.11 The electrocy-
clization of (Z,Z)-3,5-octadiene-1,7-diyne (8), obtained by
desilylation of the precursor 7, also provided a highly efficient
route to 1a via an initial formation of 1,2,4,5,7-cyclooctapen-
taene (9) followed by a second electrocyclization (Scheme 3).12
The reaction occurs at room temperature with a half-life of ca.
10 min, and the dimer 3a was isolated in 85% yield. However
with the exception of one additional case of using 7 for the
preparation of 1,2-bis(trimethylsilyl)benzocyclobutadiene,9 this
facile route has not been adopted for the synthesis of other
benzocyclobutadienes. This is due mainly to the lack of
geometric selectivity in producing predominantly the E,Z-isomer
(35%) and the E,E-isomer (15%) of 7 instead of the desired
Z,Z-isomer (35%) by dehydrobromination of 1,8-bis(trimeth-
ylsilyl)-4,5-dibromo-1,7-octadiyne.
We recently reported a facile synthesis of (Z)-1-buten-3-
ynes,13 (Z)-3-hexene-1,5-diynes (enediynes),13 and (Z)-1,2,4-
heptatrien-6-ynes (enyne-allenes)14 by condensation of the
nonconjugated aldehydes, the conjugated acetylenic and the
allenic aldehydes with γ-(trialkylsilyl)allenylboranes, followed
by the elimination step of the Peterson olefination reaction. We
envisioned that by using the conjugated (Z)-enynyl aldehydes
for condensation, dienediynes having the Z geometry for the
two central carbon-carbon double bonds could thus be syn-
thesized. We now report a successful extension of this method
to the synthesis of a variety of (Z,Z)-dienediynes and their
electrocyclizations to the corresponding benzocyclobutadienes
leading to unusual polycyclic compounds.
Table 1. Stereoselective Synthesis of Enynyl Alcohols 14 and
Dienediynes 15
isolated
yield, %
isolated yield,
R
14a
15
% (Z:E)
Mg3Si
Bu
phenyl
methylethenyl
1-cyclohexenyl
14a
14b
14c
14d
14e
61
65
56
54
55
15a
15b
15c
15d
15e
78 (93:7)
80 (92:8)
83 (95:5)
81 (95:5)
81 (94:6)
a The preferential formation of the Z isomer 15 by treatment of 14
with potassium hydride indicates that the SR/RS pair were produced
predominantly.
cross coupling between 2-bromo-1-cyclopentenecarboxalde-
hyde17 and terminal alkynes.18 Subsequent condensation of 11,
prepared in situ, with 12 furnished, after treatment with
2-aminoethanol, the condensation adducts 14 with high dias-
tereoselectivity (Table 1). The high diastereoselectivity in
forming 14 can be attributed to the preference of the tert-
butyldimethylsilyl group and the Z-enynyl group of aldehydes
in adopting the opposite sides of the six-membered transition
states 13 in order to minimize nonbonded steric interactions as
observed in the earlier cases.13 Conversion of 14 to dienediynes
15 was carried out with potassium hydride in diethyl ether at 0
°C under a nitrogen atmosphere.13 Dienediynes 15 having two
substituents at the alkynyl terminus were stable enough to allow
purification by column chromatography at room temperature
and were produced with high geometric purity (Z:E g 92:8)
(Table 1).
Results and Discussion
Treatment of the readily available 3-(tert-butyldimethylsilyl)-
1-(trimethylsilyl)-1-propyne (10)15 with n-butyllithium followed
by B-methoxy-9-borabicyclo[3.3.1]nonane (B-MeO-9-BBN) and
4/3BF3‚OEt216 gave allenylborane 11 (Scheme 4).13 The requisite
(Z)-enynyl aldehydes 12 were prepared by a Pd(PPh3)4-catalyzed
Treatment of 15a with tetrabutylammonium fluoride (TBAF)
produced the corresponding desilylated dienediyne 16a, which
then underwent electrocyclization reactions to benzocyclobuta-
diene 17a, giving rise to the four possible angular dimers 18a-d
with nearly equal proportions in 66% isolated yield (Scheme
5). Apparently there was little preference for the four possible
orientations during dimerization.
(11) (a) Straub, H. Liebigs Ann. Chem. 1978, 1675-1701. (b) Straub,
H. Angew. Chem., Int. Ed. Engl. 1974, 13, 405-406.
(12) Mitchell, G. H.; Sondheimer, F. J. Am. Chem. Soc. 1969, 91, 7520-
7521.
(13) Wang, K. K.; Wang, Z.; Gu, Y. Z. Tetrahedron Lett. 1993, 34,
8391-8394.
(14) (a) Andemichael, Y. W.; Gu, Y. G.; Wang, K. K. J. Org. Chem.
1992, 57, 794-796. (b) Andemichael, Y. W.; Huang, Y.; Wang, K. K. J.
Org. Chem. 1993, 58, 1651-1652. (c) Wang, K. K.; Wang, Z.; Sattsangi,
P. D. J. Org. Chem. 1996, 61, 1516-1518.
(15) (a) Furuta, K.; Ishiguro, M.; Haruta, R.; Ikeda, N.; Yamamoto, H.
Bull. Chem. Soc. Jpn. 1984, 57, 2768-2776. (b) Yamakado, Y.; Ishiguro,
M.; Ikeda, N.; Yamamoto, H. J. Am. Chem. Soc. 1981, 103, 5568-5570.
(16) Brown, H. C.; Sinclair, J. A. J. Organomet. Chem. 1977, 131, 163-
169.
(17) (a) Arnold, Z.; Holy, A. Collect. Czech. Chem. Commun. 1961, 26,
3059-3073. (b) Robertson, I. R.; Sharp, J. T. Tetrahedron 1984, 40, 3095-
3112.
(18) (a) Saito, I.; Yamaguchi, K.; Nagata, R.; Murahashi, E. Tetrahedron
Lett. 1990, 31, 7469-7472. (b) Wang, K. K.; Liu, B.; Lu, Y.-d. Tetrahedron
Lett. 1995, 36, 3785-3788.

6862 J. Am. Chem. Soc., Vol. 118, No. 29, 1996
Wang et al.
Scheme 5
Figure 1. ORTEP drawing of the crystal structure of the dimer 24.
Scheme 6
Scheme 7
leading to the formation of 21. This selectivity is reminiscent
of the results observed with 1b-d which produced the angular
dimers 3b-d.3 Treatment of 15b with TBAF in the presence
of an excess of cyclopentadiene also furnished the Diels-Alder
adduct 22 in 50% isolated yield along with 20% of the dimer
21.
It was difficult to separate the four isomers 18a-d by column
1
chromatography. Fortunately, the H NMR spectrum of the
Interestingly when 15c having a phenyl substituent was
treated with TBAF, the dimer 23 was formed initially (Scheme
7). As the reaction mixture was stirred at room temperature
for 12 h to allow the reaction to go to completion, the linear
dimer 24 slowly appeared as observed previously in the cases
of 4a and 4b.3a,4 Fortunately, it was possible to separate a portion
of 23 (19%) cleanly by column chromatography to allow
structural elucidation by 1H and 13C NMR spectroscopy. Only
one set of NMR signals due to either the syn or the anti isomer
of 23 were observed. A second portion containing a mixture
of 23 and 24 (40:60) was also isolated in 36% yield. The dimer
23 was unstable at room temperature and slowly rearranged to
24. On heating at 60 °C, 23 was converted to 24 within 3 h in
89% yield after purification by column chromatography. The
structure of 24 was established by an X-ray structure determi-
nation (Figure 1).
While the stereochemistry of 23 was not determined, the facile
isomerization of 23 to 24 even at room temperature appears to
suggest that the syn isomer of 23 was formed, consistent with
the earlier observation that the syn isomer 5b is prone to thermal
isomerization.4b The anti isomer of 5b is thermally much more
stable,4b and a similar thermal stability could also be expected
for the anti isomer of 23.
mixture exhibited clearly seven sets of signals from δ 6.43 to
6.08 with the signal at δ 6.08 having ca. twice the intensity of
the other peaks with nearly equal intensities among them. These
signals can be attributed to the eight vinylic hydrogens of the
four angular dimers with an overlap of two hydrogens at δ 6.08.
The 13C NMR spectrum also showed eight well-separated signals
from δ 43.58 to 42.28 due to the eight sp3 carbons on the four-
membered rings of the four isomers. In addition, 31 signals
(one less than expected because of an overlap at δ 142.42) due
to the 32 quaternary sp2 carbons and 23 signals (two peaks
overlapping at δ 123.73) due to the 24 CH sp2 carbons were
clearly discernible. Moreover, 8 signals from δ 25.74 to 25.00
due to the 8 central CH2 carbons on the five-membered rings
along with 10 signals due to the remaining CH2 carbons (several
overlapping peaks) were also observed. Attempts to separate
18a-d by HPLC yielded a fraction which contained predomi-
nantly only one of the four isomers, permitting a more definitive
analysis of the 1H and 13C NMR spectra for structural elucida-
1
tion. Indeed, its H and 13C NMR spectra are consistent with
the structural features of the angular dimers.
In addition to dimerization, it was possible to capture 17a
with cyclopentadiene to form the endo isomer of the Diels-
Alder adduct 19 in 80% isolated yield by treating 15a with
TBAF in the presence of an excess of cyclopentadiene (Scheme
5). The endo configuration of 19 was supported by the NOE
measurements.
On the other hand, when 15b was treated with TBAF only
one angular dimer 21 was isolated in 61% yield (Scheme 6).
Apparently, the butyl group on the four-membered ring in 17b
directs the preferential formation of 20 during dimerization
The resonance stabilizing effect of a 1-phenyl substituent may
significantly alter the electronic structure of benzocyclobuta-
diene, causing the formation of the linear dimer. Since a
1-alkenyl substituent could also exert a similar stabilizing effect,
it is therefore of interest to study the chemical behaviors of such
benzocyclobutadiene derivatives. Surprisingly when 15d was
treated with TBAF, the anticipated dibenzocyclooctadiene 25
was not isolated. Instead, a formal [4 + 4] cycloaddition of

Electrocyclizations of (Z,Z)-3,5-Octadiene-1,7-diynes
J. Am. Chem. Soc., Vol. 118, No. 29, 1996 6863
Scheme 9
Figure 2. ORTEP drawing of the crystal structure of the dimer 26.
Scheme 8
pathway involving an initial head-to-head dimerization to form
diradical 30 followed by an intramolecular radical-radical
combination could account for the formation of the trans isomer
26 (Scheme 9). There are several possible orientations of the
two monomers in the transition state of the first step for
dimerization that could lead to 30, including the one depicted
in Scheme 9 with the exo approach and the cisoid encounter of
the two diene components in the s-cis conformation. It was
previously established that dimerizations of the benzene-based21a
and the furan-based o-quinodimethanes21b proceed through the
exo approach.
Alternatively, an initial tail-to-tail dimerization to form
diradical 31 followed by the exo approach for the intramolecular
radical-radical combination could also furnish 26. When
compared with the pathway through diradical 30, the route
through an initial tail-to-tail connection is favored by less steric
interactions in joining the two tails together. Dimerization of
the furan-based o-quinodimethanes was found to be sensitive
to steric hindrance at the diene terminus.20b On the other hand,
diradical 31 still retains the reactive benzocyclobutadiene
moieties which could make the pathway less favorable. Clearly,
17d occurred, producing the 1,5-cyclooctadiene 26 having the
trans geometry in 36% isolated yield (Scheme 8). The structure
of 26 was unequivocally established by an X-ray structure
determination (Figure 2).
The formation of the trans geometry in 26 was particularly
unexpected. If the cis isomer 28 had been isolated, a pathway
involving an initial formation of the syn dimer 27 as in the case
of 23 followed by a facile Cope rearrangement could have
accounted for its formation (eq 1). However, such a pathway
to the trans isomer 26 would require an initial formation of the
anti dimer 29 followed by a nonconcerted rearrangement
(eq 2).
additional studies will be needed to probe into the details of
the dimerization mechanism. Nevertheless, it is interesting to
note that the absence of the head-to-tail [4 + 4] dimer and the
[4 + 2] dimers closely resemble those of the dimerization of
2,3-dimethylene-2,3-dihydrofuran which produces the head-to-
head [4 + 4] dimer exclusively.20
(19) (a) Errede, L. A. J. Am. Chem. Soc. 1961, 83, 949-954. (b)
Trahanovsky, W. S.; Chou, C.-H.; Fischer, D. R.; Gerstein, B. C. J. Am.
Chem. Soc. 1988, 110, 6579-6581.
(20) (a) Trahanovsky, W. S.; Cassady, T. J.; Woods, T. L. J. Am. Chem.
Soc. 1981, 103, 6691-6695. (b) Leung, M.-k.; Trahanovsky, W. S. J. Am.
Chem. Soc. 1995, 117, 841-851. (c) Chou, C.-H.; Trahanovsky, W. S. J.
Am. Chem. Soc. 1986, 108, 4138-4144.
(21) (a) Trahanovsky, W. S.; Macias, J. R. J. Am. Chem. Soc. 1986,
108, 6820-6821. (b) Trahanovsky, W. S.; Chou, C.-H.; Cassady, T. J. J.
Org. Chem. 1994, 59, 2613-2615.
Perhaps the chemical behavior of 17d resembles those of the
benzene-based19 and the furan-based o-quinodimethanes20 in
producing 26 via a two-step diradical mechanism. A radical

6864 J. Am. Chem. Soc., Vol. 118, No. 29, 1996
Wang et al.
Scheme 10
via a formal [4 + 4] cycloaddition to polycyclic compounds
having unusual carbon frameworks.
Experimental Section
General procedures for manipulation of organoboranes and other
organometallic reagents were described previously.22 All reactions were
conducted in oven-dried (120 °C) glassware under a nitrogen atmo-
sphere. Tetrahydrofuran (THF) and diethyl ether were distilled from
benzophenone ketyl prior to use. n-Butyllithium (2.5 M) in hexanes,
tetrabutylammonium fluoride (TBAF) (1.0 M in THF), and 1-(trim-
ethylsilyl)-1-propyne were purchased from Aldrich Chemical Co., Inc.
and were used as received. Potassium hydride (35 wt % dispersion in
mineral oil) was also purchased from Aldrich, and mineral oil was
removed by washing with pentane prior to use. Silica gel (70-230
mesh) for column chromatography was also purchased from Aldrich.
B-Methoxy-9-borabicyclo[3.3.1]nonane (B-MeO-9-BBN),23 3-(tert-bu-
tyldimethylsilyl)-1-(trimethylsilyl)-1-propyne,15 and enynyl aldehydes
1218 were prepared according to the reported procedures. 1H (270 MHz)
and 13C (67.9 MHz) NMR spectra were recorded in CDCl3 or C6D6
using Me4Si, CHCl3 (1H δ 7.26), CDCl3 (13C δ 77.02), C6D5H (1H δ
7.15), or C6D6 (13C δ 128.00) as internal standard. The isomer ratios
Similarly, when 15e was treated with TBAF, the dimer 32
having nine fused rings was isolated in 37% yield (Scheme 10).
Of the six possible diastereomers of 32, only two were present
as indicated by two singlet signals in the 1H NMR spectrum at
δ 3.97 and 3.92 in a ratio of 35:65 attributable to the hydrogen
atoms on the four-membered ring as observed in the case of
26. If one assumes the preferential formation of the trans
junction at the ends of the four-membered ring as in the case
of 26, then the number of possible diastereomers is reduced to
three as depicted in 32a-c. Both 32a and 32b have a C2
symmetry, while no symmetry exists with 32c. The observation
of two singlets with unequal intensities at δ 3.97 and 3.92 in
1
were determined by integration of the H NMR spectra.
2-(Trimethylsilylethynyl)-1-cyclopentenecarboxaldehyde (12a).
The following procedure for the preparation of 12a is representative.
A flask containing 0.867 g (0.75 mmol) of Pd(PPh3)4 and 0.29 g (1.50
mmol) of CuI was evacuated and then purged with nitrogen. To a
second flask were added 2.68 g (15.0 mmol) of 2-bromo-1-cyclopen-
tenecarboxaldehyde, 5.82 g (45.0 mmol, 7.8 mL) of ethyldiisopropy-
lamine, and 20 mL of DMF. The mixture was degassed by three cycles
of freeze-thaw before it was transferred via cannula into the first flask
at room temperature. Then a degassed solution of trimethylsilylacety-
lene (2.26 g, 23.0 mmol) in 10 mL of DMF was introduced into the
reaction mixture. After 12 h, the mixture was poured into 200 mL of
an aqueous NH4Cl solution, filtrated, and extracted with pentane (3 ×
30 mL). The combined organic layers were washed with water (3 ×
30 mL), dried over Na2SO4, and concentrated. The residue was purified
by column chromatography (silica gel/5% diethyl ether in hexanes) to
furnish 2.79 g (97%) of 12a as a light yellow oil: IR (neat) 2138,
1672, 1593, 1250, 1199, 992, 852, 761, 689 cm-1; 1H NMR (C6D6) δ
10.32 (1 H, s), 2.36 (2 H, tt, J ) 7.7 and 2.3 Hz), 2.25 (2 H, tt, J ) 7.7
and 2.3 Hz), 1.32 (2 H, quintet, J ) 7.7 Hz), 0.13 (9 H, s); 13C NMR
(C6D6) δ 187.45, 149.85, 141.29, 106.43, 99.16, 38.67, 29.81, 21.98,
-0.37; MS (m/e) 192 (M+), 191, 177, 164, 149, 117, 103, 83, 75, 73.
2-(1-Hexynyl)-1-cyclopentenecarboxaldehyde (12b): isolated in
97% yield as a light yellow oil; IR (neat) 2211, 1668, 1596, 1466,
1
the H NMR spectrum along with only 18 sp3-carbon and 14
(two signals less than expected presumably due to overlaps)
sp2-carbon signals strongly suggest that the two isolated isomers
are 32a and 32b. Because of the lack of symmetry in 32c, the
1
1430, 1383, 1352, 1224, 730 cm-1; H NMR (CDCl3) δ 10.01 (1 H,
s), 2.65 (2 H, tt, J ) 7.7 and 2.2 Hz), 2.57 (2 H, t, J ) 7.6 Hz), 2.43
(2 H, t, J ) 6.9 Hz), 1.92 (2 H, quintet, J ) 7.6 Hz), 1.62-1.5 (2 H,
m), 1.5-1.35 (2 H, m), 0.92 (3 H, t, J ) 7.2 Hz); 13C NMR (CDCl3)
δ 189.23, 146.94, 144.71, 103.24, 74.99, 39.21, 30.42, 29.28, 21.99,
21.96, 19.51, 13.54; MS (m/e) 176 (M+), 175, 161, 147, 134, 117, 105,
91, 77.
presence of 32c as one of the two isomers would have produced
a more complex pattern at ca. δ 3.9 in the H NMR spectrum
2-(Phenylethynyl)-1-cyclopentenecarboxaldehyde (12c): isolated
in 84% yield as a yellow solid; IR (KBr) 2197, 1670, 1603, 1488, 1441,
1355, 1245, 754, 689 cm-1; 1H NMR (CDCl3) δ 10.16 (1 H, s), 7.50-
7.48 (2 H, m), 7.38-7.34 (3 H, m), 2.80 (2 H, tt, J ) 7.6 and 2.2 Hz),
1
and additional 13C signals. As in the case of 26, an initial step
involving the exo approach of two molecules of 17e with the
cisoid encounter of the two diene components in the s-cis
conformation followed by the two diastereomeric exo ap-
proaches for the intramolecular radical-radical combination
could produce the two trans junctions in 32a and 32b.
2.66 (2 H, tt, J ) 7.7 and 2.2 Hz), 2.00 (2 H, quintet, J ) 7.7 Hz); 13
C
NMR (CDCl3) δ 188.65, 147.79, 142.94, 131.72, 129.26, 128.40,
121.92, 100.58, 83.17, 38.79, 29.49, 22.00; MS (m/e) 196 (M+), 167,
153, 152, 139, 115.
2-(3-Methyl-3-buten-1-ynyl)-1-cyclopentenecarboxaldehyde
(12d): isolated in 84% yield as a light yellow oil; IR (neat) 2194,
1670, 1614, 1587, 1434, 1384, 1353, 1304, 1247, 1198, 904, 724 cm-1
;
Conclusions
1H NMR (CDCl3) δ 10.02 (1 H, s), 5.40 (1 H, br s), 5.35 (1 H, quintet,
J ) 1.6 Hz), 2.69 (2 H, tt, J ) 7.4 and 2.2 Hz), 2.58 (2 H, tt, J ) 7.7
and 2.2 Hz), 1.99-1.87 (5 H, m); 13C NMR (CDCl3) δ 188.82, 147.78,
143.07, 126.06, 124.10, 101.71, 82.09, 38.80, 29.47, 22.99, 22.03; MS
(m/e) 160 (M+), 159, 145, 131, 117, 115, 103, 91, 77.
The ability to synthesize a variety of (Z,Z)-3,5-octadiene-
1,7-diynes provides an easy access to the reactive benzocy-
clobutadienes with diverse structures. Benzocyclobutadienes
with no substituent on the four-membered ring or with a 1-alkyl
or a 1-phenyl substituent dimerize by the pathways observed
previously.1 On the other hand, benzocyclobutadienes substi-
tuted with a 1-alkenyl group exhibit a new reaction pathway
(22) Brown, H. C.; Kramer, G. W.; Levy, A. B.; Midland, M. M. Organic
Syntheses Via Boranes; Wiley-Interscience: New York, 1975.
(23) Kramer, G. W.; Brown, H. C. J. Organomet. Chem. 1974, 73, 1-15.

Electrocyclizations of (Z,Z)-3,5-Octadiene-1,7-diynes
J. Am. Chem. Soc., Vol. 118, No. 29, 1996 6865
2-(1-Cyclohexenylethynyl)-1-cyclopentenecarboxaldehyde
(12e): isolated in 98% yield as a yellow oil; IR (neat) 2183, 1667,
1584, 1434, 1386, 1350, 1238, 919, 842, 799, 737, 645 cm-1; 1H NMR
(C6D6) δ 10.38 (1 H, s), 6.09 (1 H, tt, J ) 4.1 and 2.0 Hz), 2.44 (2 H,
tt, J ) 7.6 and 2.2 Hz), 2.31 (2 H, tt, J ) 7.7 and 2.2 Hz), 2.05-2.0
(2 H, m), 1.82-1.7 (2 H, m), 1.46-1.2 (6 H, m); 13C NMR (C6D6) δ
187.69, 147.65, 142.35, 137.58, 120.77, 102.77, 81.72, 38.89, 29.87,
29.15, 25.96, 22.34, 22.09, 21.50; MS (m/e) 200 (M+), 199, 185, 171,
157, 129, 128, 115, 108, 91, 77.
(Z)-4-(Trimethylsilyl)-1-[2-(trimethylsilylethynyl)-1-cyclopente-
nyl]-1-buten-3-yne (15a). The following procedure for the synthesis
of 15a is representative. To a dispersion of 0.16 g (4.0 mmol) of KH
in 10 mL of diethyl ether at 0 °C under a nitrogen atmosphere was
added 0.418 g (1.0 mmol) of 14a in 6 mL of diethyl ether. After 45
min of stirring, the reaction mixture was filtrated to remove excess
KH, washed with an aqueous solution of NH4Cl, dried over MgSO4,
and concentrated. The residue was purified by column chromatography
(silica gel/hexanes) to furnish 0.223 g (78%) of 15a (Z:E ) 93:7) as a
yellow solid: mp 69-71 °C; IR (KBr) 2139, 2130, 1438, 1249, 1173,
(1S,2R)-2-(tert-Butyldimethylsilyl)-4-(trimethylsilyl)-1-[2-(tri-
methylsilylethynyl)-1-cyclopentenyl]-3-butyn-1-ol (14a). The fol-
lowing procedure for the synthesis of 14a is representative. To a
solution of 0.68 g (3.00 mmol) of 3-(tert-butyldimethylsilyl)-1-
(trimethylsilyl)-1-propyne in 10 mL of THF was added 1.2 mL (3.00
mmol) of a 2.5 M solution of n-butyllithium in hexanes at -10 °C.
After 30 min at -10 °C, 0.5 mL of B-MeO-9-BBN (0.46 g, 3.00 mmol)
was introduced with a syringe. After an additional 45 min at 0 °C, 0.5
mL of BF3‚OEt2 (0.57 g, 4.0 mmol) was added and the mixture was
stirred at 0 °C for 20 min to form the 3-(tert-butyldimethylsilyl)-1-
(trimethylsilyl)-1-allenylborane 11 before 0.576 g of the enynyl
aldehyde 12a (3.00 mmol) in 5 mL of THF was introduced. The
mixture was allowed to warm to room temperature and stirred for 6 h.
THF and hexanes were evaporated at reduced pressure and pressure
was then restored with nitrogen. Hexanes (20 mL) was added followed
by 0.5 mL of 2-aminoethanol, and a precipitate was formed almost
immediately. After 15 min of stirring, the precipitate was removed by
filtration, and the filtrate was washed with water, dried over MgSO4,
and concentrated. The residue was purified by column chromatography
(silica gel/5% diethyl ether in hexanes) to afford 0.76 g (1.82 mmol,
61%) of 14a as a yellow oil: IR (neat) 3532, 2157, 2136, 1674, 1593,
1
1007, 986, 850, 758 cm-1; H NMR (C6D6) δ 7.06 (1 H, d, J ) 11.7
Hz), 5.48 (1 H, d, J ) 11.9 Hz), 3.07 (2 H, t, J ) 7.4 Hz), 2.39 (2 H,
t, J ) 7.6 Hz), 1.59 (2 H, quintet, J ) 7.6 Hz), 0.19 (9 H, s), 0.14 (9
H, s); 13C NMR (CDCl3) δ 148.89, 134.94, 126.96, 108.23, 104.33,
103.16, 101.96, 101.35, 36.38, 33.70, 23.15, 0.08, -0.32; MS (m/e)
286 (M+), 271, 255, 213, 197, 175, 128, 73. A minor set of 1H NMR
(C6D6) signals at δ 7.50 (1 H, d, J ) 16 Hz) and 5.57 (1 H, d, J ) 16
Hz) attributable to the E isomer (7%) were also observed. The E isomer
was separated by column chromatography.
(Z)-1-[2-(1-Hexynyl)-1-cyclopentenyl]-4-(trimethylsilyl)-1-buten-
3-yne (15b): a yellow liquid; IR (neat) 2136, 1588, 1464, 1249, 1180,
1
1000, 842, 760 cm-1; H NMR (C6D6) δ 7.05 (1 H, d, J ) 11.7 Hz),
5.50 (1 H, d, J ) 11.7 Hz), 3.13 (2 H, t, J ) 7.4 Hz), 2.43 (2 H, t, J
) 7.4 Hz), 2.21 (2 H, t, J ) 6.6 Hz), 1.66 (2 H, quintet, J ) 7.5 Hz),
1.39-1.24 (4 H, m), 0.77 (3 H, t, J ) 7.1 Hz), 0.16 (9 H, s).; 13C
NMR (C6D6) δ 146.17, 135.74, 128.88, 107.67, 105.31, 101.35, 99.52,
77.76, 37.13, 33.93, 31.21, 23.19, 22.23, 19.78, 13.68, -0.23; MS (m/
e) 270 (M+), 196, 167, 141, 115, 73. Anal. Calcd for C18H26Si: C,
1
79.93; H, 9.69. Found: C, 79.79; H, 9.55. A minor set of H NMR
(CDCl3) signals at δ 7.10 (1 H, d, J ) 16 Hz) and 5.53 (1 H, d, J )
16 Hz) attributable to the E isomer (8%) were also observed. The E
isomer was separated by column chromatography.
1
1470, 1249, 842, 759 cm-1; H NMR (CDCl3) δ 4.76 (1 H, d, J ) 9
Hz), 2.65-2.5 (2 H, m), 2.5-2.4 (2 H, m), 2.47 (1 H, d, J ) 9 Hz),
2.21 (1 H, d, J ) 3.1 Hz), 1.9-1.8 (2 H, m), 0.96 (9 H, s), 0.17 (12
H, s), 0.13 (9 H, s), 0.12 (3 H, s); 13C NMR (CDCl3) δ 155.81, 118.07,
105.65, 101.14, 100.00, 90.30, 68.50, 36.73, 32.83, 27.06, 26.94, 22.57,
17.71, 0.07, 0.04, -6.60, -6.63.
(Z)-1-[2-(Phenylethynyl)-1-cyclopentenyl]-4-(trimethylsilyl)-1-
buten-3-yne (15c): a yellow liquid; IR (neat) 2135, 1599, 1489, 1442,
1412, 1249, 1180, 1018, 977, 846, 755 cm-1; 1H NMR (CDCl3) δ 7.44-
7.40 (2 H, m), 7.28-7.25 (3 H, m), 6.85 (1 H, d, J ) 11.7 Hz), 5.52
(1 H, d, J ) 11.9 Hz), 3.02 (2 H, t, J ) 7.4 Hz), 2.58 (2 H, t, J ) 7.6
Hz), 1.93 (2 H, quintet, J ) 7.6 Hz), 0.17 (9 H, s); 13C NMR (CDCl3)
δ 147.66, 135.06, 131.41, 128.28, 128.20, 127.09, 123.33, 107.93,
104.42, 101.87, 97.89, 85.89, 36.44, 33.76, 23.16, -0.32; MS (m/e)
290 (M+), 275, 259, 247, 231, 215, 202, 173, 159, 145, 135, 121, 105,
73.
(1S,2R)-2-(tert-Butyldimethylsilyl)-1-[2-(1-hexynyl)-1-cyclopente-
nyl]-4-(trimethylsilyl)-3-butyn-1-ol (14b): a yellow oil; IR (neat)
1
3463, 2157, 1464, 1249, 841 cm-1; H NMR (C6D6) δ 4.97 (1 H, dd,
J ) 8.6 and 3.1 Hz), 2.75-2.4 (4 H, m), 2.55 (1 H, d, J ) 8.6 Hz),
2.43 (1 H, d, J ) 3.1 Hz), 2.17 (2 H, t, J ) 6.8 Hz), 1.75-1.64 (2 H,
m), 1.48-1.26 (4 H, m), 1.04 (9 H, s), 0.79 (3 H, t, J ) 7.0 Hz), 0.37
(3 H, s), 0.25 (3 H, s), 0.16 (9 H, s); 13C NMR (C6D6) δ 152.73, 118.79,
107.09, 96.48, 89.55, 77.28, 69.45, 37.53, 33.23, 31.26, 27.45, 27.25,
22.77, 22.25, 19.59, 17.88, 13.72, 0.17, -6.17, -6.42.
(Z)-1-[2-(3-Methyl-3-buten-1-ynyl)-1-cyclopentenyl]-4-(trimeth-
ylsilyl)-1-buten-3-yne (15d): a yellow liquid; IR (neat) 2135, 1612,
1
1438, 1250, 1180, 1025, 995, 893, 843, 760 cm-1; H NMR (CDCl3)
(1S,2R)-2-(tert-Butyldimethylsilyl)-1-[2-(phenylethynyl)-1-cyclo-
pentenyl]-4-(trimethylsilyl)-3-butyn-1-ol (14c): a yellow oil; IR
δ 6.76 (1 H, d, J ) 11.7 Hz), 5.51 (1 H, d, J ) 11.7 Hz), 5.32 (1 H,
m), 5.25 (1 H, quintet, J ) 1.6 Hz), 3.01 (2 H, t, J ) 7.5 Hz), 2.52 (2
H, t, J ) 7.7 Hz), 1.98-1.87 (2 H, m), 1.94 (3 H, dd, J ) 1.6 and 1.0
Hz), 0.19 (9 H, s); 13C NMR (CDCl3) δ 147.46, 135.01, 127.10, 126.88,
121.76, 107.81, 104.42, 101.77, 99.09, 84.84, 36.42, 33.70, 23.53, 23.13,
-0.33; MS (m/e) 254 (M+), 239, 223, 211, 195, 181, 179, 165, 153,
141, 128, 115, 97, 83, 73. A minor set of 1H NMR (CDCl3) signals at
δ 7.08 (1 H, d, J ) 16 Hz) and 5.58 (1 H, d, J ) 16 Hz) attributable
to the E isomer (5%) were also observed.
1
(neat) 3545, 2155, 1489, 1469, 1249, 841, 755, 690 cm-1; H NMR
(CDCl3) δ 7.41-7.37 (2 H, m), 7.29-7.24 (3 H, m), 4.84 (1 H, dd, J
) 9.0 and 3.3 Hz), 2.60 (4 H, m), 2.57 (1 H, d, J ) 9.0 Hz), 2.28 (1
H, d, J ) 3.3 Hz), 1.92 (2 H, quintet, J ) 7.5 Hz), 0.95 (9 H, s), 0.18
(3 H, s), 0.14 (3 H, s), 0.13 (9 H, s); 13C NMR (CDCl3) δ 154.27,
131.27, 128.24, 127.97, 123.48, 118.21, 105.64, 95.12, 90.37, 85.41,
68.53, 36.83, 32.83, 27.07, 27.01, 22.55, 17.62, 0.04, -6.53, -6.65.
(1S,2R)-2-(tert-Butyldimethylsilyl)-1-[2-(3-methyl-3-buten-1-ynyl)-
1-cyclopentenyl]-4-(trimethylsilyl)-3-butyn-1-ol (14d): a yellow oil;
(Z)-1-[2-(1-Cyclohexenylethynyl)-1-cyclopentenyl]-4-(trimethyl-
silyl)-1-buten-3-yne (15e): a yellow liquid; IR (neat) 2136, 1583, 1552,
1
1
IR (neat) 3540, 2156, 1673, 1605, 1470, 1249, 839 cm-1; H NMR
1437, 1249, 1017, 842, 759 cm-1; H NMR (C6D6) δ 7.07 (1 H, d, J
(CDCl3) δ 5.24 (1 H, m), 5.20 (1 H, quintet, J ) 1.7 Hz), 4.74 (1 H,
dm, J ) 9 Hz), 2.64-2.46 (5 H, m), 2.23 (1 H, d, J ) 3.3 Hz), 1.99-
1.83 (2 H, m), 1.89 (3 H, t, J ) 1.5 Hz), 0.96 (9 H, s), 0.16 (3 H, s),
0.13 (9 H, s), 0.12 (3 H, s); 13C NMR (CDCl3) δ 154.02, 126.87, 121.28,
118.14, 105.65, 96.33, 90.34, 84.38, 68.50, 36.79, 32.80, 27.02, 23.57,
22.54, 17.64, 0.05, -6.61, -6.66.
(1S,2R)-2-(tert-Butyldimethylsilyl)-1-[2-(1-cyclohexenylethynyl)-
1-cyclopentenyl]-4-(trimethylsilyl)-3-butyn-1-ol (14e): a yellow oil;
IR (neat) 3538, 2156, 1470, 1249, 1030, 838 cm-1; 1H NMR (C6D6) δ
6.12 (1 H, tt, J ) 4 and 2 Hz), 4.99 (1 H, br s), 2.86-2.40 (4 H, m),
2.45 (1 H, d, J ) 3.1 Hz), 2.18-2.12 (2 H, m), 1.86-1.77 (2 H, m),
1.75-1.65 (2 H, m), 1.44-1.24 (4 H, m), 1.02 (9 H, s), 0.36 (3 H, s),
0.25 (3 H, s), 0.16 (9 H, s); 13C NMR (C6D6) δ 153.71, 134.35, 121.44,
118.59, 107.02, 97.88, 89.62, 83.52, 69.50, 37.38, 33.48, 29.75, 27.26,
27.17, 25.85, 22.85, 22.56, 21.74, 17.87, 0.17, -6.16, -6.35.
) 11.7 Hz), 6.13 (1 H, tt, J ) 4.1 and 1.9 Hz), 5.51 (1 H, d, J ) 11.9
Hz), 3.13 (2 H, t, J ) 7.5 Hz), 2.44 (2 H, t, J ) 7.6 Hz), 2.16-2.10
(2 H, m), 1.87-1.80 (2 H, m), 1.65 (2 H, quintet, J ) 7.5 Hz), 1.42-
1.26 (4 H, m), 0.16 (9 H, s); 13C NMR (C6D6) δ 146.78, 135.66, 135.05,
128.45, 121.51, 107.93, 105.35, 101.66, 100.76, 84.11, 36.93, 34.10,
29.68, 25.94, 23.28, 22.56, 21.73, -0.23; MS (m/e) 294 (M+), 220,
191, 165, 115, 73. Anal. Calcd for C20H26Si: C, 81.57; H, 8.90.
Found: C, 80.96; H, 9.03. A minor set of 1H NMR (C6D6) signals at
δ 7.60 (1 H, d, J ) 16 Hz) and 5.63 (1H, d, J ) 16 Hz) attributable
to the E isomer (6%) were also observed.
Dimers 18a-d. To a solution containing 0.334 g (1.17 mmol) of
15a in 10 mL of THF and 5 mL of ethanol was added 6 mL (6.0 mmol)
of a 1.0 M solution of TBAF in THF at room temperature under a
nitrogen atmosphere. After 4 h of stirring, 100 mL of diethyl ether
was added. The mixture was washed with water (3 × 30 mL), dried

6866 J. Am. Chem. Soc., Vol. 118, No. 29, 1996
Wang et al.
over MgSO4, and concentrated. The residue was purified by column
chromatography (silica gel/hexanes) to afford 0.11 g (66%) of 18a-d
as a white solid: 1H NMR (CDCl3) δ 7.2-6.8 (4 H, m) [6.43 (dm, J
) 9.9 and 1 Hz), 6.41 (dm, J ) 10.1 and 1 Hz), 6.31 (dd, J ) 9.9 and
1 Hz), 6.29 (dd, J ) 9.9 and 1 Hz), 6.20 (dd, J ) 10.1 and 4.5 Hz),
6.19 (dd, J ) 9.9 and 4.5 Hz), 6.08 (dd, J ) 9.9 and 4.4 Hz), signals
due to the eight vinylic hydrogens of the four isomers with one overlap
at δ 6.08 which has ca. twice the intensity of the other signals with
nearly equal intensities among them], 4.83 (1 H, m), 4.41 (1 H, m),
3.3-2.6 (8 H, m), 2.3-1.9 (4 H, m); 13C NMR (CDCl3) δ [148.46,
146.94, 146.82, 146.42, 144.79, 144.66, 144.62, 144.50, 144.42, 144.23,
144.18, 144.13, 143.99, 143.24, 142.42 (2 carbons), 142.26, 142.16,
141.21, 141.10, 137.93, 137.78, 136.82, 136.78, 132.66, 131.77, 130.82,
129.78, 129.72, 129.19, 126.96, 126.70 (31 signals due to the 32
quaternary sp2 carbons of the four isomers)], [127.37, 126.46, 126.36,
126.31, 126.24, 126.20, 126.12, 125.95, 125.60, 124.76, 123.90, 123.77,
123.73 (2 carbons), 123.42, 123.14, 123.00, 122.66, 122.40, 122.25,
119.33, 118.72, 117.94, 117.78 (23 signals due to the 24 CH sp2
carbons)], [43.58, 43.48, 42.98, 42.88, 42.76, 42.68, 42.48, 42.24 (8
signals of the 8 sp3 carbons on the four-membered rings)], [32.96, 32.93,
32.79, 32.72, 32.56, 31.33, 30.85, 30.72, 30.18, 29.29 (benzylic CH2
carbons on the five-membered rings with several overlapping peaks)],
[25.74, 25.57, 25.55, 25.38, 25.35, 25.16, 25.03, 25.00 (8 signals due
to the 8 central CH2 carbons on the five-membered rings)]; MS (m/e)
284 (M+), 269, 256, 241, 239, 228, 214, 167, 152, 142, 128, 114.
Attempts to separate 18a-d by HPLC produced a fraction which
contained predominantly only one of the four isomers. Its NMR
spectral data are consistent with the structural features of the angular
dimers: 1H NMR (CDCl3) δ 7.16 (1 H, d, J ) 8 Hz), 7.13 (1 H, d, J
) 7 Hz), 7.10 (1 H, d, J ) 8 Hz), 6.93 (1 H, d, J ) 7.5 Hz), 6.41 (1
H, dd, J ) 10.1 and 1 Hz), 6.19 (1 H, dd, J ) 10.1 and 4.5 Hz), 4.85
(1 H, d, J ) 6.0 Hz), 4.42 (1 H, t, J ) 5 Hz), 2.95-2.75 (8 H, m),
2.15-1.95 (4 H, m); 13C NMR (CDCl3) δ 146.93, 144.66, 144.17,
142.44, 141.24, 137.97, 131.74, 127.38, 126.94, 126.36, 123.74, 123.16,
122.66, 117.96, 43.45, 42.86, 32.80, 32.56, 30.73, 29.28, 25.39, 25.01.
139.61, 138.20, 136.24, 136.20, 129.85, 125.75, 125.58, 123.65, 123.14,
118.50, 51.56, 48.57, 40.77, 36.40, 35.40, 32.52, 32.37, 32.24, 29.67,
27.89, 25.45, 25.38, 23.11, 22.38, 14.11, 14.03; MS (m/e) 396 (M+),
339, 312, 297, 283; HRMS calcd for C30H36 396.2819, found 396.2794.
Diels-Alder Adduct 22. To a solution of 15b (320 mg, 1.2 mmol)
and 2.1 g (32 mmol) of cyclopentadiene in a mixture of 10 mL of
THF and 10 mL of ethanol was added 3.0 mL of a 1.0 M solution of
TBAF (3.0 mmol) in THF at room temperature under a nitrogen
atmosphere. After 10 h, 100 mL of pentane was added and the mixture
was washed with water (3 × 30 mL), dried over MgSO4, and
concentrated. The residue was purified by column chromatography
(silica gel/hexanes) to afford 157 mg (50%) of 22 as a light yellow
oil: IR (neat) 1454, 1438, 1378, 1335, 1251, 906, 808, 756, 732, 716
1
cm-1; H NMR (CDCl3) δ 7.02 (1 H, d, J ) 7.3 Hz), 6.70 (1 H, d, J
) 7.3 Hz), 5.80 (1 H, dd, J ) 5.5 and 3.1 Hz), 5.65 (1 H, dd, J ) 5.5
and 3.1 Hz), 3.37 (1 H, d, J ) 4.6 Hz), 3.00 (1 H, br s), 2.87 (2 H, t,
J ) 7.3 Hz), 2.79 (2 H, t, J ) 8.0 Hz), 2.71 (1 H, br s), 2.13-2.02 (2
H, m), 1.99-1.86 (4 H, m), 1.55-1.34 (4 H, m), 0.96 (3 H, t, J ) 7.2
Hz); 13C NMR (CDCl3) δ 146.42, 143.92, 142.85, 137.76, 134.24,
133.27, 122.61, 119.47, 58.06, 53.93, 51.35, 47.53, 44.92, 35.63, 32.75,
30.88, 30.01, 25.56, 23.53, 14.15; MS (m/e) 264 (M+), 249, 235, 221,
207, 193, 179, 165, 152, 115, 91; HRMS calcd for C20H24 264.1879,
found 264.1890. In addition, the dimer 21 (47 mg) was also isolated
in 20% yield.
Dimer 23. To a solution of 15c (162 mg, 0.559 mmol) in a mixture
of THF (10 mL) and ethanol (2 mL) was added 1.7 mL of a 1.0 M
solution of TBAF (1.7 mmol) in THF at room temperature under a
nitrogen atmosphere and the reaction was followed by TLC. Im-
mediately after the addition of TBAF, only one major spot, presumably
the desilylated product of 15c, appeared on TLC. After 30 min of
stirring, a second spot, the dimer 23, appeared followed by the
appearance of a third spot due to the dimer 24 with a very close Rf
value 2 h later. As the mixture was stirred for 12 h, the desilylated
product of 15c completely disappeared. Diethyl ether (100 mL) was
added and the mixture was washed with water (3 × 10 mL), dried
over MgSO4, and concentrated. The residue was purified by column
chromatography (silica gel/hexanes) to afford 23 mg (19%) of the dimer
23 as a light yellow solid and 44 mg (36%) of a mixture of 23 and 24
(23:24 ) 40:60). Dimer 23: 1H NMR (CDCl3) δ 7.18-7.05 (10 H,
m), 6.88-6.84 (4 H, m), 4.07 (2 H, s), 2.75 (4 H, t, J ) 7 Hz), 2.64-
2.55 (2 H, m), 1.93-1.83 (2 H, m), 1.64-1.53 (4 H, m); 13C NMR
(CDCl3) δ 144.03, 143.62, 143.03, 140.63, 140.59, 127.42, 127.17,
126.00, 124.04, 119.71, 68.07, 51.94, 32.88, 30.83, 25.86; MS (m/e)
436 (M+), 421, 408, 359, 345, 331, 258, 218, 202. The dimer 23 was
thermally labile and slowly rearranged to 24 even at room temperature.
Dimer 24. A solution containing 22.0 mg (0.0504 mmol) of 23 in
0.5 mL of C6D6 was heated at 60 °C in an NMR tube, and the
rearrangement of 23 to 24 was found to be complete in 3 h as indicated
1
The additional H NMR decoupling experiments further support the
assigned structures.
Diels-Alder Adduct 19. To a solution containing 0.30 g (1.05
mmol) of 15a and 1.32 g (20.0 mmol) of cyclopentadiene in 10 mL of
THF and 10 mL of ethanol at 0 °C under a nitrogen atmosphere was
added 3.0 mL (3.00 mmol) of a 1.0 M solution of TBAF in THF. The
resulting mixture was stirred at 0 °C for 3 h. Pentane (100 mL) was
added, and the mixture was washed with water (3 × 20 mL), dried
over MgSO4, and concentrated. The residue was purified by column
chromatography (silica gel/hexanes) to afford 0.175 g (80%) of 19 as
a colorless oil: IR (neat) 1454, 1437, 1338, 1294, 1268, 1294, 1195,
1
1101, 1071, 906, 806, 733 cm-1; H NMR (CDCl3) δ 6.96 (1 H, d, J
) 7.3 Hz), 6.66 (1 H, d, J ) 7.5 Hz), 5.65 (2 H, d, J ) 1.5 Hz), 3.64
(2 H, m), 2.93 (2 H, m), 2.80 (2 H, t, J ) 7.4 Hz), 2.73-2.58 (2 H,
m), 2.00 (2 H, quintet, J ) 7.4 Hz), 1.93 (1 H, d, J ) 8.2 Hz), 1.63 (1
H, d, J ) 8.2 Hz); 13C NMR (CDCl3) δ 145.57, 143.41, 143.00, 138.09,
132.46, 132.14, 122.66, 119.52, 54.75, 45.49, 44.97, 43.58, 43.21, 32.70,
29.70, 25.41; MS (m/e) 208 (M+) 193, 179, 165, 152, 142, 115, 102,
89. Anal. Calcd for C16H16: C, 92.26; H, 7.74. Found: C, 91.59; H,
7.77. The endo configuration of 19 was supported by the NOE
measurements by irradiating the hydrogens on the four-membered ring
at δ 3.64 and the methylene hydrogens on the bridge at δ 1.63 (syn to
the four-membered ring) and at δ 1.93 (anti to the four-membered ring).
1
by H NMR. Benzene was removed and the residue was purified by
column chromatography (silica gel/hexanes) to afford 19.6 mg (89%)
1
of 24 as a yellow solid: IR (CDCl3) 718, 651 cm-1; H NMR (C6D6)
δ 7.33 (4 H, d, J ) 7.1 Hz), 6.97-6.85 (10 H, m), 6.79 (2 H, d, J )
7.9 Hz), 2.69 (2 H, dt, J ) 15.9 and 7.2 Hz), 2.81 (2 H, dt, J ) 15.9
and 7.6 Hz), 2.55 (2 H, dt, J ) 15.7 and 7.7 Hz), 2.41 (2 H, dt, J )
15.7 and 7.1 Hz), 1.76 (4 H, quintet, J ) 7.5 Hz); 13C NMR (C6D6) δ
143.36, 142.07, 141.39, 140.83, 140.47, 135.53, 133.88, 131.05, 128.01,
126.82, 126.28, 122.93, 33.27, 32.88, 25.52; MS (m/e) 436 (M+), 408,
359, 345, 331, 315, 302, 258, 239, 165; HRMS calcd for C34H28
436.2192, found 436.2179. The dimer 24 was recrystallized from a
1:1 mixture of hexanes and chloroform for the X-ray structure
determination.
Dimer 26. To a solution of 15d (120 mg, 0.47 mmol) in a mixture
of THF (10 mL) and ethanol (2 mL) was added 1.5 mL of a 1.0 M
solution of TBAF (1.5 mmol) in THF at room temperature under a
nitrogen atmosphere. After 16 h, 50 mL of diethyl ether was added
and the reaction mixture was washed with water (3 × 10 mL), dried
over MgSO4, and concentrated. The residue was purified by column
chromatography (silica gel/hexanes) and HPLC (silica/hexanes) to
furnish 31 mg (36%) of the dimer 26 as a white solid: 1H NMR (CDCl3)
δ 7.18 (2 H, d, J ) 7.3 Hz), 7.10 (2 H, d, J ) 7.3 Hz), 3.89 (2 H, s),
3.14-3.00 (4 H, m), 2.91 (4 H, t, J ) 7.5 Hz), 2.58 (2 H, dt, J ) 12.5
and 3 Hz), 2.16 (2 H, dt, J ) 12 and 3 Hz), 2.13 (6 H, s), 2.10 (4 H,
Dimer 21. To a solution containing 0.091 g (0.34 mmol) of 15b in
a mixture of 15 mL of THF and 5 mL of ethanol at room temperature
under a nitrogen atmosphere was added 1.02 mL (1.02 mmol) of a 1.0
M solution of TBAF in THF. The resulting mixture was stirred for 18
h. Pentane (100 mL) was added, and the mixture was washed with
water, dried over MgSO4, and concentrated. The residue was purified
by column chromatography (silica gel/hexanes) to afford 0.041 g (61%)
1
of 21 as a colorless oil: IR (neat) 1453, 1378, 1118, 808 cm-1; H
NMR (CDCl3) δ 7.38 (1 H, d, J ) 7.7 Hz), 7.12 (1 H, d, J ) 7.7 Hz),
7.07 (1 H, d, J ) 7.5 Hz), 6.86 (1 H, d, J ) 7.3 Hz), 6.01 (1 H, d, J
) 6.0 Hz), 3.88 (1 H, d, J ) 6.0 Hz), 3.20-3.04 (2 H, m), 2.95-2.66
(8 H, m), 2.48-2.30 (2 H, m), 2.04 (4 H, quintet, J ) 7.3 Hz), 2.00-
1.75 (2 H, m), 1.52-1.18 (6 H, m), 0.93 (3 H, t, J ) 7.0 Hz), 0.85 (3
H, t, J ) 7.0 Hz); 13C NMR (CDCl3) δ 149.38, 144.05, 143.34, 143.06,

Electrocyclizations of (Z,Z)-3,5-Octadiene-1,7-diynes
J. Am. Chem. Soc., Vol. 118, No. 29, 1996 6867
m); 13C NMR (CDCl3) δ 146.01, 144.92, 140.50, 136.97, 134.92,
132.17, 123.54, 119.71, 55.07, 34.59, 33.15, 32.95, 25.54, 22.83; MS
(m/e) 364 (M+), 349, 334, 219, 166, 152, 145, 139; HRMS calcd for
C28H28 364.2192, found 364.2203. The dimer 26 was recrystallized
from chloroform for the X-ray structure determination.
They are attributable to the sp3 carbons on the four-membered rings of
32a and 32b as observed in the case of 26. In addition, the two 13C
NMR signals at δ 51.97 and 45.40 were also found to be CH carbons
by the DEPT experiment and are attributable to the CH carbons on the
six-membered rings of 32a and 32b.
Dimer 32. The same procedure was repeated as described for 26
except that 186 mg (0.63 mmol) of 15e was used to furnish 52 mg
(37%) of 32 (two isomers, 35:65) as a white solid. The two isomers
tentatively assigned to 32a and 32b were inseparable by HPLC (silica/
hexanes). IR (CDCl3) 736, 651 cm-1; 1H NMR (CDCl3) δ 7.20-7.09
(4 H, m), 3.97 and 3.92 (2 H, two singlets, 35 : 65), 3.25-2.85 (10 H,
m), 2.60-1.40 (20 H, m); 13C NMR (CDCl3) δ 145.98, 144.93, 144.74,
144.41, 140.81, 140.25, 139.81, 135.27, 134.72, 134.67, 133.74, 123.32,
119.65, 119.53, 55.20, 54.47, 51.97, 45.40, 37.30, 36.88, 33.30, 33.14,
33.06, 33.02, 30.58, 28.29, 27.22, 25.53, 25.48, 25.39, 23.81, 22.64;
MS (m/e) 444 (M+), 401, 387, 375, 363, 222, 193, 179, 165, 149, 129,
115, 105; HRMS calcd for C34H36 444.2819, found 444.2825. The
Acknowledgment. We thank Professor Peter Gannett (School
of Pharmacy, West Virginia University) for performing the NOE
experiments to confirm the endo structure of 19. The financial
support of the National Science Foundation (CHE-9307994) to
K. K. W. is gratefully acknowledged. J. L. P. acknowledges
the financial support provided by the Chemical Instrumentation
Program of the National Science Foundation (CHE-9120098)
for the acquisition of a Siemens P4 X-ray diffractometer by the
Department of Chemistry at West Virginia University.
Supporting Information Available: 1H and 13C NMR
spectra of 12a-e, 14a-e, 15a-e, 18, 19, 21, 22, 23, 24, 26,
and 32 and tables of crystallographic data for the X-ray
diffraction analyses of 24 and 26 (59 pages). See any current
masthead page for ordering and Internet access instructions.
1
two singlets in the H NMR spectrum at δ 3.97 and 3.92 could be
attributed to the hydrogen atoms on the four-membered rings of 32a
and 32b as observed in the case of 26. These two signals could not be
attributed to the presence of only one isomer without symmetry in its
structure, such as 32c, which could be expected to exhibit two AB
pattern doublets with equal intensities. The two 13C NMR signals at δ
55.20 and 54.47 were found to be CH carbons by the DEPT experiment.
JA960506A