Formation of 2-[1-(Trimethylsilyl)alkylidene]-4-cyclopentene-1,3-dione from Lewis Acid-Catalyzed Reaction of Cyclobutenedione Monoacetal with Alkynylsilane: Novel Cationic 1,2-Silyl Migrative Ring Opening and Subsequent 5-Exo-Trig Ring Closure

Article abstract of DOI:10.1021/jo961552w

An ethoxycarbenium ion intermediate, which was produced by the catalytic action of a Lewis acid on a cyclobutenedione monoacetal, reacted with phenyl(trimethylsilyl)acetylene to give a normal electrophilic substitution product. In sharp contrast, the same

Full text of DOI:10.1021/jo961552w

1
292
J . Org. Chem. 1997, 62, 1292-1298
F or m a tion of
2
-[1-(Tr im eth ylsilyl)a lk ylid en e]-4-cyclop en ten e-1,3-d ion e fr om
Lew is Acid -Ca ta lyzed Rea ction of Cyclobu ten ed ion e Mon oa ceta l
w ith Alk yn ylsila n e: Novel Ca tion ic 1,2-Silyl Migr a tive Rin g
Op en in g a n d Su bsequ en t 5-Exo-Tr ig Rin g Closu r e
†
Yoshihiko Yamamoto, Masashi Noda, Masatomi Ohno, and Shoji Eguchi*
Institute of Applied Organic Chemistry, Faculty of Engineering, Nagoya University,
Chikusa, Nagoya 464-01, J apan
Received August 9, 1996X
An ethoxycarbenium ion intermediate, which was produced by the catalytic action of a Lewis acid
on a cyclobutenedione monoacetal, reacted with phenyl(trimethylsilyl)acetylene to give a normal
electrophilic substitution product. In sharp contrast, the same catalytic reaction with bis-
(trimethylsilyl)acetylene afforded a 2-methylene-4-cyclopentene-1,3-dione derivative as a ring
expansion product instead of an alkynylation product. Butyl(trimethylsilyl)acetylene showed
reactivity between the aforementioned compounds as a result of the formation of both types of
products. In the reactions of such alkyl-substituted silylacetylenes, both E- and Z-isomers of 2-(1-
silylalkylidene)cyclopentenediones were obtained in ratios dependent on the reaction temperature
and the amount of Lewis acid. This rearrangement resulted from unprecedented cationic 1,2-silyl
migration on the alkynylsilane and subsequent ring expansion promoted by the formed vinyl cation
intermediate. A detailed mechanism of the novel ring-expansion route is discussed with the aid of
PM3 calculations, especially for the reclosure step, which is explained by a 5-exo-trig cyclization
rather than a pentadienyl cation electrocyclization.
2
-Alkylidene-4-cyclopentene-1,3-dione, an attractive
Recently, the 1,2-silyl migration has been documented
from a synthetic point of view as a key step for ring
construction. The [3 + 2] cycloaddition of allenylsilanes
leading to various 5-membered carbo- and heterocyclic
compounds as developed by Danheiser involved the 1,2-
1
building block for synthesis of cyclopentanoids, is ac-
cessible from ring-expansion routes starting from squaric
acid. Liebeskind and co-workers have developed pal-
ladium- and mercury-catalyzed rearrangement of 4-
alkynyl-4-hydroxycyclobutenones to a variety of 2-alky-
lidene-4-cyclopentene-1,3-dione derivatives.2 We have
also found a similar type of reaction to furnish 2-(iodo-
methylene)-4-cyclopentene-1,3-diones, in which ionic re-
arrangement of a hypoiodite intermediate is believed to
be involved.3 These reactions are based on 1,2-acyl
migration at the ring-expansion step. On the other hand,
Liebeskind demonstrated an alternative [4 + 1] cyclo-
addition route using the reaction of cobaltacyclopentene-
dione with terminal acetylenes.4 Moore et al. also
reported a limited thermal rearrangement route involv-
ing a (2-alkynylethenyl)ketene.5
In the course of our search for electrophilic addition
reactions of cyclobutenedione monoacetal with alkynyl-
silanes, we found a novel rearrangement to 2-[1-(tri-
methylsilyl)alkylidene]-4-cyclopentene-1,3-dione. It was
induced by cationic 1,2-silyl migration after addition of
the monoacetal to the alkynylsilane and followed by the
formed vinyl cation-induced ring opening and reclosure.
silicon shift of an intermediate vinyl cation to accomplish
the subsequent annulation.6 A similar 1,2-silyl migrative
[
3 + 2] cycloaddition reaction of allylsilane was first
7
reported by Kn o¨ lker. In this case, a normal Sakurai
reaction product was also formed as a result of competi-
tive desilylation; therefore, a sterically hindered silyl
group has to be used to suppress a nucleophilic attack
on the silicon atom.8 In addition to these [3 + 2]
cycloadditions, [2 + 1] cycloaddition of 2-selenovinyl-
silanes with R,â-unsaturated ketones and aldehydes was
recently demonstrated by Yamazaki.9 This cyclopropa-
nation showed that a cationic 1,2-silicon shift was as-
sisted by the adjacent selenyl group. Thus, the 1,2-silyl
migration is favored when a cyclization pathway is
advanced by the migration and a competing desilylation
pathway is suppressed by steric bulkiness of the silyl
group. In our case, this migration was observed for the
first time as a key step for ring expansion based on
†
Research Fellow of the J apan Society for the Promotion of Science.
Abstract published in Advance ACS Abstracts, February 15, 1997.
(6) (a) Danheiser, R. L.; Carini, D. J .; Basak, A. J . Am. Chem. Soc.
1981, 103, 1604. (b) Danheiser, R. L.; Carini, D. J .; Fink, D. M.; Basak,
A. Tetrahedron 1983, 39, 935. (c) Danheiser, R. L.; Stoner, E. J .;
Koyama, H.; Yamashita, D. S. J . Am. Chem. Soc. 1989, 111, 4407. (d)
Danheiser, R. L.; Fink, D. M. Tetrahedron Lett. 1985, 25, 2513. (e)
Danheiser, R. L.; Kwasigroch, C. A.; Tasi, Y.-M. J . Am. Chem. Soc.
1985, 107, 7233. (f) Danheiser, R. L.; Becker, D. A. Heterocycles 1987,
25, 277. (g) Becker, D. A.; Danheiser, R. L. J . Am. Chem. Soc. 1988,
111, 389.
(7) Kn o¨ lker, H.-J .; J ones, P. G.; Pannek, J .-B. Synlett 1990, 429.
(8) Kn o¨ lker, H.-J .; Wanzl, G. Synlett 1995, 378 and references cited
therein. For an allylstannane case, see: Herndon, J . W. J . Am. Chem.
Soc. 1987, 109, 3165.
X
(1) (a) Kikuchi, H.; Tsukitani, Y.; Iguchi, K.; Yamada, Y. Tetrahedron
Lett. 1982, 23, 25. (b) Iguchi, K.; Kaneta, S.; Mori, K.; Yamada, Y.;
Honda, A.; Mori, Y. Tetrahedron Lett. 1985, 26, 5787. (c) Baker, B.
J .; Okuda, R. K.; Yu, P. T. K.; Scheuer, P. J . J . Am. Chem.Soc. 1985,
1
07, 2976.
2) (a) Liebeskind, L. S.; Mitchell, D.; Foster, B. S. J . Am. Chem.
(
Soc. 1987, 109, 7908. (b) Mitchell, D.; Liebeskind, L. S. J . Am. Chem.
Soc. 1990, 112, 291. (c) Liebeskind, L. S.; Bombrun, A. J . Org. Chem.
1
5
1
994, 59, 1149.
3) Yamamoto, Y.; Ohno, M.; Eguchi, S. Tetrahedron Lett. 1995, 36,
539.
(
(
4) Liebeskind, L. S.; Chidambaram, R. J . Am. Chem. Soc. 1987,
(9) (a) Yamazaki, S.; Tanaka, M.; Yamaguchi, A.; Yamabe, S. J . Am.
Chem. Soc. 1995, 60, 6546. (b) Yamazaki, S.; Tanaka, M.; Inoue, T.;
Morimoto, N.; Kumagai, H.; Yamamoto, K. J . Org. Chem. 1995, 60,
6546.
09, 5025.
(
5) Foland, L. D.; Karlsson, J . O.; Perri, S. T.; Schwabe, R.; Xu, S.
L.; Patil, S.; Moore, H. W. J . Am. Chem. Soc. 1989, 111, 975.
S0022-3263(96)01552-6 CCC: $14.00 © 1997 American Chemical Society

Reaction of Cyclobutenedione Monoacetal with Alkynylsilane
J . Org. Chem., Vol. 62, No. 5, 1997 1293
Sch em e 1
Sch em e 2
alkynylsilanes. We report here the above cationic rear-
rangement in relation to the silyl-migration aptitude of
various alkynylsilanes and a two-step process of ring
opening and reclosure. The present method is of interest
as an alternative formal [4 + 1] cycloaddition route to
Sch em e 3
2
,4
2
-alkylidene-4-cyclopentene-1,3-dione.
Resu lts
We previously reported that catalytic action of a Lewis
Ta ble 1. Syn th esis of Cyclop en ten ed ion es 8a -e fr om
Aceta ls 2a -e
acid on monoacetal 2 available from squaric acid (1)10-12
produced an ethoxycarbenium ion species 3, which sub-
sequently reacted with allylsilane, silyl enol ether, and
Lewis acid
(equiv)
acetylene time
8
entry
2
R1
R2
(equiv)
(h) yield (%)
a
b
c
d
e
f
g
h
i
2a Me
2a Me
2a Me
2a Me
2a Me
2a Me
2b Ph
2c PhCdC OEt SnCl
2d Me
2e Me
OEt BF
OEt BF
OEt BF
OEt BF
OEt TiCl (1.2)
OEt SnCl
OEt SnCl
3
3
3
3
‚OEt
‚OEt
‚OEt
‚OEt
2
2
2
2
(1.2)
(2)
(2)
3
3
6
6
3
3
3
3
3
3
24 8a (13)a
24 8a (36)a
24 8a (37)
silyl ketene acetal regioselectivity to afford the desired
substitution products 4 (Scheme 1).13,14
a
This type of electrophilic reaction using silylacetylenes
has now been studied. First examined was the phenyl-
substituted case. Thus, (phenylethynyl)trimethylsilane
8a (50)a
(3)
24
1
1
3
1
4
8a (41)
8a (85)
8b (63)
8c (35)
8d (57)
4
4
4
4
4
(1.2)
(1.2)
(1.2)
(1.2)
(1.2)
(
5a ) (3 equiv) was allowed to react with monoacetal 2a
in the presence of Et O‚BF (1.2 equiv) at 0 °C for 24 h,
2
3
Me SnCl
Ph SnCl
1
j
18 8e (19)a
and the expected 4-(phenylethynyl)-4-ethoxycyclobuten-
one 6 was obtained in 29% yield after standard workup
and chromatographic separation (Scheme 2). The yield
was increased up to 81% by using more excess of 5a (5
equiv), but not improved by shorter reaction time (1 h)
and higher (room) temperature. The structural deter-
mination was based on the spectral inspection: MS peak
a
Deacetalized cyclobutene-1,2-diones were recovered in 66%
entry a), 49% (entry b), 32% (entry c), 30% (entry d), and 35%
(
(entry j) yields, respectively.
-1
alkynyl group and at 1769 and 1626 cm due to an enone
1
group, and H NMR signals of two ethoxy groups and a
phenyl group at δ 7.29-7.36 and 7.44-7.50. The ob-
tained adduct 6 was transformed to an oxaspiro[2.5]-
+
-1
at m/z 270 (M ), IR absorption at 2224 cm due to an
5
heptadienone 7 according to the known process.
(
10) West, R. Oxocarbons; Academic Press: New York, 1980.
In sharp contrast, the reaction of 2a with bis(trimeth-
ylsilyl)acetylene (5b) gave no alkynylation product such
as 6 under the same conditions. Instead, a less polar
product (TLC analysis) was separated by chromatogra-
(11) For recent examples of synthetic application, see: (a) Koo, S.;
Liebeskind, L. S. J . Am. Chem. Soc. 1995, 117, 3389. (b) Sun, L.;
Liebeskind, L. S. J . Org. Chem. 1995, 60, 8194. (c) Turnbull, P.; Moore,
H. W. J . Org. Chem. 1995, 60, 644. (d) Lee, K. H.; Moore, H. W. J .
Org. Chem. 1995, 60, 735. (e) Turnbull, P.; Moore, H. W. J . Org. Chem.
1
1
1
3
1
995, 60, 3274. (f) Xiong, Y.; Xia, H.; Moore, H. W. J . Org. Chem.
995, 60, 6460. (g) Santora, V. J .; Moore, H. W. J . Am. Chem. Soc.
995, 117, 8486. (h) Taing, M.; Moore, H. W. J . Org. Chem. 1996, 61,
29. (i) Paquette, L. A.; Morwick, T. J . Am. Chem. Soc. 1995, 117,
451. (j) Paquette, L. A.; Doyon, J . J . Am. Chem. Soc. 1995, 117, 6799.
phy and analyzed by spectroscopy. The mass spectrum
+
(M , m/z 310) and the elemental analysis supported not
a desilylated but a deacetalized product. The IR spec-
trum showed no alkynyl absorption but a carbonyl
(k) Wilson, P. D.; Friedrich, D.; Paquette, L. A. J . Chem. Soc., Chem.
-1
absorption at 1682 cm , which was no longer ascribable
Commun. 1995, 1351. (l) Morwick, T.; Doyon, J .; Paquette, L. A.
Tetrahedron Lett. 1995, 36, 2369. (m) Yamamoto, Y.; Ohno, M.;
Eguchi, S. J . Am. Chem. Soc. 1995, 117, 9653. (n) Varea, T.; Grancha,
A.; Asensio, G. Tetrahedron 1995, 51, 12373. (o) Paquette, L. A.;
Morwick, T. M.; Negri, J . T. Tetrahedron 1996, 52, 3075. (p) Schmidt,
A. H.; Thiel, S. H.; Gaschler, O. J . Chem. Soc., Perkin Trans. 1 1996,
1
to a four-membered cyclic ketone. The H NMR spectrum
revealed the presence of two different trimethylsilyl
groups together with only one ethoxy group. More
importantly, the 13C NMR spectrum consisted of all sp -
carbon signals (δ 134.8, 150.2, 167.0, 174.8, 188.3, and
2
4
95.
12) For review of synthetic application, see: (a) Liebeskind, L. S.
(
1
91.0 ppm) except signals due to substituents. These
Tetrahedron 1989, 45, 3053. (b) Moore, H. W.; Yerxa, B. R. Chemtracts:
Org. Chem. 1992, 5, 273.
data allowed us to assign the structure as 2-[bis(tri-
methylsilyl)methylene]-4-ethoxy-5-methyl-4-cyclopentene-
(
13) (a) Yamamoto, Y.; Ohno, M.; Eguchi, S. Chem. Lett. 1995, 525.
b) Yamamoto, Y.; Ohno, M.; Eguchi, S. Bull. Chem. Soc. J pn. 1996,
9, 1353.
14) For other examples of the reaction of squaric acid derivatives
(
6
1
,3-dione (8a ) (Scheme 3). Table 1 shows optimization
of the reaction conditions. While increasing the amount
of Et O‚BF and silylacetylene 5b improved the yield
from 13% to 50% (Table 1, entries a-d), choice of the
catalyst was critical; TiCl (Table 1, entry e) and, more
efficiently, SnCl raised the yield up to 85% (Table 1,
(
with unsaturated organosilanes, see: (a) Ohno, M.; Yamamoto, Y.;
Eguchi, S. J . Chem. Soc., Perkin Trans. 1 1991, 2272. (b) Ohno, M.;
Yamamoto, Y.; Shirasaki, Y.; Eguchi, S. J . Chem. Soc., Perkin Trans.
2
3
1
1
1
1993, 263. (c) Ohno, M.; Yamamoto, Y.; Eguchi, S. Tetrahedron Lett.
993, 34, 4807. (d) Yamamoto, Y.; Ohno, M.; Eguchi, S. Tetrahedron
994, 50, 7783. (e) Yamamoto, Y.; Nunokawa, K.; Ohno, M.; Eguchi,
4
4
entry f). Under these catalytic conditions, phenyl- and
phenylethynyl-substituted monoacetals 2b and 2c gave
S. Synlett 1993, 781. (f) Yamamoto, Y.; Nunokawa, K.; Okamoto, K.;
Ohno, M.; Eguchi, S. Synthesis 1995, 571.

1
294 J . Org. Chem., Vol. 62, No. 5, 1997
Yamamoto et al.
Sch em e 4
Sch em e 5
Sch em e 6
Ta ble 2. Rea ction of Mon oa ceta l 2a a n d Alk yn ylsila n e
5
c
Lewis
acid (equiv)
T
(°C)
time
(h)
9 yield (%)
[E/Z]
10
yield (%)
a
entry
a
b
c
d
e
f
BF3‚OEt2 (1.2)
BF3‚OEt2 (1.2)
BF3‚OEt2 (1.2)
BF3‚OEt2 (2.0)
BF3‚OEt2 (2.0)
BF3‚OEt2 (2.0)
SnCl4 (1.2)
0
5
10
0
10
20
0
24
24
24
24
24
24
1
19 [81/19]
23 [56/44]
24 [41/59]
32 [28/72]
31 [30/70]
29 [29/71]
49 [39/61]
15
19
18
25
21
11
13
g
a
1
The E/Z ratio of 9 was determined by the H-NMR spectrum.
the corresponding cyclopentenediones 8b and 8c (Table
, entries g and f), respectively, in moderate yields. The
1
reaction of dimethyl-substituted monoacetal 2d affirmed
the above structural assignment (Table 1, entry i); the
1
H NMR spectrum of the anticipated symmetrical product
8
4 5
d exhibited only two singlet signals due to C - and C -
methyl groups and double trimethylsilyl groups, and the
pentenediones 12 and 13,15 respectively, in fair yields,
but the reactions with bromomethyl-substituted silyl-
acetylene 5g and unsubstituted silylacetylene 5h resulted
in a low yield, or worse, no product was isolable. In these
cases, the alkynylation byproducts were uncharacterized
because they could not be obtained in sufficient amounts
for analysis.
The stereochemistry of 9 was deduced by the relative
chemical shifts of trimethylsilyl and allylic methylene
groups. As indicated in Scheme 6, standard (E)- and (Z)-
1
3
3
C NMR spectrum revealed only two sp -carbon signals
2
and four sp -carbon signals. The reaction of 3-phenyl-
substituted 2e resulted in low yield even after prolonged
reaction time, probably because of the steric effect of a
3
phenyl group at C (Table 1, entry i).
The above results showed the difference in rearrange-
ment aptitude between phenyl(trimethylsilyl)acetylene
(5a ) and bis(trimethylsilyl)acetylene (5b). Obviously, a
substituent of the acetylene influenced the switching of
the reaction course. This fact prompted us to investigate
the reaction with various alkyl-substituted silylacety-
lenes. Typically, the acetal 2a was reacted with butyl-
substituted silylacetylene 5c (3 equiv) in the presence of
2
-[1-(trimethylsilyl)-3-butenylidene]cyclopentenediones 17
(ratio: 79/21) were prepared from 4-hydroxy-4-[(trimeth-
ylsilyl)ethynyl]cyclobutenone 16 by the established
procedure2 and compared with 9 obtained in entry a
a
1
Et
2
O‚BF
3
(1.2 equiv) at 0 °C for 24 h. In this case, the
(Table 2). In the H NMR spectra of these definite E/Z
reactivity was shown to be between 5a and 5b; a ring-
expansion product 9 with E/Z ratio of 81/19 and an
alkynylation product 10 were obtained in 19% and 15%
yields, respectively (Scheme 4, Table 2). The stereo-
isomerism of the exo-olefin was influenced by the reaction
isomers, the relative chemical shifts due to major and
minor isomers of 17 accorded well with those of the
corresponding isomers of 9.16 Likewise, the E/Z relation-
ships of the other products 11-14 were deduced and are
noted in Scheme 5.
temperature and amount of Et
2
3
O‚BF ; from 0 to 10 °C
with 1.2 equiv of the catalyst, the population of the
Z-isomer was gradually increased (Table 2, entries a-c).
The E/Z ratio was inversed and reached an almost
constant ratio ca. 3/7 at 0-20 °C with 2 equiv of the
catalyst (Table 2, entries d-f). Nearly the same reaction
Discu ssion
Scheme 7 shows the plausible mechanism for the
formation of alkynylation products 6 and 10 and ring-
expansion products 8, 9, and 11-14. The electrophilic
addition of ethoxycarbenium ion 3 to silylacetylene 5
initially produces a vinyl cation intermediate 18. Sub-
4
was attained by use of 1.2 equiv of SnCl (Table 2, entry
g), where the best yield (49%) was recorded for the ring-
expansion product 9 as in the case with 5b (cf. Table 1).
(
15) E- or Z-isomer was formed selectively. However, because of
no data for relative chemical shifts, the stereochemistry could not be
confirmed.
In this manner, the reaction of 2a with 3-butenyl-
substituted silylacetylene 5d afforded the corresponding
product 11 in a comparable yield and E/Z ratio (Scheme
(
16) This was observed more consistently in the partially hydroge-
nated products of (E)- and (Z)-17 (side-chain conversion from an allyl
to a propyl group with H /Pd-BaSO in hexane for 12 h): the chemical
shifts obtained were 0.25 ppm (TMS) and 2.95 ppm (allylic CH ) for
E-isomer and 0.24 ppm (TMS) and 3.01 ppm (allylic CH ) for Z-isomer.
2
4
5
). Similarly, the reaction with methoxymethyl- and
2
benzyl-substituted silylacetylenes 5e and 5f gave cyclo-
2

Reaction of Cyclobutenedione Monoacetal with Alkynylsilane
J . Org. Chem., Vol. 62, No. 5, 1997 1295
Sch em e 7
F igu r e 1. Schematic energy diagram for ring opening of 22
and cyclization of resulted 24. Indicated values show relative
energies, ∆∆H in kcal/mol.
ate 20 to a strain-relieved and delocalized cation inter-
mediate 21 and subsequent ring closure to 8, 9, and 11-
1
7
1
4. In fact, semiempirical calculations (PM3 ) support
1
8
this option; for simplicity, these two steps were calcu-
lated by using an unsubstituted system. Structures of a
vinyl cation 22 and a ring-opening cation 24, transition
states 23, 25, and a product cation 26 were fully
optimized by the EF routine with the keyword PRECISE.
The resulting transition structures were subjected to a
vibrational analysis, and in each case, only one imaginary
frequency was found. The energy profile is outlined in
Figure 1. It was revealed that the process involving two
steps is exothermic by 39.1 kcal/mol, and the ring
expansion evolves from the extremely low energy barrier
(2.6 kcal/mol) of the ring-opening step. The ring-opened
24 has a U-shaped structure and is considered as a
ketenyl- and allenyl-substituted cation. From this cation
24 to the product cation 26, the cyclization proceeds with
an energy barrier of 15.7 kcal/mol. Thus, the two-step
process proposed above seems to be a reasonable route
for the present ring-expansion reaction.
In the above mechanism, the final ring closure of 24,
which is regarded as a pentadienyl cation, is associated
with the Nazarov cyclization, and it may therefore involve
the conrotatory electrocyclization to a cyclopentenyl
cation 26.19 If such a pericyclic cyclization mechanism
is operative in our case, the stereochemistry of the
kinetically formed product might be controlled by a
torquoselectivity at this stage.20 In order to obtain more
detailed information on the reclosure step, the intrinsic
reaction coordinate (IRC) calculation21 was further at-
tempted. Figure 2 shows optimized structures of the
starting cation 24, the transition state 25, the cyclized
cation 26, and representative intermediates 27 and 28.
sequent 1,2-silyl migration depends on an acetylenic
substituent R, which determines the course of reaction
(path a vs path b). In the case of phenyl-substituted vinyl
cation 18 (R ) Ph), the benzylic stabilization allows
straightforward desilylation (path a) to give 4-(phenyl-
ethynyl)cyclobutenone 6. In contrast, trimethylsilyl-
3
substituted vinyl cation 18 (R ) SiMe ) can isomerize to
the other vinyl cation 20 (path b) because the rearranged
cation 20 is more stabilized by the â-effect of two silyl
groups than the primarily formed cation 18. Then 20,
formed after 1,2-silyl migration, induces the subsequent
ring expansion to [bis(trimethylsilyl)methylene]cyclo-
pentenedione 8 via two possible routes: simple and
simultaneous 1,2-acyl shift and a two-step process of ring
opening and reclosure (see below). On the other hand,
the behavior of the butyl-substituted vinyl cation 18 (R
)
Bu) is between the above two extremes. The negligible
difference in stability of the intermediate cations 18 and
9 results in the formation of both the ring-expansion
1
product 9 and the alkynylation product 10.
The reaction of unsymmetrical acetylene (e.g., 5c)
afforded stereoisomeric mixtures of (1-trimethylsilyl-
alkylidene)cyclopentenediones. As described above, the
E/Z ratio of the product 9 varied depending on the
reaction conditions. When acetal 2a was reacted with
5
c, the E-isomer of 9 was formed predominantly on
employing 1.2 equiv of Et
O‚BF at 0 °C, whereas the
Z-isomer is favored at higher temperature, in excess
2
3
(
17) Stewart, J . J . P. J . Comput. Chem. 1989, 10, 209.
amounts of Et
such as SnCl
2
O‚BF
3
, and with a stronger Lewis acid
These observations suggest that the
(18) Semiempirical calculations were carried out using MOPAC
4
.
version 94.10 packaged in the CAChe Version 3.7.
(19) Fleming, I. Frontier Orbitals and Organic Chemical Reactions;
J ohn Wiley & Sons: New York, 1976.
E-isomer was initially produced and converted into the
thermodynamically favored Z-isomer by subsequent Lewis
acid-catalyzed geometrical isomerization. In fact, the
control experiment in entry c (Table 2) indicated that the
(20) The torquoselectivity in electrocyclic ring opening of cyclobutene
derivatives has been extensively studied: (a) Kirmse, W.; Houk, K. N.
J . Am. Chem. Soc. 1984, 106, 7989. (b) Rondan, N. G.; Houk, K. N. J .
Am. Chem. Soc. 1985, 107, 2099. (c) Rudolf, K.; Spellmeyer, D. C.;
Houk, K. N. J . Org. Chem. 1987, 52, 3708. (d) Houk, K. N.; Spellmeyer,
D. C.; J efford, C. W.; Rimbault, C. G.; Wang, Y.; Miller, R. D. J . Org.
Chem. 1988, 53, 2127. (e) Buda, A. B.; Wang, Y.; Houk, K. N. J . Org.
Chem. 1989, 54, 2264. (f) Niwayama, S.; Houk, K. N. Tetrahedron
Lett. 1992, 33, 883. (g) Niwayama, S. J . Org. Chem. 1996, 61, 640.
1
E/Z ratio of 9 ( H NMR measurement) changed from 62/
3
8 (1 h) to 49/51 (6 h) and nearly reached the equilibrium
ratio of 33/67 (48 h). Because the Z-isomer should be
formed primarily via the simultaneous stereospecific 1,2-
acyl migration through a silyl-bridged cation 19, a direct
route can be ruled out.
Another ring-expansion pathway could be rationalized
by considering ring opening of the vinyl cation intermedi-
(
h) Niwayama, S.; Kallel, E. A.; Sheu, C.; Houk, K. N. J . Org. Chem.
1996, 61, 2517. (i) Niwayama, S.; Kallel, E. A.; Spellmeyer, D. C.;
Sheu, C.; Houk, K. N. J . Org. Chem. 1996, 61, 2813.
(21) (a) Fukui, K.; Kato, S.; Fujimoto, H. J . Am. Chem. Soc. 1975,
97, 1. (b) Fukui, K. Acc. Chem. Res. 1981, 14, 363.

1
296 J . Org. Chem., Vol. 62, No. 5, 1997
Yamamoto et al.
F igu r e 2. Calculated structures and reaction coordinates (R
c
) of representative intermediates on the IRC of cyclization of 24.
F igu r e 4. Plot of dihedral angle Si(9)-C(6)-C(5)-C(1) [-O-]
and heat of formation [-4-] vs reaction coordinate.
Sch em e 8
F igu r e 3. Plot of dihedral angle H(6)-C(1)-C(5)-C(4) [-O-]
and heat of formation [-4-] vs reaction coordinate.
Initially (reaction coordinate: R
4 has a delocalized U-shaped structure with the C(1)
and C(5) atoms almost sp-hybridized (∠ C(2)-C(1)-O(7)
177.2°, ∠ C(4)-C(5)-O(6) ) 175.6°) and the two silyl
c
) 3.048 Å), the cation
2
)
groups perpendicular to the molecular plane (∠ Si(9)-
C(6)-C(5)-C(1) ) 91.9°). As the cyclization proceeds,
contrast indicates that new bond formation results from
an interaction of in-plane p-orbitals of sp-C(1) and -C(5),
but not out-of-plane orbitals of these carbons, which is
associated with the C(5)-C(6) bond length to be stretched
from 1.265 Å of 24 to 1.354 Å of 28. Thus, these data
support our postulate that the reclosure step is performed
by intramolecular addition of an acyl cation to a silyla-
llene in a 5-exo-trig mode (Scheme 8).
On the basis of this mechanism, the stereochemistry
of the products can be explained as in Scheme 9. The
transition structure TS-31 leading to 9Z is less favorable
than the transition structure TS-32 leading to 9E because
of the steric repulsion between the trimethylsilyl and the
ethoxy groups. Therefore, the E isomer was obtained
2
the sp-C(5) atom is gradually rehybridized to sp with
the rotation at the C(5) atom. This motion seemingly
agrees with the Nazarov cyclization mechanism. How-
ever, the rotation at the C(5) atom was estimated to occur
differently between the cation 24 and the prototypical
pentadienyl cation 29. Figure 3 shows the gradual
rotation expected for the electrocyclization of 29. In
contrast, no appreciable rotational change was observed
in 24 before and after the transition state except for
drastic changes at the earliest and the latest stages; as
shown in Figure 4, the dihedral angle Si(9)-C(6)-C(5)-
C(1) is almost constant (ca. 130°) at the C(1)-C(5) bond-
forming stage (distance of 2.5-1.6 Å). The observed

Reaction of Cyclobutenedione Monoacetal with Alkynylsilane
J . Org. Chem., Vol. 62, No. 5, 1997 1297
Sch em e 9
Davison BW-300) eluted with mixed solvents [hexane (H),
ethyl acetate (A)]. Dichloromethane was dried over CaCl ,
2
distilled, and stored over 4 Å molecular sieves. Squaric acid
was supplied by Kyowa Hakko Kogyo Co. Ltd.
Syn th esis of Cyclobu ten ed ion e Mon oa ceta l. According
to the reported procedure,23 cyclobutenedione monoacetal 2d
was synthesized as follows: to a solution of monoacetal 2a
(
2.13 g, 10.0 mmol) in dry THF (50 mL) was added methyl
lithium (30.0 mL, 31 mmol; 1.04 M solution in ether) at -78
C under a nitrogen atmosphere, and the solution was stirred
°
for 30 min. To this solution was added trifluoroacetic anhy-
dride (2.1 mL, 15.0 mmol). The reaction mixture was stirred
for 30 min, quenched with 10% NaHCO
extracted with ether (20 mL × 3). The extracts were washed
with brine (30 mL), dried (Na SO ), and evaporated to dryness.
3
(20 mL), and
2
4
Flash chromatography of the residue (Elution H-A 40:1) gave
monoacetal 2d (850 mg, 46%) as a colorless oil. Monoacetals
a -c,e were reported in our previous paper.1
3b
2
4
,4-Dieth oxy-1,2-dim eth yl-2-cyclobu ten on e (2d): IR (neat
-
1 1
1
765, 1644 cm ; H NMR δ 1.23 (6 H, t, J ) 7.0 Hz), 1.75 (3
H, q, J ) 1.0 Hz), 2.17 (3 H, q, J ) 1.0 Hz), 3.74 (4 H, q, J )
7
1
1
6
.0 Hz); 13C NMR δ 7.2, 11.5, 15.5 (2C), 61.1 (2C), 115.6, 152.7,
+
79.8, 195.7; MS (EI) m/z (rel intensity) 184 (M , 2), 155 (25),
39 (13), 127 (100), 111 (18). Anal. Calcd for C10
5.19; H, 8.75. Found: C, 65.50; H, 8.44.
16 3
H O : C,
predominantly as a kinetic product (Table 2). Assuming
that BF -complexed intermediates 33Z and 33E are
3
produced by coordination with the more Lewis basic
carbonyl group,22 33E inversely becomes less favorable
than 33Z because of the steric repulsion between the
Syn th esis of 4-Alk yn ylcyclobu ten on e 6 a n d Its Con -
ver sion to Oxa sp ir o[2.5]octa d ien on e 7. To a solution of
a (129 mg, 0.57 mmol) and silylacetylene 5a (316 mg, 2.50
2
mmol) in dry dichloromethane (2 mL) was added Et O‚BF
2
3
(0.091 mL, 0.72 mmol) at 0 °C under a nitrogen atmosphere.
After being stirred for 13 h, the reaction mixture was quenched
3
trimethylsilyl and the BF -coordinated carbonyl groups.
with 10% NaHCO
3
(5 mL) and extracted with dichloromethane
SO ) and evaporated
Thus, the thermodynamically more favored complex 33Z
affords the major isomer 9Z under Lewis acid-catalyzed
conditions.
(5 mL × 3). The extracts were dried (Na
2
4
to dryness. Flash chromatography of the residue (Elution
H-A 8:1) gave 4-alkynylcyclobutenone 6a (125 mg, 81%) as a
pale-yellow oil. Thermal rearrangement of 6 to 7 was carried
out in the reported manner.4
Con clu sion s
Cationic 1,2-silyl migration, which has so far been
observed in cycloaddition reactions using allenyl-, allyl-,
3
,4-Dieth oxy-2-m eth yl-4-(ph en yleth yn yl)-2-cyclobu ten -
-
1
1
on e (6): IR (neat) 2224, 1769, 1626 cm ; H NMR δ 1.28 (3
H, t, J ) 7.0 Hz), 1.50 (3 H, t, J ) 7.0 Hz), 1.73 (3 H, s), 3.86
and 3.93 (each 1 H, dq, J ) 9.2, 7.0 Hz), 4.54 and 4.61 (each
6
-9
and vinyltrialkylsilanes,
emerged in the Lewis acid-
catalyzed reaction of silylacetylenes with cyclobutene-
dione monoacetals, leading to 2-alkylidene-1,3-dione. To
our knowledge, the present 1,2-silyl migration is the first
example in alkynylsilane chemistry for the ring-expan-
sion reaction. This was significant in the reaction of bis-
1
H, dq, J ) 10.0, 7.0 Hz), 7.29-7.36 (3 H, m), 7.44-7.50 (2
13
H, m); C NMR δ 6.6, 15.3, 15.6, 63.1, 69.4, 82.2, 88.3, 90.9,
1
(
(
22.3, 125.5, 128.7 (2 C), 129.3, 132.3 (2 C), 179.9, 186.8; MS
+
EI) m/z (rel intensity) 270 (M , 12), 241 (37), 224 (73), 213
100), 185 (59). Anal. Calcd for C17 : C, 75.53; H, 6.71.
18 3
H O
(trimethylsilyl)acetylene since one of the two silyl groups
Found: C, 75.57; H, 6.67.
facilitated the migration of the other. On the other hand,
phenyl-substituted silylacetylene gave a simple alkyny-
lated product without the rearrangement and butyl-
substituted silylacetylene gave both the alkynylated
product and the rearranged product with the E/Z ratio
dependent on the reaction temperature and the Lewis
acid employed. Control experiments showed that the
E-isomer was initially formed and converted into the
Z-isomer by Lewis acid-catalyzed geometrical isomeriza-
tion. This result ruled out the possibility of ring expan-
sion via simultaneous silyl and acyl migrations through
a silicon-bridged cation. PM3 calculations supported
another two-step process of ring-opening and reclosure
involving an intramolecular addition of an acyl cation to
silylallene rather than a pentadienyl cation electro-
cyclization.
4-E t h oxy-2,5-d im et h yl-7-p h en yl-1-oxa sp ir o[2.5]oct a -
4,7-d ien -6-on e (7): 49% (ca. 15:1 diastereomer mixture); oil;
-
1
1
IR (neat) 1649, 1615 cm ; H NMR (signals due to a minor
isomer are indicated in bracket) δ 1.35 [1.37] (3 H, t, J ) 7.0
Hz), 1.54 [1.72] (3 H, d, J ) 5.4 Hz), 1.97 [1.99] (3 H, s), 3.93
[
1
3.92] (1 H, q, J ) 5.4 Hz), 3.98 and 4.08 [4.05 and 4.18] (each
H, dq, J ) 9.4, 7.0 Hz), 6.57 [6.37] (1 H, s), 7.34-7.45 (5 H,
13
m); C NMR δ 9.7, 14.5 [13.0], 15.8 [15.6], 58.1 [59.2], 61.6
[63.7], 70.3 [70.0], 127.7 [128.1], 128.5 [128.6] (2 C), 128.6
[128.3], 129.3 [129.1] (2 C), 136.1 [135.6], 138.6, 144.1 [143.8],
+
1
2
7
64.5, 186.8; MS (EI) m/z (rel intensity) 270 (M , 36), 254 (100),
41 (53), 214 (29), 197 (89). Anal. Calcd for C17
5.53; H, 6.71. Found: C, 75.49; H, 6.75.
18 3
H O : C,
Typ ica l P r oced u r e for Rea ction of Cyclobu ten ed ion e
Mon oa ceta l 2 a n d Silyla cetylen e 5. To a solution of
monoacetal 2a (146 mg, 0.68 mmol) and bis(trimethylsilyl)-
acetylene (5b) (348 mg, 2.04 mmol) in dry dichloromethane (2
mL) was added SnCl
nitrogen atmosphere. After being stirred for 1 h, the solution
was quenched with 10% NaHCO (5 mL) and extracted with
4
(0.096 mL, 0.82 mmol) at 0 °C under a
Exp er im en ta l Section
Gen er a l Meth od s. 1H and 13C NMR spectra were obtained
3
dichloromethane (5 mL × 3). The extracts were dried (Na -
2
4
SO ) and evaporated to dryness. Flash chromatography of the
in CDCl
3 4
solution with SiMe as an internal standard. Flash
residue (Elution H-A 30:1) gave cyclopentenedione 8a (179
mg, 85%) as a bright yellow oil. According to the above
procedure, other cyclopentenediones 8b-e, 9, and 11-14 were
obtained as an oil unless otherwise noted. Isolated yields are
chromatography was performed with a silica gel column (Fuji-
(
22) AM1 (Dewar, M. J . S.; Zeobisch, E. G.; Healy, E. F.; Stewart,
J . J . P. J . Am. Chem. Soc. 1985, 107, 3902.) calculations suggested
that the BF coordination to 9 is thermodynamically more favorable
3
at the carbonyl group of the vinylogous ester than that of the vinylogous
ketone. Also see: Rauk, A.; Hunt, I. R.; Keay, B. A. J . Org. Chem.
(23) Gayo, L.; Winters, M. P.; Moore, H. W. J . Org. Chem. 1992, 57,
1
994, 59, 6808.
6896.

1
298 J . Org. Chem., Vol. 62, No. 5, 1997
Yamamoto et al.
shown in Scheme 5 and Tables 1 and 2, in which the reactions
of 2a -e with 5b and of 2a with 5c under various conditions
are recorded. See the Supporting Information for spectral data
of 8b (mp 82-85 °C), 8c (mp 100-104 °C), 9, and 11-14.
under a nitrogen atmosphere. The reaction mixture was
stirred for 30 min, quenched with 10% NaHCO
extracted with ether (10 mL × 3). The extracts were washed
with brine (20 mL), dried (Na SO ), and evaporated to dryness.
3
(10 mL), and
2
4
2
-[Bis(tr im eth ylsilyl)m eth ylen e]-4-eth oxy-5-m eth yl-4-
Flash chromatography of the residue (Elution H-A 7:1) gave
16 (878 mg, 74%) as a yellow oil.
cyclop en ten e-1,3-d ion e (8a ): IR (neat) 1682, 1620, 1248, 844
-
1 1
cm ; H NMR δ 0.25 (9 H, s), 0.26 (9 H, s), 1.41 (3 H, t, J )
.0 Hz), 1.99 (3 H, s), 4.72 (2 H, q, J ) 7.0 Hz); 13C NMR δ 2.0
3 C), 2.2 (3 C), 7.1, 15.9, 68.1, 134.8, 150.2, 167.0, 174.8, 188.3,
3
-E t h oxy-4-h yd r oxy-2-m et h yl-4-[(t r im et h ylsilyl)et h -
7
yn yl]-2-cyclobu ten on e (16): IR (neat) 3337, 2166, 1763,
(
1
-1 1
1
)
624, 1252, 845 cm ; H NMR δ 0.18 (9 H, s), 1.49 (3 H, t, J
7.0 Hz), 1.67 (3 H, s), 4.10 (1 H, br s), 4.59 (2 H, q, J ) 7.0
+
91.0; MS (EI) m/z (rel intensity) 310 (M , 21), 295 (70), 281
(
100), 267 (53), 251 (36). Anal. Calcd for C15 Si : C,
H
26
O
3
2
13
Hz); C NMR δ -0.4 (3 C), 6.5, 15.2, 69.5, 83.4, 95.7, 99.3,
25.6, 180.7, 187.5; MS (EI) m/z (rel intensity) 238 (M , 26),
23 (22), 209 (100), 195 (76), 181 (12). Anal. Calcd for
5
8.02; H, 8.44. Found: C, 58.45; H, 8.20.
-[Bis(tr im eth ylsilyl)m eth ylen e]-4,5-d im eth yl-4-cyclo-
+
1
2
2
-1
p en ten e-1,3-d ion e (8d ): IR (neat) 1686, 1640, 1246, 849 cm
;
C
12
H
18
O
3
Si: C, 60.47; H, 7.61. Found: C, 60.74; H, 7.34.
According to the reported procedure,2a ring expansion of
cyclobutenones 16 was carried out as follows: a solution of 16
238 mg, 1.0 mmol), allyl bromide (0.87 mL, 10 mmol),
propylene oxide (1.75 mL, 25 mmol), and Pd(II) trifluoroacetate
17 mg, 0.05 mmol) in dry dichloromethane (8 mL) was stirred
at ambient temperature under a nitrogen atmosphere for 1 h.
The reaction mixture was quenched with saturated NH Cl (10
mL) and extracted with dichloromethane (5 mL × 3). The
extracts were washed with brine (10 mL), dried (Na SO ), and
evaporated to dryness. Flash chromatography of the residue
1
13
H NMR δ 0.26 (18 H, s), 2.05 (6 H, s); C NMR δ 2.0 (6 C),
9
.2 (2 C), 149.0, 154.5 (2 C), 177.8, 193.4 (2 C); MS (EI) m/z
+
(
rel intensity) 280 (M , 97), 265 (100), 250 (9), 237 (4). Anal.
Calcd for C14 Si : C, 59.94; H, 8.62. Found: C, 60.24; H,
.24.
-[Bis(tr im eth ylsilyl)m eth ylen e]-4-m eth yl-5-p h en yl-4-
(
H
24
O
2
2
8
(
2
cyclop en ten e-1,3-d ion e (8e): mp 67-70 °C; IR (neat) 1686,
620, 1246, 845 cm-1; H NMR δ 0.30 (9 H, s), 0.31 (9 H, s),
1
4
1
2
1
3
.24 (3 H, s), 7.45-7.57 (5 H, m); C NMR δ 2.0 (3 C), 2.1 (3
2
4
C), 10.5, 128.9 (2 C), 129.9, 130.1 (2 C), 130.3, 149.1, 152.2,
+
1
1
53.6, 180.7, 192.0, 193.5; MS (EI) m/z (rel intensity) 342 (M ,
2
00), 327 (88), 312 (11), 299 (4). Anal. Calcd for C19 Si :
(
7
Elution H-A 30:1) gave ring-expansion product 17 (197 mg,
H
26
O
2
1%) as a yellow oil.
C, 66.61; H, 7.65. Found: C, 66.85; H, 7.41.
,4-Die t h oxy-4-(1-h e xyn yl)-2-m e t h yl-2-cyclob u t e n -
on e (10). This was obtained with elution H-A 10:1 as a
3
4-Eth oxy-5-m eth yl-2-[1-(tr im eth ylsilyl)-3-bu ten yliden e]-
4-cyclop en ten e-1,3-d ion e (17): IR (neat) 1680, 1626, 1246,
-
1
-1
1
byproduct after elution of 9: IR (neat) 2232, 1771, 1634 cm
;
847 cm ; H NMR (signals due to E-isomer are indicated in
brackets) δ 0.25 [0.24] (9 H, s), 1.41 [1.42] (3 H, t, J ) 7.0 Hz),
1.98 (3 H, s), 3.79 [3.87] (2 H, dt, J ) 6.0, 1.6 Hz), 4.70 (2 H,
1
H NMR δ 0.91 (3 H, t, J ) 7.4 Hz), 1.23 (3 H, t, J ) 7.0 Hz),
1
2
7
.46 (3 H, t, J ) 7.0 Hz), 1.32-1.55 (4 H, m), 1.68 (3 H, s),
.29 (2 H, q, J ) 7.0 Hz), 3.74 and 3.82 (each 1 H, dq, J ) 9.2,
.0 Hz), 4.48 and 4.55 (each 1 H, dq, J ) 9.8, 7.0 Hz); 13C NMR
1
3
q, J ) 7.0 Hz), 4.90-5.06 (2 H, m), 5.70-5.90 (1 H, m);
C
NMR δ -0.1 [-0.4] (3 C), 7.0 [7.1], 15.9 [15.8], 34.8 [35.3], 67.9
[67.8], 116.5, 132.0 [134.1], 135.5, 135.8 [135.2], 165.7 [165.2],
166.6 [166.0], 189.0 [189.6], 192.4 [192.2]; MS (EI) m/z (rel
δ 6.5, 13.6, 15.2, 15.5, 18.8, 22.0, 30.5, 62.6, 69.1, 73.2, 88.0,
+
9
3
2.4, 125.0, 180.5, 187.7; MS (EI) m/z (rel intensity) 250 (M ,
+
6), 221 (100), 205 (8), 193 (57), 165 (28). Anal. Calcd for
intensity) 278 (M , 61), 263 (37), 249 (100), 233 (43). Anal.
C
15
H
20
O
3
: C, 71.97; H, 8.86. Found: C, 71.99; H, 8.84.
Calcd for C15
7.61.
22 3
H O Si: C, 64.71; H, 7.96. Found: C, 65.06; H,
P d -Ca t a lyzed R in g E xp a n sion of 4-H yd r oxy-4-[(t r i-
m eth ylsilyl))eth yn yl]cyclobu ten on e 16. According to the
reported procedure, starting 16 was prepared as follows: to a
solution of (trimethylsilyl)acetylene (0.85 mL, 6.0 mmol) in dry
Su p p or tin g In for m a tion Ava ila ble: Spectral data for
b,c, 9, and 11-14 (2 pages). This material is contained in
libraries on microfiche, immediately follows this article in the
microfilm version of the journal, and can be ordered from the
ACS; see any current masthead page for ordering information.
8
n
THF (5 mL) was added BuLi (3.71 mL, 6.0 mmol; 1.6 M in
hexane) at 0 °C under a nitrogen atmosphere, and the solution
was stirred for 30 min. The resulting solution was then
transferred to a solution of 3-ethoxy-4-methyl-3-cyclobutene-
1
,2-dione (701 mg, 5.0 mmol) in dry THF (20 mL) at -78 °C
J O961552W