(Trialkylsilyl)vinylketenes: Synthesis and application as diene components in Diels-Alder cycloadditions

Article abstract of DOI:10.1021/jo981289u

New strategies for the synthesis of (trialkylsilyl)vinylketenes ('TAS- vinylketenes') are described based on the photochemical Wolff rearrangement of α'-silyl-α'-diazo-α,β-unsaturated ketones and the 4π electrocyclic ring opening of cyclobutenones. These remarkably robust vinylketenes undergo highly regioselective [4 + 2] cycloadditions with reactive olefinic and acetylenic dienophiles to produce highly substituted cyclohexenones and phenols in which the ketene carbonyl dominates in controlling the regiochemical course of the reaction. The stereochemical course of these cycloadditions follows the Alder endo rule, as illustrated in the reaction of nitropropene with TAS-vinylketene 22.

Full text of DOI:10.1021/jo981289u

8380
J . Org. Chem. 1998, 63, 8380-8389
(Tr ia lk ylsilyl)vin ylk eten es: Syn th esis a n d Ap p lica tion a s Dien e
Com p on en ts in Diels-Ald er Cycloa d d ition s
J ennifer L. Loebach, Dawn M. Bennett, and Rick L. Danheiser*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received J uly 2, 1998
New strategies for the synthesis of (trialkylsilyl)vinylketenes (“TAS-vinylketenes”) are described
based on the photochemical Wolff rearrangement of R′-silyl-R′-diazo-R,â-unsaturated ketones and
the 4π electrocyclic ring opening of cyclobutenones. These remarkably robust vinylketenes undergo
highly regioselective [4 + 2] cycloadditions with reactive olefinic and acetylenic dienophiles to
produce highly substituted cyclohexenones and phenols in which the ketene carbonyl dominates
in controlling the regiochemical course of the reaction. The stereochemical course of these
cycloadditions follows the Alder endo rule, as illustrated in the reaction of nitropropene with TAS-
vinylketene 22.
In tr od u ction
The utility of ketenes in organic synthesis is well-
established.1 Vinylketenes (R,â-unsaturated alkenyl-
ketenes) have the capacity to function as exceptionally
versatile four-carbon building blocks for the assembly of
a variety of carbocyclic systems. [2 + 2] Cycloadditions
of vinylketenes with alkenes and alkynes are especially
valuable transformations which provide efficient access
to 2-vinylcyclobutanone and cyclobutenone derivatives.
Prior work in our laboratory has demonstrated that these
cycloadditions can serve as triggering steps for pericyclic
cascades leading to the formation of six-2 and eight-
membered3 carbocyclic compounds (Figure 1). In these
reactions, vinylketenes function in their “normal mode”
of reactivity: as electron-deficient 2π components for [2
+ 2] cycloadditions. It occurred to us that intriguing new
synthetic transformations might be possible if this nor-
mal reaction pathway could be suppressed, allowing
vinylketenes to express their underlying reactivity as
electron-rich conjugated dienes. In this case, vinylketenes
might participate as four-carbon components in Diels-
Alder and related cycloadditions, opening up new av-
enues for the synthesis of six-membered carbocyclic and
heterocyclic compounds.
F igu r e 1. Annulation methods based on pericyclic cascades
triggered by [2 + 2] cycloadditions of vinylketenes.
generally precludes their direct use as [4 + 2] enophiles.5
Vinylketene acetals have found use as vinylketene sur-
rogates; however, the scope of this methodology is
somewhat limited because of the sensitivity of these
compounds and the fact that the s-cis conformation
required for cycloaddition is usually disfavored in these
Z-substituted dienes.4c,6
The extension of the Diels-Alder reaction to include
highly functionalized dienes has greatly expanded the
utility of this important synthetic method.4 An objective
of continuing interest in this area has been the develop-
ment of vinylketene equivalents capable of participating
as diene components in Diels-Alder cycloadditions. The
tendency of vinylketenes to form only [2 + 2] cycloadducts
with alkenes and the intrinsic instability of these ketenes
The remarkable ability of silyl substituents to stabilize
ketenes and suppress their tendency to dimerize is well-
documented and has been attributed to a combination
of σ donation by the electropositive silyl group and
hyperconjugative σ-π interactions.7 Several years ago,
we reported the synthesis and isolation of (trimethylsi-
lyl)vinylketene (2), a moderately stable, fully characteriz-
able compound which can be stored for several weeks at
(1) (a) Tidwell, T. T. Ketenes; Wiley: New York, 1991. (b) Schau-
mann, E.; Scheiblich, S. In Methoden der Organischen Chemie (Houben-
Weyl); Kropf, E., Schaumann, E., Eds.; Thieme: Stuttgart, Germany,
1993; Vol. E15, Parts 2, 3.
(2) (a) Danheiser, R. L.; Martinez-Davila, C.; Sard, H. Tetrahedron
1981, 37, 3943. (b) Danheiser, R. L.; Gee, S. K. J . Org. Chem. 1984,
49, 1672. (c) Danheiser, R. L.; Brisbois, R. G.; Kowalczyk, J . J .; Miller,
R. F. J . Am. Chem. Soc. 1990, 112, 3093.
(3) Danheiser, R. L.; Gee, S. K.; Sard, H. J . Am. Chem. Soc. 1982,
104, 7670.
(4) (a) Fringuelli, F.; Taticchi, A. Dienes in the Diels-Alder Reaction;
Wiley: New York, 1990. (b) Petrzilka, M.; Grayson, J . I. Synthesis 1981,
753. (c) Danishefsky, S. Acc. Chem. Res. 1981, 14, 400.
(5) Only
a few cases of vinylketenes participating in [4 + 2]
cycloadditions have been reported. For examples, see: (a) Day, A. C.;
McDonald, A. N.; Anderson, B. F.; Bartczak, T. J .; Hodder, O. J . R. J .
Chem. Soc., Chem. Commun. 1973, 247. (b) Berge, J . M.; Rey, M.;
Dreiding, A. S. Helv. Chim. Acta 1982, 65, 2230. (c) Collomb, D.;
Doutheau, A. Tetrahedron Lett. 1997, 38, 1397.
(6) 1-Methoxy-1-(trimethylsiloxy)-1,3-butadienes undergo Diels-
Alder reactions in poor yield unless the diene is substituted with
additional electron-donating substituents. For example, see: Savard,
J .; Brassard, P. Tetrahedron 1984, 40, 3455.
(7) For
a discussion, see ref 1a and the following: Gong, L.;
McAllister, M. A.; Tidwell, T. T. J . Am. Chem. Soc. 1991, 113, 6021.
10.1021/jo981289u CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/21/1998

(Trialkylsilyl)vinylketenes
J . Org. Chem., Vol. 63, No. 23, 1998 8381
0 °C in solution.8,9 As predicted, 2 fails to enter into
typical ketene [2 + 2] cycloadditions and instead partici-
pates as a diene component in [4 + 2] cycloadditions with
electron-deficient alkenes and alkynes (e.g., eq 1). Un-
Sch em e 1
route to TAS-vinylketenes (Scheme 1). We recognized,
however, that the Wolff rearrangement of R-substituted-
R-diazo ketones such as 5 might not prove to be a
straightforward process. The photochemical Wolff rear-
rangement is generally thought to proceed via the singlet
excited state of the diazo ketone by way of a concerted
rearrangement of the migrating group with backside
displacement of nitrogen.10 This mechanism is possible
only from the conformation in which the migrating bond
and C-N bond have an antiperiplanar relationship. The
requisite conformation of 5 (depicted in Scheme 1) is
expected to be disfavored due to steric repulsion between
the bulky trialkylsilyl group and the vinyl moiety,
suggesting that concerted rearrangement of 5 might not
be an efficient process. Fortunately, however, Maas and
co-workers have shown that saturated alkyl(TAS)ketenes
can be generated by means of the Wolff rearrangement,12
suggesting that alternative mechanisms for this trans-
formation may be possible. For example, dissociation of
nitrogen might produce an acylcarbene that could rear-
range to the desired ketene via the conformation depicted
in eq 2.
fortunately, the scope of these reactions is restricted by
the limited thermal stability of this vinylketene, which
must be handled and stored in solution. In addition,
although 2 is conveniently prepared by dehydrohaloge-
nation of the R,â-unsaturated acyl chloride 1, we were
not able to extend this approach to the efficient synthesis
of more highly substituted vinylketene derivatives.
In this paper we report two new strategies for the
synthesis of substituted (trialkylsilyl)vinylketenes (“TAS-
vinylketenes”) based on (a) the photochemical Wolff
rearrangement of R′-silyl-R′-diazo-R,â-unsaturated ke-
tones and (b) the 4π electrocyclic ring opening of cy-
clobutenones. Access to a variety of substituted TAS-
vinylketenes via these methods has permitted a systematic
investigation of their Diels-Alder reactions with alkene
and alkyne dienophiles, the details of which are disclosed
herein.
Resu lts a n d Discu ssion
Although dehydrohalogenation provides convenient
access to (trimethylsilyl)vinylketene (2), this strategy is
not well-suited for the synthesis of more highly substi-
tuted TAS-vinylketenes. In some cases the requisite
precursors are not readily available, and regiochemical
ambiguities can arise in dehydrohalogenation (1,4-
elimination) reactions involving â,â-disubstituted-R,â-
unsaturated carboxylic acid derivatives. Recently, we
have demonstrated that the photochemical Wolff rear-
rangement provides an efficient method for the synthesis
of vinylketenes.2c,10 In these transformations, the vi-
nylketene is not isolated, but is trapped in situ with an
alkyne, initiating a cascade of pericyclic reactions leading
ultimately to the formation of a highly substituted
aromatic system.11 The efficiency of this process sug-
gested that the rearrangement of R′-silyl-R′-diazo-R,â-
unsaturated ketones (5) might provide the basis for a new
Syn th esis of TAS-vin ylk eten es by P h otoch em ica l
Wolff Rea r r a n gem en t. The R-diazo ketones required
for our study were all prepared using the “detrifluoro-
acetylative” diazo transfer strategy previously developed
in our laboratory.13 Thus, the methyl ketone precursor
to 4 is treated with 1.1 equiv of lithium hexamethyldisi-
lazide in THF at -78 °C to produce the lithium enolate,
which is acylated by exposure to 1.2 equiv of trifluoro-
ethyl trifluoroacetate at -78 °C for 5-10 min. Treatment
of the resulting R-trifluoroacetyl ketone with 1.5 equiv
of MsN3 in the presence of 1.0 equiv of water and 1.5
equiv of Et3N in acetonitrile (25 °C, 4 h) then affords the
desired diazo ketones in good to excellent yield after
chromatographic purification. It should be noted that R′-
diazo-R,â-unsaturated ketones generally cannot be pre-
pared by addition of diazomethane to acyl chlorides
because of competing side reactions involving dipolar
cycloadditions to the conjugated double bond.14
(8) Danheiser, R. L.; Sard, H. J . Org. Chem. 1980, 45, 4810.
(9) For Diels-Alder reactions of (silyl)vinylketenimines, see: Dif-
ferding, E.; Vandevelde, O.; Roekens, B.; Van, T. T.; Ghosez, L.
Tetrahedron Lett. 1987, 28, 397.
(10) For reviews of the photo Wolff rearrangement, see: (a) Regitz,
M.; Maas, G. Diazo CompoundssProperties and Synthesis; Academic
Press: New York, 1986. (b) Doyle, M. P.; McKervey, M. A.; Ye, T.
Modern Catalytic Methods for Organic Synthesis with Diazo Com-
pounds; Wiley: New York, 1998. (c) Zollinger, H. Diazo Chemistry II:
Aliphatic, Inorganic, and Organometallic Compounds; VCH: New
York, 1995.
(11) For applications of this aromatic annulation strategy in the total
synthesis of natural products, see: (a) Danheiser, R. L.; Casebier, D.
S.; Loebach, J . L. Tetrahedron Lett. 1992, 33, 1149. (b) Danheiser, R.
L.; Casebier, D. S.; Huboux, A. H. J . Org. Chem. 1994, 59, 4844. (c)
Danheiser, R. L.; Helgason, A. L. J . Am. Chem. Soc. 1994, 116, 9471.
(d) Danheiser, R. L.; Trova, M. P. Synlett 1995, 573. (e) Danheiser, R.
L.; Casebier, D. S.; Firooznia, F. J . Org. Chem. 1995, 60, 8341.
(12) (a) Maas, G.; Bru¨ckmann, R. J . Org. Chem. 1985, 50, 2801. (b)
Bru¨ckmann, R.; Schneider, K.; Maas, G. Tetrahedron 1989, 45, 5517.
(13) (a) Danheiser, R. L.; Miller, R. F.; Brisbois, R. G.; Park, S. Z.
J . Org. Chem. 1990, 55, 1959. (b) Danheiser, R. L.; Miller, R. F.;
Brisbois, R. G. Org. Synth. 1995, 73, 134.

8382 J . Org. Chem., Vol. 63, No. 23, 1998
Loebach et al.
Ta ble 1. Syn th esis of TAS-vin ylk eten es via
Silylation of the diazo ketones was accomplished using
a modification of the method of Maas and co-workers.15
As reported by Maas, the silylated R-diazo ketones have
a tendency to undergo protodesilylation during their
preparation and isolation caused by the presence of acidic
trialkylammonium triflate byproducts. We found that
this problem can be minimized by employing a solvent
system consisting of equal parts Et2O and hexane. The
ammonium salts have reduced solubility in this medium,
and the desired silylation products are obtained in 10-
30% higher yield as compared to reaction in Et2O alone.
A variety of R′-trialkylsilyl-R′-diazo-R,â-enones can be
prepared in good yield by employing this protocol (Table
1). Diazo ketones 14-21 were isolated as yellow to
orange oils that can be stored in solution at 0 °C for
several days without significant decomposition. In the
case of several of the less substituted substrates (entries
4-7), byproducts were obtained that appear to be the
result of a 1,3-(CfO) silyl shift to form a diazo alkene
which then undergoes dipolar cycloadditions with the R,â-
unsaturated carbonyl compounds. This side reaction can
be suppressed by carrying out the reaction at lower
concentration (0.02 instead of 0.1 M) and by employing
an inverse addition protocol in which the base and diazo
ketone are added to a solution of the trialkylsilyl triflate.
Table 1 summarizes the results of our investigation of
the photochemical Wolff rearrangement of these R′-silyl-
R′-diazo-R,â-enones. Best results were obtained by ir-
radiation of the diazo compound in degassed benzene at
300 nm using a Rayonet RPR-100 photochemical reactor.
It is important that the internal temperature of the
reactor chamber be maintained at 30-35 °C by use of a
cooling fan. In addition to benzene, 1,2-dichloroethane,
hexane, and toluene are all suitable solvents for the
reaction. As indicated in Table 1, better yields are
obtained in the synthesis of more highly substituted
vinylketenes, possibly because of the greater stability of
the diazo ketones in these cases. In addition, irradiation
of 21 produced none of the desired ketene; in this case,
an unidentifiable product was isolated in low yield.
As anticipated, these TAS-vinylketenes are remarkably
robust substances, in dramatic contrast to typical vi-
nylketenes. The triisopropylsilyl derivatives 22, 24, and
26 were recovered unchanged after heating in C6D6 at
80 °C for 4 days, although some decomposition of the
(triethylsilyl)vinylketene 23 was observed after heating
at this temperature for 10 h. Also notable is the
observation that these ketenes can be purified by con-
ventional silica gel chromatography without any detect-
able decomposition!
P h otoch em ica l Wolff Rea r r a n gem en t
a
For preparation of 7, 8, and 12, see ref 13. The following diazo
ketones were prepared by the method described in ref 13: 9 (74-
86%), 10 (80-97%), 11 (85-94%), and 13 (62-66%). The diazo
TAS-vinylketenes exhibit a number of interesting
spectral characteristics. The IR spectra of the TAS-
vinylketenes show the expected strong diagnostic stretch
near 2100 cm-1 resulting from the symmetric stretching
modes of the ketene backbone (CdCdO). The 13C NMR
spectrum has two notable features. As expected, the C-1
(CdCdO) carbons of TAS-vinylketenes are extensively
deshielded and give a low field signal near 180 ppm,
while the C-2 carbon (CdCdO) exhibits an unusually
high field signal near 20 ppm.
b
ketone is treated with 1.0 equiv of the trialkylsilyl triflate and
1.0 equiv of i-Pr2EtN in 1:1 Et2O-hexane at 0-25 °C for 15 min
to 4 h. c The diazo ketone is irradiated at 300 nm in benzene at
d
30-35 °C for 2-4 h. Isolated yield.
on the application of vinylketenes in annulation routes
to six-2 and eight-membered3 rings, we have shown that
the electrocyclic ring opening of cyclobutenones provides
an especially attractive method for the generation of
these reactive species.16 It therefore appeared likely that
Syn th esis of TAS-vin ylk eten es by Electr ocyclic
Clea va ge of Cyclobu ten on es. In our previous studies
(16) For reviews on the chemistry of cyclobutenones, see: (a) Bellus,
D.; Ernst, B. Angew. Chem., Int. Ed. Engl. 1988, 27, 797. (b) Wong, H.
N. C.; Lau, K.-L.; Tam, K.-F. Top. Curr. Chem. 1986, 133, 83. (c) Moore,
H. W.; Yerxa, B. R. In Advances in Strain in Organic Chemistry;
Halton, B., Ed.; J ai Press: London, 1995; pp 81-162.
(14) See references cited in ref 13a.
(15) See ref 12 and the following: Bru¨ckmann, R.; Maas, G. Chem.
Ber. 1987, 120, 635.

(Trialkylsilyl)vinylketenes
Sch em e 2
J . Org. Chem., Vol. 63, No. 23, 1998 8383
Ta ble 2. Cycloa d d ition s of TAS-vin ylk eten es w ith
DMADa
2-trialkylsilylcyclobutenones might afford TAS-vinyl-
ketenes in good yield upon heating. In fact, Tidwell has
recently shown that silylated bisketenes can be generated
in a similar fashion by the thermal or photochemical ring
opening of cyclobutenediones.17 Scheme 2 outlines the
application of this strategy to the synthesis of TAS-
vinylketene 36. Addition of dichloroketene to (trialkyl-
silyl)acetylenes 30 and 31 according to our previously
reported procedure18 provides the desired dichlorocy-
clobutenones 32 and 33 in high yield. As previously
noted, the phenyl group dominates in directing the
regiochemical course of the [2 + 2] cycloaddition, whereas
in the case of (trialkylsilyl)acetylenes with alkyl substit-
uents, mixtures of regioisomeric cycloadducts are gener-
ally obtained. Dechlorination of 4,4-dichlorocyclobuten-
ones does not proceed smoothly under conventional
conditions, but can be accomplished in good yield by
employing the protocol developed independently in our
laboratory18b,c and that of Dreiding.19 Upon heating in
benzene at 60 °C for 4.5 h, 34 undergoes smooth conver-
sion to the desired TAS-vinylketene 36. As expected, this
(trimethylsilyl)vinylketene proved less stable to chroma-
tography than the triethyl- and triisopropylsilyl deriva-
tives 22-28 and could only be obtained in 37% yield after
purification on Florisil. For this reason, it ultimately
proved advantageous to generate 36 in situ for [4 + 2]
cycloadditions rather than to subject this sensitive com-
pound to prior isolation and purification.
[4 + 2] Cycloa d d ition s of TAS-vin ylk eten es. Pre-
vious work in our laboratory had established the ability
of (trimethylsilyl)vinylketene to function as a diene
component in Diels-Alder reactions, although the scope
of this process is somewhat limited by the thermal
instability of the vinylketene. Preliminary experiments
indicated that substituted TAS-vinylketenes, especially
triisopropylsilyl derivatives, are more robust compounds,
and we consequently expected that these derivatives
would engage in [4 + 2] cycloadditions with a broader
range of substrates, providing a useful synthetic route
to highly substituted cyclohexenones and phenols. We
therefore undertook a systematic investigation of Diels-
Alder reactions of these TAS-vinylketenes, focusing on
the scope, regiochemistry, and stereochemical course of
the process.
a
Cycloadditions were conducted in degassed toluene at 150 °C
for 3-22 h (entries 1-3); or at 110 °C for 42-55 h (entries 4 and
5). Isolated yield.
b
The relative reactivity of different TAS-vinylketenes
was explored initially by examining their cycloadditions
with dimethylacetylene dicarboxylate (DMAD) (Table 2).
In a typical reaction, a degassed toluene solution of TAS-
vinylketene 22 and 1.0 equiv of DMAD was heated at
150 °C for 16 h in a threaded Pyrex tube sealed with a
Teflon cap. Concentration and purification by chroma-
tography on silica gel provided the expected phenol 37
in nearly quantitative yield. Protodesilylation of this
cycloadduct was achieved readily (80% yield) by exposure
to 20 equiv of TFA in dichloromethane at 25 °C for 2 h.
Interestingly, cycloaddition with the corresponding tri-
ethylsilyl derivative 23 afforded a mixture of products
tentatively assigned as the silyl ether 42 and phenol 43
(eq 3). In this case, the acidity of the phenolic product
apparently promotes desilylation of the cycloadduct at
the elevated reaction temperature. This process is not
as facile in the case of the triisopropylsilyl derivative 37
because of the steric bulk of the silyl group.
As illustrated in entries 4 and 5, heating 2-silylcy-
clobutenones 34 and 35 in toluene at reflux in the
presence of DMAD affords phenols 40 and 41 in good
yield, presumably via the in situ electrocyclic ring open-
(17) Allen, A. D.; Lai, W.-Y.; Ma, J .; Tidwell, T. T. J . Am. Chem.
Soc. 1994, 116, 2625. Zhao, D.-C.; Allen, A. D.; Tidwell, T. T. J . Am.
Chem. Soc. 1993, 115, 10097.
(18) (a) Danheiser, R. L.; Sard, H. Tetrahedron Lett. 1983, 24, 23.
(b) Danheiser, R. L.; Savariar, S. Tetrahedron Lett. 1987, 28, 3299. (c)
Danheiser, R. L.; Savariar, S.; Cha, D. D. Organic Syntheses; Wiley:
New York, 1993; Collect. Vol. VIII, pp 82-86.
(19) Ammann, A. A.; Rey, M.; Dreiding, A. S. Helv. Chim. Acta 1987,
70, 321.

8384 J . Org. Chem., Vol. 63, No. 23, 1998
Loebach et al.
ing of 34 and 35 to generate TAS-vinylketenes which
then undergo [4 + 2] cycloaddition. Lower yields of the
desired phenols were obtained when the reactions were
conducted at higher temperatures and butenolide side
products were isolated.20
Regioch em ica l Cou r se of TAS-vin ylk eten e Cy-
cloa d d it ion s. Prior studies with (trimethylsilyl)vi-
nylketene8 had confirmed our expectation that [4 + 2]
cycloadditions with this vinylketene would be highly
regioselective. The carbonyl oxygen of the vinylketene
is predicted to function as an electron donor substituent,
while trialkylsilyl groups are known to exert only a weak
directing effect on the regiochemical course of the Diels-
Alder reaction of silyl-substituted dienes.21
stereochemistry of the major isomer could not be unam-
biguously identified.
Ster eoch em ica l Cou r se of TAS-vin ylk eten e Cy-
cloa d d ition s. Since ethyl cyanoacrylate bears two
electron-withdrawing groups, the cycloadditions of this
dienophile were not informative with regard to the
stereochemical course of the Diels-Alder reactions of
TAS-vinylketenes. Simple acrylate derivatives proved
insufficiently reactive (vide infra), so we turned our
attention to reactions of nitroalkenes. As shown in eq 6,
The regiochemical course of Diels-Alder reactions with
substituted TAS-vinylketenes was initially probed by
examining the reaction of 22 with ethyl cyanoacrylate.
As outlined in eq 4, this cycloaddition proceeds readily
TAS-vinylketene 22 combines with nitroethylene23 and
2-nitropropene24 in regioselective Diels-Alder reactions
to produce 47 and 48, respectively. Because of the high
acidity of the doubly activated R′ proton in 47a and 47b,
these products are expected to undergo equilibration, so
that the identity of the major product does not necessarily
provide insight into the stereochemical course of the
cycloaddition step. No such ambiguity is associated with
the reaction of 2-nitropropene, and indeed this cycload-
dition produces a single cycloadduct whose structure was
established as 48 by analysis of 1H NMR coupling
constant data and an NOE study. This result indicates
that, as in other Diels-Alder reactions, cycloadditions
involving TAS-vinylketenes follow the Alder endo rule
and prefer transition states in which the dienophile
activating group adopts an endo orientation (49) relative
to the diene system (eq 7).
at room temperature to afford a 2:1 mixture of two
cycloadducts in nearly quantitative yield. 1H NMR
analysis clearly indicates that both products have the
predicted regiochemistry, and analysis of coupling con-
stant data suggests that the major adduct (44) has the
cyano group cis to the methyl group on the new six-
membered ring.22 TAS-vinylketene 26 reacts with ethyl
cyanoacrylate at room temperature in a similar fashion,
albeit more slowly, to afford an inseparable 4:1 mixture
of cycloadducts 46a and 46b (eq 5). In this case the
Scop e of TAS-vin ylk eten e Cycloa d d ition s. Disap-
pointingly, vinylketene 22 failed to react with less
reactive dienophiles such as N-phenylmaleimide and
chloroacrylonitrile; in each case, complex mixtures of
products were obtained. However, monoactivated allenyl
dienophiles proved excellent partners for the reaction,
providing access to phenolic products that could other-
wise be produced only by cycloadditions with less reactive
acetylene dienophiles. The reaction outlined in Scheme
3 is representative. Thus, addition of cyanoallene25 to
TAS-vinylketene 22 in toluene at 150 °C gave a single
product in good yield, which NOE studies revealed to be
(20) The butenolide byproducts most likely result from reaction of
the TAS-vinylketene with oxygen. For a related reaction, see: Zhao,
D.-C.; Tidwell, T. T. J . Am. Chem. Soc. 1992, 114, 10980.
(21) (a) Fleming, I.; Percival, A. J . Chem. Soc., Chem. Commun.
1976, 681. (b) Fleming, I.; Percival, A. J . Chem. Soc., Chem. Commun.
1978, 178. (c) J ung, M. E.; Gaede, B. Tetrahedron 1979, 35, 621. (d)
Batt, D. G.; Ganem, B. Tetrahedron Lett. 1978, 3323. (e) For a review
on the application of silyl-substituted conjugated dienes in synthesis,
see: Luh, T.-Y.; Wong, K.-T. Synthesis 1993, 349.
(22) The 1H NMR spectrum for the major diastereomer 44 exhibits
a large geminal coupling constant (J ab ) 13.9 Hz) between the protons
Ha and Hb at C-5, and Ha and Hb couple to the C-4 proton Hc with
coupling constant values of similar magnitude (J ac ) 6.4 Hz and J bc
)
6.0 Hz). These values are consistent with typical equatorial-equatorial
and equatorial-axial values. For the minor diastereomer 45, the 1H
NMR spectrum shows a large geminal coupling constant (J ab ) 14.2
Hz) between Ha and Hb; Hb is coupled to Hc with J bc ) 5.4 Hz, and a
large axial-axial coupling constant (J ac ) 10.7 Hz) is observed between
protons Ha and Hc.
(23) Ranganathan, D.; Rao, C. B.; Ranganathan, S.; Mehrotra, A.
K.; Iyengar, R. J . Org. Chem. 1980, 45, 1185.
(24) Miyashita, M.; Yanami, T.; Yoshikoshi, A. Organic Syntheses;
Wiley: New York, 1990; Collect. Vol. VII, pp 396-397.
(25) Brandsma, L.; Verkruijsse, H. D. Synthesis of Acetylenes,
Allenes, and Cumulenes; Elsevier: New York, 1981; pp 173-175.

(Trialkylsilyl)vinylketenes
Sch em e 3
J . Org. Chem., Vol. 63, No. 23, 1998 8385
hetero Diels-Alder reactions of TAS-vinylketenes is also
under investigation.
Exp er im en ta l Section
Ma ter ia ls. Commercial grade reagents and solvents were
used without further purification, except as indicated below.
Benzene, diethyl ether, DME, DMSO, and THF were distilled
from sodium benzophenone ketyl or dianion. Acetonitrile,
dibromomethane, dichloroethane, dichloromethane, diisopro-
pylamine, diisopropylethylamine, DMAD, HMDS, toluene,
triethylamine, triisopropylsilyl chloride, and triethylsilyl tri-
fluoromethanesulfonate were distilled from calcium hydride.
1-Acetyl-1-cyclohexene, 4-(2,6,6-trimethyl-1-cyclohexen-1-yl)-
3-buten-2-one, 4-(2,6,6-trimethyl-1-cyclohexen-2-yl)-3-buten-
2-one, and trans-3-penten-2-one were purified by distillation.
Alkyllithium reagents were titrated in THF or hexane at 0 °C
with sec-butanol, using 1,10-phenanthroline as an indicator.28
Gen er a l P r oced u r es. All reactions were performed in
flame-dried glassware under a positive pressure of argon and
stirred magnetically unless otherwise indicated. Air- and
moisture-sensitive liquids and solutions were transferred via
syringe or cannula and were introduced into reaction vessels
through rubber septa. Reaction product solutions were con-
centrated by using a Bu¨chi rotary evaporator at 15-20 mmHg.
Column chromatography was performed on EM Science silica
gel 60 (35-75 µm). Photochemical Wolff reactions were
carried out in a Rayonet photochemical reactor model RPR-
100 containing 16 300-nm, low-pressure mercury vapor bulbs
(Southern New England Ultraviolet Company).
not the expected o-(triisopropylsilyl)phenol but rather the
isomeric silyl ether 50. As shown in Scheme 3, isomer-
ization of the presumed initial cycloadduct 51 to 50 may
proceed via the intermediacy of the R-silyl ketone 53. 1,3-
Silyl shifts of R-silyl ketones to form silyl enol ethers are
well-known processes.26
In an attempt to expand the scope of the [4 + 2]
cycloadditions of TAS-vinylketenes to include dienophiles
of low reactivity, the application of Lewis acids as
catalysts for the reaction was examined. It should be
noted that Lewis acids have found considerable use as
catalysts for [2 + 2] cycloadditions of TAS-ketenes with
aldehydes.27 Unfortunately, attempts to promote the
cycloaddition of vinylketene 22 with methyl acrylate
using either Me2AlCl or BF3‚OEt2 under a variety of
conditions proved unsuccessful. In all cases, extensive
decomposition of the 22 was observed and none of the
desired cycloadduct was detected.
Gen er a l P r oced u r e for Dia zo Tr a n sfer . (E)-1-Dia zo-
6-p h en yl-3-h exen -2-on e (9). A 50-mL, three-necked, round-
bottomed flask equipped with a rubber septum, argon inlet
adapter, and glass stopper was charged with HMDS (1.00 mL,
0.760 g, 4.33 mmol) in 16 mL of THF and then cooled at 0 °C
while n-BuLi (2.53 M in hexane, 1.70 mL, 4.33 mmol) was
added rapidly dropwise over 2 min. After 10 min, the solution
was cooled at -78 °C while a solution of trans-6-phenyl-3-
hexen-2-one29 (0.687 g, 3.94 mmol) in 12 mL of THF was added
dropwise over 5 min. The reaction mixture was stirred at -78
°C for 30 min, and then 2,2,2-trifluoroethyl trifluoroacetate
(TFETFA, 0.63 mL, 0.922 g, 4.70 mmol) was added in one
portion. After 10 min, the reaction mixture was poured into
a separatory funnel containing 20 mL of 5% aqueous HCl
solution and 25 mL of Et2O. The aqueous phase was extracted
with two 15-mL portions of Et2O, and the combined organic
phases were then washed with 20 mL of saturated NaCl
solution, dried over Na2SO4, filtered, and concentrated to give
a yellow-orange oil, which was immediately dissolved in 14
mL of CH3CN and transferred to a 50-mL, three-necked,
round-bottomed flask equipped with a rubber septum, argon
inlet adapter, and glass stopper. Water (0.071 mL, 0.071 g,
3.94 mmol), Et3N (0.82 mL, 0.600 g, 5.91 mmol), and meth-
anesulfonyl azide (MsN3, 0.75 mL, 0.782 g, 6.45 mmol) were
added rapidly dropwise in that order. The resulting solution
was stirred at room temperature for 2.5 h, then diluted with
50 mL of Et2O and extracted with three 15-mL portions of 10%
aqueous NaOH solution, three 15-mL portions of water, and
20 mL of saturated NaCl solution, dried over MgSO4, filtered,
and concentrated to afford 1.43 g of an orange oil. Column
chromatography on 50 g of silica gel (gradient elution with
0-20% EtOAc-hexane) provided 0.680 g (86%) of diazo ketone
Con clu sion s
The photochemical Wolff rearrangement of R′-silyl-R′-
diazo-R,â-unsaturated ketones and the 4π electrocyclic
ring opening of 2-silylcyclobutenones provide useful
methods for the preparation of a variety of substituted
TAS-vinylketenes. These remarkably robust vinylketenes
undergo highly regioselective [4 + 2] cycloadditions with
reactive olefinic and acetylenic dienophiles to produce
highly substituted cyclohexenones and phenols in which
the ketene carbonyl dominates in controlling the regio-
chemical course of the reaction. The stereochemical
course of these cycloadditions follows the Alder endo rule,
as illustrated in the reaction of nitropropene with vi-
nylketene 22. Studies are currently underway in our
laboratory to develop routes to TAS-vinylketenes bearing
electron-donating substituents; these derivatives are
expected to exhibit increased reactivity, expanding the
scope of this methodology to include less activated
dienophiles. The extension of this chemistry to include
the syntheses of oxygen and nitrogen heterocycles via
9 as a yellow oil: IR (CCl4) 3024, 2924, 2099, and 1655 cm-1
;
1H NMR (300 MHz, CDCl3) δ 7.26-7.32 (m, 2H), 7.16-7.23
(m, 3H), 6.84 (dt, J ) 15.4, 6.8 Hz, 1H), 5.99 (d, J ) 15.5 Hz,
1H), 5.27 (s, 1H), 2.77 (t, J ) 7.6 Hz, 2H), 2.52 (dt, J ) 7.1,
7.3 Hz, 2H); 13C NMR (75 MHz, CDCl3) δ 184.6, 143.9, 140.8,
128.5, 128.4, 127.7, 126.2, 55.2, 34.5, and 34.0; UV (hexane)
λ
max, nm (ꢀ) 247 (5700). Anal. Calcd for C12H12ON2: C, 71.97;
H, 6.04; N, 13.99. Found: C, 71.72; H, 5.85; N, 13.85.
(26) See: Munschauer, R.; Maas, G. Angew. Chem., Int. Ed. Engl.
1991, 30, 306 and references therein.
(27) For examples, see: (a) Brady, W. T.; Saidi, K. J . Org. Chem.
1979, 44, 733. (b) Maruoka, K.; Concepcion, A. B.; Yamamoto, H.
Synlett 1992, 31.
(28) Watson, S. C.; Eastham, J . F. J . Organomet. Chem. 1967, 9,
165.
(29) Liedtke, R. J .; Gerrard, A. F.; Diekman, J .; Djerassi, C. J . Org.
Chem. 1972, 37, 776.

8386 J . Org. Chem., Vol. 63, No. 23, 1998
Loebach et al.
1-Dia zo-4-(2,6,6-tr im eth yl-1-cycloh exen -1-yl)-3-bu ten -
2-on e (10). Reaction of 4-(2,6,6-trimethyl-1-cyclohexen-1-yl)-
3-buten-2-one (2.1 mL, 2.00 g, 10.4 mmol) with LiHMDS (11.4
mmol) and TFETFA (1.7 mL, 2.45 g, 12.5 mmol) in 50 mL of
THF according to the general procedure provided a yellow-
orange oil, which was then treated with H2O (0.19 mL, 0.187
g, 10.4 mmol), Et3N (2.2 mL, 1.58 g, 15.6 mmol), and MsN3
(1.8 mL, 1.89 g, 15.6 mmol) in 36 mL of CH3CN at 25 °C for 3
h to yield 3.96 g of an orange oil. Column chromatography on
50 g of silica gel (gradient elution with 0-10% EtOAc-hexane)
provided 2.18 g (97%) of diazo ketone 10 as a yellow-orange
oil.
1-Dia zo-4-(2,6,6-tr im eth yl-2-cycloh exen -1-yl)-3-bu ten -
2-on e (11). Reaction of 4-(2,6,6-trimethyl-2-cyclohexen-1-yl)-
3-buten-2-one (2.2 mL, 2.00 g, 10.4 mmol) with LiHMDS (11.4
mmol) and TFETFA (1.7 mL, 2.45 g, 12.5 mmol) in 30 mL of
THF according to the general procedure provided a yellow-
orange oil, which was then treated with H2O (0.19 mL, 0.190
g, 10.4 mmol), Et3N (2.2 mL, 1.57 g, 15.6 mmol), and MsN3
(1.8 mL, 1.89 g, 15.6 mmol) in 36 mL of CH3CN at 25 °C for
4.5 h to yield 3.32 g of a yellow-orange oil. Column chroma-
tography on 80 g of silica gel (gradient elution with 0-10%
EtOAc-hexane) provided 2.17 g (94%) of diazo ketone 11 as a
yellow-orange oil.
3-Cycloh exylid en e-1-d ia zop r op a n -2-on e (13). Reaction
of 1-cyclohexylidenepropan-2-one30 (0.216 g, 1.56 mmol) with
LiHMDS (1.75 mmol) and TFETFA (0.26 mL, 0.381 g, 1.91
mmol) in 5 mL of THF according to the general procedure
provided a yellow oil, which was then treated with H2O (0.029
mL, 0.029 g, 1.59 mmol), Et3N (0.33 mL, 0.240 g, 2.39 mmol),
and MsN3 (0.28 mL, 0.292 g, 2.39 mmol) in 6.5 mL of CH3CN
at room temperature for 2 h to yield 0.280 g of a yellow oil.
Column chromatography on 10 g of silica gel (gradient elution
with 0-10% EtOAc-hexane) provided 0.170 g (66%) of diazo
ketone (13) as a yellow oil.
gel (gradient elution with 0-2% EtOAc-hexane) provided
0.620 g (75%) of diazo ketone 16 as a yellow oil.
Gen er a l P r oced u r e for Silyla tion of Dia zo Keton es:
Meth od B. (E)-1-Dia zo-6-p h en yl-1-(tr iisop r op ylsilyl)-3-
h exen -2-on e (17). A 250-mL, three-necked, round-bottomed
flask equipped with a rubber septum, magnetic stir bar, glass
stopper, and argon inlet adapter was charged with TIPSOTf
(0.67 mL, 0.760 g, 2.48 mmol) in 90 mL of a 1:1 solution of
Et2O-hexane and then cooled at 0 °C in an ice-water bath.
A 50-mL, two-necked, round-bottomed flask equipped with a
rubber septum, magnetic stir bar, and argon inlet adapter was
charged with diazo ketone 9 (0.496 g, 2.48 mmol) in 20 mL of
ether and cooled at 0 °C in an ice-water bath while i-Pr2EtN
(0.43 mL, 0.320 g, 2.48 mmol) was added dropwise over 10 s.
The diazo ketone solution was then transferred dropwise via
cannula over 5 min to the triflate solution. The resulting
mixture was stirred for 1.5 h while the ice-water bath was
allowed to warm to room temperature and then filtered
through Celite with the aid of 5 mL of Et2O. Concentration
of the filtrate afforded 2.80 g of an orange oil. Column
chromatography on silica gel (gradient elution with 0-50%
benzene-hexane) provided 0.340 g (38%) of diazo ketone 17 as
an orange oil.
1-Dia zo-1-(tr iisop r op ylsilyl)-4-(2,6,6-tr im eth yl-1-cyclo-
h exen -1-yl)-3-bu ten -2-on e (18). Reaction of TIPSOTf (0.19
mL, 0.217 g, 0.707 mmol) with diazo ketone 10 (0.150 g, 0.687
mmol) and i-Pr2EtN (0.12 mL, 0.088 g, 0.689 mmol) in 35 mL
of Et2O-hexane at 0-25 °C for 1.5 h according to the general
procedure (method B) gave 0.270 g of an orange oil. Column
chromatography on 10 g of silica gel (gradient elution with
0-3% EtOAc-hexane) provided 0.230 g (87%) of diazo ketone
18 as an orange oil.
1-Dia zo-1-(tr iisop r op ylsilyl)-4-(2,6,6-tr im eth yl-2-cyclo-
h exen -1-yl)-3-bu ten -2-on e (19). Reaction of diazo ketone 11
(0.152 g, 0.690 mmol), i-Pr2EtN (0.12 mL, 0.089 g, 0.69 mmol),
and TIPSOTf (0.19 mL, 0.217 g, 0.707 mmol) in 3 mL of Et2O-
hexane at 0-25 °C for 1 h according to the general procedure
(method A) gave 0.280 g of an orange oil. Column chroma-
tography on 10 g of silica gel (gradient elution with 0-2%
EtOAc-hexane) provided 0.190 g (72%) of diazo ketone 19 as
a yellow oil.
(E)-1-Dia zo-4-p h en yl-1-(t r iisop r op ylsilyl)-3-b u t en -2-
on e (20). Reaction of TIPSOTf (0.78 mL, 0.889 g, 2.90 mmol),
diazo ketone 12 (0.509 g, 2.90 mmol), and i-Pr2EtN (0.51 mL,
0.380 g, 2.90 mmol) in 130 mL of Et2O-hexane at 0-25 °C
for 15 min according to the general procedure (method B) gave
1.08 g of a red-orange oil. Column chromatography on 40 g
silica gel (gradient elution with 0-5% EtOAc-hexane) pro-
vided 0.370 g (39%) of diazo ketone 20 as an orange oil.
Gen er a l P r oced u r e for Silyla tion of Dia zo Keton es:
Meth od A. 1-Dia zo-3-m eth yl-1-(tr iisop r op ylsilyl)-3-p en t-
en -2-on e (14). A 100-mL, three-necked, round-bottomed flask
equipped with a rubber septum and argon inlet adapter was
charged with diazo ketone 7 (0.854 g, 6.87 mmol) in 40 mL of
a 1:1 solution of Et2O-hexane and then cooled at 0 °C using
an ice-water bath while i-Pr2EtN (1.2 mL, 0.890 g, 6.87 mmol)
was added dropwise over 1 min. After 5 min, TIPSOTf (1.8
mL, 2.05 g, 6.70 mmol) was added dropwise over 1 min, and
the resulting solution was stirred for 2.5 h while the ice-water
bath warmed to 25 °C. The reaction mixture was filtered
through Celite with the aid of 10 mL of Et2O, and the filtrate
was concentrated to afford 2.09 g of an orange oil. Column
chromatography on 25 g of silica gel (elution with 5% EtOAc-
hexane) provided 1.71 g (89%) of diazo ketone 14 as a yellow
3-Cycloh exyliden e-1-diazo-1-(tr iisopr opylsilyl)pr opan -
2-on e (21). Reaction of diazo ketone 13 (0.110 g, 0.670 mmol),
i-Pr2EtN (0.11 mL, 0.0816 g, 0.632 mmol), and TIPSOTf (0.18
mL, 0.200 g, 0.660 mmol) in 6 mL of Et2O-hexane at 0-25
°C for 2 h according to the general procedure (method A) gave
0.330 g of a yellow oil. Column chromatography on 10 g of
silica gel (gradient elution with 0-5% EtOAc-hexane) pro-
vided 0.200 g (93%) of diazo ketone 21 as a yellow oil.
oil: IR (CCl4) δ 2940, 2860, 2065, and 1610 cm-1 1H NMR
;
(300 MHz, CDCl3) δ 6.13 (q, J ) 6.6 Hz, 1H), 1.83 (s, 3H), 1.78
(d, J ) 6.8 Hz, 3H), 1.37 (sept, J ) 6.7 Hz, 3H), and 1.10 (d,
J ) 7.9 Hz, 18H); 13C NMR (75 MHz, CDCl3) δ 196.9, 136.3,
129.9, 49.8, 18.4, 13.6, 13.1, and 11.5; UV (CH3CN) λmax, nm
(ꢀ) 292 (3000), 238 (6200), and 221 (6300); HRMS m/z calcd
for C15H28ON2Si 280.1971, found 280.1963.
1-Dia zo-3-m eth yl-1-(tr ieth ylsilyl)-3-p en ten -2-on e (15).
Reaction of diazo ketone 7 (0.201 g, 1.62 mmol), i-Pr2EtN (0.28
mL, 0.208 g, 1.61 mmol), and TESOTf (0.36 mL, 0.430 g, 1.61
mmol) in 8 mL of Et2O-hexane at 0-25 °C for 3 h according
to the general procedure (method A) gave an orange oil.
Column chromatography on 10 g of silica gel (gradient elution
with 0-5% EtOAc-hexane) provided 0.326 g (84%) of diazo
ketone 15 as a yellow oil.
1-Dia zo-2-(1-cycloh e xe n yl)-1-(t r iisop r op ylsilyl)-2-
eth en on e (16). Reaction of diazo ketone 8 (0.403 g, 2.69
mmol), i-Pr2EtN (0.47 mL, 0.349 g, 2.70 mmol), and TIPSOTf
(0.72 mL, 0.820 g, 2.69 mmol) in 20 mL of Et2O-hexane at
0-25 °C for 4 h according to the general procedure (method
A) gave a yellow oil. Column chromatography on 10 g of silica
Gen er a l P r oced u r e for P h otoch em ica l Wolff Rea r -
r a n gem en t. (E)-2-(1-Meth yl-1-p r op en yl)-2-(tr iisop r op yl-
silyl)k eten e (22). A solution of diazo ketone 14 (0.760 g, 2.71
mmol) in 27 mL of benzene was distributed evenly between
two 25-cm Vycor tubes fitted with rubber septa. A second
rubber septum (inverted) was secured with wire to each tube
to ensure a good seal, and the reaction mixtures were degassed
(three freeze-pump-thaw cycles at -196 °C, <0.5 mmHg) and
then irradiated with 300-nm light for 4 h in a Rayonet reactor.
The resulting solutions were combined and concentrated to
afford 0.710 g of a yellow oil. Column chromatography on 30
g of silica gel (elution with hexane) provided 0.550 g (80%) of
ketene 22 as a viscous yellow oil: IR (film) 2941, 2881, and
1
2081 cm-1; H NMR (300 MHz, CDCl3) δ 5.18 (q, J ) 6.7 Hz,
1H), 1.81 (s, 3H), 1.62 (d, J ) 6.7 Hz, 3H), 1.15-1.27 (m, 3H),
1.10 (d, J ) 6.4 Hz, 18H); 13C NMR (75 MHz, CDCl3) δ 184.7,
(30) Normant, H.; Sturtz, G. C. R. Hebd. Seances Acad. Sci. 1963,
256, 1800.
123.9, 119.5, 22.4, 18.8, 18.6, 14.0, and 12.5; UV (hexane) λmax
,

(Trialkylsilyl)vinylketenes
J . Org. Chem., Vol. 63, No. 23, 1998 8387
nm (ꢀ) 229 (9000); HRMS m/z calcd for C15H28OSi 252.1909,
found 252.1907.
12 min. The reaction mixture was stirred for 15 min, and then
the ice-bath was removed and the reaction mixture was
stirred for 3 h at 25 °C. The resulting mixture was filtered
through Celite with the aid of 80 mL of 1:1 Et2O-pentane.
The filtrate was extracted with 100 mL of 1 M aqueous HCl
solution, 100 mL of water, 80 mL of saturated NaHCO3, and
80 mL of saturated NaCl solution, dried over MgSO4, filtered,
and concentrated to give 0.409 g of a yellow liquid. Column
chromatography on 20 g of silica gel (gradient elution with
0-4% EtOAc-hexane) provided 0.257 g (74%) of 34 as a yellow
oil: IR (film) 2950 and 1785 cm-1; 1H NMR (300 MHz, CDCl3)
δ 7.59-7.61 (m, 2H), 7.50-7.51 (m, 3H), 3.71 (s, 2H), and 0.32
(s, 9H); 13C NMR (75 MHz, CDCl3) δ 191.0, 176.7, 131.4, 129.2,
(E)-2-(1-Meth yl-1-pr open yl)-2-(tr ieth ylsilyl)keten e (23).
Reaction of diazo ketone 15 (0.419 g, 1.76 mmol) in 18 mL of
benzene for 4 h according to the general procedure gave a
yellow-orange oil. Column chromatography on 10 g of silica
gel (elution with hexane) provided 0.270 g (73%) of ketene 23
as a viscous yellow oil.
2-(1-Cycloh exen yl)-2-(tr iisopr opylsilyl)keten e (24). Re-
action of diazo ketone 16 (1.16 g, 3.78 mmol) in 38 mL of
benzene for 4 h according to the general procedure gave 2.20
g of an orange-brown oil. Column chromatography on 15 g of
silica gel (elution with hexane) provided 0.940 g (89%) of
ketene 24 as a viscous yellow oil.
(E )-2-(4-P h e n y l-1-b u t e n y l)-2-(t r i i s o p r o p y ls i ly l)-
k eten e (25). Reaction of diazo ketone 17 (0.290 g, 0.81 mmol)
in 8 mL of benzene for 2 h according to the general procedure
gave 0.300 g of a red oil. Column chromatography on 10 g of
silica gel (elution with hexane) provided 0.110 g (41%) of
ketene 25 as a viscous yellow oil.
(E)-2-(2-(2,6,6-Tr im eth yl-1-cycloh exen yl)eth en yl)-2-(tr i-
isop r op ylsilyl)k eten e (26). Reaction of diazo ketone 18
(0.230 g, 0.630 mmol) in 6 mL of benzene for 2 h according to
the general procedure gave 0.210 g of a yellow-brown oil.
Column chromatography on 10 g of silica gel (elution with
hexane) provided 0.130 g (61%) of ketene 26 as a viscous yellow
oil.
(E)-2-(2-(2,6,6-Tr im eth yl-2-cycloh exen yl)eth en yl)-2-(tr i-
isop r op ylsilyl)k eten e (27). Reaction of diazo ketone 19
(0.160 g, 0.430 mmol) in 4 mL of benzene for 2.5 h according
to the general procedure gave 0.160 g of a red oil. Column
chromatography on 10 g of silica gel (elution with hexane)
provided 0.120 g (81%) of ketene 27 as a viscous yellow oil.
(E)-2-(2-P h en yleth en yl)-2-(tr iisopr opylsilyl)keten e (28).
Reaction of diazo ketone 20 (0.370 g, 1.13 mmol) in 12 mL of
benzene for 3 h according to the general procedure gave 0.250
g of a red oil. Column chromatography on 10 g of silica gel
(elution with hexane) provided 0.120 g (35%) of ketene 28 as
a viscous yellow oil.
128.7, 128.6, 126.4, 54.6, and -1.2; HRMS m/z calcd for C13H16
-
OSi 216.0970, found 216.0970.
3-P h en yl-2-(tr ieth ylsilyl)-2-cyclobu ten on e (35). Reac-
tion of zinc dust (1.14 g, 17.5 mmol), TMEDA (2.6 mL, 17.5
mmol), acetic acid (1.00 mL, 17.5 mmol), and cyclobutenone
33 (0.99 g, 3.0 mmol) in 29 mL of ethanol at 0-25 °C for 4.5
h according to the general procedure gave 0.848 g of a yellow
liquid. Column chromatography on 25 g of silica gel (gradient
elution with 0-10% ether-pentane) provided 0.428 g (55%)
1
of 35 as a yellow oil: IR (film) 2070 and 1740 cm-1; H NMR
(300 MHz, CDCl3) δ 7.63-7.66 (m, 2H), 7.51-7.54 (m, 3H),
3.75 (s, 2H), 0.99 (t, J ) 6.8 Hz, 9H), and 0.86 (q, J ) 6.8 Hz,
6H); 13C NMR (75 MHz, CDCl3) δ 191.5, 178.1, 147.2, 133.6,
131.4, 129.0, 128.7, 52.1, 7.4, and 3.4; HRMS m/z calcd for
C
16H22OSi 258.1440, found 258.1441.
2-(1-P h en yleth en yl)-2-(tr im eth ylsilyl)k eten e (36).
A
25-mL, two-necked, round-bottomed flask equipped with a
glass stopper and reflux condenser was charged with cy-
clobutenone 34 (0.106 g, 0.497 mmol) in 18 mL of benzene and
then heated at 60 °C for 4.5 h. The resulting mixture was
allowed to cool to room temperature and then concentrated to
give 0.113 g of a yellow oil. Column chromatography (twice)
on 2.0 g of Florisil (elution with hexane) provided 0.039 g (37%)
of ketene 36 as a pale yellow oil: IR (film) 2080 and 1740 cm-1
;
1H NMR (300 MHz, CDCl3) δ 7.41-7.43 (m, 2H), 7.30-7.40
(m, 3H), 4.98 (s, 2H), and 0.28 (s, 9H); 13C NMR (75 MHz,
CDCl3) δ 180.9, 142.7, 139.4, 128.3, 128.0, 126.8, 110.9, 26.0,
and -0.6; HRMS m/z calcd for C13H16OSi 216.0970, found
216.0970.
4,4-Dich lor o-3-p h en yl-2-(t r iet h ylsilyl)-2-cyclob u t en -
on e (33). A 500-mL, three-necked, round-bottomed flask
equipped with a rubber septum, reflux condenser, and pres-
sure-equalizing addition funnel was charged with activated
zinc31 (7.26 g, 111 mmol) and a solution of (triethylsilyl)-
phenylacetylene32 (4.01 g, 37.0 mmol) in 80 mL of ether. The
resulting solution was heated at reflux while a solution of
trichloroacetyl chloride (4.2 mL, 37.0 mmol) in 125 mL of ether
was added dropwise over 3 h. The reaction mixture was
heated at reflux for 14 h and then allowed to cool to room
temperature and filtered. The filtrate was extracted with two
100-mL portions of saturated NaHCO3, two 100-mL portions
of water, and two 100-mL portions of saturated NaCl, dried
over MgSO4, filtered, and concentrated to give 6.67 g of a red-
brown liquid. Column chromatography on 40 g of silica gel
(elution with 0-2% ether-pentane) provided 5.55 g (92%) of
Gen er a l P r oced u r e for Diels-Ald er Rea ction w ith
DMAD. Dim eth yl 5,6-Dim eth yl-3-h yd r oxy-4-(tr iisop r o-
p ylsilyl)p h th a la te (37). A threaded Pyrex tube (20 mm o.d.;
16 mm i.d.) was charged with a solution of vinylketene 22
(0.092 g, 0.370 mmol) and DMAD (0.052 g, 0.370 mmol) in 0.4
mL of toluene. The reaction mixture was degassed (three
freeze-pump-thaw cycles at -196 °C, <0.5 mmHg), and then
the tube was sealed with a threaded Teflon cap. The reaction
mixture was heated at 150 °C for 24 h and then allowed to
cool to room temperature and concentrated to give 0.200 g of
an orange oil. Column chromatography on 10 g of silica gel
(gradient elution with 0-70% benzene-hexane) provided 0.140
g (95%) of 37 as white crystals: mp 73-73.5 °C; IR (CCl4) 2950,
2870, 1740, and 1670 cm-1; 1H NMR (300 MHz, CDCl3) δ 11.57
(s, 1H), 3.90 (s, 3H), 3.88 (s, 3H), 2.37 (s, 3H), 2.10 (s, 3H),
cyclobutenone 33 as a yellow oil: IR (film) 2950 and 1770 cm-1
;
1H NMR (300 MHz, CDCl3) δ 7.91-7.94 (m, 2H), 7.57-7.60
(m, 3H), and 0.85-1.00 (m, 15H); 13C NMR (75 MHz, CDCl3)
δ 183.4, 181.9, 149.8, 133.1, 129.9, 129.7, 129.2, 91.0, 7.2, and
3.1.
1.58 (sept, J ) 7.5 Hz, 3H), and 1.08 (d, J ) 7.2 Hz, 18H); 13
C
NMR (75 MHz, CDCl3) δ 170.4, 170.2, 165.7, 135.2, 128.3,
126.5, 124.5, 104.9, 52.6, 52.2, 22.1, 19.3, 16.9, and 13.6; UV
(CH3CN) λmax, nm (ꢀ) 322 (9700), 257 (9900), and 220 (32 000).
Anal. Calcd for C21H34O5Si: C, 63.92; H, 8.69. Found: C,
63.88; H, 8.67.
Gen er a l P r oced u r e for Red u ctive Dech lor in a tion .
3-P h en yl-2-(tr im eth ylsilyl)-2-cyclobu ten on e (34). A 50-
mL, three-necked, round-bottomed flask equipped with a
rubber septum, 25-mL pressure-equalizing addition funnel,
and argon inlet adapter was charged with zinc dust (0.626 g,
9.53 mmol), TMEDA (1.44 mL, 9.53 mmol), and 8 mL of
ethanol and then cooled at 0 °C using an ice-water bath.
Acetic acid (0.55 mL, 9.53 mmol) was added over 2 min, and
then a solution of cyclobutenone 32 (0.463 g, 1.64 mmol) in 5
mL of ethanol was added dropwise via the addition funnel over
Dim eth yl 5,6-Dim eth yl-3-h yd r oxyp h th a la te (43). A 10-
mL, two-necked, round-bottomed flask equipped with a rubber
septum and argon inlet adapter was charged with a solution
of phthalate 37 (0.200 g, 0.507 mmol) and 0.6 mL of dichlo-
romethane. Trifluoroacetic acid (0.78 mL, 1.16 g, 10.1 mmol)
was added dropwise to the reaction mixture over 1 min, and
the resulting solution was stirred at room temperature for 2
h. The reaction mixture was concentrated to afford 0.290 g of
a brown oil. Column chromatography on 15 g of silica gel
(gradient elution with 0-10% EtOAc-hexane) provided 0.092
g (76%) of phthalate 43 as a colorless oil: IR (CHCl3) 3700,
(31) Brady, W. T.; Liddell, H. G.; Vaughn, W. L. J . Org. Chem. 1966,
31, 626.
(32) J un, C.-H.; Crabtree, R. H. J . Organomet. Chem. 1993, 447,
177.
1
3640, 3040, 2990, 1730, and 1660 cm-1; H NMR (300 MHz,

8388 J . Org. Chem., Vol. 63, No. 23, 1998
Loebach et al.
CDCl3) δ 10.76 (s, 1H), 6.84 (s, 1H), 3.88 (s, 6H), 2.25 (s, 3H),
and 2.07 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 170.0, 169.3,
159.7, 146.2, 134.6, 124.7, 119.7, 106.7, 52.8, 52.3, 20.8, and
15.6.
and 0.120 g (40%) of a mixture of 44 and 45 as white solids
(total yield, 98%). For the major diastereomer 44: IR (CCl4)
1
2940, 2870, 2240, 1750, and 1690 cm-1; H NMR (300 MHz,
CDCl3) δ 4.29 (dq, J ) 10.8, 7.1 Hz, 1H), 4.18 (dq, J ) 10.7,
7.1 Hz, 1H), 2.89 (dd, J ) 13.9, 6.0 Hz, 1H), 2.72 (app q, J )
6.7 Hz, 1H), 2.15 (dd, J ) 13.9, 6.4 Hz, 1H), 2.08 (s, 3H), 1.44
(sept, J ) 7.5 Hz, 3H), 1.35 (d, J ) 7.1 Hz, 3H), 1.31 (t, J )
7.1 Hz, 3H), 1.06 (d, J ) 7.5 Hz, 12H), and 1.04 (d, J ) 7.4
Hz, 6H); 13C NMR (75 MHz, CDCl3) δ 189.7, 174.6, 165.0,
132.1, 117.4, 63.9, 55.1, 37.3, 35.9, 24.6, 21.0, 19.4, 19.3, 14.2,
13.1, and 13.0. Anal. Calcd for C21H35O3NSi: C, 66.79; H,
9.34; N, 3.71. Found: C, 66.57; H, 9.42; N, 3.62. For the minor
Dim et h yl 3-H yd r oxy-5,6,7,8-t et r a h yd r o-4-(t r iisop r o-
p ylsilyl)n a p h th a len e-1,2-d ica r boxyla te (38). Reaction of
ketene 24 (0.154 g, 0.550 mmol) and DMAD (0.076 g, 0.540
mmol) in 0.4 mL of toluene at 150 °C for 22 h according to the
general procedure gave 0.290 g of a brown oil. Column
chromatography on 10 g of silica gel (gradient elution with
0-80% benzene-hexane) afforded 0.170 g (73%) of phenol 38
as a pale yellow oil: IR (CCl4) 2950, 2870, 1740, and 1670 cm-1
;
1H NMR (300 MHz, CDCl3) δ 11.45 (s, 1H), 3.90 (s, 3H), 3.89
(s, 3H), 2.86 (t, J ) 5.9 Hz, 2H), 2.59 (t, J ) 6.4 Hz, 2H), 1.60-
1.71 (m, 4H), 1.54 (sept, J ) 7.3 Hz, 3H), and 1.06 (d, J ) 7.3
Hz, 18H); 13C NMR (75 MHz, CDCl3) δ 170.0, 169.9, 165.0,
155.1, 135.7, 125.6, 125.3, 105.1, 52.8, 52.3, 31.8, 26.3, 22.1,
21.7, 19.5, and 13.8.
diastereomer 45: IR (CCl4) 2950, 2870, 1755, and 1695 cm-1
;
1H NMR (300 MHz, CDCl3) δ 4.39 (dq, J ) 10.8, 7.2 Hz, 1H),
4.30 (dq, J ) 12.0, 7.2 Hz, 1H), 2.81 (ddq, J ) 5.4, 10.7, 7.0
Hz, 1H), 2.52 (dd, J ) 14.2, 5.4 Hz, 1H), 2.25 (dd, J ) 14.2,
10.7 Hz, 1H), 2.10 (s, 3H), 1.43 (sept, J ) 7.5 Hz, 3H), 1.34 (t,
J ) 7.1 Hz, 3H), 1.27 (d, J ) 7.0 Hz, 3H), and 1.08 (app t, J )
7.5 Hz, 18H); 13C NMR (75 MHz, CDCl3) δ 191.0, 172.5, 164.9,
132.3, 115.9, 63.1, 55.0, 37.1, 36.0, 23.4, 20.2, 19.1, 18.9, 14.0,
and 12.7. Anal. Calcd for C21H35O3NSi: C, 66.79; H, 9.34; N,
3.71. Found: C, 66.86; H, 9.41; N, 3.62.
Dim eth yl 6-(2-P h en yleth yl)-3-h yd r oxy-4-(tr iisop r op yl-
silyl)p h th a la te (39). Reaction of ketene 25 (0.090 g, 0.270
mmol) and DMAD (0.058 g, 0.410 mmol) in 0.3 mL of toluene
at 150 °C for 3 h according to the general procedure gave 0.120
g of a brown oil. Column chromatography on 10 g of silica gel
(elution with 50% dichloromethane-petroleum ether) provided
0.079 g (61%) of phenol 39 as a white solid: mp 77.5-79.0 °C;
IR (CCl4) 2943, 2860, 1740, and 1670 cm-1; 1H NMR (300 MHz,
CDCl3) δ 11.19 (s, 1H), 7.12-7.29 (m, 6H), 3.91 (s, 3H), 3.90
(s, 3H), 2.79-2.85 (m, 4H), 1.43 (sept, J ) 7.6 Hz, 3H), and
1.04 (d, J ) 7.6 Hz, 18 H); 13C NMR (75 MHz, CDCl3) δ 170.0,
169.6, 165.0, 144.6, 141.1, 134.9, 128.5, 128.5, 128.4, 126.3,
126.0, 107.7, 52.8, 52.3, 37.7, 35.0, 18.8, and 11.5; UV (CH3-
CN) λmax, nm (ꢀ) 317 (7500), 310 (5900), and 219 (32 000). Anal.
Calcd for C27H38O5Si: C, 68.90; H, 8.14. Found: C, 68.80; H,
8.17.
Gen er a l P r oced u r e for Diels-Ald er Rea ction w ith
Cyclobu ten on es. Dim eth yl 3-Hyd r oxy-5-p h en yl-4-(tr i-
m eth ylsilyl)p h th a la te (40). A 25-mL, two-necked, round-
bottomed flask equipped with a glass stopper and reflux
condenser was charged with a solution of cyclobutenone 34
(0.109 g, 0.511 mmol), DMAD (0.063 mL, 0.511 mmol), and 7
mL of toluene. The resulting solution was heated at 110 °C
for 42 h and then allowed to cool to room temperature.
Concentration of the resulting mixture afforded 0.260 g of an
orange oil. Column chromatography on 26 g of silica gel
(elution with 20% EtOAc-hexane) provided 0.116 g (63%) of
phenol 40 as a white solid: mp 51-52 °C; IR (CCl4) 1740 and
1660 cm-1; 1H NMR (300 MHz, CDCl3) δ 11.21 (s, 1H), 7.33-
7.35 (m, 3H), 7.22-7.24 (m, 2H), 6.79 (s, 1H), 3.91 (s, 3H), 3.85
(s, 3H), and -0.04 (s, 9H); 13C NMR (75 MHz, CDCl3) δ 170.0,
169.4, 166.5, 156.0, 143.0, 135.1, 129.3, 128.7, 127.9, 127.8,
121.1, 106.9, 52.8, 52.5, and 0.8; HRMS m/z calcd for C19H22O5-
Si 358.1236, found 358.1235.
6-Cyan o-6-(eth oxycar bon yl)-2-(tr iisopr opylsilyl)-4-(2,6,6-
tr im eth yl-1-cycloh exen -1-yl)-2-cycloh exen -1-on e (46a ,b).
A 10-mL, two-necked, round-bottomed flask equipped with a
rubber septum and argon inlet adapter was charged with a
solution of ketene 18 (0.209 g, 0.600 mmol) and ethyl cy-
anoacrylate (0.118 g, 0.940 mmol) in 2.2 mL of toluene. Five
additional portions of ethyl cyanoacrylate (0.054 g, 0.430 mmol)
were added at intervals of ca. 15 h. After 93 h, the reaction
mixture was concentrated to afford 0.750 g of a pale yellow
oil. Column chromatography on 10 g of silica gel (gradient
elution with 0-3% EtOAc-hexane) provided 0.120 g (43%) of
a 4:1 mixture of diastereomers 46a and 46b as a yellow oil.
For the mixture of diastereomers: IR (CCl4) 2940, 2860, 1740,
and 1680 cm-1. For the major diastereomer: 1H NMR (300
MHz, CDCl3) δ 7.07 (s, 1H), 4.29 (dq, J ) 9.0, 7.1 Hz, 1H),
4.26 (dq, J ) 10.8, 7.2 Hz, 1H), 3.53 (dd, J ) 11.2, 3.1 Hz,
1H), 2.84 (dd, J ) 14.0, 5.0 Hz, 1H), 2.66 (dd, J ) 14.0, 11.7
Hz, 1H), 1.89-1.96 (m, 2H), 1.57 (s, 3H), 1.43-1.65 (m, 4H),
1.26-1.41 (m, 3H), 1.33 (t, J ) 7.1 Hz, 3H), 1.05 (s, 6H), and
1.04 (d, J ) 7.5 Hz, 18H); HRMS m/z calcd for C28H45O3NSi
471.3169, found 471.3164.
cis-6-Nitr o-3,4-tr im eth yl-2-(tr iisop r op ylsilyl)-2-cyclo-
h exen -1-on e (47a ) a n d tr a n s-6-Nitr o-3,4-tr im eth yl-2-(tr i-
isop r op ylsilyl)-2-cycloh exen -1-on e (47b). A 10-mL, two-
necked, round-bottomed flask equipped with a rubber septum
and argon inlet adapter was charged with a solution of ketene
22 (0.208 g, 0.822 mmol), two crystals of BHT, and 0.8 mL of
benzene. A solution of nitroethylene23 (0.99 M in benzene, 0.83
mL, 0.820 mmol) was added, and the resulting mixture was
stirred at room temperature. Two additional portions of
nitroethylene solution were added (0.83 mL, 0.820 mmol) after
12 and 20 h. The reaction mixture was stirred for a total of
39 h and then concentrated to give 0.290 g of a pale yellow
oil. Column chromatography on 30 g of silica gel (gradient
elution with 0-10% EtOAc-hexane) provided 0.230 g (85%)
of a 2:1 mixture of diastereomers 47a and 47b as a yellow oil.
For the mixture of diastereomers: IR (CCl4) 2930, 2850, 1735,
and 1670 cm-1. For the major diastereomer: 1H NMR (300
MHz, CDCl3) δ 5.42 (dd, J ) 4.2, 12.5 Hz, 1H), 2.88 (dd, J )
5.7, 13.1 Hz, 1H), 2.53-2.75 (m, 1H), 2.27-2.41 (m, 1H), 2.13
(s, 3H), 1.43 (sept, J ) 7.6 Hz, 3H), 1.34 (d, J ) 7.2 Hz, 3H),
and 1.05 (d, J ) 7.4 Hz, 18H). For the minor diastereomer:
1H NMR (300 MHz, CDCl3) δ 5.37 (dd, J ) 4.7, 12.4 Hz, 1H),
2.80 (dd, J ) 6.0, 13.7 Hz, 1H), 2.53-2.73 (m, 1H), 2.27-2.41
(m, 1H), 2.08 (s, 3H), 1.54 (sept, J ) 7.5 Hz, 3H), 1.29 (d, J )
6.9 Hz, 3H), and 1.08 (d, J ) 7.4 Hz, 18H). For the mixture
of diastereomers: 13C NMR (75 MHz, CDCl3) δ 191.9, 191.6,
174.6, 172.7, 132.6, 132.1, 88.9, 86.5, 37.6, 37.5, 34.3, 33.6, 24.5,
23.3, 20.9, 19.1, 19.0, 18.9, 18.5, 12.6, 12.5, and 11.4.
Dim eth yl 3-Hyd r oxy-5-p h en yl-4-(tr ieth ylsilyl)p h th a -
la te (41). Reaction of cyclobutenone 35 (0.111 g, 0.429 mmol)
and DMAD (0.053 mL, 0.429 mmol) in 7 mL of toluene at 110
°C for 55 h according to the general procedure gave 0.167 g of
an orange oil. Column chromatography on 10 g of silica gel
(gradient elution with 0-70% benzene-hexane) afforded 0.095
g (55%) of phenol 41 as a white solid: mp 61-61.5 °C; IR (CCl4)
1
1730 and 1660 cm-1; H NMR (300 MHz, CDCl3) δ 11.28 (s,
1H), 7.32-7.36 (m, 5H), 6.79 (s, 1H), 3.93 (s, 3H), 3.87 (s, 3H),
0.78 (t, J ) 7.8 Hz, 9H), and 0.48 (q, J ) 7.8 Hz, 6H); 13C NMR
(75 MHz, CDCl3) δ 170.1, 169.5, 166.8, 157.2, 143.1, 135.0,
128.7, 127.8, 127.7, 127.0, 121.3, 106.6, 52.8, 52.5, 7.8, and
4.2; HRMS m/z calcd for C22H28O5Si 400.1706, found 400.1707.
6-Cya n o-3,4-d im eth yl-6-(eth oxyca r bon yl)-2-(tr iisop r o-
p ylsilyl)-2-cycloh exen -1-on e (44, 45). A 10-mL, two-necked,
round-bottomed flask equipped with a rubber septum and
argon inlet adapter was charged with a solution of ketene 22
(0.201 g, 0.800 mmol) and ethyl cyanoacrylate (0.120 g, 0.960
mmol) in 0.8 mL of toluene. The reaction mixture was stirred
for 24 h and then concentrated to give 0.390 g of a pale yellow
oil. Column chromatography on 10 g of silica gel (gradient
elution with 0-20% EtOAc-hexane) provided 0.113 g (38%)
of 44 (mp 81.5-82 °C), 0.625 g (20%) of 45 (mp 65-66.5 °C),
6-Nitr o-3,4,6-tr im eth yl-2-(tr iisopr op ylsilyl)-2-cycloh ex-
en -1-on e (48). A threaded Pyrex tube was charged with a
solution of ketene 22 (0.154 g, 0.610 mmol), nitropropene24
(0.104 g, 1.20 mmol), and one crystal of BHT in 0.6 mL of

(Trialkylsilyl)vinylketenes
J . Org. Chem., Vol. 63, No. 23, 1998 8389
1
toluene. The reaction mixture was degassed (three freeze-
pump-thaw cycles at -196 °C, <0.5 mmHg) and tightly sealed
with a threaded Teflon cap. The reaction mixture was heated
at 110 °C for 16 h and then concentrated to give 0.230 g of a
brown oil. Column chromatography on 15 g of silica gel
(gradient elution with 0-10% EtOAc-hexane) provided 0.073
g (35%) of 48 as a white solid: mp 82-84 °C; IR (CCl4) 2930,
cm-1; H NMR (300 MHz, CDCl3) δ 6.55 (s, 1H), 2.43 (s, 3H),
2.26 (s, 3H), 2.11 (s, 3H), 1.32 (sept, J ) 6.9 Hz, 3H), and 1.13
(d, J ) 7.0 Hz, 18H); 13C NMR (75 MHz, CDCl3) δ 159.9, 142.5,
140.7, 127.9, 117.7, 117.0, 103.5, 21.6, 18.9, 18.0, 15.1, and
13.0; UV (CH3CN) λmax, nm (ꢀ) 222 (470); HRMS m/z calcd for
C
19H31ONSi 317.2175, found 317.2165.
1
2860, and 1670 cm-1; H NMR (300 MHz, CDCl3) δ 2.71 (dd,
Ack n ow led gm en t. We are grateful to Dr. Kazuhiko
J ) 13.5, 9.8 Hz, 1H), 2.52 (m, 1H), 2.27 (dd, J ) 13.5, 5.8 Hz,
1H), 2.08 (s, 3H), 1.78 (s, 3H), 1.43 (sept, J ) 7.4 Hz, 3H),
1.28 (d, J ) 7.1 Hz, 3H), and 1.04 (app t, J ) 7.6 Hz, 18H);
13C NMR (75 MHz, CDCl3) δ 194.9, 171.8, 132.2, 93.1, 40.4,
37.0, 23.4, 20.9, 20.4, 19.1, 19.0, 12.7, and 12.5.
2-[(Tr iisop r op ylsilyl)oxy]-4,5,6-t r im e t h ylb e n zon i-
tr ile (50). A threaded Pyrex tube was charged with a solution
of ketene 22 (0.153 g, 0.605 mmol) and cyanoallene (0.132 g,
2.03 mmol) in 0.6 mL of toluene. The reaction mixture was
degassed (three freeze-pump-thaw cycles at -196 °C, <0.5
mmHg) and tightly sealed with a threaded Teflon cap. The
reaction mixture was heated at 150 °C for 31 h and concen-
trated to provide a brown oil. Column chromatography on 10
g of silica gel (gradient elution with 0-45% dichloromethane-
petroleum ether) provided 0.130 g (67%) of 50 as white
crystals: mp 58-59 °C; IR (CCl4) 2950, 2870, 2225, and 1730
Takahashi for exploratory experiments on the photo-
chemical Wolff rearrangement. We thank the National
Institutes of Health (GM 28273) for generous financial
support. J .L.L. and D.M.B. were supported in part as
National Science Foundation Predoctoral Fellows.
Su p p or tin g In for m a tion Ava ila ble: Characterization
data for compounds 10, 11, 13, 15-21, and 23-28 and 1H
NMR spectra for all new compounds (39 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.
J O981289U