Regioselective formation of silylated cyclobutenes by the photochemical [2+2] cycloaddition of 2(5H)-furanones to trialkylsilylacetylenes

Article abstract of DOI:10.1002/ejoc.201101332

Enantiopure 5-alkyl-4-methyl-2(5H)-furanones reacted by [2+2] photochemical cycloaddition with several trialkylsilylacetylenes to afford regioselectively 3-oxabicyclo[3.2.0]hept-6-en-2-ones silylated at the vinylic C-7 atom. The possible reasons for such regioselectivity are discussed. The anti/syn stereoselectivity of the photocycloaddition depends on the nature of the alkyl group on the silicon atom in the starting silylacetylenes. The photochemical cycloaddition of 2(5H)-furanones to trialkylsilylacetylenes regioselectively afforded cyclobutenes fused with furanone systems. Copyright

Full text of DOI:10.1002/ejoc.201101332

FULL PAPER
DOI: 10.1002/ejoc.201101332
Regioselective Formation of Silylated Cyclobutenes by the Photochemical [2+2]
Cycloaddition of 2(5H)-Furanones to Trialkylsilylacetylenes
Andrea D’Annibale,*[a] Maurizio D’Auria,*[b] Giovanna Mancini,[c] Alessio Daniel Pace,[a]
and Rocco Racioppi[b]
Keywords: 2(5H)-Furanones / Cyclobutenes / Photochemistry / Silanes / Cycloaddition
Enantiopure 5-alkyl-4-methyl-2(5H)-furanones reacted by
[2+2] photochemical cycloaddition with several trialkylsilyl-
acetylenes to afford regioselectively 3-oxabicyclo[3.2.0]hept-
6-en-2-ones silylated at the vinylic C-7 atom. The possible
reasons for such regioselectivity are discussed. The anti/syn
stereoselectivity of the photocycloaddition depends on the
nature of the alkyl group on the silicon atom in the starting
silylacetylenes.
when a stereocenter was present at C-5 of butenolide, the
byciclic cyclobutene was obtained as an anti/syn mixture,
and diastereomeric ratios did not exceed 7:3.
Introduction
Substituted four-membered carbocyclic rings are com-
mon subunits in many important bioactive compounds, and
they can be significant intermediates en route to natural
products.[1] One of the most exploited methods for their
preparation is the [2+2] photochemical cycloaddition of
alkenes to α,β-unsaturated carbonyl compounds[2] and ket-
enes,[3] which are both very good reaction partners for a
wide variety of alkenes. In particular, cycloaddition reac-
tions of α,β-unsaturated carbonyl compounds to alkenes
and alkynes represent an important way to generate small
rings,[4] which is extensively used in the synthesis of natural
products.[5] Studies on stereoselective and enantioselective
cycloaddition reactions have been performed.[6]
We have been involved in the synthesis of lactone systems
by alkene metathesis of allyl and homoallyl acrylates.[10] In
particular, we have explored the ring closing metathesis
(RCM) of methallyl acrylates 1, which have both a conju-
gated, electron-poor double bond and a trisubstituted ole-
fin. The RCM of 1 afforded 5-alkyl-4-methyl-substituted
2(5H)-furanones 2 in good yields (Scheme 1). As we were
particularly interested in the possible synthetic manipula-
tions of 2, we decided to study their photochemical [2+2]
cycloaddition with unsymmetrical trialkylsilylacetylenes 4,
and we report our results here.
Among the unsaturated carbonyl compounds used in
such reactions, several cases involving 2(5H)-furanones
have been reported.[7] Under photochemical irradiation,
such conjugated butenolides have been shown to react eas-
ily with alkenes to afford fused bicyclic cyclobutanes, which
are useful intermediates in the synthesis of important bioac-
tive compounds such as grandisol.[8] The butenolide photo-
chemical cycloaddition has been extended to use ethyne as
a reaction partner to afford cyclobutenes in modest yields.[9]
As regards the stereochemistry of such [2+2] cycloadditions,
Scheme 1. Synthesis of 2. (a) Acryloyl chloride, Et3N, CH2Cl2,
0 °C; (b) first-generation Grubbs catalyst (5mol-%), CH2Cl2, re-
flux.
To the best of our knowledge, no examples of the cyclo-
addition of unsymmetrical silylated alkynes on butenolides
that bear an alkyl group on the double bond have been
described. The choice of such trialkylsilylacetylenes was
made in order to study the regioselectivity in the formation
of silylated cyclobutene products. Indeed, the methyl group
on C-4 of the starting butenolides 2, together with the steric
bulk of alkyl groups on the silicon atom, could significantly
affect the regiochemistry of the cycloaddition. In this work,
we have also explored the diastereoselectivity of the cyclo-
addition using enantiopure 4-methylbutenolides 2, which
have a stereogenic center at C-5, as the substrates. We have
compared the stereochemical results obtained from using
[a] Dipartimento di Chimica, Università “La Sapienza”,
P.le Aldo Moro 5, 00185 Roma, Italy
Fax: +39-064-991-3628
E-mail: andrea.dannibale@uniroma1.it
[b] Dipartimento di Chimica “A. M. Tamburro”, Università della
Basilicata,
Via dell’Ateneo Lucano 10, 85100 Potenza, Italy
E-mail: maurizio.dauria@unibas.it
[c] Istituto CNR di Metodologie Chimiche, Dipartimento di
Chimica, Università “La Sapienza”,
P.le Aldo Moro 5, 00184 Roma, Italy
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101332.
Eur. J. Org. Chem. 2012, 785–791
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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different alkyl chain substituents on C-5. Furthermore, we
have investigated the effect exerted by trialkylsilylacetylenes
with different steric bulk on the diastereoselectivity of their
cycloaddition with chiral 2.
Results and Discussion
Scheme 2. Possible regiochemical outcome and products of [2+2]
cycloadditions of 2 with 4a.
The initial step was the preparation of the enantiomer-
ically pure 5-alkyl-4-methylbutenolides 2. Their synthesis
was carried out from chiral acrylate esters 1[7a] by RCM
(Scheme 1). The optically active methallyl alcohols (R)-3,
which were precursors of 1, were obtained by Sharpless ki-
netic resolution of the corresponding racemates (see Sup-
porting Information for procedures and yields).
Table 1. [2+2] Cycloaddition of 2a–d with 4a.
Substrate
R
Product Yield [%][a]
syn/anti ratio[b]
2a
2b
2c
2d
n-C4H9
n-C5H11
n-C10H21
n-C12H25
5a
5b
5c
5d
30
42
44
32
80:20
57:43
62:38
81:19
Once substrates 2 were prepared, we chose the best con-
ditions to let them react with trialkylsilylacetylenes 4. Pre-
liminary attempts led to the best experimental conditions [a] Calculated from the isolated product. [b] Calculated from the
chromatographically separated stereoisomers.
for our cycloadditions, that is, irradiation of the reaction
mixtures at 254 nm in quartz vessels with acetonitrile as the
solvent at room temperature for 40 h. We first investigated ucts in nonnegligible amounts. Thus, only compounds 5
the reactivity of 2 towards trimethylsilylacetylene (4a). In were purified after quenching the reaction. As regards the
principle, from such a photochemical cycloaddition we yields, we did not observe a definite trend that correlated
could afford both the diastereomeric syn/anti couples of the their values with the alkyl group on C-5 of 2. Indeed, in-
regioisomeric products 5 and 6, arising from head-to-head creasing the length of the alkyl chain at C-5 seemed to have
(HH) and head-to-tail cycloaddition modes, respectively a positive effect on the yield for 2a–c; however, 2d, which
(Scheme 2).
bears an n-dodecyl substituent, afforded 5d in low yields.
The cycloaddition was performed on butenolides 2a–d,
The regioselective outcome of the reaction was clarified
which bear an alkyl chain substituent on C-5, and the re- by NOESY experiments that were carried out on both dia-
sults are reported in Table 1. stereomers of 5b, which were easily separated. Positive NOE
As regards the regioselectivity of the cycloaddition, the interactions between the methyl groups on the silicon atom
reaction afforded only the HH products 5 as syn/anti mix- (δ = 0.11 ppm in both diastereomers) and the proton on the
tures selectively.
ring bridgehead tertiary carbon atom (δ = 3.16 ppm) were
The yields of 5 were not high, despite the complete con- a common feature of both the diastereomers 5b, together
version of 2 in the reaction mixture upon irradiation. This with a correlation between the olefinic proton (δ = 6.71, syn
was due to the formation of a complex mixture of byprod- isomer, and δ = 6.79 ppm, anti isomer) and the methyl
Figure 1. NOE interactions in 5b-anti, which account for the regioselectivity.
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Regioselective Formation of Silylated Cyclobutenes
Figure 2. Molecular orbitals involved in the photochemical reaction of 2 with 4.
group on the quaternary bridgehead carbon atom (δ = 1.30, more, the role of frontier-orbital control can be understood
syn isomer, and δ = 1.36 ppm, anti isomer, Figure 1). The considering that, if we calculate the total energy of the re-
same interactions were shown by other compounds 5, which gioisomeric biradical intermediates B (precursor of 5-syn)
gave similar NOESY spectra.
and C (precursor of 6-syn, not found in the reaction mix-
The preference for the HH product 5 was explained by ture) (Figure 4), C is more stable than B by 0.86 kcalmol–1.
analysis of molecular orbitals involved in the photochemi-
cal reaction (Figure 2). We performed calculations of the
optimized structures of the reagents at the DFT/B3LYP/6-
31-G+(d,p) level with Gaussian09.[11] The butenolide 2 that
bears a methyl group at C-5 was used as a model in the
calculations. The only interaction that can account for the
photochemical addition is that involving the butenolide
highest singly occupied molecular orbital (HSOMO) and
the highest occupied molecular orbital (HOMO) of 4a (Fig-
ure 2). According to this analysis, the cycloaddition reac-
tion cannot proceed through a concerted pathway, and the
Figure 4. Structure of the possible biradical intermediates involved
in the photochemical reaction of 2 and 4.
NOESY spectra of the separated diastereomers allowed
us to understand the stereochemical outcome of the reac-
tion. Indeed, their relative stereochemistry was recognized
by the peculiar NOE correlation between 5-H and the vin-
ylic proton in the anti isomers, which was absent in the
NOESY spectra of the syn compounds (an example is given
in Figure 5).
The data shown in Table 1 indicate that syn isomers were
the prevalent products. The change of the length of the
alkyl group at C-5 did not significantly change the stereose-
lectivity of the reaction. To explain this behavior, we exam-
ined the relative stability of the diradical intermediate in-
volved in the reaction. We considered the hypothesis that
the stereochemistry of the reaction could be determined by
the initial attack of acetylenic C-2 at C-4 of the butenolide,
which depends on the relative stability of the intermediate
thus obtained. We calculated the optimized structures for
the biradical intermediates A and B (Figure 4). The biradi-
cal B is more stable than A by 0.52 kcalmol–1. This moder-
presence of a methyl group at C-4 does not allow the antara-
facial approach of the reagents. Conversely, a diradical in-
termediate is likely. Moreover, the evaluation of atomic co-
efficients shows values of 0.36 and 0.39 for C-1 and C-2,
respectively, for the HOMO of 4a.
The lactone HSOMO has values of –0.35 for C-3 and
0.40 for C-4. Due to such values, the only possible interac-
tion between these molecular orbitals can be that between
butenolide C-4 and C-2 of 4a (Figure 3).
Figure 3. Interaction between the HSOMO of 2 and the HOMO
of 4.
This type of interaction allowed us to explain the regi- ate difference can explain the low diastereoselectivity ob-
ochemical behavior of the cycloaddition reaction. Further- served.
Eur. J. Org. Chem. 2012, 785–791
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Table 2. Photochemical reaction of 2a–d with 4b.
Substrate
R
Product Yield [%][a]
syn/anti ratio[b]
2a
2b
2c
2d
n-C4H9
n-C5H11
n-C10H21
n-C12H25
7a
7b
7c
7d
44
43
27
24
59:41
55:45
34:66
35:65
[a] Calculated from isolated product. [b] Calculated from chro-
matographically separated stereoisomers.
The results shown in Table 1 from the reaction with 4a
indicated that compounds 5 were obtained in moderate
yields. In the reaction with 4b, the yields were more coher-
ent and seemed to decrease as the alkyl substituent became
larger. Again, regio- and stereochemical assignments were
made by NOESY experiments. The peculiar NOE interac-
tions detected for 5 were also present in 7, which again
proved the reaction to be highly regioselective (Figure 6).
According to the data shown in Table 2, the syn isomer
was prevalent only when R was n-butyl or n-pentyl. The
increase in the length of the alkyl group at C-5 led to a
change in selectivity, which afforded the anti isomer as the
major product. In all cases, the diastereomeric ratio for 7
was lower than that of 5. The reason for this lower stereose-
lectivity can be understood by considering the relative sta-
bility of the biradical intermediates that are involved in the
formation of the two stereoisomers. Indeed, we calculated
Figure 5. Peculiar NOE interaction in 5b-anti.
Thus, we decided to use triisopropylsilylacetylene (4b) as
a reaction partner to verify if bulkier groups on the silicon
atom would significantly affect the relative diastereomeric
ratios of the expected products 7 (Scheme 3). The results
are shown in Table 2.
Figure 7. Biradical intermediates in the reaction of a model methyl-
substituted lactone 2 with 4b.
Scheme 3. Cycloaddition of 2 with 4b.
Figure 6. Peculiar NOE interaction in 7a-anti, which accounts for regio- and stereoselectivity.
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Regioselective Formation of Silylated Cyclobutenes
Scheme 4. Hypothetical mechanism for the [2+2] cycloaddition between 1 and 4a.
(CDCl3: δH = 7.25 ppm, δC = 77.0 ppm). IR spectra were recorded
the energy of the biradical intermediates D and E (which
lead to anti and syn isomers, respectively) that could be ob-
tained from a 5-methyl-substituted model butenolide (Fig-
ure 7).
with a Shimadzu IR 470 in CHCl3 by using NaCl cells. GC–MS
analyses were performed with an HP 5890 gas chromatograph
linked to an HP 5791 mass spectrometer. ESI HRMS spectra were
recorded with a QTOF Micro spectrometer (Waters) operating in
positive ion mode. Chemicals were commercially available and used
without further purification. Acetonitrile was dried by distillation
with Na under argon. Photochemical reactions were carried out
inside a Rayonet RPR 100 apparatus, which was equipped with Hg
lamps irradiating at 254 nm, with quartz cuvettes as the reaction
vessels. All the reactions were monitored by TLC on silica gel plates
60 F254. Column chromatography was performed with silica gel 60
(Merck, 70–230 mesh).
The calculations carried out on this model showed that
the energy difference between
D and E was only
0.1 kcalmol–1, which is in agreement with the low stereose-
lectivity observed experimentally.
Conclusions
The photochemical cycloaddition of chiral butenolides
to trialkylsilylacetylenes has been shown to be highly re-
gioselective and leads exclusively to HH products. The ex-
planation of such a selectivity led us to propose a mecha-
nism based on a biradical adduct intermediate (Scheme 4).
As regards the stereochemistry, the nature of the alkyl
group on C-5 of 2 did not seem to affect the diastereofacial
selection, which generally led to syn products. The moderate
ratios between the syn and anti diastereoisomers could be
due to the small difference in the energy of the triplet inter-
mediates. However, the isolation of the anti isomer for 7c
and 7d suggests that other factors should also be consid-
ered. Indeed, the energy difference between the biradical
intermediates only accounts for the different populations of
the biradicals. Nevertheless, the effective distribution of the
diastereoisomers in principle depends also on various ki-
netic factors, of which the first is the rate of the retrocleav-
age reaction of the biradical to give the starting material
(k–1, Scheme 4). The efficiency of the intersystem crossing
(kISC) and the rate of coupling of the resultant singlet birad-
ical (k2) also have to be taken into account.
General Procedure for the Photochemical [2+2] Cycloaddition of
Butenolides 2. Preparation of Compounds 5 and 7: In a 5 mL quartz
cuvette, butenolide
2 (0.5 mmol) and trialkylsilylacetylene 4
(2.5 mmol) were dissolved in acetonitrile (1.5 mL). After irradiating
for 40 h, the solvent was evaporated under reduced pressure to af-
ford an oily residue. Purification of the residue by column
chromatography (light petroleum ether/Et2O, 9:1) afforded 5 and 7
as colorless oils.
4-n-Butyl-5-methyl-7-trimethylsislyl-cis-3-oxabicyclo[3.2.0]hept-6-
en-2-one (5a-anti): Yield: 6%. [α]2D5 = –41.6 (c = 0.3, CHCl3). 1H
NMR (300 MHz, CDCl3): δ = 0.12 (s, 9 H, 9-H), 0.91 (t, J =
7.2 Hz, 3 H, 13-H), 1.10–1.65 (m, 6 H, 10–12-H), 1.31 (s, 3 H, 8-
H), 3.17 (s, 1 H, 1-H), 4.33 (dd, J1 = 2.4, J2 = 9.3 Hz, 1 H, 4-H),
6.79 (s, 1 H, 6-H) ppm. 13C NMR (75 MHz, CDCl3): δ = – 2.42,
13.9, 16.8, 22.5, 27.9, 32.3, 51.0, 53.4, 82.4, 156.0, 156.8,
Our future efforts will be devoted to synthesizing chemi-
cal structures that are able to give a higher energy difference
between the triplet biradicals. This can probably be
achieved by increasing the size of the substituent at C-4.
The interesting change in diastereoselectivity, which was
observed when a bulkier trialkylsilylacetylene was em-
ployed, will be the object of future investigations.
176.0 ppm. IR (NaCl): ν = 1065, 1427, 1753, 2976, 3060 cm–1
.
˜
HRMS (ES): calcd. for C14H24O2NaSi [M + Na]+ 275.1443; found
275.1438.
4-n-Butyl-5-methyl-7-trimethylsislyl-cis-3-oxabicyclo[3.2.0]hept-6-
en-2-one (5a-syn): Yield: 24%. [α]2D5 = 62.6 (c = 0.2, CHCl3). 1H
NMR (300 MHz, CDCl3): δ = 0.12 (s, 9 H, 9-H), 0.91 (t, J =
6.6 Hz, 3 H, 13-H), 1.15–1.70 (m, 6 H, 10–12-H), 1.36 (s, 3 H, 8-
H), 3.15 (s, 1 H, 1-H), 4.05 (dd, J1 = 4.5, J2 = 9.3 Hz, 1 H, 4-H),
6.71 (s, 1 H, 6-H) ppm. 13C NMR (75 MHz, CDCl3): δ = – 2.4,
13.9, 20.3, 22.5, 28.6, 30.5, 51.9, 54.2, 84.4, 153.1, 157.2,
Experimental Section
General: 1H and 13C NMR spectra were recorded in CDCl3 with
300 MHz Mercury Varian and Avance 2 Bruker spectrometers. The
calibration of the spectra was carried out on solvent signals
175.7 ppm. IR (NaCl): ν = 1035, 1445, 1744, 2967, 3086 cm–1
.
˜
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HRMS (ES): calcd. for C15H26O2NaSi [M + Na]+ 289.1600; found
275.1441.
4-n-Butyl-5-methyl-7-triisopropylsislyl-cis-3-oxabicyclo[3.2.0]hept-6-
1
en-2-one (7a-anti): Yield: 18%. [α]2D5 = – 63.2 (c = 0.8, CHCl3). H
NMR (300 MHz, CDCl3): δ = 0.90 (t, J = 7.2 Hz, 3 H, 13-H), 1.05
and 1.08 (2 d, J = 6.6 Hz, 6 H, 9Ј-H), 1.00–1.20 (m, 3 H, 9-H),
1.00–1.60 (m, 6 H, 10-H, 11-H, 12-H), 1.33 (s, 3 H, 8-H), 3.19 (s,
1 H, 1-H), 4.34 (dd, J1 = 2.4, J2 = 9.3 Hz, 1 H, 4-H), 6.92 (s, 1 H,
6-H) ppm. 13C NMR (75 MHz, CDCl3): δ = 10.7, 13.9, 17.2, 18.6,
22.5, 27.9, 32.3, 51.6, 55.2, 82.3, 152.6, 160.5, 175.8 ppm. IR
5-Methyl-4-n-pentyl-7-trimethylsislyl-cis-3-oxabicyclo[3.2.0]hept-6-
1
en-2-one (5b-anti): Yield: 18%. [α]2D5 = –85.3 (c = 0.8, CHCl3). H
NMR (300 MHz, CDCl3): δ = 0.11 (s, 9 H, 9-H), 0.88 (t, J =
7.2 Hz, 3 H, 14-H), 1.21–1.64 (m, 8 H, 10–13-H), 1.30 (s, 3 H, 8-
H), 3.16 (s, 1 H, 1-H), 4.33 (dd, J1 = 2.4, J2 = 9.3 Hz, 1 H, 4-H),
6.79 (s, 1 H, 6-H) ppm. 13C NMR (75 MHz, CDCl3): δ = – 2.4,
14.0, 16.8, 22.5, 25.5, 31.6, 32.6, 51.0, 53.4, 82.4, 156.0, 156.9,
(NaCl): ν = 943, 1525, 1741, 2842, 2978 cm–1. HRMS (ES): calcd.
˜
for C20H36O2NaSi [M + Na]+ 359.2382; found 359.2377.
176.2 ppm. IR (NaCl): ν = 1233, 1442, 1527, 1739, 2965,
˜
3075 cm–1. HRMS (ES): calcd. for C15H26O2NaSi [M + Na]+
289.1600; found 289.1606.
4-n-Butyl-5-methyl-7-triisopropylsislyl-cis-3-oxabicyclo[3.2.0]hept-6-
en-2-one (7a-syn): Yield: 26%. [α]2D5 = 76.8 (c = 0.9, CHCl3). 1H
NMR (300 MHz, CDCl3): δ = 0.90 (t, J = 7.2 Hz, 3 H, 13-H), 1.05
and 1.08 (2 d, J = 6.6 Hz, 9 H, 9Ј-H), 1.00–1.20 (m, 3 H, 9-H),
1.00–1.70 (m, 6 H, 10-H, 11-H, 12-H), 1.38 (s, 3 H, 8-H), 3.19 (s,
1 H, 1-H), 4.03 (dd, J1 = 4.5, J2 = 9.3 Hz, 1 H, 4-H), 6.84 (s, 1 H,
6-H) ppm. 13C NMR (75 MHz, CDCl3): δ = 10.7, 13.8, 18.6, 20.7,
22.5, 28.5, 30.8, 52.5, 56.0, 84.2, 153.7, 156.9, 175.3 ppm. IR
5-Methyl-4-n-pentyl-7-trimethylsislyl-cis-3-oxabicyclo[3.2.0]hept-6-
1
en-2-one (5b-syn): Yield: 24%. [α]2D5 = 134.3 (c = 0.6, CHCl3). H
NMR (300 MHz, CDCl3): δ = 0.11 (s, 9 H, 9-H), 0.89 (t, J =
6.6 Hz, 3 H, 14-H), 1.21–1.70 (m, 8 H, 10–13-H), 1.36 (s, 3 H, 8-
H), 3.15 (s, 1 H, 1-H), 4.05 (dd, J1 = 4.5, J2 = 9.3 Hz, 1 H, 4-H),
6.71 (s, 1 H, 6-H) ppm. 13C NMR (75 MHz, CDCl3): δ = – 2.4,
14.0, 20.3, 22.5, 26.1, 30.8, 31.6, 51.9, 54.2, 84.5, 153.2, 157.1,
(NaCl): ν = 923, 1535, 1738, 2845, 2983 cm–1. HRMS (ES): calcd.
˜
for C20H36O2NaSi [M + Na]+ 359.2382; found 359.2384.
175.9 ppm. IR (NaCl): ν = 935, 1438, 1737, 2985, 3065 cm–1
.
˜
HRMS (ES): calcd. for C15H26O2NaSi [M + Na]+ 289.1600; found
289.1558.
5-Methyl-4-n-pentyl-7-triisopropylsislyl-cis-3-oxabicyclo[3.2.0]-
hept-6-en-2-one (7b-anti): Yield: 19%. [α]2D5 = – 68.3 (c = 1.5,
1
4-n-Decyl-5-methyl-7-trimethylsislyl-cis-3-oxabicyclo[3.2.0]hept-6-
CHCl3). H NMR (300 MHz, CDCl3): δ = 0.89 (t, J = 7.2 Hz, 3
1
en-2-one (5c-anti): Yield: 17%. [α]2D5 = –65.9 (c = 0.2, CHCl3). H
H, 14-H), 1.05 and 1.08 (2 d, J = 6.6 Hz, 9 H, 9Ј-H), 1.00–1.20 (m,
3 H, 9-H), 1.00–1.60 (m, 8 H, 10–13-H), 1.33 (s, 3 H, 8-H), 3.20
(s, 1 H, 1-H), 4.34 (dd, J1 = 2.4, J2 = 9.3 Hz, 1 H, 4-H), 6.92 (s, 1
H, 6-H) ppm. 13C NMR (75 MHz, CDCl3): δ = 10.7, 13.9, 17.2,
18.6, 22.5, 25.5, 31.6, 32.6, 51.6, 55.2, 82.3, 152.7, 160.5,
NMR (300 MHz, CDCl3): δ = 0.11 (s, 9 H, 9-H), 0.87 (t, J =
7.2 Hz, 3 H, 19-H), 1.16–1.61 (m, 18 H, 10–18-H), 1.30 (s, 3 H, 8-
H), 3.16 (s, 1 H, 1-H), 4.33 (dd, J1 = 2.4, J2 = 9.3 Hz, 1 H, 4-H),
6.79 (s, 1 H, 6-H) ppm. 13C NMR (75 MHz, CDCl3): δ = – 2.42,
14.1, 16.8, 22.7, 25.8, 26.9, 29.3, 29.4, 29.5, 29.6, 31.9, 32.6, 51.0,
175.7 ppm. IR (NaCl): ν = 935, 1530, 1745, 2840, 2910, 2950 cm–1.
˜
HRMS (ES): calcd. for C21H38O2NaSi [M + Na]+ 373.2539; found
373.2538.
53.4, 82.4, 156.0, 156.9, 176.0 ppm. IR (NaCl): ν = 1035, 1370,
˜
1723, 2880, 2972 cm–1. HRMS (ES): calcd. for C20H36O2NaSi [M
+ Na]+ 359.2382; found 359.2379.
5-Methyl-4-n-pentyl-7-triisopropylsislyl-cis-3-oxabicyclo[3.2.0]-
hept-6-en-2-one (7b-syn): Yield: 24%. [α]2D5 = 111.1 (c = 1.2, CHCl3).
1H NMR (300 MHz, CDCl3): δ = 0.89 (t, J = 7.2 Hz, 3 H, 14-H),
1.06 and 1.08 (2 d, J = 6.6 Hz, 9 H, 9Ј-H), 1.00–1.20 (m, 3 H, 9-
H), 1.00–1.75 (m, 8 H, from 10–13-H), 1.39 (s, 3 H, 8-H), 3.19 (s,
1 H, 1-H), 4.03 (dd, J1 = 4.5, J2 = 9.3 Hz, 1 H, 4-H), 6.84 (s, 1 H,
6-H) ppm. 13C NMR (75 MHz, CDCl3): δ = 10.7, 13.9, 18.6, 20.7,
22.5, 26.1, 31.1, 31.6, 52.5, 56.0, 84.2, 153.7, 156.9, 175.3 ppm. IR
4-n-Decyl-5-methyl-7-trimethylsislyl-cis-3-oxabicyclo[3.2.0]hept-6-
en-2-one (5c-syn): Yield: 27%. [α]2D5 = 39.4 (c = 0.2, CHCl3). 1H
NMR (300 MHz, CDCl3): δ = 0.12 (s, 9 H, 9-H), 0.87 (t, J =
6.6 Hz, 3 H, 19-H), 1.20–1.72 (m, 18 H, 10–18-H), 1.36 (s, 3 H, 8-
H), 3.15 (s, 1 H, 1-H), 4.05 (dd, J1 = 4.5, J2 = 9.3 Hz, 1 H, 4-H),
6.71 (s, 1 H, 6-H) ppm. 13C NMR (75 MHz, CDCl3): δ = – 2.41,
14.1, 20.3, 22.6, 25.8, 26.5, 29.3, 29.4, 29.5, 29.6, 30.9, 31.9, 51.9,
54.2, 84.4, 153.1, 157.2, 175.7 ppm. IR (NaCl): ν = 1160, 1530,
˜
(NaCl): ν = 862, 1520, 1744, 2820, 2940 cm–1. HRMS (ES): calcd.
˜
1745, 2860, 2930, 3090 cm–1. HRMS (ES): calcd. for C20H36O2-
NaSi [M + Na]+ 359.2382; found 359.2376.
for C21H38O2NaSi [M + Na]+ 373.2539; found 373.2542.
4-n-Decyl-5-methyl-7-triisopropylsislyl-cis-3-oxabicyclo[3.2.0]hept-
6-en-2-one (7c-anti): Yield: 18%. [α]2D5 = – 55.5 (c = 1.6, CHCl3).
1H NMR (300 MHz, CDCl3): δ = 0.88 (t, J = 7.2 Hz, 3 H, 19-H),
1.05 and 1.08 (2 d, J = 6.6 Hz, 9 H, 9Ј-H), 1.00–1.20 (m, 3 H, 9-
H), 1.10–1.65 (m, 18 H, 10–18-H), 1.33 (s, 3 H, 8-H), 3.20 (s, 1 H,
1-H), 4.34 (dd, J1 = 2.4, J2 = 9.3 Hz, 1 H, 4-H), 6.92 (s, 1 H, 6-H)
ppm. 13C NMR (75 MHz, CDCl3): δ = 10.7, 14.1, 17.2, 18.6, 22.7,
25.8, 29.3, 29.4, 29.5, 29.6, 29.6, 31.9, 32.7, 51.6, 55.2, 82.3, 152.6,
4-n-Dodecyl-5-methyl-7-trimethylsislyl-cis-3-oxabicyclo[3.2.0]hept-
1
6-en-2-one (5d-anti): Yield: 6%. [α]2D5 = –24.1 (c = 0.4, CHCl3). H
NMR (300 MHz, CDCl3): δ = 0.11 (s, 9 H, 9-H), 0.88 (t, J =
7.2 Hz, 3 H, 21-H), 1.11–1.61 (m, 22 H, 10–20-H), 1.30 (s, 3 H, 8-
H), 3.16 (s, 1 H, 1-H), 4.33 (dd, J1 = 2.4, J2 = 9.3 Hz, 1 H, 4-H),
6.80 (s, 1 H, 6-H) ppm. 13C NMR (75 MHz, CDCl3): δ = – 2.42,
14.1, 16.8, 22.7, 25.8, 29.3, 29.4, 29.5, 29.5, 29.6, 29.6, 29.7, 31.9,
32.6, 51.0, 53.4, 82.4, 156.0, 156.9, 176.1 ppm. IR (NaCl): ν = 1045,
˜
1422, 1744, 2880, 2934 cm–1. HRMS (ES): calcd. for C22H40O2-
NaSi [M + Na]+ 386.2695; found 386.2691.
160.5, 175.8 ppm. IR (NaCl): ν = 913, 1464, 1740, 2876, 2970,
˜
2985 cm–1. HRMS (ES): calcd. for C26H48O2NaSi [M + Na]+
443.3321; found 443.3325.
4-n-Dodecyl-5-methyl-7-trimethylsislyl-cis-3-oxabicyclo[3.2.0]hept-
1
6-en-2-one (5d-syn): Yield: 26%. [α]2D5 = 89.7 (c = 0.6, CHCl3). H
4-n-Decyl-5-methyl-7-triisopropylsislyl-cis-3-oxabicyclo[3.2.0]hept-
6-en-2-one (7c-syn): Yield: 9%. [α]2D5 = 60.5 (c = 1.3, CHCl3). 1H
NMR (300 MHz, CDCl3): δ = 0.12 (s, 9 H, 9-H), 0.87 (t, J =
6.6 Hz, 3 H, 21-H), 1.06–1.69 (m, 22 H, 10–20-H), 1.35 (s, 3 H, 8- NMR (300 MHz, CDCl3): δ = 0.88 (t, J = 7.2 Hz, 3 H, 19-H), 1.05
H), 3.15 (s, 1 H, 1-H), 4.04 (dd, J1 = 4.5, J2 = 9.3 Hz, 1 H, 4-H), and 1.08 (2 d, J = 6.6 Hz, 9 H, 9Ј-H), 1.00–1.20 (m, 3 H, 9-H),
6.71 (s, 1 H 6-H) ppm. 13C NMR (75 MHz, CDCl3): δ = – 2.39, 1.00–1.75 (m, 18 H, 10–18-H), 1.39 (s, 3 H, 8-H), 3.19 (s, 1 H, 1-
14.1, 20.3, 22.7, 26.5, 29.3, 29.4, 29.5, 29.6, 29.6, 29.7, 30.9, 31.9,
H), 4.03 (dd, J1 = 4.5, J2 = 9.3 Hz, 1 H, 4-H), 6.84 (s, 1 H, 6-H)
32.6, 51.9, 54.2, 84.4, 153.1, 157.2, 175.7 ppm. IR (NaCl): ν = 1540,
ppm. 13C NMR (75 MHz, CDCl3): δ = 10.7, 14.1, 18.6, 20.7, 22.7,
˜
1724, 2910, 2976, 3005 cm–1. HRMS (ES): calcd. for C22H40O2- 26.4, 29.3, 29.4, 29.4, 29.5, 29.6, 31.1, 31.9, 52.5, 56.0, 84.2, 153.7,
NaSi [M + Na]+ 386.2695; found 386.2694.
156.9, 175.3 ppm. IR (NaCl): ν = 935, 1464, 1730, 2840, 2900,
˜
790
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 785–791

Regioselective Formation of Silylated Cyclobutenes
3030 cm–1. HRMS (ES): calcd. for C26H48O2NaSi [Na + Na]+
443.3321; found 443.3323.
of Organic Photochemistry and Photobiology (Eds.: W. Hor-
spool, F. Lenci), CRC Press, Boca Raton, 2004, p. 72; c) J. P.
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pp. 171–216.
4-n-Dodecyl-5-methyl-7-triisopropylsislyl-cis-3-oxabicyclo[3.2.0]-
hept-6-en-2-one (7d-anti): Yield: 16%. [α]2D5 = – 35.4 (c = 0.7,
1
CHCl3). H NMR (300 MHz, CDCl3): δ = 0.88 (t, J = 7.2 Hz, 3
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H, 21-H), 1.05 and 1.08 (2 d, J = 6.6 Hz, 9 H, 9Ј-H), 1.00–1.20 (m,
3 H, 9-H), 1.00–1.60 (m, 22 H, 10–20-H), 1.33 (s, 3 H, 8-H), 3.20
(s, 1 H, 1-H), 4.34 (dd, J1 = 2.4, J2 = 9.3 Hz, 1 H, 4-H), 6.92 (s, 1
H, 6-H) ppm. 13C NMR (75 MHz, CDCl3): δ = 10.7, 14.1, 17.2,
18.6, 19.3, 22.7, 25.8, 29.3, 29.4, 29.5, 29.5, 29.6, 29.6, 31.9, 32.7,
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51.6, 55.2, 82.3, 152.7, 160.5, 175.7 ppm. IR (NaCl): ν = 936, 1454,
˜
1745, 2864, 2923, 2985 cm–1. HRMS (ES): calcd. for C28H52O2-
NaSi [M + Na]+ 471.3634; found 471.3636.
4-n-Dodecyl-5-methyl-7-triisopropylsislyl-cis-3-oxabicyclo[3.2.0]-
hept-6-en-2-one (7d-syn): Yield: 8%. [α]2D5 = 40.7 (c = 0.3, CHCl3).
1H NMR (300 MHz, CDCl3): δ = 0.88 (t, J = 7.2 Hz, 3 H, 21-H),
1.05 and 1.08 (2 d, J = 6.6 Hz, 9 H, 9Ј-H), 1.00–1.20 (m, 3 H, 9-
H), 1.00–1.73 (m, 22 H, 10–20-H), 1.39 (s, 3 H, 8-H), 3.19 (s, 1 H,
1-H), 4.04 (dd, J1 = 4.5, J2 = 9.3 Hz, 1 H, 4-H), 6.84 (s, 1 H, 6-H)
ppm. 13C NMR (75 MHz, CDCl3): δ = 10.7, 14.1, 18.5, 18.6, 20.8,
22.7, 26.4, 29.3, 29.4, 29.4, 29.5, 29.6, 29.6, 31.1, 31.9, 52.5, 56.0,
84.2, 153.7, 156.9, 175.3 ppm. IR (NaCl): ν = 1116, 1433, 1723,
˜
2872, 2930, 2980 cm–1. HRMS (ES): calcd. for C28H52O2NaSi [M
+ Na]+ 471.3634; found 471.3638.
Supporting Information (see footnote on the first page of this arti-
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3; NMR spectra of 5 and 7, NOESY spectra of 5b and 7a: DFT
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Received: September 12, 2011
Published Online: November 30, 2011
Eur. J. Org. Chem. 2012, 785–791
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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