Ring-closing metathesis for the synthesis of novel 9- and 10-membered silicon-containing benzo-fused heterocycles

Article abstract of DOI:10.1016/j.tetlet.2008.10.067

A ring-closing metathesis (RCM) strategy afforded a number of novel 9- and 10-membered benzo-fused compounds containing at least one silicon atom as part of the heterocyclic portion. In this manner, the following compounds containing heterocyclic rings of 9-10 atoms were synthesized: (Z)-2,2-dimethyl-7-[(4-methylphenyl)sulfonyl]-2,3,6,7-tetrahydrobenzo[h][1,7,2]oxazasilonine, (Z)-2,2-dimethyl-3,6-dihydro-2H-benzo[h][1,7,2]dioxasilonine, (Z)-8-isopropoxy-9-methoxy-3,3-dimethyl-1,3,4,7-tetrahydrobenzo[g][1,2]oxasilonine and (Z)-2,2,7,7-tetramethyl-2,3,6,7-tetrahydrobenzo[i][1,8,2,7]dioxadisilecine.

Full text of DOI:10.1016/j.tetlet.2008.10.067

Tetrahedron Letters 49 (2008) 7403–7405
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Tetrahedron Letters
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Ring-closing metathesis for the synthesis of novel 9- and 10-membered
silicon-containing benzo-fused heterocycles
*
Stefania M. Scalzullo, Rafique Ul Islam, Garreth L. Morgans, Joseph P. Michael, Willem A. L. van Otterlo
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, PO Wits, 2050 Johannesburg, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
A ring-closing metathesis (RCM) strategy afforded a number of novel 9- and 10-membered benzo-fused
compounds containing at least one silicon atom as part of the heterocyclic portion. In this manner, the
following compounds containing heterocyclic rings of 9–10 atoms were synthesized: (Z)-2,2-dimethyl-
7-[(4-methylphenyl)sulfonyl]-2,3,6,7-tetrahydrobenzo[h][1,7,2]oxazasilonine, (Z)-2,2-dimethyl-3,6-di-
hydro-2H-benzo[h][1,7,2]dioxasilonine, (Z)-8-isopropoxy-9-methoxy-3,3-dimethyl-1,3,4,7-tetrahydro-
benzo[g]-[1,2]oxasilonine and (Z)-2,2,7,7-tetramethyl-2,3,6,7-tetrahydrobenzo[i][1,8,2,7]dioxadisilecine.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 8 September 2008
Revised 6 October 2008
Accepted 14 October 2008
Available online 18 October 2008
Medicinal chemistry regularly makes use of benzo-fused com-
pounds as templates for the investigation of pharmaceutical scaf-
folds.1 In addition, the use of silicon as an isostere in drug
discovery has also been seen to increase;2 this would make benz-
annulated compounds containing silicon atoms in their backbone
interesting candidates for biological evaluation. Over the past
few decades, benzo-fused heterocycles containing silicon in the
heterocyclic portion of the molecule have seen sporadic investiga-
tion.3 Examples include compounds which comprise 6,6-, 6,7- and
6,8-fused systems, such as the generalized structures 1a,4 1b,5 1c,6
benzoxasilepin-2(3H)-one 27 and benzazasilocine 3,8 respectively
(Fig. 1).
decade,9 very few examples resulting in the synthesis of com-
pounds such as those represented in Figure 1 have been reported.10
To the best of our knowledge only Rodríguez-García and co-work-
ers have been active in this area, in their elegant pursuit of the
pterocarpans and related compounds.11 Amongst other examples,
this group demonstrated that the application of RCM to com-
pounds 5 and 8, synthesized from substrates 4 and 7, readily affor-
ded 2,3-dihydro-1,2-benzoxasilepine 6 and 3,6-dihydro-2H-1,2-
benzoxasilocine 9, respectively (Scheme 1).
Our research group has been heavily involved in the application
of RCM to the synthesis of benzo-fused bicyclic molecules.12 It was
thus decided to investigate the application of metathesis and isom-
erization-metathesis strategies towards the synthesis of other sili-
con-containing benzo-fused heterocycles. This communication
describes the successes of our investigations as well as some of
the limitations of this approach.
Rather surprisingly, although ring-closing metathesis (RCM) has
seen a tremendous increase in synthetic applications over the last
In the first experimental work performed, it was decided to re-
peat the work described by Rodríguez-García and co-workers
(Scheme 1),11 with the difference that the second-generation Grub-
bs’ catalyst 11 was used in preference to 10.13 The reason for this
was because one of the goals of this research was to extend the
methodology to other potentially electron-rich substrates.
The initial results were very promising, and it was thus decided
to investigate the RCM reaction for the formation of other annu-
lated heteroatom-containing silacycles, the first of which con-
tained nitrogen and oxygen atoms. To this end, substituted
phenol 1314 was silylated with reagent 1215 to afford the diallyl
species 14 in a good yield of 80% (Scheme 2). A RCM reaction with
catalyst 11 proved to be a straightforward process and afforded the
9-membered benzoxazasilonine 15 in good yield (78%).16
Si
Z
Y
1a Y = SO2 or CO, Z = NH
1b Y = NMe, NEt or NBn, Z = CH2
1c Y = CH2, Z = NMe, NEt or NBn
Si
Si
O
N
H
2
O
3
Figure 1. Examples of previously prepared dimethylsilyl-containing bicyclic benzo-
Rodríguez-García and co-workers successfully employed an
‘isomerization-RCM’ strategy to obtain compounds with the skele-
ton 9. It was thus decided to investigate whether it was possible to
utilize a similar strategy involving an O-vinyl ether obtained from
fused compounds.
* Corresponding author.
E-mail address: Willem.vanOtterlo@wits.ac.za (W. A. L. van Otterlo).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.10.067

7404
S. M. Scalzullo et al. / Tetrahedron Letters 49 (2008) 7403–7405
OH
OH
OH
OH
7
8
4
5
16
i
i
OH
O
OH
O
i
O
O
Si
Si
18
ii
17
iv
Si
Si
ii
O Si
ii
O
O
O
O
O
N
Si
21
19
iii
v
9
6
Si
O
O Si
O
PCy3
Ru
N
Cl
Mes
Ph
Mes
Cl
Cl
Cl
Ru
O
Ph
10
20
22
PCy3
11
PCy3
Scheme 3. Reagents and conditions: (i) 3% catalyst [RuClH(CO)(PPh3)3], CH2Cl2,
reflux, N2, 20 h, 56%; (ii) Et3N, CH2Cl2, allylchlorodimethylsilane 12, rt, N2, 5 h, 73%;
(iii) 5% catalyst 11, CH2Cl2, reflux, N2, 24 h; (iv) Et3N, CH2Cl2, allylchlorodimeth-
ylsilane 12, rt, N2, 66 h, 76%; (v) 5% catalyst 11, CH2Cl2, reflux, N2, 18 h, 22%.
Scheme 1. Rodríguez-García’s work.11 Reagents and conditions: (i) Et3N, CH2Cl2,
allylchlorodimethylsilane 12; (ii) 5% catalyst 10.
metathesis methodology then gratifyingly afforded (Z)-8-iso-
propoxy-9-methoxy-3,3-dimethyl-1,3,4,7-tetrahydrobenzo[g][1,2]-
oxasilonine 2522 in a good yield of 82%.
Si
O
OH
i
N
N
Ts
Ts
13
14
ii
Si
O
OH
i
Si
O
MeO
MeO
Si
Cl
OiPr
OiPr
12
23
24
N
Ts
15
ii
O
Scheme 2. Reagents and conditions: (i) Et3N, CH2Cl2, allylchlorodimethylsilane 12,
0 °C–rt, N2, 15 h, 80%; (ii) 5% catalyst 11, CH2Cl2, rt, N2, 12 h, 78%.
Si
MeO
the corresponding O-allyl ether.17,18 Catechol 16 was thus mono-
allylated to afford compound 17 (Scheme 3). Subsequent treatment
of this substance with the ruthenium hydride complex [RuClH-
(CO)(PPh3)3]19 successfully afforded the vinyl ether 18, as a
mixture of E/Z isomers. Conversion into compound 19 with allyl-
chlorodimethylsilane 12 then occurred without problem. However,
we were unable to initiate the metathesis reaction to afford the 8-
membered heterocycle 20, a benzodioxasilocine, and are unable to
offer a rationalization for this result as our previous experience
with the RCM of similar propenyl ethers has been excellent.18
Unfortunately, even under harsher conditions, only the unreacted
starting material was obtained after chromatography. However, a
RCM strategy utilizing the unisomerized scaffold 17 gave more
promising results: treatment of the allyl ether 17 with 12 afforded
the bis-allyl product 21 in good yield (Scheme 3). This compound
then underwent cyclization in the presence of catalyst 11, produc-
ing the benzodioxasilonine 22,20 albeit only in a low, unoptimized
22% yield.
OiPr
25
Scheme 4. Reagents and conditions: (i) Et3N, CH2Cl2, allylchlorodimethylsilane 12,
rt, N2, 40 h, 85%; (ii) 5% catalyst 11, CH2Cl2, reflux, N2, 8 h, 82%.
Si
OH
OH
O
O
i
Si
26
ii
16
Si
O
O
A search for other potential scaffolds for the RCM identified the
substituted benzyl alcohol 23, a substrate utilized previously in our
studies.21 This compound was readily converted into silylated
compound 24 in a yield of 85% (Scheme 4). Application of the
Si
27
Scheme 5. Reagents and conditions: (i) Et3N, CH2Cl2, allylchlorodimethylsilane 12,
rt, N2, 66 h, 76%; (ii) 5% catalyst 11, CH2Cl2, reflux, N2, 18 h, 96%.

S. M. Scalzullo et al. / Tetrahedron Letters 49 (2008) 7403–7405
7405
(b) Panayides, J.-L.; Pathak, R.; Panagiotopoulos, H.; Davids, H.; Fernandes, M.
A.; de Koning, C. B.; van Otterlo, W. A. L. Tetrahedron 2007, 63, 4737–4747; (c)
Coyanis, E. M.; Panayides, J.-L.; Fernandes, M. A.; de Koning, C. B.; van Otterlo,
W. A. L. J. Organomet. Chem. 2006, 691, 5222–5239.
Finally, we investigated whether larger benzo-fused ring-
systems containing silicon could also be synthesized utilizing the
RCM strategy. Catechol 16 was thus treated with allylchlorodi-
methylsilane 12, and the disilylated compound 26 was isolated
after chromatography in good yield (Scheme 5). Subsequent reac-
tion of 26 with catalyst 11 then gave compound 27.23 Product 27
contains a 10-membered benzo-fused ring, a system that tradition-
ally is challenging to synthesize by way of a metathesis strategy.15a
We have thus shown that the versatile RCM reaction with Grub-
bs’ second-generation catalyst 11 can be applied to the synthesis of
a number of silicon-containing benzo-fused heterocycles. The
extension of this work to the synthesis of other interesting systems
is currently under investigation, and will be reported in due course.
13. Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18–29.
14. Details of the synthesis of this compound will be reported in
a future
publication.
15. For representative examples of RCM with the –OSi(Me)2 allyl group see: (a)
Chang, D.; Grubbs, R. H. Tetrahedron Lett. 1997, 38, 4757–4760; (b) Meyer, C.;
Cossy, J. Tetrahedron Lett. 1997, 38, 7861–7864; (c) Hoshi, T.; Yasuda, H.; Sanji,
T.; Sakurai, H. Bull. Chem. Soc. Jpn. 1999, 72, 821–827; (d) Brouard, I.; Hanxing,
L.; Martín, J. D. Synthesis 2000, 883–892; (e) Ahmed, M.; Atkinson, C. E.; Barrett,
A. G. M.; Malagu, K.; Procopiou, P. A. Org. Lett. 2003, 5, 669–672; (f) Miles, S. M.;
Marsden, S. P.; Leatherbarrow, R. J.; Coates, W. J. J. Org. Chem. 2004, 69, 6874–
6882; (g) Akindele, T.; Marsden, S. P.; Cumming, J. G. Org. Lett. 2005, 7, 3685–
3688; (h) Li, F.; Miller, M. J. J. Org. Chem. 2006, 71, 5221–5227; (i) Creary, X.;
O’Donnell, B. D.; Vervaeke, M. J. Org. Chem. 2007, 72, 3360–3368.
16. (Z)-2,2-Dimethyl-7-[(4-methylphenyl)sulfonyl]-2,3,6,7-tetrahydrobenzo[h][1,7,2]-
oxazasilonine 15: Benzenesulfonamide 14 (0.10 g, 0.25 mmol) was dissolved in
CH2Cl2 under N2 and catalyst 11 (0.011 g, 0.013 mmol, 5 mol %) was added. The
reaction mixture was stirred at rt for 12 h. After evaporation of the solvent, the
residue was subjected to column chromatography (8% EtOAc/hexane) to yield
the pure ring-closed product 15 (0.072 g, 78%). Mp: 75–77 °C; mmax (thin film/
cmÀ1): 1596, 1487, 1465; 1H NMR (300 MHz; CDCl3): d 7.61 (d, 2H, J = 8.2 Hz,
2 Â ArH), 7.26 (d, 2H, J = 8.2 Hz, 2 Â ArH), 7.16–7.11 (m, 1H, ArH), 6.91–6.76
(m, 3H, 3 Â ArH), 5.72–5.63 (m, 1H, NCH2CH@CH), 5.08–5.00 (m, 1H,
CH@CHCH2Si), 4.13 (br d, 2H, J = 4.5 Hz, NCH2), 2.42 (s, 3H, ArCH3), 1.77 (br
s, 2H, SiCH2), 0.22 [s, 6H, Si(CH3)2]; 13C NMR (75 MHz; CDCl3, assignments with
same superscript may be interchanged): d 154.1 (C), 143.1 (C), 135.4 (C), 132.1
(CH), 130.2 (C), 129.6 (CH), 129.2 (2 Â CH), 128.9 (CH), 128.0 (2 Â CH), 122.0
(CH), 121.8 (CH), 120.2 (CH), 46.9 (NCH2), 21.5 (ArCH3),a 20.6 (SiCH2),a À1.4
[Si(CH3)2]; m/z (EI): 373 (M+, 0.34%), 319 (72), 218 (21), 203 (22), 202 (16), 164
(100), 91 (12); HRMS: Calculated for C19H23NO3SSi: 373.1168, found:
373.1162.
Acknowledgements
This work was supported by the National Research Foundation
(NRF, GUN 2053652), Pretoria, and the University of the Witwa-
tersrand (University and Science Faculty Research Committees).
Mr. S. Mthembu and Mr. T. Tshabalala are thanked for initial repe-
tition of the Rodríguez-García work. Professor C. B. de Koning is
thanked for helpful discussions. Dr. R. Ul Islam gratefully acknowl-
edges the Poliomyelitis Research Foundation, South Africa for
financial support. Messrs R. Mampa (NMR), T. van der Merwe
(MS), M. Brits (MS) and Dr. A. Dinsmore (MS) are also thanked
for providing the spectroscopic services.
17. For
a review highlighting the connection between isomerizations and
metathesis see: Schmidt, B. Eur. J. Org. Chem. 2004, 1865–1880.
References and notes
18. For representative examples of our work utilizing the RCM of O-vinyl ethers
see: (a) van Otterlo, W. A. L.; Morgans, G. L.; Madeley, L. G.; Kuzvidza, S.;
Moleele, S. S.; Thornton, N.; de Koning, C. B. Tetrahedron 2005, 61, 7746–7755;
(b) van Otterlo, W. A. L.; Ngidi, E. L.; Kuzvidza, S.; Morgans, G. L.; Moleele, S. S.;
de Koning, C. B. Tetrahedron 2005, 61, 9996–10006; (c) van Otterlo, W. A. L.;
Ngidi, E. L.; de Koning, C. B. Tetrahedron Lett. 2003, 44, 6483–6486.
1. For reviews see: (a) Souers, A. J.; Ellman, J. A. Tetrahedron 2001, 57, 7431–7448;
(b) Thompson, L. A.; Ellman, J. A. Chem. Rev. 1996, 96, 555–600.
2. (a) Franz, A. K. Curr. Opin. Drug Discov. Devel. 2007, 10, 654–671; (b) Pooni, P. K.;
Showell, G. A. Mini Rev. Med. Chem. 2006, 6, 1169–1177; (c) Mills, J. S.; Showell,
G. A. Expert Opin. Investig. Drugs 2004, 13, 1149–1157; (d) Bains, W.; Tacke, R.
Curr. Opin. Drug Discov. Devel. 2003, 6, 526–543; (e) Showell, G. A.; Mills, J. S.
Drug Discov. Today 2003, 8, 551–556.
´
19. See the following references and citations listed therein: (a) Kuznik, N.;
Krompiec, S. Coord. Chem. Rev. 2007, 251, 222–233; (b) Krompiec, S.; Kuz´nik,
N.; Urbala, M.; Rzepa, J. J. Mol. Catal. A 2006, 248, 198–209; (c) Krompiec, S.;
Kuz´nik, N.; Krompiec, M.; Penczek, R.; Mrzigod, J.; Tórz, A. J. Mol. Catal. A 2006,
253, 132–146.
3. For two reviews see: (a) Rousseau, G.; Blanco, L. Tetrahedron 2006, 62, 7951–
7993; (b) Ando, W. Bull. Chem. Soc. Jpn. 1996, 69, 1–16.
20. (Z)-2,2-Dimethyl-3,6-dihydro-2H-benzo[h][1,7,2]dioxasilonine 22: mmax (thin
film/cmÀ1): 2927, 1639, 1495, 1249; 1H NMR (300 MHz; CDCl3): d 6.75–6.62
(m, 1H, ArH), 6.60–6.42 (m, 3H, 3 Â ArH), 5.64–5.50 (m, 1H, HC@CH), 5.20–5.06
(m, 1H, HC@CH), 4.43–4.35 (m, 2H, CH2O), 1.58–1.44 (m, 2H, CH2Si), –0.05 [br
s, 6H, Si(CH3)2]; 13C NMR (75 MHz; CDCl3): d 148.9 (C), 147.4 (C), 133.5 (CH),
123.3 (CH), 122.1 (CH), 121.8 (CH), 121.5 (CH), 119.6 (CH), 68.6 (CH2), 21.4
(CH2), À0.5 [Si(CH3)2]; m/z (EI): 221 (M++H, 12%), 205 (5), 170 (10), 169 (64),
166 (100), 151 (56), 136 (30), 108 (2), 77 (3), 63 (4); HRMS: Calculated for
C12H16O2Si: 220.0920, found: 220.0914.
4. Mignani, S.; Damour, D.; Doble, A.; Labaudinière, R.; Malleron, J.-L.; Piot, O.;
Gueremy, C. Bioorg. Med. Chem. Lett. 1993, 3, 1913–1918. Of interest was that
compounds 1a and 1b were tested as potential serotonin uptake inhibitors but
were found to lack potency.
5. Aoyama, T.; Sato, Y.; Suzuki, T.; Shirai, H. J. Organomet. Chem. 1978, 153, 193–
207.
6. Sato, Y.; Fukami, Y.; Shirai, H. J. Organomet. Chem. 1974, 78, 75–81.
7. Mironov, V. F.; Kozlikov, V. L.; Fedotov, N. S.; Khatuntsev, G. D.; Sheludyakov, V.
D. Zh. Obshch. Khim. 1972, 42, 1365–1371.
21. (a) Pathak, R.; Panayides, J.-L.; Jeftic, T. D.; de Koning, C. B.; van Otterlo, W. A. L.
S. Afr. J. Chem. 2007, 60, 1–7. http://www.journals.co.za/sajchem/; (b) van
Otterlo, W. A. L.; Pathak, R.; de Koning, C. B. Synlett 2003, 1859–1861.
22. (Z)-8-Isopropoxy-9-methoxy-3,3-dimethyl-1,3,4,7-tetrahydrobenzo[g][1,2]oxasilo-
nine 25: mmax (thin film/cmÀ1): 1601, 1487, 1435, 1270, 1195; 1H NMR
(300 MHz; CDCl3): d 6.94 (d, 1H, J = 8.3 Hz, ArH), 6.72 (d, 1H, J = 8.3 Hz, ArH),
5.59–5.43 (m, 2H, HC@CH), 4.73 (s, 2H, ArCH2OSi), 4.61–4.52 [m, 1H, CH(CH3)2],
3.85 (s, 3H, OCH3), 3.59 (br d, 2H, J = 7.2 Hz, ArCH2CH), 1.76 (br d, 2H, J = 7.8 Hz,
CH2Si), 1.31 [d, 6H, J = 6.2 Hz, CH(CH3)2], 0.19 [s, 6H, Si(CH3)2]; 13C NMR
(75 MHz; CDCl3): d 152.8 (C), 145.2 (C), 134.3 (C), 131.1 (C), 126.2 (CH), 125.5
(CH), 123.0 (CH), 109.5 (CH), 74.3 (CH), 64.5 (CH2), 55.5 (OCH3), 25.4 (CH2), 22.4
(2 Â CH3), 17.8 (CH2), À1.67 [Si(CH3)2]. m/z (EI): 307 (M+H+, <1%), 177 (100),
145 (40), 117 (46), 115 (82), 91 (38), 55 (36); HRMS: Calculated for C17H27O3Si
(M+H+): 307.1729, found: 307.1723.
8. Barluenga, J.; González, R.; Fañanás, F. J.; Yus, M.; Foubelo, F. J. Chem. Soc., Perkin
Trans. 1 1994, 1069–1077.
9. For recent reviews see the following papers: (a) Chattopadhyay, S. K.;
Karmakar, S.; Biswas, T.; Majumdar, K. C.; Rahaman, H.; Roy, B. Tetrahedron
2007, 63, 3919–3952; (b) Conrad, J. C.; Fogg, D. E. Curr. Org. Chem. 2006, 10,
185–202; (c) Schmidt, B.; Hermanns, J. Curr. Org. Chem. 2006, 10, 1363–1396;
(d) Deiters, A.; Martin, S. F. Chem. Rev. 2004, 104, 2199–2238; (e) McReynolds,
M. D.; Dougherty, J. M.; Hanson, P. R. Chem. Rev. 2004, 104, 2239–2258.
10. For
a comprehensive review on the application of RCM to produce Si-
containing ring systems see: Marciniec, B.; Pietraszuk, C. Curr. Org. Chem.
2003, 7, 691–735.
11. (a) Jiménez-González, L.; García-Muñoz, S.; Álvarez-Corral, M.; Muñoz-Dorado,
M.; Rodríguez-García, I. Chem. Eur. J. 2007, 13, 557–568; (b) Álvarez-Corral, M.;
López-Sánchez, C.; Jiménez-González, L.; Rosales, A.; Muñoz-Dorado, M.;
Rodríguez-García, I. Beilstein J. Org. Chem. 2007, 3, 5; (c) Jiménez-González,
L.; García-Muñoz, S.; Álvarez-Corral, M.; Muñoz-Dorado, M.; Rodríguez-García,
I. Chem. Eur. J. 2006, 12, 8762–8769; (d) García-Muñoz, S.; Jiménez-González,
L.; Álvarez-Corral, M.; Muñoz-Dorado, M.; Rodríguez-García, I. Synlett 2005,
3011–3013; (e) Jiménez-González, L.; Álvarez-Corral, M.; Muñoz-Dorado, M.;
Rodríguez-García, I. Chem. Commun. 2005, 2689–2691.
23. (Z)-2,2,7,7-Tetramethyl-2,3,6,7-tetrahydrobenzo[i][1,8,2,7]dioxadisilecine
27:
mmax (thin film/cmÀ1): 1631, 1499, 1451, 1252, 1158, 1107; 1H NMR
(300 MHz; CDCl3): d 6.57 (br s, 4H, 4 Â ArH), 5.25 (br t, 2H, J = 5.9 Hz, 2 Â
@CH), 1.48–1.51 (m, 4H, 2 Â CH2), À0.04 [s, 12H, 2 Â Si(CH3)2]; 13C NMR
(75 MHz; CDCl3):
d
146.7 (2 Â C), 122.6 (2 Â CH), 122.0 (2 Â CH), 121.0
(2 Â CH), 19.4 (2 Â CH2), À1.2 [2 Â Si(CH3)2]; m/z (EI): 279 (M+H+, 24%), 263
(6), 227 (3), 181 (2), 169 (8), 167 (14), 166 (100), 151 (4), 136 (3), 98 (22), 95
(11), 69 (2); HRMS: Calculated for C14H22O2Si2: 278.1158, found: 278.1153.
12. For recent examples from our research group see: (a) Panayides, J.-L.; Pathak,
R.; de Koning, C. B.; van Otterlo, W. A. L. Eur. J. Org. Chem. 2007, 29, 4953–4961;