Synthetic studies on bafilomycin A1: First formation of the 16-membered macrolide via an intramolecular Stille reaction

Article abstract of DOI:10.1016/j.tetlet.2004.04.035

The 16-membered macrolide formation of a bafilomycin A₁ synthesis intermediate showed to be very difficult to achieve via an intramolecular Stille reaction. Complex reactions were observed, depending on the nature of the palladium source, ligan

Full text of DOI:10.1016/j.tetlet.2004.04.035

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 4539–4543
Synthetic studies on bafilomycin A1: first formation of the
16-membered macrolide via an intramolecular Stille reaction
*
ꢀ
Emmanuelle Queron and Robert Lett
Unitꢀe Mixte CNRS-AVENTIS Pharma (UMR 26) 102, route de Noisy, 93235 Romainville, France
Received 23 March 2004; accepted 7 April 2004
Abstract—The 16-membered macrolide formation of a bafilomycin A1 synthesis intermediate showed to be very difficult to achieve
via an intramolecular Stille reaction. Complex reactions were observed, depending on the nature of the palladium source, ligand,
solvent and reaction conditions. Unexpected reactions of the 2-furyl group transfer of trifurylphosphine were observed on the vinylic
iodide and (or) the vinylstannane. Best conditions were found with Pd2(dba)3/AsPh3/i-Pr2NEt in DMF, at 40 °C, to afford the
desired macrocycle in 28% yield (33% corrected), the structure of which was definitely confirmed by chemical filiation.
Ó 2004 Elsevier Ltd. All rights reserved.
In the two preceding communications, we described the
synthesis of the enantiopure C1–C11 fragment 1 of ba-
filomycin A1,1a that of the required C12–C17 subunit 2
and their intermolecular esterification product 3.1b We
herein report our results concerning the formation of the
16-membered macrolactone of bafilomycin A1, for the
first time via an intramolecular Stille coupling, thus
obtaining 5 from 4 (Scheme 1).2 The synthesis of 5
corresponds to a formal one of bafilomycin A1 via a
strategy developed by Evans and Calter for the com-
pletion.3
dard procedure,4 based on the fact that they observed
the formation of the symmetric anhydride in the con-
ditions required for the lactonization (dilution, toluene,
100 °C) and therefore used a large excess of 2,4,6-tri-
chlorobenzoyl chloride/NEt3/DMAP.3a Independently,
Yonemitsu and co-workers made analogous observa-
tions and used a similar procedure in their total syn-
thesis of hygrolidin.5 Similar procedures have been
afterwards employed by Toshima et al.6 and Roush and
co-workers.7 An acyl activation with (EDC, HCl) in the
presence of DMAP was later used by Hanessian et al.8
The success or failure of macrolactonization via an acyl
activation is also highly dependent on the nature of the
precursor and its preferred conformation(s).3–9 There-
fore, instead of an acyl activation and in order to study
an alternate solution, we decided to examine the
At the present time, the 16-membered macrolide for-
mation of bafilomycin A1 has always been achieved via
an acyl activation, first in 55–60% yield by Evans and
Calter who used a modification of Yamaguchi’s stan-
OMe
OMe
O
OMe
O
I
I
O
O
CO2H
HO
11
6
1
1
OTES
Pd°
1
OR
OH
nBu3Sn
nBu3Sn
ODMT
ODMT
ODMT
17
17
12
OMe
OMe
OMe
2
3 R = TES
4 R = H
5
Scheme 1.
Keywords: Macrolides; Intramolecular Stille reaction; Palladium and compounds; Phosphines; Tin and compounds; Solvents and solvent effects.
* Corresponding author. Tel.: +33-1-49-91-52-80; fax: +33-1-49-91-38-00; e-mail: robert.lett@aventis.com
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.04.035

4540
E. Quꢀeron, R. Lett / Tetrahedron Letters 45 (2004) 4539–4543
formation of the macrocycle via an intramolecular Stille
coupling.10 Thus, the required enantiopure precursor 3
was obtained in 89% yield by an intermolecular esteri-
fication of the enantiopure C1–C11 acid 1 with the C12–
C17 alcohol 2 (ee ¼ 80%) (1.08 equiv), in the presence of
Yamaguchi’s chloride,4 NEt3 and DMAP, at room
temperature.1b
We then decided to examine the cyclization of the 7-OH
derivative 4, since the nature of the 7-OH protective
group has been shown to affect the conformation of the
macrolide precursor by Evans and Calter.3a Later the
free 7-OH was also found to allow the lactonization by
Roush and co-workers.7
2. Preparation of 4 and intramolecular Stille coupling
(Scheme 3)
1. Attempted intramolecular Stille coupling of 3
(Scheme 2)
Hence deprotection with TBAF in THF at rt afforded 4
and the 6-epimer 8, respectively, in 87% and 5% isolated
yields, thus clearly pointing out the high sensitivity of
the 6-position to basic conditions.
We never could obtain the 16-membered macrolide via
an intramolecular Stille coupling of the 7-OTES pre-
cursor 3. Just starting material was recovered with
PdCl2(MeCN)2, in the presence of Ph3P, in DME at
reflux. Very complex reaction mixtures, with some 3
were obtained in other conditions such as
{[Pd(OAc)2 + 4Ph3P]11 (5–15 mol %), toluene, 65 °C,
overnight or in DMF, rt, overnight} or [Pd(PPh3)4
(10 mol %), toluene, at 80 °C or 110 °C]. Compared to
triphenylphosphine, tri-2-furylphosphine (TFP) and
triphenylarsine as palladium ligands have been shown to
give a large rate increase of the Stille coupling when the
transmetalation is the rate-determining step of the cat-
alytic cycle.12 However, with [Pd(OAc)2 + 4TFP]
(20 mol %), quite surprisingly we only could isolate
products resulting from the coupling of 3 with the 2-
furyl group from the phosphine, 6 and 7 in 6% and 11%
yield, respectively, and some deprotection of the 17-
ODMT ether also occurred since we isolated 14%
DMTOH (Scheme 2). To our knowledge, such transfers
of the 2-furyl group of the phosphine were seldom ob-
served,13 or even for the phenyl group of triphenyl-
arsine.13–15 In contrast, the phenyl group transfer of
triphenylphosphine as a ligand is sometimes a compet-
itive reaction in palladium-catalyzed reactions (Stille,
Heck, Suzuki).13;15;16 Facile aryl–aryl exchange between
an aryl substituted by an electron donating group at the
palladium(II) centre and the phenyl groups of triphen-
ylphosphine has been demonstrated by Kong and Chen,
the migration from palladium to phosphorus being
enhanced with electron donating substituents of the aryl
group at the Pd(II) centre and their number.17
Attempted cyclizations of 4 with the catalyst produced
from [Pd(OAc)2 + 4 TFP] (10–15 mol %), in DME
(60 °C, 3 h) or DMF (50 °C, 45 h) gave 9, which was the
only product which could be characterized in 11% and
10% yields, with reisolated starting material (29% and
42%), respectively, for each of reaction conditions. The
desired macrolide 5 could only be obtained with
[Pd2(dba)3/AsPh3]12 in DMF, in the presence of i-
Pr2NEt, at 50 °C, or even at rt (but in a too slow reac-
tion), conditions which minimized deprotection of the
DMT ether and destannylation. THF or DME as sol-
vents were unsatisfactory and mixtures of those with
DMF (10 equiv), or THF/DMF (1/1), gave inferior re-
sults when compared with those obtained in DMF with
the same catalytic system. On the other hand, reaction
of 4 (0.0005 M) in the presence of [Pd(OAc)2 + 4AsPh3]
(10 mol %) and i-Pr2NEt (10 equiv), in DMF (50 °C,
3.5 days), just gave 11% of the desired 5, and 4 was re-
isolated in 35% yield. Pd2(dba)3 (10 mol %) with AsPh3
(80 mol %), in otherwise the same conditions, gave after
24 h 5 (23%), recovered 4 (12%) and traces of the ho-
mocoupling product 10. NMP or THF/DMF (1/1) gave
inferior results in the same conditions. For comparison
also, when associated to Pd2(dba)3,11b;18 PPh3 or P(p-
MeOC6H4)3 gave only traces of 5 after 3.5 and 2 days,
respectively, starting material being partly recovered
(55% and 33%). Increasing catalyst or substrate con-
centration in order to try to get faster reactions were
OMe
O
I
O
OTES
OMe
I
O
ODMT
6
O
a
OMe
O
OTES
+
OMe
O
nBu3Sn
ODMT
O
O
OMe
3
OTES
ODMT
7
O
OMe
Scheme 2. Reagents and conditions: (a) Pd(OAc)2 (10 mol %), TFP (40 mol %), DME, rt, 2.5 h, then 3 (0.005 M), 60 °C, 11 h, followed by addition of
a preformed solution of Pd(OAc)2 (10 mol %) and TFP (40 mol %) in DME, reflux, 7 h.

E. Quꢀeron, R. Lett / Tetrahedron Letters 45 (2004) 4539–4543
4541
OMe
OMe
O
I
O
I
O
O
OH
nBu3Sn
OH
nBu3Sn
a
3
+
ODMT
ODMT
OMe
4
OMe
8
87%
5%
I
OMe
OH
O
I
O
MeO
OMe
O
O
OH
DMTO
ODMT
ODMT
O
O
OH OMe
OMe
9
10
O
I
OMe
O
OMe
OMe
O
OMe
O
I
O
O
HO
b
O
OH
nBu3Sn
OH
nBu3Sn
+
ODMT
ODMT
ODMT
11
4
5
OMe
OMe
OMe
1.10-3
M
28%
15%
Scheme 3. Reagents and conditions: (a) TBAF 1 M in THF (1.1 equiv), 3 (1 M), rt, 2 h, then addition of TBAF (1.1 equiv), rt, 2 h; (b) Pd2(dba)3
(10 mol %), AsPh3 (80 mol %), i-Pr2NEt (10 equiv), DMF, rt, then 4 (0.001 M), 40 °C, 30 h.
unsatisfactory; thus
4
(0.005 M) with Pd2(dba)3
acyl activation procedure.6;21 Examination of the X-rays
22
(20 mol %)/AsPh3 (160 mol %)/i-Pr2NEt (10 equiv), in
DMF (50 °C, 2.5 h) afforded 5 in only 13% yield and
significantly increased 10 (15%). After examination of
temperature and dilution effects, the best conditions we
found gave the desired macrolide 5 in 28% yield, and the
vinylic iodide reduction product 11 (15%) with still some
starting material 4 (15%) after chromatography.
structures of bafilomycin A1 and concanamycin A23
show different conformations for the 16- and 18-mem-
bered macrolides and that the steric interactions are
1
much more severe for bafilomycin A1. Consistently, H
and 13C NMR studies, and circular dichroism have
shown much more conformational flexibility for con-
canamycin A24 in solution than for bafilomycin A1 16-
membered lactone ring, that of the latter being very
limited in solution and conformation being very similar
to that found in the crystal structure.25 As appearing
either from the macrolactonizations by acyl activation
or here by the intramolecular coupling, it is probably
more difficult to find among the conformations of the
macrocycle precursor, which are in dynamic equilib-
rium, those which allow the formation of the macrolide
or of the intermediate palladamacrocycle in the case of
bafilomycin than for concanamycin. The problem is still
more acute for intramolecular Stille or Suzuki cou-
plings, since dilution is required for avoiding intermo-
lecular reactions, thus slowing down the catalytic
process. Indeed here a successful macrocyclization not
only requires a good turn-over of the catalyst, that is
that the absolute rates of each individual step of the
catalytic cycle have to be of the same order of magni-
tude, but also that those rates have to be compatible
with the rates of interconversion between the different
conformations of the precursor in order to get those
allowing the macrocycle formation.
More recently, a thorough study of the Stille coupling of
phenyl iodide with CH2@CH–SnBu3, in DMF, cata-
lyzed by Pd(dba)2/AsPh3 has shown the complex role of
the constituents of the system for the oxidative addition
and the transmetalation.19 It is worth also to mention
that when we tried to facilitate the transmetalation step
in a different manner, by formation of insoluble
Ph2PO2SnBu3 as SnBu3 scavenger,20 the macrolide 5
was obtained in only 10% yield by reaction of 4
(0.0005 M) with Pd2(dba)3 (10 mol %)/Ph2POꢀ2 , þNBu4
(1.5 equiv)/i-Pr2NEt (10 equiv) in DMF (50 °C, 24 h),
with recovered 4 (11%).
Thus, an intramolecular Stille coupling affords the 16-
membered ring macrolide of bafilomycin A1 in a sig-
nificantly lower yield (28%, 33% corrected yield) than in
the related formation of concanolide A, which was ob-
tained in 72% yield in quite comparable conditions by
Toshima and co-workers.21 The same significant differ-
ence is observed for macrolactonization via the same

4542
E. Quꢀeron, R. Lett / Tetrahedron Letters 45 (2004) 4539–4543
OMe
O
OMe
O
OMe
O
O
O
HO
DEIPSO
DEIPSO
a
b
O
ODMT
ODMT
OH
5
13
12
OMe
OMe
OMe
88%
62%
DEIPS = Et2iPrSi
Scheme 4. Reagents and conditions: (a) DEIPSOTf (6 equiv), i-Pr2NEt (14 equiv), DMF, rt, 3 h; (b) PPTS (1 equiv), MeOH, rt, 4 h.
10. Duncton, M. A. J.; Pattenden, G. J. Chem. Soc., Perkin
Trans. 1 1999, 1235–1246.
3. Chemical filiation with Toshima’s intermediate
(Scheme 4)
11. (a) Amatore, C.; Jutand, A.; M’Barki, M. A. Organomet-
allics 1992, 11, 3009–3013; (b) Amatore, C.; Jutand, A. J.
Organomet. Chem. 1999, 576, 254–278.
12. (a) Farina, V.; Krishnan, B. J. Am. Chem. Soc. 1991, 113,
9585–9595; (b) Farina, V.; Roth, G. P. In Advances in
Metal-Organic Chemistry; Liebeskind, L. S., Ed.; JAI,
1996. Vol. 5, p 1–53.
13. Segelstein, B. E.; Butler, T. W.; Chenard, B. L. J. Org.
Chem. 1995, 60, 12–13.
14. Crisp, G. T.; Glink, P. T. Tetrahedron 1994, 50, 3213–
3234.
In order to get a definite proof of the structure for the
macrolide 5, a chemical filiation was achieved with 13,
intermediate involved in the total synthesis of bafilo-
mycin A1 of Toshima et al.6 (Scheme 4). Therefore,
formation of the 7-ODEIPS derivative 12 and sub-
sequent DMT ether cleavage afforded the macrolide 13,
which was definitely identical to the previously described
compound, as shown by unambiguous comparison of
the NMR data.26
15. Morita, D. K.; Stille, J. K.; Norton, J. R. J. Am. Chem.
Soc. 1995, 117, 8576–8581.
16. (a) O’Keefe, D. F.; Dannock, M. C.; Marcuccio, S. M.
Tetrahedron Lett. 1992, 33, 6679–6680; (b) Hunt, A. R.;
Stewart, S. K.; Whiting, A. Tetrahedron Lett. 1993, 34,
3599–3602; (c) Wallow, T. I.; Novak, B. M. J. Org. Chem.
1994, 59, 5034–5037.
17. Kong, K.-C.; Cheng, C.-H. J. Am. Chem. Soc. 1991, 113,
6313–6315.
18. Amatore, C.; Jutand, A. Coord. Chem. Rev. 1998, 178–
180, 511–528.
19. (a) Amatore, C.; Bucaille, A.; Fuxa, A.; Jutand, A.;
Meyer, G.; Ndedi Ntepe, A. Chem. Eur. J. 2001, 7,
2134–2142; (b) Amatore, C.; Bahsoun, A. A.; Jutand, A.;
Meyer, G.; Ndedi Ntepe, A.; Ricard, L. J. Am. Chem. Soc.
2003, 125, 4212–4222.
Acknowledgements
We thank the staff of the Analytical Department of the
Research Center at Romainville and also Roussel Uclaf,
Hoechst Marion Roussel and the CNRS for a Ph.D.
grant to E. Queron, the Direction des Recherches
Chimiques of Roussel Uclaf and HMR for support of
this work.
ꢀ
References and notes
20. (a) Srogl, J.; Allred, G. D.; Liebeskind, L. S. J. Am. Chem.
Soc. 1997, 119, 12376–12377; (b) Arimoto, H.; Nishimura,
K.; Kuramato, M.; Uemura, D. Tetrahedron Lett. 1998,
39, 9513–9516; (c) Smith, A. B., III; Ott, G. R. J. Am.
Chem. Soc. 1998, 120, 3935–3948; (d) Toshima, K.; Arita,
T.; Kato, K.; Tanaka, D.; Matsumura, S. Tetrahedron
Lett. 2001, 42, 8873–8876.
ꢀ
1. (a) Queron, E.; Lett, R. Tetrahedron Lett., 2004, 45,
preceding paper doi:10.1016/j.tetlet.2004.04.033; (b)
ꢀ
Queron, E.; Lett, R. Tetrahedron Lett., 2004, 45, preceding
paper doi:10.1016/j.tetlet.2004.04.034.
ꢀ
2. Queron, E. Ph.D. thesis, Paris VI University, 2000.
3. (a) Calter, M. A. Ph.D. thesis, Harvard University, 1993
(b) Evans, D. A.; Calter, M. A. Tetrahedron Lett. 1993, 34,
6871–6874.
21. Jyojima, T.; Miyamoto, N.; Katohno, M.; Nakata, M.;
Matsumura, S.; Toshima, K. Tetrahedron Lett. 1998, 39,
6007–6010.
4. Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yama-
guchi, M. Bull. Chem. Soc. Jpn. 1979, 52, 1989–1993.
5. Makino, K.; Nakajima, N.; Hashimoto, S.; Yonemitsu, O.
Tetrahedron Lett. 1996, 37, 9077–9080.
6. Toshima, K.; Jyojima, T.; Yamaguchi, H.; Noguchi, Y.;
Yoshida, T.; Murase, H.; Nakata, M.; Matsumura, S. J.
Org. Chem. 1997, 62, 3271–3284.
7. (a) Scheidt, K. A.; Tasaka, A.; Bannister, T. D.; Wendt,
M. D.; Roush, W. R. Angew. Chem., Int. Ed. 1999, 38,
1652–1655; (b) Scheidt, K. A.; Bannister, T. D.; Tasaka,
A.; Wendt, M. D.; Savall, B. M.; Fegley, G. J.; Roush, W.
R. J. Am. Chem. Soc. 2002, 124, 6981–6990.
22. Baker, G. H.; Brown, P. J.; Dorgan, R. J. J.; Everett, J. R.;
Ley, S. V.; Slawin, A. M. Z.; Williams, D. J. Tetrahedron
Lett. 1987, 28, 5565–5568.
23. (a) Westley, J. W.; Liu, C.-M.; Sello, L. H.; Evans, R. H.;
Troupe, N.; Blount, J. F.; Chiu, A. M.; Todaro, L. J.;
Miller, P. A. J. Antibiotics 1984, 37, 1738–1740; (b) Nakai,
H.; Matsutani, S. Acta Cryst. 1992, C48, 1519–1521.
24. Bindseil, K. U.; Zeeck, A. Liebigs Ann. Chem. 1994, 305–
312.
25. (a) Baker, G. H.; Brown, P. J.; Dorgan, R. J. J.; Everett, J.
R. J. Chem. Soc., Perkin Trans. 2 1989, 1073–1079; (b)
Everett, J. R.; Baker, G. H.; Dorgan, R. J. J. J. Chem.
Soc., Perkin Trans. 2 1990, 717–724.
26. Compound 5: pale yellow oil; 1H NMR (300 MHz,
CDCl3): d/TMS 7.46 (d, 2H, Ha, JHa;Hb ¼ 9), 7.34 (d,
4H, Hd, JHd;He ¼ 9), 7.29 (m, 2H, Hb), 7.19 (m, 1H, Hc),
8. Hanessian, S.; Ma, J.; Wang, W.; Gai, Y. J. Am. Chem.
Soc. 2001, 123, 10200–10206.
9. Marshall, J. A.; Adams, N. D. J. Org. Chem. 2002, 67,
733–740.

E. Quꢀeron, R. Lett / Tetrahedron Letters 45 (2004) 4539–4543
4543
6.81 (d, 4H, He, JHd;He ¼ 9), 6.60 (s, 1H, H3), 6.40 (dd, 1H,
H12, J12;13 ¼ 15, J12;11 ¼ 11), 5.85 (d, 1H, H11, J11;12 ¼ 11),
Compound 13: pale yellow oil; 1H NMR (250 MHz,
CDCl3): d/TMS 6.67 (s, 1H, H3), 6.47 (dd, 1H, H12,
J12;13 ¼ 15, J11;12 ¼ 11), 5.92 (d, 1H, H11, J11;12 ¼ 11), 5.88
5.77 (dq, 1H, H5, J5;6 ¼ 7, JCH ;5 ¼ 1), 5.24 (dd, 1H, H13,
3
J13;12 ¼ 15, J13;14 ¼ 7), 4.99 (dd, 1H, H15, J15;14 ¼ 6,
J15;16 ¼ 6), 3.82 (m, 1H, H14), 3.78 (s, 6H, OCH3), 3.62
(s, 3H, CH3, C2OCH3), 3.39 (m, 1H, H7), 3.17 (m, 1H,
(dq, 1H, H5, J5;6 ¼ 9, JCH ;5 ¼ 1), 5.39 (dd, 1H, H13,
3
J13;12 ¼ 15, J13;14 ¼ 5), 5.12 (dd, 1H, H15, J15;14 ¼ 5,
J15;16 ¼ 5), 3.94 (dd, 1H, H14, J14;13 ¼ 5, J14;15 ¼ 5), 3.67
(s, 3H, CH3, C2OCH3), 3.62 (dd, 1H, H7, J7;6 ¼ 3,
J7;8 ¼ 3), 3.53 (dd, 1H, H17a, J17a;17b ¼ 9, J17a;16 ¼ 6), 3.37
(dd, 1H, H17b, J17a;17b ¼ 9, J17b;16 ¼ 8), 3.29 (s, 3H, CH3,
HC14OCH3), 3.00 (m, 1H, OH), 2.40–2.55 (m, 2H, H6,
H
17a), 3.16 (s, 3H, CH3, HC14OCH3), 3.01 (dd, 1H, H17b,
J17a;17b ¼ 9, J17b;16 ¼ 6), 2.49 (m, 1H, H6), 2.18 (m, 1H,
H9a), 2.16 (m, 1H, H16), 2.02 (br s, 1H, H9b), 1.95 (d, 3H,
CH3, JCH ;5 ¼ 1), 1.87 (m, 1H, H8), 1.74 (s, 3H, CH3), 1.49
3
(d, 1H, OH, JOH;7 ¼ 7), 1.04 (d, 3H, CH3, J16;CH ¼ 7), 1.07
H9a), 2.15 (m, 1H, H16), 1.94 (d, 3H, CH3, JCH ;5 ¼ 1),
3
3
(d, 3H, CH3, J6;CH ¼ 7), 0.97 (d, 3H, CH3, J8;CH ¼ 7); 13
C
1.70–1.85 (m, 2H, H8, H9b), 1.68 (s, 3H, CH3), 0.98–1.13
(m, 19H, SiCH(CH3)2, SiCH(CH3)2, Si(CH2CH3)2,
3
3
NMR (50.3 MHz, CDCl3): 164.2 (C1), 158.3 (C@COCH3),
145.0 (C10), 141.8 and 136.4 (Cq Ar), 141.2 and 132.1 (C2,
C4), 140.8 (C5), 131.7 (C3), 131.1 (C13), 130.1, 128.3, 127.7
and 126.6 (CH Ar), 126.0 and 125.2 (C11, C12), 113.0
(C@COCH3), 85.9 (CqO), 82.8 (C15), 81.2 (C7), 76.7 (C14),
65.4 (C17), 59.8 (C2OCH3), 55.8 (HC14OCH3), 55.2
(OCH3), 41.5 (C9), 38.3, 37.7 and 36.5 (C6, C8, C16),
23.2, 18.4, 18.3, 13.8 and 13.6 (CH3); C45H56O8 ¼ 724.39;
HRMS/EI: Mþ calcd: 724.3975, found: 724.3972.
HC8CH3, HC6CH3), 0.90 (d, 3H, CH3, J16;CH ¼ 7),
3
0.64–0.83 (m, 4H, Si(CH2CH3)2); 13C NMR (50.3 MHz,
CDCl3): 165.7 (C1), 143.7 (C10), 141.2 (C5), 141.1 and
131.4 (C2, C4), 133.3 (C13), 130.8 (C3), 125.3 and 124.7
(C11, C12), 82.6 (C15), 80.8 (C7), 77.1 (C14), 64.6 (C17), 60.2
(C2OCH3), 56.3 (HC14OCH3), 41.5 (C9), 40.1, 38.0 and
37.7 (C6, C8, C16), 24.8, 18.4, 17.8, 13.6 and 11.5 (CH3), 7.4
(SiCH(CH3)2), 4.8 and 4.3 (Si(CH2CH3)2).