Synthetic studies towards the marine natural product palmerolide A: Synthesis of the C3-C15 and C16-C23 fragments

Article abstract of DOI:10.1055/s-2007-992366

The synthesis of the C3-C15 and C16-C23 fragments of the marine natural product palmerolide A has been achieved. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-992366

LETTER
2983
Synthetic Studies towards the Marine Natural Product Palmerolide A:
Synthesis of the C3–C15 and C16–C23 Fragments
Synthetic Studies
t
u
owards Palm
i
erolide
l
A
laume Cantagrel, Christophe Meyer,* Janine Cossy*
Laboratoire de Chimie Organique, associé au CNRS, ESPCI, 10 Rue Vauquelin, 75231 Paris Cedex 05, France
Fax +33(1)40794660; E-mail: christophe.meyer@espci.fr; E-mail: janine.cossy@espci.fr
Received 11 September 2007
alkene by steric hindrance as the latter olefin could also
participate to a RCM with the C2–C3 alkene leading to a
six-membered ring rather than to the desired macrolide.
Abstract: The synthesis of the C3–C15 and C16–C23 fragments of
the marine natural product palmerolide A has been achieved.
Key words: palmerolide A, natural products, diastereoselectivity,
The C3–C23 fragment of type A was further disconnected
at C15–C16 and C9–C10. A palladium-catalyzed cross-
coupling involving a trisubstituted (E)-alkenyl iodide of
type D (C16–C23 fragment) and (E)-alkenyl metal, gener-
ated from the corresponding alkyne (C14–C15) of type C,
could be used to build the C15–C16 bond. The formation
of the C9–C10 bond was initially planned by nucleophilic
addition of an (E)-alkenyl metal at C9 to the aldehyde at
C10. The appropriate choice of the protecting group for
the alcohol at C11 should enable to control the diastereo-
selectivity of this reaction.7,8 However, we also ques-
tioned the configurational assignment initially made for
palmerolide A when our work was undertaken. For this
reason, the formation of the C9–C10 bond was rather en-
visaged by nucleophilic addition of an alkynyl metal de-
rived from the terminal alkyne of type B (C3–C9 subunit)
to the aldehyde of type C (C10–C15 fragment), followed
by stereoselective reduction of the disubstituted triple
bond. According to this approach, the absolute configura-
tion at C7 and C10 carbons should be controlled and eas-
ily tuned-up by the highly face-selective Noyori’s
reduction of the corresponding acetylenic keto groups
(Scheme 1).9
Noyori reduction
The marine natural product palmerolide A was recently
isolated from the extracts of the marine tunicate Synoicum
adareanum collected around Anvers Island in the Antarc-
tic Peninsula.1 Screening assays by the National Cancer
Institute (NCI) in the United States revealed that palm-
erolide A exhibits a rather potent and selective cytotoxic-
ity against the melanoma cell-line UACC-62 (LC50 = 18
nM). As suggested by its activity profile and in vitro ex-
periments, palmerolide A is a potent inhibitor of vacuolar
ATPase.1 The structure of palmerolide A was deduced
from NMR spectroscopic analysis. The relative and abso-
lute configuration of palmerolide A was initially assigned
as 7R,10R,11R,19R,20R by NMR experiments.1 Due to its
challenging structure and its unique biological activity
profile, palmerolide A has already elicited considerable
synthetic interest. A first report dealing with the total syn-
thesis of one diastereomer and the enantiomer of palm-
erolide A revealed that the configuration of the natural
product had to be revised to 7S,10S,11S,19R,20R
(Scheme 1).2
The preparation of the C3–C9 fragment was carried out
from the commercially available 5-hexenoic acid 1. After
activation as a mixed anhydride (i-BuOCOCl, excess
NMM, CH2Cl2, –10 °C), addition of (MeO)MeNH·HCl
(–10 °C to r.t.) afforded the Weinreb amide 2 (92%).10
Condensation of the Weinreb amide 2 with trimethylsilyl-
ethynyllithium (generated from trimethylsilylacetylene
and n-BuLi, THF, –78 °C to r.t.) led to the acetylenic ke-
tone 3 (98%).11 The latter compound underwent enanti-
oselective reduction in the presence of Noyori’s catalyst
(S,S)-I (5 mol%, i-PrOH, r.t.)9 to afford the optically ac-
tive propargylic alcohol 4 (80%, 98% ee).12 After protec-
tion of the secondary alcohol 4 as a tert-butyldiphenysilyl
ether (TBDPSCl, imidazole, DMF, r.t.), the acetylenic si-
lyl group was selectively removed (K2CO3, MeOH, r.t.) to
deliver the terminal alkyne 5 (97%, two steps from 4).13
The latter compound, which constitutes the C3–C9 sub-
unit of palmerolide A, was prepared in five steps from the
commercially available carboxylic acid 1 with an overall
yield of 70% (Scheme 2).
Recent reports on the first total synthesis of palmerolide
A,3 as well as synthetic studies towards the C1–C9 and
C15–C21 fragments,4 urged us to disclose our results on
our ongoing total synthesis of palmerolide A. Herein, we
report a synthetic approach towards this natural product
including the preparation of the C3–C15 and C16–C23
subunits.
In our retrosynthetic analysis of palmerolide A, the instal-
lation of the dienamide functionality was planned at a late
stage in the synthesis by a copper-catalyzed cross-
coupling5 between 3,3-dimethylacrylamide and an (E)-
alkenyl iodide generated by Takai olefination of an alde-
hyde at C23. The formation of the 20-membered macro-
cyclic lactone was envisaged by ring-closing metathesis
(RCM) involving an acrylate at C19 and a terminal alkene
at C3.6 We postulated that bulky protecting groups for the
hydroxyl groups at C7 and C10 would mask the C8–C9
SYNLETT 2007, No. 19, pp 2983–2986
x
x
.x
x
.2
0
0
7
Advanced online publication: 08.11.2007
DOI: 10.1055/s-2007-992366; Art ID: G28907ST
© Georg Thieme Verlag Stuttgart · New York

2984
G. Cantagrel et al.
LETTER
RCM
[Cl3CC(=NH)OPMB, cat. CSA, CH2Cl2, r.t.] to give 9
(76%) and subsequent deprotection of the primary alcohol
(cat. TsOH, MeOH, r.t.)15 afforded compound 10 which
was contaminated with p-methoxybenzyl alcohol (15%).
The latter byproduct could not be easily separated by flash
chromatography but underwent chemoselective oxidation
with MnO2 (CH2Cl2, r.t.). After separation of p-anisalde-
hyde, the primary alcohol 10 was isolated in 58% yield
(two steps from 9).16 Oxidation of alcohol 10 was then
achieved under Swern conditions [DMSO, (COCl)2, Et3N,
CH2Cl2, –78 °C to r.t.] to afford aldehyde 11, which was
not purified to avoid partial racemization. Aldehyde 11,
corresponding to the C10–C15 subunit of palmerolide A,
was prepared in eight steps from the commercially avail-
able alcohol 6 in 36% overall yield (Scheme 3).
O
1
Cu(I)-catalyzed Takai
cross-coupling olefination
3
O
H
21
N
19
7
24
17
O
OH
palmerolide A
HO
14
O
10
O
NH2
O
1
PG = appropriate
protecting group
O
3
21
23
PGO
19
7
17
OPG
Pd(0)-catalyzed
cross-coupling
PGO
14
nucleophilic
addition
10
A
TMS
OPG
OH
TMS
21
23
15
3
O
10
10
OH
O
O
c,d
a,b
PO
19
OTr
OH
10
93%
O
88%
11
16
D
11
TMS
O
11
I
7
6
7
8
15
H
OPG
e
76%
10
TMS
O
TMS
B
TMS
9
15
15
15
OPG
11
h
f,g
H
OTr
OH
C
10
10
10
58%
Scheme 1 Retrosynthetic analysis of palmerolide A
OPMB
OPMB
11
OPMB
11
11
11
10
9
3
3
3
Scheme 3 Reagents and conditions: a) PPh3, imidazole, I2, THF,
0 °C to r.t.; b) TMSC≡CLi, THF–HMPA, –25 °C to r.t.; c) AcOH–
H2O (4:1), 60 °C; d) TrCl, DMAP (10 mol%), pyridine, r.t.;
e) Cl3CC(=NH)OPMB, CSA·H2O (20 mol%), CH2Cl2, r.t.; f) TsOH
(9 mol%), MeOH, r.t.; g) MnO2, CH2Cl2, r.t.; h) DMSO, (COCl)2,
CH2Cl2, –78 °C then add 10 and Et3N, –78 °C to r.t.; PMB =
p-MeOC6H4CH2; Tr = Ph3C; CSA = 10-camphorsulfonic acid.
a
b
92%
98%
7
7
7
MeO
N
HO
O
O
O
1
2
3
Me
TMS
9
c
80%
3
3
Ts
Ph
Ph
The synthesis of the C16–C23 fragment was achieved
from but-3-yn-1-ol (12). This compound was subjected to
a zirconium-catalyzed carboalumination (Me3Al, H2O,
cat. Cp2ZrCl2, CH2Cl2, –20 °C to r.t.) followed by iodinol-
ysis (I2, Et2O, 0 °C to r.t.)17 to give the trisubstituted
(E)-alkenyl iodide 13 (70%) which was oxidized with
Dess–Martin periodinane (DMP, NaHCO3, CH2Cl2, r.t.)
to provide aldehyde 14 that was directly engaged in the
next step.18 As in the previous reported synthetic ap-
proaches towards palmerolide A,2–4 the syn relative orien-
tation of the hydroxyl at C19 and the methyl at C20 also
led us to consider an aldol reaction to control the configu-
ration of these two asymmetric carbons. Thus, the boron
enolate derived from N-propionyloxazolidinone (S)-II
(n-Bu2BOTf, i-Pr2NEt, CH2Cl2, 0 °C) was condensed
with aldehyde 14 at –78 °C to afford the corresponding
syn-aldol 15 (44%, dr ≥ 95:5).19 After transamidation
[Me3Al, (MeO)MeNH·HCl, THF, 0 °C to r.t.), the result-
ing b-hydroxy Weinreb amide 16 (75%) was reduced
(DIBAL-H, CH2Cl2, 0 °C to r.t.) to the corresponding sen-
sitive b-hydroxyaldehyde 17 which was directly subjected
to a Wittig olefination [EtO2C(Me)C=PPh3, CH2Cl2, r.t.]
to provide the trisubstituted E-configured a,b-unsaturated
ester 18 (50%, two steps from 16). After reduction of 18
N
d,e
Ru
N
7
7
97%
H
OH
OTBDPS
9
5
4
(S,S)-I
TMS
9
98% ee
Scheme 2 Reagents and conditions: a) i-BuOCOCl, NMM, CH2Cl2,
–10 °C then (MeO)MeNH·HCl, –10 °C to r.t.; b) TMSC≡CLi (from
TMSC≡CH, n-BuLi, THF, –78 °C), –78 °C to r.t.; c) catalyst (S,S)-I
(5 mol%), i-PrOH, r.t.; d) TBDPSCl, imidazole, DMF, r.t.; e) K2CO3,
MeOH, r.t.
The preparation of the C10–C15 fragment was next exam-
ined. The commercially available alcohol 6, derived from
L-malic acid, was transformed to the corresponding pri-
mary alkyl iodide (PPh3, imidazole, I2, THF, 0 °C to r.t.)
and the latter compound underwent SN2 displacement
with trimethylsilylethynylllithium (THF–HMPA, –25 °C
to r.t.) to produce the alkynylsilane 7 (93%, two steps
from 6).14 After hydrolysis of the acetonide (80% aq
AcOH, 60 °C) and subsequent protection of the primary
alcohol (TrCl, cat. DMAP, pyridine, r.t.), the trityl ether 8
was obtained (88%, two steps from 7).14,15 The secondary
alcohol at C11 was protected as a p-methoxybenzyl ether
Synlett 2007, No. 19, 2983–2986 © Thieme Stuttgart · New York

LETTER
Synthetic Studies towards Palmerolide A
2985
(DIBAL-H, CH2Cl2, –78 °C), the resulting allylic alcohol derived from 5 to aldehyde 11 is therefore in agreement
19 (93%) was protected as a tert-butyldimethylsilyl ether with the Cram-chelated model.7 The disubstituted triple
(TBSCl, cat. DMAP, Et3N, CH2Cl2, 0 °C to r.t.) to afford bond in compound 21 underwent stereoselective hydro-
compound 20 (68%). The C16–C23 subunit of palm- alumination (Red-Al, THF, 0 °C to r.t.) to give the E-alk-
erolide A has thus been prepared in eight steps from but- ene 23 (76%). The hydroxyl group at C10 was protected
3-yn-1-ol with an overall yield of 7% (Scheme 4).
as a tert-butyldimethylsilyl ether (TBSOTf, 2,6-lutidine,
CH2Cl2, –78 °C) and the acetylenic silyl group was re-
moved (K2CO3, MeOH, r.t.) to afford the terminal alkyne
24 (74%, two steps from 23) which constitutes the C3–
C15 fragment of palmerolide A (Scheme 5).21
O
OH
OH
O
O
OH
20
O
N
H
a
b
c
44%
70%
16
Bn
(2 steps
from 13)
3
3
12
13
14
15
TMS
O
I
I
I
15
d
75%
H
a
10
+
OH
19
O
OH
19
O
OH
OPMB
11
OTBDPS
OTBDPS
7
20
20
20
EtO2C
22
MeO
11
9
73%
3
f
e
H
N
19
BrMg
5
Me
50%
3
16
16
16
(2 steps
from 16)
18
17
16
I
I
I
g
93%
TMS
HO
TMS
15
O
O
OH
OH
19
15
OTBDPS
+
7
7
OTBDPS
23
20
20
HO
h
O
N
19
HO
TBSO
10
10
23
68%
OPMB
OPMB
11
11
Bn
16
20 16
19
21/21' = 70:30
(S)-II
21
21'
I
I
86%
b
Scheme 4 Reagents and conditions: a) Me3Al, H2O, Cp2ZrCl2
(22 mol%), CH2Cl2, –20 °C to r.t., then I2, Et2O, 0 °C to r.t.; b) DMP,
NaHCO3, CH2Cl2, r.t.; c) (S)-II, n-Bu2BOTf, i-Pr2NEt, CH2Cl2, 0 °C
then add 14, –78 °C; d) (MeO)MeNH·HCl, Me3Al, THF, then add 15,
0 °C to r.t.; e) DIBAL-H, CH2Cl2, 0 °C to r.t.; f) EtO2C(Me)C=PPh3,
CH2Cl2, r.t.; g) DIBAL-H, CH2Cl2, –78 °C; h) TBSCl, DMAP
(20 mol%), Et3N, CH2Cl2, 0 °C to r.t.
3
3
TMS
HO
TMS
O
15
7
15
c
OTBDPS
OTBDPS
7
91%
10
10
OPMB
OPMB
11
11
At the moment, we have investigated the coupling of the
C3–C9 and C10–C15 subunits of palmerolide A. Thus,
condensation of an excess (3 equiv) of the Grignard re-
agent generated from the terminal alkyne 5 (n-BuLi, Et2O,
0 °C then MgBr2·OEt2, 0 °C) with aldehyde 11 in the pres-
ence of MgBr2·OEt2 (Et2O, –78 °C) provided a 70:30 mix-
ture of the diastereomeric propargylic alcohols 21 and 21¢
in 73% combined yield and the excess of alkyne 5 was
easily separated and recovered after purification by flash
chromatography. Although these two diastereomeric al-
cohols could be separated by flash chromatography, the
mixture was oxidized (DMP, pyridine, CH2Cl2, r.t.) to the
corresponding acetylenic ketone 22 (86%). This ketone
was obtained as a single diastereomer and this result con-
firmed that no racemization of aldehyde 11 has occurred
and that 21 and 21¢ are epimeric at C10. The acetylenic
ketone 22 underwent reduction in the presence of
Noyori’s catalyst (R,R)-I (5 mol%, i-PrOH, r.t.) with high
diastereoselectivity (dr ≥ 95:5) to afford the major epimer-
ic alcohol 21 (91%). Due to the well-known high reagent-
controlled face selectivity of the reduction of acetylenic
ketones under these conditions,9,20 a S-configuration can
reasonably be assigned to carbon C10 in the major dia-
stereomer 21. The diastereoselection observed during the
nucleophilic addition of the acetylenic Grignard reagent
21
22
d
76%
3
3
TMS
7
OTBDPS
7
OTBDPS
TBS
O
15
15
HO
e,f
74%
10
10
OPMB
OPMB
11
11
23
24
Scheme 5 Reagents and conditions: a) alkyne 5 (3 equiv) and
n-BuLi (3 equiv), Et2O, 0 °C then MgBr2·OEt2 (3 equiv), 0 °C and add
aldehyde 11 (1 equiv) and MgBr2·OEt2 (1.5 equiv), Et2O, –78 °C;
b) DMP, pyridine, CH2Cl2, r.t.; c) (R,R)-I (5 mol%), i-PrOH, r.t.;
d) Red-Al, THF, 0 °C to r.t.; e) TBSOTf, 2,6-lutidine, CH2Cl2,
–78 °C; f) K2CO3, MeOH, r.t.
In conclusion, we have reported the preparation of the
C3–C9, C10–C15, and C16–C23 subunits of palmerolide
A. The C3–C9 and C10–C15 fragments have been cou-
pled by a nucleophilic addition of an alkynyl metal at C9
to an aldehyde at C10 followed by oxidation and a dia-
stereoselective reduction to control the configuration of
Synlett 2007, No. 19, 2983–2986 © Thieme Stuttgart · New York

2986
G. Cantagrel et al.
LETTER
33.2 (t), 26.9 (q, 3 C), 23.9 (t), 19.3 (s). MS (EI, 70 eV): m/z
(%) = 305 (9)[(M – t-Bu+], 227 (10), 207 (30), 200 (19), 199
(100), 181 (10), 147 (3), 135 (5), 123 (4), 105 (5), 77 (5).
HRMS–FAB: m/z calcd for C24H30NaOSi [M + Na+]:
385.1958; found: 385.1953.
the stereocenter at C10. Efforts are currently underway to
complete the total synthesis of palmerolide A from the
C3–C15 segment of the natural product.
(14) Yang, W.-Q.; Kitahara, T. Tetrahedron 2000, 56, 1451.
(15) Qin, D.; Byun, H.-S.; Bittman, R. J. Am. Chem. Soc. 1999,
121, 662.
Acknowledgment
G.C. thanks the MRES for a grant.
(16) (2S)-(4-Methoxybenzylxoxy)-6-trimethysilanylhex-5-yn-
1-ol (10)
References and Notes
[a]D –34.5 (c 0.50, CHCl3). IR: 3414, 2956, 2174, 1613,
1514, 1249, 1037, 842, 760 cm–1. 1H NMR (400 MHz,
CDCl3): d = 7.27 (br d, J = 8.7 Hz, 2 H), 6.88 (br d, J = 8.7
Hz, 2 H), 4.56 (d, AB syst, J = 11.2 Hz, 1 H), 4.51 (d, AB
syst, J = 11.2 Hz, 1 H), 3.80 (s, 3 H), 3.75–3.69 (m, 1 H),
3.68–3.63 (m, 1 H), 3.55–3.50 (m, 1 H), 2.40–2.27 (m, 2 H),
1.86 (br s, 1 H, OH), 1.87–1.78 (m, 1 H), 1.75–1.67 (m, 1 H),
0.15 (s, 9 H). 13C NMR (100 MHz, CDCl3): d = 159.3 (s),
130.4 (s), 129.4 (d, 2 C), 113.9 (d, 2 C), 106.6 (s), 85.0 (s),
77.9 (d), 71.6 (t), 63.9 (t), 55.2 (q), 29.9 (t), 15.9 (t), 0.00 (q,
3 C). MS (EI, 70 eV): m/z (%) = 291 (5)[M – Me+], 247 (4),
233 (8), 217 (5), 137 (9), 135 (13), 121 (100), 73 (10).
HRMS–FAB: m/z calcd for C17H26NaO3Si [M + Na+]:
329.1543; found: 329.1537.
(1) Diyabalanage, T.; Amsler, C. D.; McClintock, J. B.; Baker,
B. J. J. Am. Chem. Soc. 2006, 128, 5630.
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D. Y.-K. Angew. Chem. Int. Ed. 2007, 46, 5896.
(4) Kaliappan, K. P.; Gowrisankar, P. Synlett 2007, 1537.
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cross-coupling reactions, see: Dehli, J. R.; Legros, J.; Bolm,
C. Chem. Commun. 2005, 973.
(6) For a review on macrocyclization by RCM in the total
synthesis of natural products, see: Gradillas, A.; Pérez-
Castells, J. Angew. Chem. Int. Ed. 2006, 45, 6086.
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(12) The enantiomeric excess of alcohol 4 was determined by
super-critical fluid (CO2) chromatography analysis of the
corresponding p-nitrobenzoate derivative (4-nitrobenzoyl
chloride, Et3N, CH2Cl2, r.t., 75%). Calibration was achieved
with the racemic material obtained from ketone 3 by
reduction (DIBAL-H, CH2Cl2, –78 °C to –30 °C, 70%) and
subsequent condensation with p-nitrobenzoyl chloride.
(13) (3S)-3-(tert-Butyldiphenylsilanyloxy)oct-7-en-1-yne (5)
[a]D –37.5 (c 0.73, CHCl3). IR: 3305, 1640, 1589, 1427,
1105, 1086, 997, 911, 821, 739, 699 cm–1. 1H NMR (400
MHz, CDCl3): d = 7.75–7.67 (m, 4 H), 7.45–7.34 (m, 6 H),
5.73 (ddt, J = 16.9, 10.2, 6.7 Hz, 1 H), 4.94 (dq, J = 16.9, 1.7
Hz, 1 H), 4.93–4.89 (m, 1 H), 4.34 (ddd, J = 6.6, 5.6, 2.1 Hz,
1 H), 2.30 (d, J = 2.1 Hz, 1 H), 2.01–1.95 (m, 2 H), 1.72–
1.59 (m, 2 H), 1.58–1.45 (m, 2 H), 1.07 (s, 9 H). 13C NMR
(100 MHz, CDCl3): d = 138.5 (d), 136.0 (d, 2 C), 135.8 (d, 2
C), 133.6 (s), 133.5 (s), 129.7 (d), 129.6 (d), 127.6 (d, 2 C),
127.4 (d, 2 C), 114.6 (t), 85.0 (s), 72.6 (d), 63.5 (d), 37.6 (t),
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(21) (E)-(5S,6S,9S)-6-(tert-Butyldimethylsilanyloxy)-9-(tert-
butyldiphenylsilanyloxy)-5-(4-methoxybenzyl)tetra-
deca-7,13-dien-1-yne (24)
[a]D –37.7 (c 0.79, CHCl3). IR: 3310, 1613, 1514, 1249,
1111, 1077, 1040, 836, 776, 741, 703 cm–1. 1H NMR (400
MHz, CDCl3): d = 7.69–7.63 (m, 4 H), 7.43–7.31 (m, 6 H),
7.23 (app br d, J = 8.7 Hz, 2 H), 6.86 (app br d, J = 8.7 Hz,
2 H), 5.67 (ddt, J = 16.9, 10.2, 6.6 Hz, 1 H), 5.66 (ddd,
J = 15.5, 6.7, 1.4 Hz, 1 H), 5.45 (ddd, J = 15.5, 4.9, 0.8 Hz,
1 H), 4.92–4.85 (m, 2 H), 4.58 (d, AB syst, J = 11.2 Hz, 1 H),
4.43 (d, AB syst, J = 11.2 Hz, 1 H), 4.22–4.16 (m, 2 H), 3.80
(s, 3 H), 3.42 (ddd, J = 9.7, 5.2, 2.9 Hz, 1 H), 2.24–2.15 (m,
2 H), 1.91 (t, J = 2.6 Hz, 1 H), 1.91–1.85 (m, 2 H), 1.69–
1.61 (m, 1 H), 1.52–1.19 (m, 5 H), 1.05 (s, 9 H), 0.88 (s,
9 H), 0.02 (s, 3 H), –0.01 (s, 3 H). 13C NMR (75 MHz,
CDCl3): d = 159.1 (s), 138.8 (d), 136.0 (d, 2 C), 135.9 (d, 2
C), 134.5 (s), 134.3 (s), 133.8 (d), 131.0 (s), 129.5 (d), 129.4
(d), 129.3 (d, 2 C), 129.1 (d), 127.5 (d, 2 C), 127.4 (d, 2 C),
114.3 (t), 113.7 (d, 2 C), 84.5 (s), 80.7 (d), 73.8 (t), 72.7 (t),
72.6 (t), 68.3 (d), 55.3 (q), 37.3 (t), 33.6 (t), 28.7 (t), 27.1 (q,
3 C), 25.9 (q, 3 C), 23.8 (t), 19.3 (s), 18.2 (s), 15.0(t), –4.7
(q), –4.8 (q). C44H62O4Si2.
Synlett 2007, No. 19, 2983–2986 © Thieme Stuttgart · New York

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