Asymmetric conjugate silyl transfer in iterative catalytic sequences: Synthesis of the C7-C16 fragment of (+)-neopeltolide

Article abstract of DOI:10.1002/anie.201002916

Matched or mismatched, that is not the question! The anti,anti configuration of the C7- C16 fragment of (+)-neopeltolide is stereoselectively installed in an iterative sequence of catalyst-controlled Si group and Me group transfers, even with mismatched selectivity in the former (Si=Me2PhSi, see scheme; TBS=tert-butyldimethylsilyl).

Full text of DOI:10.1002/anie.201002916

Angewandte
Chemie
DOI: 10.1002/anie.201002916
Iterative Synthesis
Asymmetric Conjugate Silyl Transfer in Iterative Catalytic Sequences:
Synthesis of the C7–C16 Fragment of (+)-Neopeltolide**
Eduard Hartmann and Martin Oestreich*
The assembly of recurring structural motifs in complex
molecules, such as 1,3,…n-polymethylated or 1,3,…n-poly-
hydroxylated carbon chains (n = odd integers), is elegantly
realized by several unique iterative sequences.[1] These
approaches enable the stereoselective synthesis of either
Me,Me[2–5] or OH,OH[6] (I and II; Figure 1) but not Me,OH
of all isomeric Me,Si and Si,Me arrangements, we demon-
strate the feasibility of our approach in the synthesis of the
C7–C16 fragment of (+)-neopeltolide[11] (1; Figure 2).[12–14]
Protected 2 is an intermediate previously reported by
Paterson and Miller,[13a] requiring the stereoselective prepa-
ration of Me,Si,Si building block V (Figure 2).
Figure 1. Me,Me and OH,OH and Me,OH building blocks.
Si=Me2PhSi.
Figure 2. (+)-Neopeltolide (1) and its C7–C16 unit 2.
(or OH,Me) arrangements (III; Figure 1). We thus sought a
flexible strategy that would allow for the direct introduction
of either methyl or hydroxy groups into a linear carbon chain.
The idea was to merge our enantioselective conjugate silyl
transfer[7,8] into the Feringa–Minnaard approach, which
hinges upon the repeated catalyst-controlled 1,4-addition of
Grignard reagents.[4] Tamao–Fleming oxidative degradation
of the carbon–silicon bond[10] in the thus-formed Me,Si
building block IV renders it a synthetic equivalent of the
desired unit III (Figure 1).
A salient feature of the Feringa–Minnaard iterative
syntheses is the exceptional catalyst control seen in each
copper(I)-catalyzed 1,4-addition.[4] Our task at the outset was
therefore to learn about the influence of the existing
stereogenic carbon atom on stereoinduction in the possible
stereochemical scenarios of Me,Si and Si,Me building block
syntheses (Scheme 1).
*
Scenario copper(I) catalysis (Scheme 1, upper): Our
enantioselective
rhodium(I)-catalyzed
1,4-addition
yielded the Si-containing intermediate with perfect enan-
Herein, we detail the realization of a conjugate addition-
based iterative strategy with optional stereocontrolled intro-
duction of either methyl or silyl (hydroxy equivalent) groups
into a carbon chain. Aside from the systematic investigation
tiomeric excess (3!4),[7b] and the a,b-unsaturated
acceptor with
E
double-bond configuration[15] was
accessed by a conventional three-step two-carbon homo-
logation (4!5).[16a] The subsequent copper(I)-catalyzed
conjugate methyl transfer with josiphos (racemic or either
enantiomer) was not fully catalyst-controlled, with anti
(matched case, 5!anti-6) being preferred over syn relative
configuration (mismatched case, 5!syn-6).[17] The inter-
fering substrate control might be attributed to the steric
demand of the Si group.
[*] E. Hartmann, Prof. Dr. M. Oestreich
Organisch-Chemisches Institut
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Corrensstrasse 40, 48149 Mꢁnster (Germany)
Fax: (+49)251-83-36501
E-mail: martin.oestreich@uni-muenster.de
Homepage: http://www.uni-muenster.de/Chemie.oc/oestreich
*
Scenario rhodium(I) catalysis (Scheme 1, lower): The
methyl-group-containing precursor was prepared in high
enantiomeric excess according to the Feringa–Minnaard
methodology (7!8),[18] and the a,b-unsaturated acceptor
with Z double-bond configuration[15] was again available
through a straightforward three-step two-carbon homolo-
gation (8!9).[16b] The asymmetric rhodium(I)-catalyzed
[**] This research was supported by the Deutsche Forschungsgemein-
schaft (Oe 249/3-2) and the Fonds der Chemischen Industrie
(predoctoral fellowship to E.H., 2009–2011). We thank Solvias AG
(Basel, Switzerland) for the generous donation of ligands.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002916.
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agrees with the Feringa–Minnaard sequences in that no
substrate control is imposed by the existing stereogenic
tertiary carbon atom.
This survey shows that three out of four Me,Si and Si,Me
combinations are accessible with excellent isomeric purity. It
also reveals however that the silicon-bearing carbon atom is
not innocent in the next iteration, and we found this to be true
in the unselective preparation of the Si,Si array (not shown).
The problematic preparation of the Si,Si unit by consec-
utive silyl transfers brought about the issue of how to
assemble the Me,Si,Si building block V required for the
synthesis of 2 (Figure 2). This situation prompted us to test
the role of an existing protected hydroxy group in a,b-
unsaturated acceptors. Orthogonal protection of the hydroxy
groups in 2 would also be secured. We were then delighted to
see that g- and d-silyloxy-substituted compounds 11 and 12
performed well in the 1,4-addition (11!13 and 12!14,
Scheme 2), exceeding any chemical yield previously obtained
Scheme 2. Conjugate silyl transfer onto g- and d-silyloxy-substituted
a,b-unsaturated acceptors followed by conjugate methyl transfer (see
Scheme 1 for reagents and reaction conditions).
for acyclic acceptors.[7b] Standard homologation (13!15 and
14!16) was followed by purely catalyst-controlled methyl
transfer (matched cases; 15!anti-17 and 16!anti-18). The E/
Z ratios of 15 and 16 translate into the diastereomeric ratios
of anti-17 and anti-18, indicating that this time both double-
bond isomers are reactive in the 1,4-addition (cf. 5!anti-6;
Scheme 1).[17]
With these results at hand, we turned to the synthesis of
the C7–C16 fragment 2 (Figure 2 and Scheme 3). Chiral d-
silyloxy-substituted a,b-unsaturated carboxyl compound 20
was made available from the known protected b-hydroxy
carboxyl compound 19.[20] The choice of the hydroxy protect-
ing group emerged as crucial, as its size (TES, TBS, or TIPS)
had a marked influence on substrate control in the subsequent
1,4-addition. While TES was superior to TBS and TIPS in the
conjugate addition, its lability in the later oxidative degrada-
tion of the carbon–silicon bond forced us to consider larger
groups, such as TBS and TIPS. Only a few examples have
been reported for the difficult Tamao–Fleming oxidation,[21]
and control experiments verified that TBS protection is ideal.
The conjugate addition using (R)-binap proceeded smoothly
Scheme 1. Catalyst versus substrate control in the second iteration
towards Me,Si and Si,Me building blocks. Rhodium(I)-catalyzed con-
jugate silyl transfer: [Rh(cod)2]OTf (5.0 mol%), binap (10 mol%),
Me2PhSi-Bpin[9] (2.5 equiv), Et3N (1.0 equiv), 1,4-dioxane/H2O 10:1,
458C; copper(I)-catalyzed conjugate methyl or butyl transfer:
CuBr·SMe2 (5.0 mol%), josiphos (6.0 mol%), MeMgBr (1.2 equiv) or
BuMgBr (1.4 equiv), tBuOMe, ꢀ788C; three-step homologations:
a) DIBAL-H (3.5 equiv), THF, ꢀ788C; b) (COCl)2 (1.5 equiv), DMSO
(3.0 equiv), Et3N (6.0 equiv), CH2Cl2, ꢀ788C; c) for E alkene[16a]
Ph3P=CHC(O)SEt (1.4 equiv), CHCl3, D or for Z alkene[16b]
(CF3CH2O)2P(O)CH2CO2Me (1.4 equiv), KHMDS (1.4 equiv),
[18]crown-6 (2.5 equiv), THF, ꢀ788C. binap=2,2’-bis(diphenylphos-
phanyl)-1,1’-binaphthyl, cod=cycloocta-1,5-diene, DIBAL-H=diisobu-
tylaluminum hydride, DMSO=dimethylsulfoxide, josiphos=
1-[2-(diphenylphosphanyl)ferrocenyl]ethyldicyclohexylphosphine,
KHMDS=potassium hexamethyldisilazide, OTf=trifluoromethanesul-
fonate, pin=pinacolato, THF=tetrahydrofuran.
conjugate carbon–silicon bond formation then occurred
with outstanding diastereocontrol in both cases (9!anti-
10 and 9!syn-10).[19] The pronounced catalyst control
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[2] Catalyst-controlled carboalu-
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[4] Conjugate addition of RMgBr
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Scheme 3. Synthesis of the C7–C16 fragment 2 of (+)-neopeltolide (1; see Scheme 1 for reagents and
reaction conditions). TBS=tert-butyldimethylsilyl, TBDPS=tert-butyldiphenylsilyl, TES=triethylsilyl,
TIPS=triisopropylsilyl, proton sponge=1,8-bis(dimethylamino)naphthalene.
to yield the desired anti diastereomer with good mismatched
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[8] For a general procedure for enantioselective silyl transfer that
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selectivity with d.r. = 95:5 (20!anti-21, Scheme 3). In the
presence of (S)-binap, the matched isomer was formed with
d.r. > 99:1 (20!syn-21; not shown). Homologation (anti-21!
22) and anti-selective conjugate methylation (22!anti,anti-
23) afforded the fully elaborated carbon framework as a
single diastereomer. After thio-to-oxo transesterification
(23!24), the Fleming procedure[10b,c] allowed selective oxi-
dative degradation to form a lactone (24!25). Its opening to
the Weinreb amide (25!26) was followed by methyl ether
formation, and reduction to the carbonyl oxidation level
completed the synthesis of the C7–C16 fragment (26!2).[22]
The spectroscopic data were in accordance with those
reported by Paterson and Miller.[13a]
Based on alternating conjugate silyl and methyl transfer,
we accomplished a flexible iterative approach to the stereo-
controlled synthesis of mixed methylated/hydroxylated linear
carbon chains with methyl and hydroxy groups in 1,3-
relationship. All diastereomers are accessible in principle.
The 1,4-addition of nucleophilic silicon was found to be
enhanced by an existing d-silyloxy group, thus paving the way
for the formal total synthesis of (+)-neopeltolide (1).
[9] For the preparation of Me2PhSi-Bpin, see: a) M. Suginome, T.
Matsuda, Y. Ito, Organometallics 2000, 19, 4647 – 4649; for a
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[10] a) K. Tamao, T. Kakui, M. Akita, T. Iwahara, R. Kanatani, J.
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[12] For (formal) total syntheses using the C7–C16 fragment, see:
a) W. Youngsaye, J. T. Lowe, F. Pohlki, P. Ralifo, J. S. Panek,
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Received: May 14, 2010
Published online: July 20, 2010
Keywords: asymmetric catalysis · copper · iterative synthesis ·
.
rhodium · silicon
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Communications
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[19] Chemical yields were moderate however, and conjugate reduc-
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[20] Ref. [13a] provides no experimental details on the preparation of
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[21] Tamao–Fleming oxidations of 1-OSiR3,3-Si motifs are scarce and
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b) SiR3 = TBS: C. Boglio, S. Stahlke, S. Thorimbert, M. Malacria,
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[22] After submission of this manuscript, we learned that Martinez-
Solorio and Jennings had independently developed the identical
endgame: D. Martinez-Solorio, M. P. Jennings, J. Org. Chem.
2010, 75, 4095 – 4104.
[15] An E double-bond configuration is required for the copper(I)-
catalyzed methyl transfer, and a Z double-bond configuration is
essential for the rhodium(I)-catalyzed silyl transfer.
[16] E-selective Wittig olefination: a) G. E. Keck, E. P. Boden, S. A.
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