Functionalized carbosilane dendritic species as soluble supports in organic synthesis

Article abstract of DOI:10.1021/jo991726k

A new methodology, which is compatible with the use of reactive organometallic reagents, has been developed for the use of carbosilane dendrimers as soluble supports in organic synthesis. Hydroxy-functionalized dendritic carbosilanes Si[CH₂CH₂CH₂SiMe₂(C₆H₄CH(R)OH)]₄ (G₀-OH, R = H or (S)-Me) and Si[CH₂CH₂CH₂Si[CH₂CH₂CH₂SiMe₂(C₆H₄CH(R)OH)]₃]₄ (G₁-OH, R = H or (S)-Me) were prepared and subsequently converted into the esters Si[CH₂CH₂CH₂SiMe₂(C₆H₄-CH(R)OC(O)CH₂Ph)]₄ (R = H or (S)-Me) and Si[CH₂CH₂CH₂Si[CH₂CH₂CH₂SiMe₂(C₆H₄CH(R)OC-(O)CH₂C₆H₄R')]₃]₄ (R = H and R' = H or R = (S)-Me and R' = H or R = H and R' = Br). As an example the latter compound was functionalized under Suzuki conditions. The functionalized carboxylic acid was obtained in high yield after cleavage from the dendritic support. Moreover, the ester functionalized dendrimers were converted to the corresponding zinc enolates followed by a condensation reaction with an imine to a β-lactam in excellent yield and purity. Furthermore, it was demonstrated that a small combinatorial library of β-lactams could be prepared starting from a carbosilane dendrimer functionalized with different ester moieties. These results show that carbosilane dendrimers can be applied as soluble substrate carriers for the generation of low molecular weight organic molecules. In combination with nanofiltration techniques, separation and recycling of the dendrimers can be realized.

Full text of DOI:10.1021/jo991726k

6338
J . Org. Chem. 2000, 65, 6338-6344
F u n ction a lized Ca r bosila n e Den d r itic Sp ecies a s Solu ble Su p p or ts
in Or ga n ic Syn th esis
Neldes J . Hovestad, Allan Ford, J ohann T. B. H. J astrzebski, and Gerard van Koten*
Debye Institute, Department of Metal-Mediated Synthesis, Utrecht University,
Padualaan 8, 3584 CH Utrecht, The Netherlands
g.vankoten@chem.uu.nl
Received November 4, 1999
A new methodology, which is compatible with the use of reactive organometallic reagents, has
been developed for the use of carbosilane dendrimers as soluble supports in organic synthesis.
Hydroxy-functionalized dendritic carbosilanes Si[CH2CH2CH2SiMe2(C6H4CH(R)OH)]4 (G0-OH,
R ) H or (S)-Me) and Si[CH2CH2CH2Si[CH2CH2CH2SiMe2(C6H4CH(R)OH)]3]4 (G1-OH, R ) H or
(S)-Me) were prepared and subsequently converted into the esters Si[CH2CH2CH2SiMe2(C6H4-
CH(R)OC(O)CH2Ph)]4 (R ) H or (S)-Me) and Si[CH2CH2CH2Si[CH2CH2CH2SiMe2(C6H4CH(R)OC-
(O)CH2C6H4R′)]3]4 (R ) H and R′ ) H or R ) (S)-Me and R′ ) H or R ) H and R′ ) Br). As an
example the latter compound was functionalized under Suzuki conditions. The functionalized
carboxylic acid was obtained in high yield after cleavage from the dendritic support. Moreover, the
ester functionalized dendrimers were converted to the corresponding zinc enolates followed by a
condensation reaction with an imine to a â-lactam in excellent yield and purity. Furthermore, it
was demonstrated that a small combinatorial library of â-lactams could be prepared starting from
a carbosilane dendrimer functionalized with different ester moieties. These results show that
carbosilane dendrimers can be applied as soluble substrate carriers for the generation of low
molecular weight organic molecules. In combination with nanofiltration techniques, separation and
recycling of the dendrimers can be realized.
In tr od u ction
that controls the solubility of the support. Through the
special solubility properties of these devices the isolation
of the desired material from reaction mixtures can be
realized, and as a result the advantages of homogeneous-
phase chemistry can be combined with the utility of solid-
phase purification.
In pharmaceutical chemistry the use of insoluble
supports has been incorporated into numerous synthetic
methodologies.1 Central to the effectiveness of these
methods is the easy removal of (excess) reagents and
solvents from the support by washing. This allows the
use of large reagent excesses to drive reactions to
completion and so enables the efficient synthesis and
purification of resin-bound products. However, often the
synthetic and analytical methodologies used in solution
processes are not compatible with the properties of
insoluble supports. Therefore, a number of groups have
looked into the use of soluble polymeric supports.2 J anda
and co-workers3 have described the use of soluble poly-
(ethylene glycol) supports in the preparation of combi-
natorial organic libraries. A crucial feature of this method
is that the compounds of interest are attached to a device
Dendritic polymers4 are currently generating interest
as soluble supports as a result of (i) their well-defined
molecular composition that provides supports with pre-
cise spatial arrangement of the active reaction sites, (ii)
the high loading that can be achieved at the dendrimer
surface, and (iii) the possibility to apply nanofiltration
techniques as an alternative approach for the separation
of the dendritic support from products and reagents. The
application of nanofiltration techniques allows not only
the use of (large) excesses of reagents during the syn-
thesis but also easy recycling of the support. Recently,
Kim et al. described the preparation of small libraries of
indoles anchored on polyamidoamine (PAMAM) den-
drimers via a so-called dendrimer-supported combinato-
rial chemistry (DCC) approach.5 Here, the products are
* Tel: +3130 2533120. Fax: +3130 2523615.
(1) (a) Leznoff, C. C. Chem. Soc. Rev. 1974, 3, 65. (b) Thompson, L.
A.; Ellman, J . A. Chem. Rev. 1996, 96, 555. (c) Terret, N. K.; Gardner,
M.; Gordon, D. W.; Kobylecki, R. J .; Steele, J . Tetrahedron 1995, 51,
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Zechel, C. Angew. Chem., Int. Ed. Engl. 1996, 35, 2288. (e) Marx, M.
A.; Grillot, A.-L.; Louer, C. T.; Beaver, K. A.; Bartlett, P. A. J . Am.
Chem. Soc. 1997, 119, 6153. (f) Hermkens, P. H. H.; Ottenheijm, H.
C. J .; Rees, D. C Tetrahedron 1997, 53, 5643. (g) Guillermo, A. M.;
Corbett, J . W.; DeGrado, W. F. J . Org. Chem. 1998, 63, 1172-1177.
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Perspectives; VCH: Weinheim, 1996. (d) Vo¨gtle, F. Dendrimer Topics
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Fischer, M.; Vo¨gtle, F. Angew. Chem., Int. Ed. Engl. 1999, 38, 884. (f)
Knapen, J . W. J .; van der Made, A. W.; de Wilde, J . C.; van Leeuwen,
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Eng. 1997, 77, 75. (h) Hovestad, N. J .; Hoare, J . L.; J astrzebski, J . T.
B. H.; Canty, A. J .; Smeets, W. J . J .; Spek, A. L.; van Koten, G.
Organometallics 1999, 18, 2970. (i) Wells, N. J .; Basso, A.; Bradley,
M. Biopolymers 1998, 47, 381.
(2) (a) Geckeler, K.; Pillai, V. N. R.; Mutter, M. Adv. Polym. Sci.
1981, 39, 65. (b) Braca, G.; Di Girolamo, M.; Raspolli Galletti, A. M.;
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10.1021/jo991726k CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/01/2000

Functionalized Carbosilane Dendritic Species
J . Org. Chem., Vol. 65, No. 20, 2000 6339
Sch em e 1
assembled into an enlarged dendritic molecule and
separation is achieved by molecular size.
tion reaction of allyl terminated corbosilane dendrimers8
and dimethylchlorosilane. The presence of terminal SiCl
functionalities in these molecules allows the straightfor-
ward introduction of functional molecules, via an orga-
nolithium or Grignard route.9
Recently we set out to explore the use of functionalized
carbosilane dendrimers as supports6 or catalysts7 in
organic synthesis. Such dendrimers are of special interest
because of their inertness toward organometallic re-
agents. In this paper, we describe two applications: (1)
the synthesis of products at the carbosilane support and
the use of the dendritic species as a leaving group in the
last step, and (2) the multistep synthesis of products at
the dendritic surface followed by cleavage and separation
from the dendritic species.
Separation of the dendritic support from the products
can be achieved by (nano)filtration techniques, which
allows the recycling (i.e., multiple use) of the carbosilane
support. This opens up the possibility to develop methods
for the use of (soluble) dendritic supports in the continu-
ous as well as the batch-wise production of organic
compounds.
Using a similar approach we have synthesized the
4-(hydroxymethyl)phenyl substituted derivatives of 1 and
2. The alcohol moieties in the obtained products may be
regarded as the connecting functionalities to which a
variety of organic substrates can be immobilized via, e.g.,
ester formation. In this respect it should be noted that
we recently reported the synthesis of 4-(bromomethyl)-
phenyl functionalized derivatives of 1 and 2 and its
subsequent conversion into aryl ether derivatives.4h
Reaction of 1 or 2 with 4-lithiobenzyl tert-butyldim-
ethylsilyl ether or (S)-4-lithio-R-methylbenzyl tert-bu-
tyldimethylsilyl ether afforded the protected 4-(hydroxym-
ethyl)phenyl functionalized dendritic compounds 3a -6a
(see Scheme 1) in high yield. These new compounds were
fully characterized by 1H and 13C NMR, elemental
analysis, and MALDI-TOF-MS; see Experimental Sec-
tion.
Resu lts a n d Discu ssion
Syn th esis of F u n ction a lized Den d r im er s. Recently
we have shown that Me2SiCl terminated carbosilane
dendritic molecules, i.e., Si[CH2CH2CH2SiMe2Cl]4 (1) and
Si[CH2CH2CH2Si(CH2CH2CH2SiMe2Cl)3]4 (2), are valu-
able starting materials for the synthesis of functionalized
dendrimers and are readily available via a hydrosilyla-
Removal of the protecting group, according to the
reaction conditions in Scheme 1, afforded after workup
the 4-(hydroxymethyl)phenyl substituted dendrimers
3b-6b in high yield (see Experimental Section) and were
fully characterized by 1H and 13C NMR, elemental
analysis, and MALDI-TOF-MS. Subsequent reaction of
(6) Hovestad, N. J .; J astrzebski, J . T. B. H.; van Koten, G. Polym.
Mater. Sci. Eng. 1999, 80, 53.
(7) Hovestad, N. J .; Eggeling, E. B.; Heidbu¨chel, J . H.; J astrzebski,
J . T. B. H.; Kragl, U.; Keim, W.; Vogt, D.; van Koten, G. Angew. Chem.,
Int. Ed. 1999, 38, 1655.
(8) Van der Made, A. W.; van Leeuwen, P. W. M. N. J . Chem. Soc.,
Chem. Commun. 1992, 1400.
(9) Wijkens, P.; J astrzebski, J . T. B. H.; van der Schaaf, P. A.; Kolly,
R.; Hafner, A.; van Koten, G. Org. Lett. 2000, 2, 1621.

6340 J . Org. Chem., Vol. 65, No. 20, 2000
Hovestad et al.
Sch em e 2
3b-6b with phenylacetyl chloride in the presence of
pyridine in THF results in the formation of the dendritic
esters 3c-6c as light-yellow, viscous oils in high yield.
For their full characterization, see Experimental Section.
The 4-bromobenzyl ester 7c was obtained from reaction
of 5b with (4-bromophenyl)acetyl chloride according to
the reaction conditions given above.
Ap p lica tion of F u n ction a lized Den d r im er s a s
Su bstr a te Ca r r ier s in Or ga n ic Rea ction s. To dem-
onstrate the feasibility of functionalizing the dendrimer
immobilized esters according to a procedure using in-
soluble supports, e.g., the Suzuki coupling (see ref 10 for
precedents), 7c was reacted with 4-methylphenyl boronic
acid in the presence of [Pd(PPh3)4] and Na2CO3 (see
Scheme 1) to afford 8 in 80% yield (see Experimental
Section). The funcionalized carboxylic acid, i.e., 4-(4-
tolyl)phenylcarboxylic acid, 9, was obtained in quantita-
tive yield after basic hydrolysis of 8, together with the
4-(hydroxymethyl)phenylcarbosilane carrier 5b (see
Scheme 1).
least to our knowledge, there are no precedents for this
specific reaction, using solid or soluble supports.12
Deprotonation of one of the ester functionalized den-
drimers 3c-6c with LDA in THF at -78 °C and
subsequent transmetalation with ZnCl2 affords the cor-
responding zinc-enolate 3d -6d , see Scheme 2. Figure 1
shows a schematic representation of the zinc-enolate
derived from 5c. Previously we have shown that usually
zinc ester enolates are aggregated species.13 Because 3d -
6d contain several zinc-enolate moieties, at forehand it
cannot be excluded that inter- and intramolecular ag-
gregation occurs, resulting in polymeric structures. How-
ever, because we observed that under the reaction
conditions (-78 °C, THF) the resulting solutions remain
homogeneous, such phenomena most probably does not
occur. A possible explanation could be that intramolecu-
lar aggregation of suitable zinc enolate units with a
proper spatial arrangement occurs as well as of zinc
enolate units with lithium and zinc halides present in
solution.
Previously we have investigated in detail the zinc
mediated condensation of esters with imines to â-lac-
tams.11 This reaction involves several steps, and therefore
it was a challenge to perform this multistep reaction
using dendrimers as a substrate carrier, i.e., with esters
connected to the periphery of a dendrimer. Moreover, at
The in situ generated zinc-enolates were not isolated
but reacted as such with N-(trimethylsilyl)phenylimine.
The first step of this condensation reaction involves C-C
bond formation resulting in a â-amino ester that is still
(12) Recently, Ruhland et al. reported the solid-phase combinatorial
synthesis of â-lactams via a [2 + 2] cycloaddition reaction of ketenes
with resin-bound imines derived from amino acids. Ruhland, B.;
Bhandari, A.; Gordon, E. M.; Gallop, M. A. J . Am. Chem. Soc. 1996,
118, 253.
(13) (a) van der Steen F. H.; Boersma, J .; Spek, A. L.; van Koten,
G. J . Organomet. Chem. 1990, 390, C21. (b) van der Steen, F. H.;
Boersma, J .; Spek, A. L.; van Koten, G. Organometallics 1991, 10, 2468.
(c) Kleijn, H.; van Maanen, H. L.; J astrzebski, J . T. B. H.; van Koten,
G. Recl. Trav. Chim. Pays-Bas 1992, 111, 497. (d) van Maanen, H. L.;
Kleijn, H.; J astrzebski, J . T. B. H.; van Koten, G. Recl. Trav. Chim.
Pays-Bas 1994, 113, 567.
(10) (a) Backes, B. J .; Ellman, J . A. J . Am. Chem. Soc. 1994, 116,
11171. (b) Ruhland, B.; Bombrun, A.; Gallop, M. A. J . Org. Chem. 1997,
62, 7820. (c) Fenger, I.; Le Drian, C. Tetrahedron Lett. 1998, 39, 4287.
(d) Wendeborn, S.; Berteina, S.; Brill, W. K.-D.; De Mesmaeker, A.
Synlett 1998, 671.
(11) (a) J astrzebski, J . T. B. H.; van der Steen, F. H. van Koten, G.
Recl. Trav. Chim. Pays-Bas, 1987, 106, 516. (b) J astrzebski, J . T. B.
H.; van Koten, G. Bioorg. Med. Chem. Lett. 1993, 3, 2351. (c) van
Maanen, H. L.; Kleijn, H.; J astrzebski, J . T. B. H.; Verweij, J .; Kieboom,
A. P. G.; van Koten, G. J . Org. Chem. 1995, 60, 4331.

Functionalized Carbosilane Dendritic Species
J . Org. Chem., Vol. 65, No. 20, 2000 6341
F igu r e 1.
Ta ble 1. Syn th esis of tr a n s-â-La cta m s via a Solu ble
not unexpected. In a separate experiment it was shown
that reaction of the zinc-enolate derived from the enan-
tiopure 1-phenylethanol ester of phenylacetic acid affords
â-lactam 10 with identical diastereoselectivity (de 95%
trans) and enantioselectivity (30%). Furthermore, it has
been well established that the enantioselectivity of the
zinc mediated condensation of enantiopure menthyl and
bornyl esters with imines to â-lactams never exceeds
35%.15
Den d r itic G0 a n d G1 Ca r bosila n ea
entry
estera
conv.
de [%]
ee [%]b
1
2
3
4
3c, R ) H
4c, R ) Me
5c, R ) H
6c, R ) Me
95
83
80
82
>95
>95
>95
>95
31
30
a
b
See Scheme 3. HPLC using Diacel chiralcel OD column;
eluents hexane/i-PrOH, 99:1; flow 1 mL/min.
From the above-mentioned observations (see Table 1)
it might be concluded that the use of carbosilanes as
substrate carriers for the zinc mediated synthesis of
â-lactams might be an alternative for the conventional
route, i.e., the use of common esters, especially when the
recovery of the alcohol moiety is required, e.g., when an
expensive enantiopure alcohol is used. Moreover, the
developed methodology might be advantageous from an
environmental point of view, i.e., recovery and reuse (vide
infra) of material that otherwise would be treated as
chemical waste.
connected to the dendrimer backbone via the ester
linkage. In the second step a spontaneous ring-closure
reaction occurs, resulting in a â-lactam with concomitant
cleavage from the carbosilane support (see Scheme 2). It
should be noted that the alcoholate moieties present at
the peripheric sites of the carbosilane backbone might
be regarded as the leaving groups. Initially, a N-tri-
methylsilyl-â-lactam is formed and the zinc ions are
present in the form of zinc-alcoholate moieties connected
to the carbosilane dendrimer. These species, however,
were not observed, but a concomitant transmetalation
reaction, i.e., the exchange of ZnCl and Me3Si results,
after hydrolysis, in the formation of â-lactam 10 and the
trimethylsilyl derivative of 3b-6b. After separation of
the product from the trimethylsilyl functionalized sub-
strate carrier, vide infra, 10 was isolated and was
So far, for analytical purposes, separation and purifica-
tion of the products, i.e., â-lactam 10, and the trimeth-
ylsilyl ether functionalized dendrimer was performed
making use of preparative GPC techniques (Sephadex
LH-20). The application of membrane technology (nano-
filtration) would be a large improvement. It should be
noted, however, that the current commercially available
membranes with a proper permeability are not or almost
are not compatible with the applied reaction conditions,
especially the use of organic solvents. Preliminary reten-
tion measurement experiments using a MPF-60 NF
membrane, however, have shown that under proper
conditions the functionalized dendrimers 5b and 6b are
1
identified by H and 13C NMR and HPLC (see Table 1).
As expected for zinc-enolates14 the â-lactam formation
is highly trans selective, de > 95%. The moderate
enantioselectivity (approximately 30%) observed for the
reactions starting from the enantiopure ester 4c or 6c is
(14) The excellent cis-trans selectivity was rationalized via a six-
membered cyclic transition state, involving a (Z)-enolate and a (E)-
imine, analogous to the transition state models for the related aldol
reaction. See: Evans, D. A.; Nelson, J . V.; Taber, T. R. Top. Stereochem.
1982, 13, 1. Seebach, D. Angew. Chem. 1988, 100, 1685.
(15) van der Steen, H.; Kleijn, H.; Britovsek, G. J .; J astrzebski, J .
T. B. H.; van Koten, G. J . Org. Chem. 1992, 57, 3906.

6342 J . Org. Chem., Vol. 65, No. 20, 2000
Hovestad et al.
Sch em e 3
of sufficient size to be retained to a large extend. Recently
we have shown that this type of membranes can be
applied in a membrane reactor to retain a palladium
catalyst immobilized at the peripheric sites of a carbosi-
lane dendrimer to perform the hydrovinylation reaction
of styrene with ethylene in a continueous process.7
Con clu sion
We have presented a method to use functionalized
carbosilane dendrimers as soluble supports in organic
synthesis, using ester enolate-imine condensation and
preparation of functionalized carboxylic acids as ex-
amples. To our knowledge, the present method is the first
example of using a carbosilane dendritic species as a
support in organic synthesis.
The recovered trimethylsilyl ether derivatives of 3b-
6b can easily be reused as a substrate carrier because
reaction of these compounds with an appropriate acid
chloride affords the ester functionalized carbosilane
dendrimer and Me3SiCl in a straightforward reaction.
This study has shown that the use of dendritic soluble
supports is of interest in repetitive batch or continuous
processes for organic product formation. In our present
study we used carbosilane dendritic species having only
one functional group per peripheral Si-site. This approach
was chosen to ensure that each site could be indepen-
dently addressed during the subsequent synthetic step
and no interactions between sites could occur. We are
currently studying the influence on the synthetic proce-
dures when the remaining sites at each peripheral Si-
site also are used. We are also working to explore the
use of dendritic supports in organic synthesis, as support
or catalyst, and to isolate the dendrimer from the
products in a membrane reactor to allow for reuse of the
dendritic support. However, more stable nanofiltration
membranes have to become available to assist the further
development of this potentially important field in homo-
geneous phase supported organic synthesis and catalysis.
To demonstrate the feasibility to use carbosilane
dendrimers as substrate carriers in a combinatorial
approach we have synthesized an ester functionalized
dendrimer containing two different ester moieties. Reac-
tion of 5b with a 1:1 mixture of phenylacetic acid chloride
and pivaloyl chloride resulted after workup (see experi-
mental part) in the formation of 11 (Scheme 3). A
statistical mixture, n varying from 0 to 12, is expected.
MALDI-TOF-MS analysis of 11 showed the presence of
peaks that could be assigned to the presence of species
for which n ) 5, 6, and 7 (see Scheme 3).
Deprotonation of 11 with LDA and subsequent trans-
metalation with ZnCl2 afforded after reaction with a 1:1
mixture of 2-PyCHdNCH(Me)Ph and 2-PyCNdNCMe3
all four possible â-lactams, 13-16, in equal amounts with
an overall yield of 85% (see Scheme 3). A proper choice
of a combination of imines is important because only
certain combinations of ester-enolate and imine are
reactive. It appeared, for example, that the zinc-enolate
derived from pivalic acid is not reactive toward N-
(trimethylsilyl)phenylimine. On the other hand, this
“mismatch” in reactivity can be used to remove selectively
particular ester groups from ester functionalized carbosi-
lanes and opens the opportunity for the development of
carbosilane dendrimers containing different functional
groups at the periphery. Reaction of 11, according to the
reaction conditions given above with N-(trimethylsilyl)-
phenyimine affords after hydrolysis â-lactam 10 together
with 12. MALDI-TOF-MS analysis of 12 showed the
presence of peaks that could be assigned to presence of
species for which n ) 5, 6, and 7 (see Scheme 3),
indicating that the pivaloyl moieties remain unaffected.
The potential of these partly functionalized dendrimers
and the introduction of a “second” functional group, e.g.,
to control the solubility properties of these materials, is
currently under investigation.
Exp er im en ta l Section
Gen er a l. All reactions were carried out using standard
Schlenk techniques under an inert atmosphere of dry, oxygen-
free nitrogen unless otherwise stated. Et2O, THF, and
hexane were carefully dried and distilled from Na/benzophe-
none prior to use. CH2Cl2 was distilled from CaH2. Diisopro-
pylamine was distilled at atmospheric pressure and stored over
molecular sieves (3 Å). All other standard chemicals were
purchased from ACROS Chimica or Aldrich Chemical Co. and
used without further purification. Imines were prepared
according literature procedures.16 Dry ZnCl2 was prepared via
a literature procedure17 and used as a 1.0 M stock solution in
Et2O. n-Butyllithium was obtained as a 1.6 M solution in
hexanes from Aldrich. The starting materials 4-bromobenzyl
tert-butyldimethylsilyl ether18 and (S)-4-bromo-R-methyl
(16) Dayagi, S.; Degani, Y. The Chemistry of the Carbon-Nitrogen
Double Bond; Patai, S., Ed.; Interscience: London, 1970; p 61.
(17) Hamilton, R. T.; Butler, J . A. J . Chem. Soc. 1932, 2283.
(18) Brunner, J .; Richards, F. M. J . Biol. Chem. 1981, 225, 3319.

Functionalized Carbosilane Dendritic Species
J . Org. Chem., Vol. 65, No. 20, 2000 6343
benzyl tert-butyldimethylsilyl ether19 were prepared as previ-
ously described. The carbosilane dendrimers were prepared
according to literature procedures.8 GPC was performed on a
1.0 cm × 20 cm column using Sephadex LH-20 as the
stationary phase and THF as the eluent. FAB-MS spectra were
recorded either on a J EOL J MS SX/SX 102A four-sector mass
spectrometer, operated at 10 kV accelerating voltage, equipped
with a J EOL MS-FAB 10 D FAB gun operated at a 5 mA
emission current, producing a beam of 6 keV xenon atoms, or
a J EOL J MS AX 505 spectrometer, operated at 3 kV accelerat-
ing voltage, equipped with a J EOL MS-FAB 10 D FAB gun
operated at a 10 mA emission current, producing a beam of 6
keV xenon. MALDI-TOF-MS spectra were acquired using a
Voyager-DE BioSpectrometry Workstation (PerSeptive Bio-
systems Inc) mass spectrometer equipped with a nitrogen laser
emitting at 337 nm. The instrument was operated in the linear
mode at an accelerating voltage in the range 23 000-25 000
V. External calibration was performed using insulin (bovine),
and detection was done by means of a linear detector and a
digitizing oscilloscope operating at 500 MHz. Sample solutions
with ∼30 mg/mL in THF were used, and the matrix was 3,5-
dihydroxybenzoic acid in THF (36 mg/mL). A solution of
sodium acetate in THF or a solution of silver(I) trifluoroacetate
in THF was added to the sample to improve the peak
resolution. The sample solution (0.2 µL) and the matrix
solution (0.2 µL) were combined and placed on a gold MALDI
target and analyzed after evaporation of the solvents. Elemen-
tal microanalysis were obtained from Dornis und Kolbe
Mikroanalytisches Laboratorium, Mu¨lheim a.d. Ruhr, Ger-
many.
Syn th esis of Si{(CH2)3SiMe2(C6H4-4)CH2OSiMe2tBu }4
(3a ). To a solution of BrC6H4CH2OSiMe2tBu (10.00 g, 33.2
mmol) in Et2O (50 mL) was added tert-butyllithium (40.3 mL,
1.63 M solution in pentane, 65.7 mmol) at -78 °C. After the
addition was complete, the solution was stirred for 30 min at
-78 °C and then allowed to rise to -20 °C. A solution of G0-
SiMe2Cl 1 (4.68 g, 8.23 mmol) in Et2O (15 mL) was then added,
and the mixture was stirred overnight at room temperature.
The organic layer was separated, and the water layer was
extracted with Et2O (3 × 50 mL). The combined organic layers
were dried over Na2SO4, and the solvent was removed in vacuo.
The residue was purified by Kugelrohr distillation (140 °C,
0.5 mmHg) to give a slightly yellow viscous oil. Yield: 9.61 g,
7.23 mmol, 89%. Anal. Calcd for C72H132O4Si9 (1314.6): C,
65.78; H, 10.12; Si, 19.23. Found: C, 65.76; H, 10.11; Si, 19.15.
1H NMR (CDCl3; 298K): δ 7.48 (d, J ) 7.9, 8H), 7.33 (d, J )
7.8, 8H), 4.76 (s, 8H), 1.33 (m, 8H), 0.94 (s, 36H), 0.79 (t, J )
7.9, 8H), 0.52 (t, J ) 8.2, 8H), 0.25 (s, 24H), 0.13 (s, 24H).
13C{1H} NMR (CDCl3, 298K): δ 142.0, 138, 133.5, 125.4, 65.0,
26.1, 20, 18.6, 18.5, 17.5, -2.7, -5.2. MALDI-TOF-MS m/z:
1421.4 [G0-CH2OSiMe2tBu + Ag]+ (calcd 1421.7).
viscous oil was obtained, yield 3.48 g (0.84 mmol, 82%). Anal.
Calcd for C228H396O12Si29 (4168.3): C, 65.70; H, 10.16; Si, 19.54.
Found: C, 65.56; H, 10.12; Si, 19.47. H NMR (CDCl3; 298K):
δ 7.44 (d, J ) 7.9, 24H), 7.29 (d, J ) 7.7, 24H), 4.72 (s, 24H),
1.39-1.25 (m, 32H), 0.95 (s, 108H), 0.78 (t, J ) 8, 24H), 0.55
(m, 40H), 0.21 (s, 72H), 0.11 (s, 72H). 13C{1H} NMR (CDCl3,
298K): δ 142.0, 138.1, 133.5, 125.4, 64.9, 26.0, 20.7, 18.6, 18.5,
18.2, 17.7, 17.5, -2.7, -5.2. MALDI-TOF-MS m/z: 4278.4 [G1-
CH2OSiMe2tBu + Ag]+ (calcd 4277.2).
1
Syn th esis of Si{(CH2)3Si[(CH2)3SiMe2(C6H4-4)CH(Me)-
OSiMe2tBu ]3}4 (6a ). The procedure was identical to that
described for 3a , starting from (S)-BrC6H4CH(Me)OSiMe2tBu
(1.76 g, 5.6 mmol), t-BuLi (5.6 mL 1.5 M solution in pentane,
16.1 mmol), and G1-SiMe2Cl 2 (0.60 g, 0.31 mmol). A slightly
yellow viscous oil was obtained, yield 1.21 g (0.28 mmol, 90%).
Anal. Calcd for C228H396O12Si29 (4336.7): C, 66.47; H, 10.32;
Si, 18.78. Found: C, 66.31; H, 10.26; Si, 18.60. 1H NMR
(CDCl3; 298K): δ 7.42 (d, J ) 7.9, 24H), 7.29 (d, J ) 7.8, 24H),
4.86 (q, J ) 6.2), 1.43 (d, J ) 6.0, 36H), 1.38 (m, 32H), 0.92 (s,
108H), 0.80 (t, J ) 8.0, 24H), 0.55 (m, 40H), 0.22 (s, 72H),
0.06 (s, 72H), -0.02 (s, 72H). 13C{1H} NMR (CDCl3, 298K): δ
147.3, 137.7, 133.3, 124.5, 70.7, 27.2, 25.9, 20.7, 18.7, 18.3, 17.7,
17.5, -2.7, -5.2. MALDI-TOF-MS m/z: 4444.6 [G1-CH(Me)-
OSiMe2tBu + Ag]+ (calcd 4444.6). [R]20D ) -37 ° (c 4.7, CHCl3).
Syn th esis of Si{(CH2)3SiMe2(C6H4-4)CH2OH}4 (3b). To
a solution of 3a (6.22 g, 4.74 mmol) in THF (30 mL) was added
dropwise a solution of triethylamine trihydrofluoride (4.59 g,
28.4 mmol). After the addition was complete the reaction
mixture was stirred overnight at room temperature. The
volatiles were removed in vacuo, and CH2Cl2 (50 mL) and
aqueous NaOH (50 mL, 3 M) were added. The organic layer
was separated and washed with aqueous NaOH (2 × 30 mL,
3 M). The combined organic layers were extracted with H2O/
CO2 (s) (2 × 50 mL). The organic layer was dried over Na2-
SO4, and the solvent was removed in vacuo. A slightly yellow
oil was obtained. Yield: 3.00 g, 3.51 mmol, 74%. 1H NMR
(CDCl3; 298K): δ 7.48 (d, J ) 7.9 8H), 7.31 (d, J ) 7.9, 8H),
4.62 (s, 8H), 2.21 (b s, 1H), 1.29 (m, 8H), 0.76 (t, J ) 8.0, 8H),
0.50 (t, J ) 8.2, 8H), 0.24 (s, 24H). 13C{1H} NMR (CDCl3,
298K): δ 141.4, 139.1, 133.8, 126.4, 65.2, 20.6, 18.6, 17.4, -2.8.
FAB-MS m/z: 879.4 [G0-CH2OH + Na]+ (calcd 879.5). IR
(CCl4): 3330 cm-1 (OH).
Syn th esis of Si{(CH2)3SiMe2(C6H4-4)CH(Me)OH}4 (4b).
The procedure was identical to that described for 3b, starting
from 4a (2.36 g, 0.57 mmol) in THF (20 mL) and triethylamine
trihydrofluoride (1.65 g, 10.25 mmol). The reaction mixture
was heated under reflux overnight. A slightly yellow oil was
obtained. Yield: 0.85 g, 0.46 mmol, 81%. 1H NMR (CDCl3;
298K): δ 7.48 (d, J ) 7.8, 8H), 7.33 (d, J ) 7.6, 8H), 4.85 (q,
J ) 6.7, 4H), 2.27 (s, 4H), 1.47 (d, J ) 6.4, 12H), 1.31 (m, 8H),
0.79 (t, J ) 8.1, 8H), 0.52 (t, J ) 8.4, 8H), 0.25 (s, 24H). 13C-
{1H} NMR (CDCl3, 298K): δ 146.4, 138.7, 133.8, 124.6, 70.3,
25.0, 20.6, 18.6, 17.4, -2.8. FAB-MS: 935.5 [G0-CH(Me)OH
Syn t h esis
of
Si{(CH2)3SiMe2(C6H 4-4)CH (Me)OSi-
Me2tBu }4 (4a ). The procedure was identical to that described
for 3a , starting from (S)-BrC6H4CH(Me)OSiMe2tBu (3.06 g, 9.7
mmol), t-BuLi (10.7 mL, 1.5 M solution in pentane, 16.1 mmol),
and G0-SiMe2Cl 1 (1.01 g, 1.77 mmol). A slightly yellow viscous
oil was obtained, yield 1.94 g (1.42 mmol, 80%). Anal. Calcd
+ Na]+ (calcd 935.5). IR (CCl4): 3320. [R]20 ) -24° (c 2.9,
D
CHCl3).
Syn th esis of Si{(CH2)3Si[(CH2)3SiMe2(C6H4-4)CH2OH]3}4
(5b). The procedure was identical to that described for 3b,
starting from 5a (2.36 g, 0.57 mmol) in THF (20 mL) and
triethylamine trihydrofluoride (1.65 g, 10.25 mmol). A slightly
yellow oil was obtained. Yield: 0.85 g, 0.46 mmol, 81%. 1H
NMR (CDCl3; 298K): δ 7.43 (d, J ) 7.8, 24H), 7.22 (d, J )
7.6, 24H), 4.52 (s, 24H), 2.67 (s, 12H), 1.36 (m), 0.77 (t, J )
7.7, 24H), 0.52 (m, 40H), 0.21 (s, 72H). 13C{1H} NMR (CDCl3,
298K): δ 141.4, 138.8, 133.7, 126.4, 64.9, 20.6, 18.6, 18.1, 17.8,
17.4, -2.7. FAB-MS m/z: 2818.3 [G1-CH2OH + Na]+ (calcd
2820.2). IR (CCl4): 3315 cm-1 (OH).
Syn th esis of Si{(CH2)3Si[(CH2)3SiMe2(C6H4-4)CH(Me)-
OH]3}4 (6b). The procedure was similar to that described for
3b, starting from 6a (1.03 g, 0.25 mmol) in THF (20 mL) and
triethylamine trihydrofluoride (0.73 g, 4.5 mmol). The reaction
mixture was heated under reflux overnight. A slightly yellow
oil was obtained. Yield: 0.63 g, 0.21 mmol, 84%. 1H NMR
(CDCl3; 298K): δ 7.42 (d, J ) 7.9, 24H), 7.29 (d, J ) 7.8, 24H),
4.86 (q, J ) 6.2, 12H), 2.27 (s, 4H), 1.43 (d, J ) 6.0, 36H), 1.38
(m), 0.80 (t, J ) 8.0, 24H), 0.55 (m, 40H), 0.22 (s, 72H). 13C-
for
C72H132O4Si9 (1370.7): C, 66.59; H, 10.29; Si, 18.44.
1
Found: C, 66.65; H, 10.26; Si, 18.31. H NMR (CDCl3; 298K):
δ 7.45 (d, J ) 7.9, 8H), 7.32 (d, J ) 7.9, 8H), 4.88 (q, J ) 6.3,
4H), 1.42 (d, J ) 6.3, 3H), 1.34 (m, 8H), 0.94 (s, 36H), 0.79 (t,
J ) 8.1, 8H), 0.52 (t, J ) 8.2, 8H), 0.25 (s, 24H), 0.08 (s, 24H),
0.00 (s, 24H). 13C{1H} NMR (CDCl3, 298K): δ 147.4, 137.7,
133.4, 124.5, 70.7, 27.2, 25.9, 20.7, 18.6, 18.3, 17.4, -2.7, -4.7.
FAB-MS m/z: 1369.8 [G0-CH(Me)OSiMe2tBu + H]+ (calcd
1368.9). [R]20 ) -40° (c 9.2, CHCl3).
D
Syn t h esis of Si{(CH2)3Si[(CH2)3SiMe2(C6H 4-4)CH2-
OSiMe2tBu ]3}4 (5a ). The procedure was identical to that
described for 3a , starting from BrC6H4CH2OSiMe2tBu (4.37
g, 14.5 mmol), t-BuLi (18.5 mL, 1.5 M solution in pentane, 27.8
mmol), and G1-SiMe2Cl 2 (2.00 g, 1.03 mmol). A slightly yellow
(19) Chandrasekharan, J .; Ramachandran, P. V.; Brown, H. C. J .
Org. Chem. 1985, 50, 5446.

6344 J . Org. Chem., Vol. 65, No. 20, 2000
Hovestad et al.
{1H} NMR (CDCl3, 298K): δ 146.4, 138.7, 133.7, 124.7, 70.2,
25.0, 20.7, 18.7, 18.2, 17.7, 17.5, -2.7. MALDI-TOF-MS m/z:
2983.6 [G1-CH(Me)OH + Na]+ (calcd 2983.7). IR (CCl4): 3315
3.58 (s, 24H), 1.36 (m, 32H), 0.80 (m, 24H), 0.62 (m, 40H), 0.23
(s, 72H). 13C{1H} NMR (CDCl3, 298K): δ 171.0, 140.1, 136.8,
134.0, 133.0, 131.9, 131.3, 127.7, 121.4, 66.9, 40.9, 20.8, 18.9,
18.5, 18.1, 17.7, -2.5. MALDI-TOF-MS m/z: 5176.6 [G1-CH2-
OC(O)CH2C6H4Br + Na]+ (calcd 5177.9). IR (CCl4): 1765 cm-1
(CdO).
cm-1 (OH). [R]20 ) -27° (c 2.1, CHCl3).
D
Syn t h esis of Si{(CH2)3SiMe2(C6H 4-4)CH2OC(O)CH2-
C6H5}4 (3c). To a solution of 3b (0.71 g, 0.83 mmol) and
pyridine (0.42 g, 5.3 mmol) in THF (20 mL) was added
dropwise a solution of phenylacetyl chloride (0.77 g, 5.0 mmol)
in THF (10 mL). After stirring overnight at room temperature
the reaction mixture was poured onto a 4 M HCl (100 mL)
solution. The water layer was washed with CH2Cl2 (3 × 50
mL). The combined organic layers were extracted with aqueous
NaOH (3 × 50 mL, 3 M). The organic layer was dried over
Na2SO4, and the solvent was removed in vacuo. A yellow oil
was obtained. Yield: 0.91 g, 0.68 mmol, 82%. Anal. Calcd for
C80H100O8Si5 (1330.1): C, 72.24; H, 7.58; Si, 10.56. Found: C,
Syn th esis of Si{(CH2)3Si[(CH2)3SiMe2(C6H4-4)CH2OC-
(O)CH2(C6H4-4C6H4-4Me)]3}4 (8). To a solution of Pd(PPh3)4
(1.83 g, 6.0 mmol) in DMF (100 mL) was added 7c (2.1 g, 0.4
mmol), an aqueous solution of Na2CO3 (2 M, 10 mL), and
4-methylboronic acid (0.9 g, 6.6 mmol). The solution was
heated overnight at 100 °C under nitrogen atmosphere. The
solvents were removed in vacuo, and the residue was purified
by column chromatography (silica, CH2Cl2). Subsequently
drying over Na2SO4 and removal of the solvent in vacuo
afforded a slightly yellow viscous oil. Yield: 1.29 g, 0.42 mmol,
75%. 1H NMR (CDCl3, 298K): δ 7.48 (m, 48H), 7.19 (m, 48H),
5.09 (s, 24H), 3.58 (s, 24H), 1.36 (m, 32H), 0.80 (m, 24H), 0.62
(m, 40H), 0.23 (s, 72H). 13C{1H} NMR (CDCl3, 298K): δ 171.0,
140.1, 136.8, 134.0, 133.0, 131.9, 131.3, 127.7, 121.4, 66.9, 40.9,
20.8, 18.9, 18.5, 18.1, 17.7, -2.5. MALDI-TOF-MS m/z: 5176.6
[G1-CH2OC(O)CH2C6H4C6H4Me + Na]+ (calcd 5177.9). IR
(CCl4): 1765 cm-1 (CdO).
1
72.34; H, 7.58; Si, 10.52. H NMR (CDCl3; 298K): δ 7.53 (d, J
) 7.9 8H), 7.35 (m, 28H), 5.18 (s, 8H), 3.72 (s, 8H), 1.37 (m,
8H), 0.86 (t, J ) 7.9, 8H), 0.60 (t, J ) 8.1, 8H), 0.31 (s, 24H).
13C{1H} NMR (CDCl3, 298K): δ 171.5, 140.0, 136.4, 134.0,
133.9, 129.4, 128.7, 127.5, 127.2, 66.7, 41.4, 20.6, 18.7, 17.5,
-2.8. FAB-MS m/z: 1352.1 [G0-CH2OC(O)CH2Ph + Na]+ (calcd
1352.1). IR (CCl4): 1744 cm-1 (CdO).
Syn t h esis of Si{(CH 2)3SiMe2(C6H 4-4)CH (Me)OC(O)-
CH2C6H5}4 (4c). The procedure was similar to that described
for 3c, starting from 4b (1.22 g, 0.44 mmol), pyridine (0.48 g,
6.0 mmol) in THF (20 mL), and phenylacetyl chloride (0.89 g,
5.8 mmol) in THF (10 mL). A yellow oil was obtained. Yield:
1.86 g, 0.42 mmol, 96%. Anal. Calcd for C84H108O8Si5 (1386.2):
C, 72.28; H, 7.85; Si, 10.13. Found: C, 72.29; H, 7.86; Si, 10.15.
1H NMR (CDCl3; 298K): δ 7.47 (d, J ) 7.8 8H), 7.33 (m, 28H),
5.88 (q, 4H), 3.64 (s, 8H), 1.52 (d, J ) 6.2, 12H), 1.35 (m, 8H),
0.81 (t, J ) 8.0, 8H), 0.54 (t, J ) 8.2, 8H), 0.23 (s, 24H). 13C-
{1H} NMR (CDCl3, 298K): δ 170.8, 141.9, 139.4, 134.0, 133.7,
129.3, 128.5, 127.0, 125.3, 72.7, 41.6, 22.1, 20.5, 18.6, 17.5,
-2.8. FAB-MS m/z: 1408.8 [G0-CH(Me)OC(O)CH2Ph + Na]+
Aqueaus basic hydrolysis of 8 afforded acetic acid derivative
9 and 5b in quantitative yield.
Syn th esis of Si{(CH2)3Si((CH2)3SiMe2(C6H4-4){CH2O-
(C(O)CH2C6H5)n (C(O)CH2-tBu )12-n } (n ) 0-12) (11). The
synthetic procedure is identical to that described for 5c,
starting from 5b (1.02 g, 0.40 mmol), pyridine (0.42 g, 5.3
mmol) in THF (20 mL), and phenylacetyl chloride (0.39 g, 2.5
mmol) and tert-butylacetyl chloride (0.34 g, 2.5 mmol) in THF
(10 mL). A yellow oil was obtained. Yield: 1.86 g, 0.38 mmol,
1
91%. H NMR (CDCl3, 298K): δ 7.46 (d, J ) 7.8, 24H), 7.35
(m, 84H), 5.09 (s, 24H), 3.63 (s, 12H), 2.33 (s, 12H), 1.42 (m,
32H), 1.10 (s, 108H), 0.80 (t, J ) 7.7, 24H), 0.58 (m, 40H),
0.22 (s, 72H). 13C{1H} NMR (CDCl3, 298K): δ 171.4, 168.2,
139.8, 136.3, 133.7, 133.7, 129.3, 128.6, 127.4, 127.2, 66.5, 41.3,
29.7, 20.8, 20.6, 19.6, 18.7, 17.5, -2.8. MALDI-TOF-MS m/z:
4242.2 [n ) 8 + Ag]+ (calcd 4235.9), 4222.9 [n ) 7 + Ag]+ (calcd
4215.9.0), 4203.4 [n ) 6 + Ag]+ (calcd 4195.9) and 4182.3 [n
) 5 + Ag]+ (4175.9).
(calcd 1407.7). IR (CCl4): 1748 cm-1 (CdO) [R]20 ) -62°
D
(c 3.2, CHCl3).
Syn th esis of Si{(CH2)3Si[(CH2)3SiMe2(C6H4-4)CH2OC-
(O)CH2C6H5]3}4 (5c). The procedure was identical to that
described for 3c, starting from 5b (1.22 g, 0.44 mmol), pyridine
(0.48 g, 6.0 mmol) in THF (20 mL), and phenylacetyl chloride
(0.89 g, 5.8 mmol) in THF (10 mL). A yellow oil was obtained.
Gen er a l P r oced u r e for th e â-La cta m F or m a tion . A
well-stirred solution of 0.51 g (5.0 mmol) of diisopropylamine
in THF (25 mL) was cooled to -78 °C. The following reagents
were subsequently added at 10 min intervals: (i) n-butyl-
lithium (3.2 mL, 1.6 M solution in hexane, 5.0 mmol); (ii) 5.0
mmol of dendritic ester groups; (iii) 5.0 mmol of ZnCl2 (5.0 mL,
1.0 M solution in Et2O); (iv) 5.0 mmol of the appropriate imine.
The reaction mixture was stirred at -78 °C for 1 h and allowed
to rise to room temperature and stirred for another 17 h. The
reaction mixture was quenched with a saturated NH4Cl
solution. The aqueous layer was extracted with Et2O (3 × 10
mL), and the combined organic layers were dried over sodium
sulfate, filtrered, and concentrated in vacuo. The organic
Yield: 1.86 g, 0.42 mmol, 96%. Anal. Calcd for C252H324O24
-
Si17 (4214.8): C, 71.81; H, 7.75; Si, 11.33; Found: C, 71.65; H,
7.62; Si, 11.42. 1H NMR (CDCl3; 298K): δ 7.46 (d, J ) 7.8,
24H), 7.35 (m, 84H), 5.09 (s, 24H), 3.63 (s, 24H), 1.42 (m, 32H),
0.80 (t, J ) 7.7, 24H), 0.58 (m, 40H), 0.22 (s, 72H). 13C{1H}
NMR (CDCl3, 298K): δ 171.4, 139.8, 136.3, 133.7, 133.7, 129.3,
128.6, 127.4, 127.2, 66.5, 41.3, 20.6, 19.6, 18.7, 17.5, -2.8.
MALDI-TOF-MS m/z: 4234.5 [G1-CH2OC(O)CH2Ph + Na]+
(calcd 4233.0). IR (CCl4): 1752 cm-1 (CdO).
Syn th esis of Si{(CH2)3Si[(CH2)3SiMe2(C6H4-4)CH(Me)-
OC(O)CH2C6H5]3}4 (6c). The procedure was identical to that
described for 3c, starting from 6b (1.22 g, 0.44 mmol), pyridine
(0.48 g, 6.0 mmol) in THF (20 mL), and phenylacetyl chloride
(0.89 g, 5.8 mmol) in THF (10 mL). A yellow oil was obtained.
1
products were analyzed by H and 13C NMR and HPLC. The
specroscopic data of the obtained â-lactams are in agreement
with the data earlier reported.20
Yield: 1.86 g, 0.42 mmol, 96%. Anal. Calcd for C264H348O24
-
Si17 (4383.1): C, 72.34; H, 8.00; Si, 10.89. Found: C, 72.19; H,
8.11; Si, 10.76. 1H NMR (CDCl3; 298K): δ 7.46 (d, J ) 7.8,
24H), 7.35 (m, 84H), 5.88 (q, 4H), 3.63 (s, 24H), 1.52 (d, J )
6.2, 12H), 1.42 (m, 32H), 0.80 (t, J ) 7.7, 24H), 0.58 (m, 40H),
0.22 (s, 72H). 13C{1H} NMR (CDCl3, 298K): δ 171.4, 139.8,
136.3, 133.7, 133.7, 129.3, 128.6, 127.4, 127.2, 72.7, 41.3, 22.1,
20.6, 19.6, 18.7, 17.5, -2.8. MALDI-TOF-MS m/z: 4234.5
[G1-CH(Me)OC(O)CH2Ph + Na]+ (calcd 4401.2). IR (CCl4):
Ack n ow led gm en t. The authors wish to thank Mrs.
Anca van der Kerk and Mr. Kees Versluis from the
Analytical Chemical Department for measuring the
FAB-MS spectra. Dr. J . Verweij (DSM Gist) and Prof.
dr. J .G. de Vries (DSM) are kindly acknowledged for
their stimulating discussions. This work was supported
by the Council for Chemical Sciences of The Netherlands
Organization for Scientific Research (CW/NWO) and the
Dutch Technology Foundation (STW) with financial aid
from DSM Gist.
1752 cm-1 (CdO). [R]20 ) -62° (c 3.2, CHCl3).
D
Syn th esis of Si{(CH2)3Si[(CH2)3SiMe2(C6H4-4)CH2OC-
(O)CH2(C6H4-4)Br ]3}4 (7c). The procedure was similar to that
described for 3c, starting from 5b (0.93 g, 0.33 mmol), pyridine
(0.63 g, 8.28 mmol) in THF (20 mL), and 4-bromophenylacetyl
chloride (1.63 g, 6.96 mmol) in THF (10 mL). A slightly yellow
oil was obtained. Yield: 1.29 g, 0.42 mmol, 75%. 1H NMR
(CDCl3; 298K): δ 7.48 (m, 48H), 7.19 (m, 48H), 5.09 (s, 24H),
J O991726K
(20) van Maanen, H. L.; Kleijn, H.; J astrzebski, J . T. B. H.; van
Koten G. Recl. Trav. Chim. Pays-Bas 1994, 113, 567.