Mechanism of β-silyl diacyl peroxide decomposition: A mild and stereoselective synthesis of β-silyl esters

Article abstract of DOI:10.1021/jo049852y

A novel method for the formation of β-silyl esters is presented. Mechanistic studies were carried out on the formation and decomposition of β-silyl diacyl peroxides, showing that the decomposition pathway involves an ionic mechanism that is influenced by the presence of the β-silyl moiety. These studies exclude a free radical decomposition pathway as evidenced by the absence of chemically induced dynamic nuclear polarization (CIDNP) during the reaction and a strong correlation of the resulting regioisomeric product distribution to σ⁺. This reaction allows for the formation of β-silyl esters in 45-50% isolated yield with predictable regioselectivity and good to excellent diastereoselectivity. Studies demonstrate that ester products which are formed at benzylic centers have the erythro configuration, whereas ester products formed at alkyl centers have the threo configuration.

Full text of DOI:10.1021/jo049852y

Mech a n ism of â-Silyl Dia cyl P er oxid e Decom p osition : A Mild a n d
Ster eoselective Syn th esis of â-Silyl Ester s
Douglas S. Masterson and Ned A. Porter*
Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235
n.porter@vanderbilt.edu
Received J anuary 26, 2004
A novel method for the formation of â-silyl esters is presented. Mechanistic studies were carried
out on the formation and decomposition of â-silyl diacyl peroxides, showing that the decomposition
pathway involves an ionic mechanism that is influenced by the presence of the â-silyl moiety. These
studies exclude a free radical decomposition pathway as evidenced by the absence of chemically
induced dynamic nuclear polarization (CIDNP) during the reaction and a strong correlation of the
resulting regioisomeric product distribution to σ+. This reaction allows for the formation of â-silyl
esters in 45-50% isolated yield with predictable regioselectivity and good to excellent diastereo-
selectivity. Studies demonstrate that ester products which are formed at benzylic centers have the
erythro configuration, whereas ester products formed at alkyl centers have the threo configuration.
SCHEME 1. Rep r esen ta tive Ca r boxyin ver sion
Rea ction of Dia cyl P er oxid es
In tr od u ction
Organic peroxides are known to undergo diverse
transformations.1 Although free radical mechanisms
dominate the chemistry of peroxides, it has been known
for some years that diacyl peroxides undergo ionic
decomposition.2-8 Diacyl peroxides, for example, undergo
a carboxyinversion reaction when heated in a polar
solvent2,4-6 that generates products such as 2 shown in
Scheme 1. Over the last 30 years there have been only a
handful of reports dealing with the mechanism of this
transformation.
SCHEME 2. Attem p ted P r ep a r a tion of 3 w ith
Isola tion of 4 (Ar ) m -ClP h )
Although the carboxyinversion reaction has been little
studied, it is commonly accepted that diacyl peroxides
simultaneously undergo ionic and free radical decomposi-
tion processes when heated in polar solvents.6 A combi-
nation of chemically induced dynamic nuclear polariza-
tion (CIDNP) experiments and product identification has
established that the products of diacyl peroxide decom-
position arise through simultaneous electron transfer (et),
radical, and ionic pathways.2-4,6 Increasing the solvent
polarity results in more products being formed through
ionic pathways, whereas decreasing solvent polarity
results in increased production of products derived from
free radical decomposition. Carboxyinversion is believed
to occur through a polar but concerted reaction mecha-
nism as supported by studies with chiral substrates.3,7
The known Criegee rearrangement is another example
of a concerted rearrangement involving peroxy com-
pounds. The Criegee rearrangement results from a
concerted but polar rearrangement of peroxy esters. See
ref 1 for examples of the Criegee rearrangement.
We had cause to prepare the previously unknown
â-silyl diacyl peroxide compound 3 shown in Scheme 2
for possible use as a precursor to â-silyl free radicals. In
the course of attempts to prepare compounds such as 3,
we discovered that â-silyl esters 4 are instead formed in
reasonable yield and good to excellent diastereoselectiv-
ity. We report here on mechanistic studies that aim to
elucidate the mechanism for the formation of the unex-
pected â-silyl esters 4 and explain the role that silicon
plays in the decomposition of 3 (Scheme 2).
(1) Ando, W. Organic Peroxides; J ohn Wiley and Sons: Chichester,
U.K., 1992.
(2) Walling, C.; Waits, H. P.; Milovanovic, J .; Pappiaonnou, C. G.
J . Am. Chem. Soc. 1970, 92, 4927-4932.
(3) Walling, C.; Sloan, J . P. J . Am. Chem. Soc. 1979, 101, 7679-
7683.
(4) Taylor, K. G.; Govindan, C. K.; Kaelin, M. S. J . Am. Chem. Soc.
1979, 101, 2091-2099.
(5) Sigman, M. E.; Barbas, J . T.; Leffler, J . E. J . Org. Chem. 1987,
52, 1754-1757.
Resu lts a n d Discu ssion
(6) Lee, S.-G. J . Chem. Soc., Perkin Trans. 2 1993, 1361-1372.
(7) Kashiwagi, T.; Kozuka, S.; Oae, S. Tetrahedron 1970, 26, 3619-
3629.
Initial attempts to prepare and isolate compound 3
(8) Flowers, G. C.; Leffler, J . E. J . Org. Chem. 1989, 54, 3995-3998.
involved the use of standard methods for the preparation
10.1021/jo049852y CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/06/2004
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Masterson and Porter
NMR analysis was performed and revealed that the
isolated compound was in fact a mixture of two regio-
1
isomeric esters. The H NMR analysis established that
the major product was benzylic ester 6 and the minor
product was the secondary alkyl ester 7. Figure 1 shows
the structures assigned to major product 6 and minor
product 7 along with the diagnostic 1H NMR signals used
to assign the respective regioisomers. 2D NMR tech-
niques were employed to unambiguously assign struc-
tures to the isomers 6 and 7, shown in Figure 1. As
illustrated from Scheme 3 and Figure 1, a formal migra-
tion of the silyl moiety must occur in order to obtain the
major regioisomer 6. Although a pair of regioisomers was
formed during this transformation, it should be noted
that each compound was produced as a single diaster-
F IGURE 1. NMR evidence for regioisomeric esters 6 and 7
(relative stereochemistry not shown, Ar ) m-ClPh).
SCHEME 3. Attem p ted P r ep a r a tion of 3
1
eomer as determined from H and 13C NMR experiments
on the crude reaction mixture.
Three plausible mechanisms, shown in Scheme 4, could
account for the formation of esters 6 and 7 from the
reaction of 5 with thionyl chloride and m-CPBA. Mech-
anism A proceeds through an initially formed diacyl
peroxide intermediate 3 that decomposes by a free radical
mechanism to give loss of CO2 and generate a solvent
caged radical pair. The radical pair could then simply
recombine, producing ester 7. This mechanism can be
discounted since a regioisomeric mixture of â-silyl esters
is produced and there is no evidence from previous
studies that â-silyl radicals undergo silyl rearrange-
ment.13,14
of diacyl peroxides.9 The attempted synthesis of 3 made
use of the known â-silyl carboxylic acid 5,10 which was
transformed into the corresponding acid chloride by
conventional methods. The acid chloride of 5 was then
treated with a single equivalent of pure m-CPBA in
dichloromethane at -10 °C. The reaction was monitored
by thin-layer chromatography, which indicated that both
the starting acid chloride and m-CPBA had been com-
pletely consumed. Chromatography also indicated the
presence of two new products (Scheme 3), neither of
which were peroxides as determined by the use of a
peroxide-sensitive thin-layer stain.11 The crude reaction
mixture was subjected to chromatography and the more
polar of the two products was isolated and subjected to
IR, MS, and NMR spectral analysis. The IR analysis
revealed the absence of the characteristic diacyl peroxide
absorptions12 and the presence of a single strong band
at 1722 cm-1, indicative of ester functionality.12 MS
analysis of the products generated by the chemistry
described in Scheme 3 was consistent with loss of CO2
Mechanism B also makes use of intermediate 3 but
differs from mechanism A in that the initially formed
radical pair undergoes electron transfer, a step that is
reasonable on the basis both that decarboxylation of the
aryl carboxyl radical is slow and that the ion pair may
be energetically more stable than the radical pair (a
â-silyl cation has a stabilization energy of ∼38 kcal/mol,15
whereas a â-silyl radical has a stabilization energy of only
3-4 kcal/mol16,17). Following electron transfer, the result-
1
from the desired diacyl peroxide 3. Both 1D and 2D H
SCHEME 4. P ossible Mech a n ism s for th e F or m a tion of â-Silyl Ester s (Ar ) m -ClP h )
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Mild Stereoselective Synthesis of â-Silyl Esters
SCHEME 5. P r ep a r a tion of 13C-La beled 8 a n d Use in CO2 CIDNP Stu d y (Ar ) m -ClP h )
ing â-silyl cation could rearrange,18 giving the observed
regioisomeric mixture of ester products (6 and 7).
Mechanism C differs from mechanisms A and B in that
3 decomposes via a carboxyinversion process2-4,6,7 instead
of a free radical process. Loss of CO2 from the intermedi-
ate 3′ could occur, giving an ion pair. The resulting â-silyl
cation could then undergo facile rearrangement and
trapping by the carboxylate anion to produce the ob-
served regioisomeric mixture of esters. Having discounted
mechanism A on the basis of the product distribution,
we considered experiments that would discriminate
between mechanisms B and C.
Mechanisms B and C differ in that B proceeds by a
radical pair whereas C is polar-concerted and ionic. We
devised a 13C labeling experiment in order to determine
whether CIDNP signals could be observed during the
transformation. Detection of a significant 13C CIDNP
signal for the liberated CO2 would be strong evidence for
a mechanism involving radical pair intermediates as
required for mechanism B.6 Compound 8 was prepared
as shown in Scheme 5 such that the carboxylic acid was
approximately 30% enriched in 13C label. Acid 8 was
prepared by a known literature procedure10 from labeled
cinnamic acid, which was prepared by a Perkin reaction
with 13C-sodium acetate as the 13C source.19 The resulting
acid 8 was converted to the acid chloride 8′ by standard
methods.
peroxide 3. The compound appeared to be stable over the
course of acquiring the spectrum at -30 °C. The tem-
perature of the NMR probe was then raised to ambient
and a signal corresponding to 13C-labeled CO2 was
immediately observed. No evidence of an enhanced
absorption or emission CIDNP signal was detected for
the liberated CO2. Absence of a CIDNP signal for CO2
suggests the absence of free radical intermediates in the
decomposition of diacyl peroxide 3 (see Supporting In-
formation for 13C spectra).
We consistently observed 45-50% yield of ester prod-
ucts with quantitative conversion of starting materials
by both 1H NMR and TLC analysis. To account for the
remaining mass balance, a 13C experiment was devised
that would allow for straightforward detection of all
products formed during this transformation. Labeled
â-silyl acid 9 was prepared by conventional synthetic
techniques as shown in Scheme 6. Acid 9 was then
treated with m-CPBA/DCC20 and 13C NMR analysis was
performed on the crude reaction mixture.
The NMR analysis of the product mixture revealed the
presence of three products. The esters 6 and 7 had been
previously identified in earlier experiments but were
shown by this experiment to be only one diastereomeric
form. Olefin 10 was the only other product observed, and
it was found to comprise about 50% of the reaction
mixture by comparison of the NMR signals. Olefin 10 was
isolated from the reaction mixture and analyzed further
The acid chloride 8′ in dry d-chloroform was placed in
the NMR spectrometer and cooled to -30 °C. This
solution was treated with a single equivalent of m-CPBA
and the reaction was monitored by NMR at -30 °C. A
sole signal in the carbon NMR spectrum at 171.98 ppm
was observed and assigned to the suspected diacyl
1
by MS and H NMR, confirming its structure compared
to authentic material.21
The data gathered from both 13C NMR experiments are
taken as strong evidence against a free radical mecha-
nism and for a carboxyinversion-ionic decomposition
pathway. It is expected that in an ionic decomposition
the ratio of ester products would be strongly dependent
on phenyl substituents in the para position of the
aromatic ring. A series of para-substituted â-silyl acids
was therefore prepared in order to determine the depen-
dence of the ester product ratio on the phenyl substituent.
Table 1 presents product distributions from the reaction
of substituted â-silyl acids with m-CPBA, and the data
were subjected to a Hammett analysis. A strong correla-
tion for the data exists only by the use of the σ+
parameter. The Hammett analysis gives a F value of
-1.2, which is taken as strong evidence of a cationic
mechanism for the transformation (Figure 2).
(9) Fujimori, K. In Organic Peroxides; Ando, W., Ed.; J ohn Wiley &
Sons: Chichester, U.K., 1992; pp 320-379.
(10) Crump, R. A. N. C.; Fleming, I.; Hill, J . H. M.; Parker, D.;
Reddy, N. L.; Waterson, D. J . Chem. Soc., Perkin Trans. 1 1992, 3277-
3294.
(11) Smith, L. L. Cholesterol Autoxidation; Plenum Press: New
York, 1981.
(12) Pretsch, E.; Clerc, T.; Seibl, J .; Simon, W. Tables of Spectral
Data for Structure Determination of Organic Compounds, 2nd ed.;
Springer-Verlag: Berlin, 1989.
(13) Masterson, D. S.; Porter, N. A. Org. Lett. 2002, 4, 4253-4256.
(14) Porter, N. A.; Zhang, G.; Reed, A. D. Tetrahedron Lett. 2000,
41, 5773-5777.
(15) Wierschke, S. G.; Chandrasekhar, J .; J orgensen, W. L. J . Am.
Chem. Soc. 1985, 107, 1496-1500.
(16) Auner, N.; Walsh, R.; Westrup, J . J . Chem. Soc., Chem.
Commun. 1986, 207-208.
Products 6 and 7 can also be accessed by the reaction
of silyl carboxylic acid 11 with m-CPBA. Compound 11
(17) Walsh, R. Pure Appl. Chem. 1987, 59, 69-72.
(18) Suginome, M.; Takama, A.; Ito, Y. J . Am. Chem. Soc. 1998, 120,
1930-1931.
(19) J emielity, J .; Kanska, M.; Kanski, R. Isotopes Environ. Health
Stud. 1998, 34, 335-339.
(20) Greene, F. D.; Kazan, J . J . Org. Chem. 1963, 28, 2168-2171.
(21) Fleming, I.; Rowley, M. Tetrahedron 1989, 45, 413-424.
J . Org. Chem, Vol. 69, No. 11, 2004 3695

Masterson and Porter
SCHEME 6. 13C Ben zylic La belin g Exp er im en t: Id en tifica tion a n d Qu a n tifica tion of Olefin 10 a n d Ester s 6
a n d 7 (Ar ) m -ClP h )
TABLE 1. Effect of P a r a Su bstitu en ts on Regioselective Ester Ra tio
entry
substrate
% ester 6a
% ester 7a
yieldb (%)
1
2
3
4
5
6
3a
3b
3c
3d
3e
3f
85
95
86
90
56
47
15
5
14
10
44
53
50
50
42
48
39
32
a
b
Determined by 1H NMR on crude reaction mixtures; average values of at least two experiments. Determined by 1H NMR relative
to an internal standard of 1,1,2,2-tetrachloroethane; reported as the total yield of 6 and 7.
ably more 6 than that formed from the reaction of 3 with
m-CPBA under identical conditions.
Reaction of 11 with m-CPBA gave a 6:7 ester ratio of
92:8, whereas an analogous reaction of 3 gave an ester
ratio of 85:15. Both reactions gave ester products as about
50% of the product mixture, the remainder being the
vinyl silane.
Scheme 8 presents a proposed mechanism for the
conversion of 3 to esters 6 and 7 and olefin 10. The
carboxyinversion compound shown in the scheme was not
observed in our NMR studies and is therefore not a
required intermediate. Indeed, direct decomposition of
the diacyl peroxide to form the stable â-silyl carbocation-
carboxylate ion pair is a possible alternative. The car-
boxyinversion process is, however, a well-known pathway
and we include it here based upon established precedent.
Once the cationic intermediate is formed as a tight ion
pair, rearrangement, â-elimination of a proton, and ion
pair collapse produce the observed ester and olefin
products.
F IGURE 2. Hammett analysis of product ratio data from
Table 1.
was prepared and converted to its acid chloride by
conventional methods. The acid chloride of 11 was
coupled with m-CPBA and the resulting product ester
1
ratio was determined by H NMR analysis of the crude
reaction mixture as previously described (Scheme 7). The
results suggest that 6 and 7 do not fully equilibrate since
the product mixture obtained from acid 11 had measur-
The reaction illustrated in Scheme 8 produces a
mixture of ester products but each product is produced
as a single diastereomer. We prepared the acids 12 and
3696 J . Org. Chem., Vol. 69, No. 11, 2004

Mild Stereoselective Synthesis of â-Silyl Esters
SCHEME 7. P r ep a r a tion of 11 a n d Ester Ra tio fr om Cou p lin g 11 w ith m -CP BA
SCHEME 8. P r op osed Decom p osition Mech a n ism for Com p ou n d 3 (Ar ) m -ClP h )
SCHEME 9. Deter m in in g th e Con figu r a tion of th e
Ma jor Isom er of 14 via P eter son Olefin a tion (Ar )
m -ClP h )
F IGURE 3. Compounds 12 and 13 for diastereoselectivity
study.
1313 (Figure 3) to further explore stereochemical ques-
tions raised by the transformation.
Compound 12 can only form benzylic cations after
reaction with m-CPBA, while 13 can only form secondary
alkyl cations. The â-silyl acid 12 was coupled with
m-CPBA (DCC method) and the resulting esters (14)
were isolated as a 3:1 mixture of diastereomers. The ester
mixture 14 was then reduced to the corresponding â-silyl
alcohol with LAH. The resulting alcohol mixture was
immediately treated with an excess of KH to promote a
syn-specific “Peterson-like” elimination (Scheme 9).22 The
resulting stilbene mixture obtained from the Peterson
olefination reaction was analyzed by gas chromatography
and found to contain a 3:1 mixture of cis/trans stilbene.
The Peterson olefination, being syn-specific, could pro-
duce cis-stilbene as the major product only if the major
diastereomer of 14 had the erythro relative configuration
as illustrated in Scheme 9. Indeed, the major ester
produced from the sequence would have the relative
configuration shown in Scheme 9 if the stereocontrol
element in the transformation was A-strain.
treated with TBAF to promote an anti-specific olefination
reaction (Scheme 10).22 Gas chromatographic analysis of
mixture 16/17 resulted in an 8:1 ratio of cis/trans olefin
by comparison to authentic samples. The major ester
product obtained from 13 was expected to have a threo
relative configuration as shown in Scheme 9. The major
Compound 13 was coupled with m-CPBA and the
resulting alkyl ester mixture (16/17) was isolated and
(22) Bernhard, W.; Fleming, I. J . Organomet. Chem. 1984, 271, 281-
288.
J . Org. Chem, Vol. 69, No. 11, 2004 3697

Masterson and Porter
SCHEME 10. Deter m in a tion of th e Con figu r a tion of th e Ma jor Isom er of 16 a n d 17 via a n An ti-Sp ecific
Elim in a tion Sequ en ce (Ar ) m -ClP h )
ester from mixture 16/17 was shown to have the expected
configuration since the cis olefin can only be produced
from the threo configuration in an anti-specific elimina-
tion sequence (Scheme 10).
This reaction allows for the mild and stereoselective
formation of â-silyl esters from readily available materi-
als. This procedure may find use for the in situ prepara-
tion of substrates used in the study of â-silyl cations.23-26
The result of the experiments, as illustrated in Schemes
9 and 10 above, demonstrates that the configurations of
the ester products are controlled by elements that have
been established by extensive investigations in analogous
systems. In the case where the cation is trapped at a
benzylic position, the resulting ester product has the
erythro configuration that results from A-strain, which
forces the two phenyl groups to obtain spatial arrange-
ments that maximize their separation from one another
(180°). In experiments where the cation is trapped at an
alkyl position, the resulting ester product has the threo
configuration as shown in Scheme 10 that results from
minimizing interactions of the alkyl groups and the
approaching nucleophile.
Exp er im en ta l Section
Rea ction of â-Silyl Acid s w ith m -CP BA (Typ ica l
P r oced u r e). A 25 mL round-bottom flask was charged
with 0.10 g of (RR,SS)-2-methyl-3-phenyldimethylsilyl-
3-phenylpropionic acid (3) (0.35 mmol), 10 mL of dry
CDCl3, and a stir bar. The resulting solution was cooled
to -10 °C and 1 equiv of m-CPBA (60 mg) was added in
one portion. After the m-CPBA had dissolved, 1.1 equiv
of DCC (80 mg) was added. The solution was stirred for
2 h and allowed to reach 0 °C. Upon completion of the
reaction, 0.5 equiv (relative to silyl acid) of 1,1,2,2-
tetrachloroethane was added as an NMR standard and
the resulting solution was filtered directly into an NMR
tube. NMR analysis was performed immediately in order
to obtain the ratio of ester products. NMR analysis
revealed a 50% yield of ester products having an 85:15
ratio of regioisomers. Solvent was removed in vacuo and
the residue was purified by preparative TLC (5% EtOAc/
hexane trace triethylamine), giving an inseparable mix-
ture of â-silyl esters. 1H NMR (300 MHz, CD2Cl2) (major
Con clu sion
â-Silyl diacyl peroxides readily decompose to produce
â-silyl esters in moderate yield with predictable regiose-
lectivity and good to excellent diastereoselectivity. De-
composition of the â-silyl diacyl peroxide proceeds by an
ionic process. The ionic decomposition produces silyl-
stabilized cationic intermediates that readily undergo
both trapping with a carboxylate anion and elimination
producing an olefinic product. Configuration studies
demonstrate that when the cation is a benzylic ion,
trapping occurs to produce ester products of the erythro
configuration. Alkyl cations are trapped to produce ester
products having predominantly the threo configuration.
(23) Lambert, J . B.; Wang, G.-t.; Finzel, R. B.; Teramura, D. H. J .
Am. Chem. Soc. 1987, 109, 7838-7845.
(24) Lambert, J . B.; Emblidge, R. W.; Malany, S. J . Am. Chem. Soc.
1993, 115, 1317-1320.
(25) Lambert, J . B.; Zhao, Y. J . Am. Chem. Soc. 1996, 118, 7867-
7868.
(26) Lambert, J . B.; Zhao, Y.; Emblidge, R. W.; Salvador, L. A.; Liu,
X.; So, J .-H.; Chelius, E. C. Acc. Chem. Res. 1999, 32, 183-190.
3698 J . Org. Chem., Vol. 69, No. 11, 2004

Mild Stereoselective Synthesis of â-Silyl Esters
regioisomer) 0.28 (s, 3H), 0.30 (3, 3H), 0.90 (d, 3H, J ) 8
Hz), 1.84-1.95 (m, 1H), 5.81 (d, 1H, J ) 10 Hz), 7.20-
7.40 (m, 9H), 7.50-7.53 (m, 3H), 7.60-7.70 (m, 1H), 7.75
(t, 1H, J ) 2 Hz); (minor regioisomer) 0.09 (s, 3H), 0.10
(s, 3H), 1.21 (d, 3H, J ) 6 Hz), 2.84 (d, 1H, J ) 10 Hz),
5.61-5.68 (m, 1H). Aromatic protons of the minor isomer
could not be accurately assigned and integrated due to
overlap with aromatic signals of the major isomer. 13C
NMR (75 MHz, CD2Cl2) (major regioisomer) -4.1, -3.0,
12.2, 27.2, 80.9, 127.8, 128.3, 128.5, 128.8, 129.0, 129.5,
130.0, 130.2, 132.8, 133.3, 134.3, 134.7, 139, 141.3, 164.7.
HRMS C24H25ClO2SiNa (M + Na)+ 431.1204 calcd,
tube was placed into the NMR and the probe was cooled
to -30 °C. A 13C NMR (100 MHz) was taken of the acid
chloride, which gave an acid chloride signal at 177.7 ppm.
The tube was quickly removed from the NMR and 1 equiv
of m-CPBA was added along with 1 drop of pyridine-d5.
The sample was returned to the NMR and a spectrum
was acquired at -30 °C, giving a signal at 172.0 ppm
(consistent with a diacyl peroxide). The sample was then
warmed to ambient temperature, whereupon the libera-
tion of 13C-CO2 was immediately detected at 124.8 ppm.
Ack n ow led gm en t. We thank Mr. M. Voehler for
help with the low-temperature NMR experiments, Dr.
H. Yin for his help with the low-resolution MS analysis,
and Mrs. T. Masterson for reviewing the manuscript.
Financial support provided by NIH and Vanderbilt
University is greatly appreciated.
431.1197 found. IR (thin film) 1722 cm-1
.
Low -Tem p er a tu r e 13C NMR Exp er im en t. A 10 mL
round-bottom flask was charged with 17 mg of 8 (0.06
mmol), 2 mL of dry CH2Cl2, and 1 mL of SOCl2. The flask
was connected to a reflux condenser and placed under
an inert atmosphere. The solution was heated to reflux
solvent overnight. The solution was cooled to room
temperature and the solvent and excess SOCl2 were
removed in vacuo. The residue was taken up in 0.5 mL
of dry CDCl3 and placed in a dry NMR tube. The NMR
Su p p or tin g In for m a tion Ava ila ble: NMR spectra of all
new â-silyl acids and â-silyl esters 6 and 7. This material is
available free of charge via the Internet at http://pubs.acs.org.
J O049852Y
J . Org. Chem, Vol. 69, No. 11, 2004 3699