Trialkylsilyl triflimides as easily tunable organocatalysts for allylation and benzylation of silyl carbon nucleophiles with non-genotoxic reagents

Article abstract of DOI:10.1016/j.tetlet.2010.03.030

Trialkylsilyl triflimides generated in situ are unique catalysts for the electrophilic benzylation or allylation of trialkylsilylenol ethers or allyl trialkylsilanes with non-genotoxic alkylating reagents such as benzyl and allyl acetates. In most cases the reactions are fast at room temperature and yields are high. The reaction works particularly well with electron-rich benzyl donors including derivatives of pyrrole, indole and furane.

Full text of DOI:10.1016/j.tetlet.2010.03.030

Tetrahedron Letters 51 (2010) 2571–2575
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Trialkylsilyl triflimides as easily tunable organocatalysts for allylation
and benzylation of silyl carbon nucleophiles with non-genotoxic reagents
Oscar Mendoza a, Guy Rossey b, Léon Ghosez a,
*
a Institut Européen de Chimie et Biologie, Université de Bordeaux-CNRS UMR 5248, 2 rue Robert Escarpit, 33607 Pessac, France
b Sanofi-Aventis Research & Development, 371 rue du Professeur Joseph Blayac, 34184 Montpellier Cedex 04, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Trialkylsilyl triflimides generated in situ are unique catalysts for the electrophilic benzylation or allyla-
tion of trialkylsilylenol ethers or allyl trialkylsilanes with non-genotoxic alkylating reagents such as ben-
zyl and allyl acetates. In most cases the reactions are fast at room temperature and yields are high. The
reaction works particularly well with electron-rich benzyl donors including derivatives of pyrrole, indole
and furane.
Received 5 February 2010
Revised 24 February 2010
Accepted 5 March 2010
Available online 12 March 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Allylation
Benzylation
Green chemistry
Triflimide
Acid-catalyzed alkylations of carbon nucleophiles are attractive
substitutes for the classical nucleophilic SN2-type substitution
involving carbanions and alkyl halides or sulfonates.1 They require
alkylating agents (e.g., benzyl, allyl and t-alkyl) prone to undergo
SN1-type substitution reactions, most often after activation
in situ by a Brönsted or a Lewis acid. Thus alcohols, ethers or esters
which are much less genotoxic than the classical halides and sulfo-
nates have been successfully used as precursors of the alkylating
species. Acid-catalyzed benzylations and allylations of silyl
nucleophiles involving benzyl or allyl alcohols and ethers or esters
have received much attention in recent years.2 However, many of
the reported reactions suffer from major drawbacks such as
the use of toxic metal-derived Lewis acids, significant amounts of
by-products, difficulties in work-up and often experimental
conditions which are incompatible with the presence of many
functional groups. Recently several groups have proposed interest-
ing approaches to overcome these problems.3
In 1997 we and Mikami’s group independently reported that
trimethylsilyl bistrifluoromethanesulfonimide (TMSNTf2) was a
much more efficient oxophilic catalyst than the corresponding tri-
flate (TMSOTf) for Diels–Alder cycloadditions, ene reactions and
Friedel–Crafts alkylations with alkenyloxysilanes.4a,5a,b This unpre-
dicted reversal of acidity sequence in going from the protic acid to
the trimethylsilyl derivative probably resulted from the size
difference of the two anions: the higher I-strain of TMSNTf2 ther-
modynamically favours the complexation with a smaller Lewis
base such as a carbonyl group. Our group also made the remark-
able observation that the Lewis acidity of trialkylsilyl triflimides
increased with the size of the alkyl groups in contrast to what had
always been observed for all other silylating agents.4b Yamamoto
reported that in situ-generated TMSNTf2 was a strong catalyst
for the Mukayama-aldol and Sakurai–Hosomi allylation reac-
tions.6 Recently several groups have elegantly demonstrated the
efficiency of Me3SiNTf2 as a catalyst for several carbon-carbon
bond-forming reactions.7 We also reported the first significant
asymmetric inductions in the Diels–Alder reactions of dienes with
a
,b-ethylenic esters catalyzed by silylated triflimides carrying a
chiral substituent on silicon.8 Very recently List reported high
asymmetric inductions in the Mukayama-aldol reaction catalyzed
by a chiral binaphthyl-derived disulfonimide.9
We anticipated that trialkylsilyl triflimides could be attractive
catalysts for allylations and benzylations of silyl nucleophiles: (1)
they are easily prepared in situ from cheap and commercially
available starting materials, (2) they have been shown to be good
activators of esters,4a (3) their catalytic activity can be tuned by
modifying the substituents at silicon4b and (4) they are tolerated
by many functional groups. We selected the p-methoxybenzyla-
tions of trimethyl cyclohexenyloxysilane 2 and allyl trimethysilane
3 with p-methoxybenzyl acetate 1 as model reactions (Table 1).
TMSNTf2 was compared to TMSOTf and a variety of Bronsted acids.
It was gratifying to observe that TMSNTf2 efficiently catalyzed the
reaction with both enolether 2 and allysilane 3 whether generated
in situ from the instantaneous reaction of the silylated nucleophile
with HNTf2 (entries 1 and 9) or prepared before use (2 and 10).
TMSOTf was less efficient (entries 3–5) as anticipated from the
* Corresponding author. Fax: +33 5 2000 2215.
E-mail address: tetrahedron@uclouvain.be (L. Ghosez).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.03.030

2572
O. Mendoza et al. / Tetrahedron Letters 51 (2010) 2571–2575
Table 1
lower acidity of silylated triflates versus the corresponding trifli-
mides.4a,b This also confirmed that the catalysts could not be the
corresponding Brönsted acids because triflic acid is a stronger acid
than triflimide and therefore expected to show a higher catalytic
activity. The other Brönsted acids generated trimethylsilyl deriva-
tives which were unable to catalyze the reaction (entries 6–8 and
14–16).
Model reactions for the p-methoxybenzylation of silyl enolether 2 (entries 1–8) and
allylsilane 3 (entries 9–16)a
O
5 mol% cat
TMS
H3CO
O
CH2Cl2
OAc
2
4
entries 1-8
We then examined the scope and limitations of these reactions
by varying both the substituent of the acetate and the nature of the
carbon nucleophile (Table 2). A 0.5 M solution of HNTf2 in dichlo-
romethane10 was added to the mixture of the two reagents. In a
few cases (entries 22–24 and 29) additional dichloromethane
was needed to favour the desired reaction over decomposition of
the electrophile. Yields shown in Table 2 were measured by com-
parison of the integration areas of one representative proton signal
of the product against C2HCl5 as internal standard. They were
found to be reliable when compared to the isolated yields. Entries
1–5 illustrate the catalytic efficiency of in situ-generated TMSNTf2
for the p-methoxybenzylation of various silyl nucleophiles. The
reaction was not very sensitive to steric hindrance as shown by
+
OCH3
5 mol% cat
CH2Cl2
H3CO
TMS
1
3
5
entries 9-16
Entry
Catalyst
t (min)
Yieldd (%)
a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
HNTf2
5
5
5
5
120
5
5
5
5
5
5
5
120
5
5
100
100
20
16
90
0
b
TMSNTf2
HOTf
TMSOTf
TMSOTf
CH3SO3H
CF3COOH
HClc
the facile alkylation of
a,
a0-disubstituted silyl ketene acetal 17
0
0
(95%) which created a quaternary carbon atom (entry 3). Allyl tri-
methylsilane 3 reacted equally well but vinyltrimethylsilane 18
yielded a complex mixture of products.
a
HNTf2
100
100
19
17
85
<1
0
b
TMSNTf2
HOTf
Less reactive electrophiles 6 and 7 did not react and a complex
mixture of unidentified products was observed (entries 6 and 13).
This probably resulted from an unfavourable competition between
the desired benzylation reaction and the silylation of the enolether
by the highly reactive TMSNTf2 as shown by a control experiment.
The catalyst could be easily tuned to overcome this problem: using
a TIPS enolether generates TIPSNTf2 in situ which had been shown
to be a weaker electrophile (kinetically controlled reaction) than
TMSNTf2 but a stronger Lewis acid (thermodynamically controlled
TMSOTf
TMSOTf
CH3SO3H
CF3COOH
HClc
5
0
a
HNTf2 was taken from a 0.5 M solution in CH2Cl2.
TMSNTf2 was prepared in situ from the addition of HNTf2 to allyl
trimethylsilane.
b
c
HCl was taken from a 4 M solution in dioxane.
Yields based on the integration of the aromatic peaks.
d
Table 2
Electrophilic allylations and benzylations of silyl carbon nucleophiles
Entry
1a
Electrophile
Nucleophile
Product
T (°C)
t (min)
% Cat
Yieldc (%)
93 (85)
OTMS
MeO
MeO
O
2
rt
20
5% HNTf2
OAc
1
4
OTMS
MeO
MeO
O
2a
rt
rt
20
20
5% HNTf2
5% HNTf2
93 (78)
95 (83)
16
21
OTMS
Me
Me
3a
OMe
17
MeOOC
22
TMS
MeO
3
4a
5a
6a
rt
rt
rt
20
5
5% HNTf2
5% HNTf2
5% HNTf2
95 (90)
5
TMS
18
Decomposition of anisyl acetate
0
0
OTMS
Complex mixture
20
2
OAc
OTIPS
6
O
7a
8a
100
100
20
1
5% HNTf2
85 (70)
90
19
23
20% HNTf2

O. Mendoza et al. / Tetrahedron Letters 51 (2010) 2571–2575
2573
Table 2 (continued)
Entry
Electrophile
Nucleophile
Product
T (°C)
t (min)
% Cat
Yieldc (%)
OTMS
Me
COOMe
OMe
17
Me
24
9a
No reaction
Decomposition
100
100
20
90
5% HNTf2
5% HNTf2
0
0
10a
TMS
TIPS
3
11a
12a
No reaction
Decomposition
100
100
20
90
5% HNTf2
5% HNTf2
0
0
20
OTMS
F
13a
14a
Complex mixture
rt
20
1
5% HNTf2
5% HNTf2
0
2
OAc
OTIPS
7
F
O
19
100
78 (70)
25
OTMS
F
Me
Me
15a
100
20
5% HNTf2
88 (80)
OMe
17
MeOOC
26
16a
17a
No reaction
Decomposition
100
100
20
300
5% HNTf2
5% HNTf2
0
0
TMS
TIPS
3
18a
No reaction
Decomposition
100
100
20
300
5% HNTf2
5% HNTf2
0
0
19a
20
OTMS
MeO
MeO
O
2
20a
rt
rt
20
20
5% HNTf2
5% HNTf2
95 (91)
90 (82)
OAc
MeO
MeO
8
MeO
27
OTMS
MeO
2
MeO
O
MeO
21a
OAc
MeO
9
MeO
28
O
OTMS
O
O
2
22b
23b
rt
rt
60
20
1.0% HNTf2
1% HNTf2
80 (70)
OAc
10
29
Decomposition
0
TMS
3
S
OTMS
S
O
2
24b
rt
rt
60
20
1.5% HNTf2
88 (79)
96 (89)
OAc
11
30
SO2Ph
SO2Ph
OTMS
N
N
2
O
25a
5% HNTf2
OAc
Tos
12
31
OTMS
Tos
N
N
2
O
26a
rt
20
5% HNTf2
95 (76)
13
OAc
32
OTMS
OAc
14
27a
28a
Complex mixture
No reaction
rt
20
20
5% HNTf2
5% HNTf2
0
0
2
90
(continued on next page)

2574
O. Mendoza et al. / Tetrahedron Letters 51 (2010) 2571–2575
Table 2 (continued)
Entry
Electrophile
Nucleophile
Product
T (°C)
t (min)
% Cat
Yieldc (%)
OTIPS
19
OTMS
O
OAc
15
29b
rt
20
5% HNTf2
80
2
33
a
b
c
Reaction conditions: 0.25 mmol of electrophile, 0.3 mmol of nucleophile.
Reaction conditions: 0.25 mmol of electrophile, 0.5 mmol of nucleophile, in 1 mL CH2Cl2.10
Yields calculated from the 1H NMR spectra, based; in brackets yields of pure product after chromatography.
reaction): this was expected to favour the reversible complexation
of the carbonyl ester over the silylation of the nucleophilic carbon
atom of the enolether. It was indeed gratifying to observe that the
reaction of 19 with 6 and 7 led to good yields of benzylated prod-
ucts (entries 7 and 14). Steric hindrance favoured the less bulky
benzyl electrophile: indeed the benzylation reaction worked well
References and notes
1. Reviews: (a) Reetz, M. T. Angew. Chem., Int. Ed. Engl. 1982, 21, 96–108; (b)
Kobayashi, S.; Wanabe, K.; Ishitani, H.; Matsuo, J.-I.. In Science of Synthesis,
Houben–Weyl Methods of Molecular Transformations; Fleming, I., Ed.; Thieme:
Stuttgart, 2002; Vol. 4, pp 317–369; (c) Fleming, I. Pure Appl. Chem. 1983, 55,
1707–1713; (d) Hosomi, A. Acc. Chem. Res. 1988, 21, 200–206; (e) Sarkar, T. K..
In Science of Synthesis, Houben–Weyl Methods of Molecular Transformations;
Fleming, I., Ed.; Thieme: Stuttgart, 2002; Vol. 4, See also: (f) Mayr, H.; Pock, R.
Tetrahedron 1986, 42, 4211–4214; (g) Dau-Schmidt, J. P.; Mayr, H. Chem. Ber.
1994, 127, 205; (h) Mayr, H.; Gorath, G.; Bauer, B. Angew. Chem., Int. Ed. Engl.
1994, 33, 788–789.
with
a,
a0-disubstituted silyl ketene acetal 17 (entries 8 and 15).
However, the less reactive allyl tri-i-propyl and allyl trimethylsil-
anes did not react at room temperature and yielded a complex
mixture at higher temperatures. (entries 9–12 and 16–19). As ex-
pected the presence of additional activating methoxy groups on
2. AgOTf: (a) Takeda, K.; Ayabe, A.; Kawashima, H.; Harigaya, Y. Tetrahedron Lett.
1992, 33, 951–952; (b) Black, T.; Stubbs, K. A.; Stick, R. V.; Weibel, J. M.; Pale, P.
e-EROS Encyclopedia of Reagents for Organic Synthesis, 2007.; BF3: (c) Cella, J. A. J.
Org. Chem. 1982, 47, 2125–2130; B(C6F5)3: (d) Rubin, M.; Gevorgyan, V. Org.
Lett. 2001, 3, 2705–2707; (e) Schwier, T.; Rubin, M.; Gevorgyan, V. Org. Lett.
2004, 6, 1999–2001; (f) De, S. K.; Gibbs, R. A. Tetrahedron Lett. 2005, 46, 8345–
8350; HN(SO2F)2: (g) Kaur, G.; Kaushik, M.; Trehan, S. Tetrahedron Lett. 1997,
38, 2521–2524; InBr3: (h) Kim, S. H.; Shin, C.; Pai, A. N.; Koh, H. K.; Chang, M. H.;
Chung, B. Y.; Cho, Y. S. Synthesis 2004, 1581–1584; InCl3 (i) Yasuda, M.; Saito,
T.; Ueba, M.; Baba, A. Angew. Chem., Int. Ed. 2004, 43, 1414–1416; (j) Saito, T.;
Nishimoto, Y.; Yasuda, M.; Baba, A. J. Org. Chem 2006, 71, 8516–8522; (k) Saito,
T.; Nishimoto, Y.; Yasuda, M.; Baba, A. J. Org. Chem. 2007, 72, 8588–8590;
LiClO4: (l) Ying, B.; Trogden, B. G.; Kohlman, D. T.; Liang, S. X.; Xu, Y. Org. Lett.
2004, 6, 1523–1526; Mg(ClO4)2: (m) Miyachi, H.; Nomura, M.; Tanase, T.;
Takahashi, Y.; Ide, T.; Tsunoda, M.; Murakami, K.; Awano, K. Bioorg. Med. Chem.
Lett. 2002, 12, 77–80; (n) Nomura, M.; Tanase, T.; Ide, T.; Tsunoda, M.; Suzuki,
M.; Uchiki, H.; Murakami, K.; Miyachi, H. J. Med. Chem. 2003, 46, 3581–3599;
(o) Usui, S.; Fujieda, H.; Suzuki, T.; Yoshida, N.; Nakagawa, H.; Miyata, N. Bioorg.
Med. Chem. Lett. 2006, 16, 3249–3254; (p) Usui, S.; Fujieda, H.; Suzuki, T.;
Yoshida, N.; Nakagawa, H.; Ogura, M.; Makishima, M.; Miyata, N. Chem. Pharm.
Bull. 2007, 55, 1053–1059; Organotin perfluorooctanesulfonates: (q) An, L.;
Peng, Z.; Orita, A.; Kurita, A.; Man-e, S.; Ohkubo, K.; Li, X.; Fukuzumi, S.; Otera, J.
Chem. Eur. J. 2006, 12, 1642–1647; rhodium complex: (r) Soga, T.; Takenoshita,
H.; Yamada, M.; Mukaiyama, T. Bull. Chem. Soc. Jpn. 1990, 63, 3122–3131; SnCl4/
ZnCl2: (s) Hayashi, M.; Inubushi, A.; Mukaiyama, T. Chem. Lett. 1987, 1975–
1978; ZrCl4: (t) Sharma, G. V. M.; Reddy, K. L.; Lakshmi, P. S.; Ravi, R.; Kunwar,
A. C. J. Org. Chem. 2006, 71, 3967–3969; ZnI2: (u) Reetz, M. T.; Walz, P.;
Huebner, F.; Huettenhain, S. H.; Heimbach, H. Chem. Ber. 1984, 117, 322–335;
(v) Collins, D. J.; Jacobs, H. A. Aust. J. Chem. 1986, 39, 2095–2110.
the phenyl ring led to excellent yields of
nones (entries 20 and 21).
a-benzylated cyclohexa-
Acetates derived from electron-rich heteroarene carbinols 10–
13 were also appropriate reagents for the delivery of an HetAr-
CH2 fragment on a silyl enolether (entries 22–26). Since acetates
derived from
a-hydroxymethyl pyrrole and b-hydroxymethyl in-
dole were rather unstable, we used the more stable N-sulfonylated
derivatives.
A simple allyl group could not be transferred (entries 27 and 28)
even if one uses a TIPS-derived carbon nucleophile at 90 °C. On the
other hand isoprenyl acetate reacted well under standard condi-
tions (entry 29).
We believe that these results demonstrated the efficiency of tri-
alkylsilyl triflimides as a powerful class of catalyst for the benzyla-
tion and allylation of various classes of silyl carbon nucleophiles.
The catalysts are generated in situ from the reaction of the com-
mercially available triflimide (1–5%) with the nucleophile. Interest-
ingly the catalytic activity can be tuned up by choosing the most
appropriate alkyl substituent on silicon. Yields are high and
work-up is easy. The precursors of the alkylating species are esters
which are non-genotoxic. In a control experiment we showed that
the corresponding benzyl alcohols could not be used since they de-
stroy the catalyst. Also the reactions do not involve any toxic me-
tal-containing catalyst. The reactions work particularly well with
electron-rich aromatic substituents. This is interesting since the
corresponding halides or tosylates are highly genotoxic. In most
cases the reactions can be performed without solvent.10 We believe
that this procedure should appeal to the synthetic chemists looking
for practical, safe and environmentally acceptable synthetic
methods.11
3. (a) Grieco, P. A.; Handy, S. T. Tetrahedron Lett. 1997, 38, 2645–2648; (b) Iovel, I.;
Mertins, K.; Kischel, J.; Zapf, A.; Beller, M. Angew. Chem., Int. Ed. 2005, 44, 238–
242. Angew. Chem., Int. Ed. 2005, 44, 3913–3917; (c) De Surya, K.; Richard, A. G.
Tetrahedron Lett. 2005, 46, 8345–8350; (d) Motokura, K.; Nakagiri, N.;
Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Org. Chem. 2007, 72, 6006–6015; (e)
Rubenbauer, P.; Herdtweck, E.; Strassner, T.; Bach, T. Angew. Chem., Int. Ed.
2008, 47, 10106–10109.
4. (a) Mathieu, B.; Ghosez, L. Tetrahedron Lett. 1997, 38, 5497–5500; (b) Mathieu,
B.; de Fays, L.; Ghosez, L. Tetrahedron Lett. 2000, 41, 9561–9564; See also: (c)
Simchen, G.; Jonas, S. J. Prakt. Chem. 1998, 340, 506–512.
5. (a) Ishii, A.; Kotera, O.; Saeki, T.; Mikami, K. Synlett 1997, 1145–1146; (b)
Mikami, K.; Kotera, O.; Motoyama, Y.; Tanaka, M. Inorg. Chem. Commun. 1998, 1,
10–11.
6. (a) Ishihara, K.; Hiraiwa, Y.; Yamamoto, H. Synlett 2001, 1851–1854; (b)
Ishihara, K.; Hiraiwa, Y.; Yamamoto, H. Chem. Commun. 2002, 1564–1565; (c)
Ishihara, K.; Hiraiwa, Y. Eur. J. Org. Chem. 2006, 1837–1844; See also: (d) Hara,
K.; Akiyama, R.; Sawamura, M. Org. Lett. 2005, 7, 5621–5623.
7. (a) Kuhnert, N.; Peverley, J.; Robertson, J. Tetrahedron Lett. 1998, 39, 3215–
3216; (b) Cossy, J.; Lutz, F.; Alauze, V.; Meyer, C. Synlett 2002, 45–48; (c) Yu, C.-
M.; Lee, J.-Y.; So, B.; Hong, J. Angew. Chem., Int. Ed. 2002, 41, 161–163; (d)
Mathieu, B.; Ghosez, L. Tetrahedron 2002, 58, 8219–8226; (e) Inanaga, K.;
Takasu, K.; Ihara, M. J. Am. Chem. Soc. 2005, 127, 3668–3669; (f) Othman, R. B.;
Bousquet, T.; Othman, M.; Dalla, V. Org. Lett. 2005, 7, 5335–5337; (g) Tranchant,
M. J.; Moine, C.; Othman, R. B.; Bousquet, T.; Othman, M.; Dalla, V. Tetrahedron
Lett. 2006, 47, 4447–4480; (h) Takasu, K.; Hosokawa, N.; Inanaga, K.; Ihara, M.
Acknowledgements
Funding by Sanofi-Aventis Research and Development and IECB
is gratefully acknowledged.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.03.030.

O. Mendoza et al. / Tetrahedron Letters 51 (2010) 2571–2575
2575
Tetrahedron Lett. 2006, 47, 6053–6056; (i) Jung, M. E.; Ho, D. G. Org. Lett. 2007,
9, 375–378; (j) Dudnick, A. S.; Gevorgyan, V. Angew. Chem., Int. Ed. 2007, 46,
5195–5197; (k) Takasu, K.; Miyakawa, Y.; Ihara, M.; Tokuyama, H. Chem. Pharm.
Bull. 2008, 56, 1205–1206; (l) Turks, M.; Vogel, P. J. Org. Chem. 2009, 74, 435–
437; (m) Commandeur, M.; Commandeur, C.; De Paolis, M.; Edmunds, A. J. F.;
Maienfisch, P.; Ghosez, L. Tetrahedron Lett. 2009, 50, 3359–3362.
8. Tang, Z.; Mathieu, B.; Tinant, B.; Dive, G.; Ghosez, L. Tetrahedron 2007, 63,
8449–8462. See also Ref. 2b.
9. Garcia-Garcia, P.; Lay, F.; Garcia-Garcia, P.; Rabalakos, C.; List, B. Angew. Chem.,
Int. Ed. 2009, 48, 4363–4366.
10. In our small-scale experiments we found more practical to use a solution of
HNTf2 in dichloromethane. For large-scale experiments, triflimide was
introduced as a solid. Therefore, one can say that in most cases, the reaction
can be performed in the absence of solvent.
11. Representative procedures: (a) Reaction of anisyl acetate with
allyltrimethylsilane: 250 L of a 0.5 M solution of HNTf2 in CH2Cl2 were added
to 5-mL round-bottomed flask equipped with magnetic stirbar and
containing 410 of anisyl acetate (2.5 mmol) and 480 of
l
a
a
lL
lL
allyltrimethylsilane (3.0 mmol). After 5 min the solvent was evaporated and
the crude residue was purified by column chromatography (silicagel, eluent:
cyclohexane/ethyl acetate 9:1) to obtain a pure yellow oil in 92% yield.
(b) Reaction of anisyl acetate with trimethyl cyclohexenyloxysilane: 250
0.5 M solution of HNNf2 in CH2Cl2 were added into a 5-mL round-bottomed
flask equipped with a magnetic stirbar, 410 L of anisyl acetate (2.5 mmol) and
583 L of trimethyl cyclohexenyloxysilane (3.0 mmol). After 5 min the solvent
lL of a
l
l
was evaporated and the crude mixture was purified by column
chromatography (silicagel, eluent: cyclohexane/ethyl acetate 9:1) to obtain a
pure colourless oil in 85% yield.