DMAP-organocatalyzed O-silyl-O-(or C-)-benzoyl interconversions by means of benzoyl fluoride

Article abstract of DOI:10.1055/s-2007-968028

A mild and efficient transprotection of alcohols from silyl ethers 1a-f to benzoates 2a-f is reported in fair to good yields (50-98%). This silyl-acyl exchange reaction proceeds readily in acetonitrile at room temperature in the presence of benzoyl fluoride and DMAP as an acyl transfer catalyst. A two-step 'one-pot' DMAP-catalyzed silylcyanation-transprotection sequence which gives the corresponding O-benzoyl cyanohydrines 2g-l in high yields (72-98%) from various benzaldehyde and ketone derivatives is also reported. This original organocatalytic acyl transfer process was also found to be effective in the O-benzoylation of trimethysilyl enolates 1m-o, providing enol esters 2m-o. Lastly, the potential of this strategy is also illustrated by a DMAP-mediated Claisen condensation between ketene silyl acetals 1p-r and benzoyl fluoride. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-968028

LETTER
381
DMAP-Organocatalyzed O-Silyl–O-(or C-)-Benzoyl Interconversions by
Means of Benzoyl Fluoride
D
MAP-Organo
h
catalyzed
O
-
o
Silyl–
O
-(or
C
m
-)-Benzoyl Interc
oa
nversions s Poisson,a Vincent Dalla,b Cyril Papamicaël,a Georges Dupas,a Francis Marsais,a Vincent Levacher*a
a
Laboratoire de Chimie Organique Fine et Hétérocyclique, UMR 6014, IRCOF, CNRS, Université et INSA de Rouen, B.P. 08, 76131
Mont-Saint-Aignan Cédex, France
Fax +33(2)35522962; E-mail: vincent.levacher@insa-rouen.fr
b
Laboratoire de Chimie, URCOM, EA 3221, Faculté des Sciences et Techniques de l’Université du Havre, 25, Rue Philippe Lebon,
BP 540, 76058 Le Havre Cédex, France
Received 14 November 2006
Although the common use of such a transprotection
strategy may represent a real advance in synthesis, only a
few reports address this issue, with the most represen-
tative ones referring to transacetalisation of dithianes to
Abstract: A mild and efficient transprotection of alcohols from si-
lyl ethers 1a–f to benzoates 2a–f is reported in fair to good yields
(50–98%). This silyl–acyl exchange reaction proceeds readily in
acetonitrile at room temperature in the presence of benzoyl fluoride
and DMAP as an acyl transfer catalyst. A two-step ‘one-pot’ O,O-acetals,2 conversion of N-benzyl to N-carbamate
derivatives,3 direct conversion of N-[(fluorenyl-
DMAP-catalyzed silylcyanation–transprotection sequence which
gives the corresponding O-benzoyl cyanohydrines 2g–l in high
methoxy)carbonyl] (Fmoc) to N-(tert-butoxycarbonyl)
yields (72–98%) from various benzaldehyde and ketone derivatives
(Boc),4 and interconversion of silyl ethers into acetates.5
is also reported. This original organocatalytic acyl transfer process
Obviously, the scarcity of literature reports does not
was also found to be effective in the O-benzoylation of trimethysilyl
match the great benefit that such interconversions could
have in synthesis. In this context, we turned our interest in
the one-step conversion of alcohols from silyl ethers into
acetates. The reported catalytic procedures make use of
Lewis acids such as Tin(II) bromide,5a Cu(OTf)2,5b
FeCl3,5c Sc(OTf)3,5e and FeCl3.5f Many of these methods
suffer from modest yields and lack of selectivity. With the
aim of developing a new catalytic strategy under mild,
neutral and more environmentally friendly metal-free
conditions, we sought to examine the use of 4-(dimethyl-
amino)pyridine (DMAP) as an organocatalyst in silyl–
acyl exchange reactions. DMAP is routinely used as
nucleophilic acyl transfer catalyst in the acylation of alco-
hols and related transformations.6 The currently accepted
mechanism involves the formation of a N-acylpyridinium
salt as the key reactive intermediate.6a,7 It is well known
that during acylation of alcohols, DMAP is more effective
when used in combination with anhydrides rather than
with acid chlorides, pointing out a probable assistance of
the acetate counterion in the deprotonation of the alco-
hol.6a,7 With this scenario in mind, we questioned whether
the reactivity of these electrophilic N-acylpyridinium salts
could be controlled by tuning the nature of the counterion.
As a result from the high affinity of silicon for the fluoride
ion,8 we indeed anticipated a specific activation of the ‘na-
ked-alcoholate’ silyl ether 1 through the uncommon use of
an acid fluoride as the acylating agent. This specific oxa-
activation mode should ultimately promote a selective
silyl–acyl exchange reaction (Figure 2).
enolates 1m–o, providing enol esters 2m–o. Lastly, the potential of
this strategy is also illustrated by a DMAP-mediated Claisen con-
densation between ketene silyl acetals 1p–r and benzoyl fluoride.
Key words: organocatalysis, DMAP, transprotection, acid fluo-
rides, silyl ethers, benzoylation, alcohols, cyanohydrins, Claisen
condensation
The need for highly selective transformations in organic
synthesis has stimulated chemists to develop a wide range
of protecting groups for the preparation of multifunctional
molecules.1 Since a given protective group can, on rare
occasions, survive several synthetic steps, recurrent costly
and time-consuming deprotection–reprotection sequences
are frequently required in the construction of sophisticat-
ed molecules. Straightforward methodologies enabling
the transformation of a protecting group into another one
in mild, and preferably, one-pot procedures are therefore
highly appealing tools in organic synthesis due to the sim-
ple fact that both deprotection–reprotection steps, gener-
ally needed in a classical approach, are replaced by a
single transformation (Figure 1).1
F
deprotection
F-PG1
reprotection
F-PG2
transprotection
Figure 1 Transprotection methodology: a useful alternative to the
On the basis of this working hypothesis, various benzoyl-
ating agents were first examined to ascertain the crucial
role played by the fluoride counterion of the N-acyl
pyridinium. Thus, silyl ether 1a9 was reacted separately
with benzoyl fluoride, benzoic anhydride and benzoyl
chloride in the presence of DMAP (10 mol%) in acetoni-
trile at room temperature (Table 1). While transprotection
deprotection–reprotection classical sequence
SYNLETT 2007, No. 3, pp 0381–0386
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5.
0
2.
2
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0
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Advanced online publication: 07.02.2007
DOI: 10.1055/s-2007-968028; Art ID: G33706ST
© Georg Thieme Verlag Stuttgart · New York

382
T. Poisson et al.
LETTER
Benzoate 2a (%)
100
N
R1COF
ROSiR3
10% of DMAP
5% of DMAP
1
DMAP
N+
F–
O
SiR3
80
R1
O
R
SiR3F
ROCOR1
2
DMAP
(X mol%)
60
40
20
1a
2a
Figure 2 DMAP-organocatalyzed silyl–acyl exchange reaction
PhCOF
(1 equiv)
MeCN
proceeded smoothly with benzoyl fluoride, no reaction
could be detected when using benzoyl chloride, whereas
the use of benzoic anhydride led to an incomplete conver-
sion within the same period of time (Table 1, entries 1–3).
These observations confirm the expected requirement of a
fluoride counterion for the specific activation of silyl
ethers. No reaction took place in the absence of DMAP,
even after prolonged reaction time (>6 h), indicating that
this was indeed an organocatalyzed reaction.
1% of DMAP
time (h)
2
4
6
8
12
Figure 3 Influence of DMAP loading on the rate of the reaction
Benzoate 2a (%)
Table 1 Silyl–Acyl Exchange Reaction from 1a by Means of
Various Benzoylating Agents
MeCN
100
80
OCOPh
OTMS
PhCOX (1 equiv),
r.t., MeCN
DMAP
(10 mol%)
CH2Cl2
2a
1a
Acetone
60
40
20
Entry
1
PhCOX
Reaction time (h)
Yield of 2a (%)a
DMAP
(10 mol%)
PhCOF
1
3
5.5
28
73
100
1a
2a
PhCOF
(1 equiv)
2
3
(PhCO)2O
PhCOCl
2
5.5
26
49
THF
time (h)
5.5
0
2
4
6
8
12
a Yield of isolated products.
Figure 4 Solvent survey for the transprotection of silyl ether 1a into
benzoate 2a
Having evidenced the catalytic efficiency of DMAP in
this silyl–benzoyl exchange reaction, we next focused on
reaction parameters that can influence the reaction rate.
We next explored the scope of this interconversion pro-
cess with various silyl ethers 1a–e under optimized condi-
tions (Table 2).11 In all the cases examined, quantitative
conversions were obtained and the resulting benzoates
2a–e were isolated in fair to excellent yields. Increasing
the steric demand of the silicon led to a significant dimi-
nution of the reaction rate as demonstrated by the use of
TES ether 1b (Table 2, entry 1, 2). Interestingly, when the
reaction was performed from bis-TMS ether 1d,
monotransprotection took place exclusively at the prima-
ry TMS ether affording 2d in 50% yield (Table 2, entry 4).
Likewise, it was observed that primary TMS ethers could
be selectively transprotected in the presence of phenolic
TMS ether (Table 2, entry 6). However, prolonged reac-
tion times led to transprotection of aryl TMS ether in good
yields as well (Table 2, entry 5).
Reducing the catalyst loading to 5 mol% did not signifi-
cantly affect the reaction time, with the silyl–acyl ex-
change being completed within 7 hours. However, the
transprotection progressed very sluggishly in the presence
of 1 mol% of DMAP, affording 2a in only 23% yield
within 5 hours (Figure 3). Further investigations on the
solvent employed indicated that higher conversion rates
were obtained in polar solvents. Whereas acetonitrile af-
forded the transprotected product 2a in quantitative yield
within 5.5 hours, less polar solvents such as THF revealed
to be poorly effective, giving 2a in only 15% yield.10 At-
tempts to use acetone and CH2Cl2 gave moderate conver-
sion rates, although still satisfactory compared to THF
(Figure 4).
We then turned our interest into the transprotection of
O-trimethylsilyl cyanohydrins. This class of compounds
is usually prepared by silylcyanation of activated carbonyl
Synlett 2007, No. 3, 381–386 © Thieme Stuttgart · New York

LETTER
DMAP-Organocatalyzed O-Silyl–O-(or C-)-Benzoyl Interconversions
383
Table 2 Scope of Silyl Ethers (1a–e)–Benzoates (2a–e) Transprotection
PhCOF (1 equiv),
r.t., MeCN
R
OSiMe3
R
OCOPh
DMAP
1a–f
(10 mol%)
2a–f
Entry
1
Silyl ether 1
Benzoate 2
Yield (%)a
96
Time (h)
5.5
OCOPh
OTMS
Me
Me
2a
2a
1a
1b
2
3
17
42
65
2
5
9
OTES
Me
OCOPh
Me
96
6
8
OCOPh
OCOPh
Me
Me
1c
2c
4
50b
OH
OTMS
OCOPh
OTMS
2d
2e
2f
1d
1e
1f
5
6
98
12
8
OTMS
OCOPh
62b
OTMS
OTMS
OCOPh
OH
a Yield of isolated products.
b Hydrolysis of the residual silyl ether protecting group occurred during the work-up of the reaction.
compounds. Various catalytic methods have been devel- Methods for clean and selective O-acylation of ketone-de-
oped including nucleophilic activation of TMSCN. rived enolates are lacking. Although silyl–acyl exchange
Recently, DMAP was found to be effective in the silyl- reaction from silyl enol ethers may afford an attractive
cyanation of aldehydes.12 Therefore, we speculated that alternative to prepare enol esters under neutral conditions,
DMAP could possibly catalyze both catalytic silyl- only a few papers have reported mild and simple proce-
cyanation–transprotection steps in a ‘one-pot’ procedure. dures.14 In this context, our catalytic conditions would
A series of aryl aldehydes were subjected to the silyl- make available a new procedure under neutral and mild
cyanation–transprotection sequence in acetonitrile in the conditions for such interesting interconversions. The
presence of 10 mol% of DMAP at room temperature. effectiveness and the future potential of this approach
After addition of TMSCN, the reaction mixture was have been illustrated by the conversion of silyl enol ethers
stirred for 1 hour and subsequent monitoring by GC-MS 1m–o into enol esters 2m–o. Employing optimal condi-
analyses revealed complete conversion for the various tions,11 DMAP was found to catalyze the desired silyl enol
aldehydes examined to the O-trimethylsilyl cyanohydrins ether–enol benzoate interconversion in 70–84% yields,
1g–k. The transprotection was next accomplished in the respectively, free from competing C-benzoylation prod-
same pot by adding benzoyl fluoride affording after 12 ucts (Table 4).15
hours the desired O-benzoyl cyanohydrins 2g–k in 72–
98% over the two steps (Table 3, entries 1–5). Under
identical reaction conditions, the less electrophilic cyclo-
In a last set of experiments, we briefly considered the
N-acyl pyridinium fluoride intermediate as a potential
acyl acceptor in the Claisen condensation with ketene silyl
acetals. Beside the conventional method using strong
hexanone gave, without apparent loss of reactivity, 2l in
85% yield (Table 3, entry 6).13
bases, alternative approaches making use of ketene silyl
Synlett 2007, No. 3, 381–386 © Thieme Stuttgart · New York

384
T. Poisson et al.
LETTER
Table 3 ‘One-pot’ Silylcyanation–Transprotection Sequence
PhCOF (1 equiv),
r.t., MeCN
TMSCN (1 equiv),
r.t., MeCN
OSiMe3
CN
OCOPh
ArCHO
Ar
Ar
CN
2g–l
DMAP
(10 mol%)
DMAP
(10 mol%)
1g–l
Entry
1
Silyl ether 1
Benzoate 2
Yield from ArCHO (%)a
80b
OTMS
OCOPh
CN
CN
1g
2g
2
3
4
5
6
72
98
79
72
85
OTMS
CN
OCOPh
CN
MeO
MeO
1h
2h
OTMS
CN
OCOPh
CN
MeOOC
MeOOC
1i
2i
OTMS
CN
OCOPh
CN
N
N
1j
2j
OTMS
OCOPh
CN
CN
S
S
1k
2k
NC
NC
OTMS
OCOPh
1l
2l
a Yield of the isolated products.
b Transprotection from enantiopure 1g provided 2g without any significant loss of optical purity, demonstrating that this silyl–acyl exchange
reaction is a non-racemizing transprotection process.
acetals and acid chlorides or anhydrides16,17a usually sion is catalyzed by DMAP and proceeds through what
afford crossed-Claisen condensation products in better appears to be an activation of the silyl ethers 1 by the flu-
yields and selectivity with respect to the undesired self- oride counterion of the N-acyl pyridinium intermediate.
Claisen condensation products which are generally This silyl–acyl exchange reaction was also found to be ef-
formed in the base-promoted approach. Our strategy fective in the conversion of silyl enol ethers 1m–o into
would therefore extend this efficient approach to the use enol esters 2m–o. Lastly, this acyl-transfer process opens
of the more stable acid fluorides under neutral conditions. the route to the development of new Claisen condensation
Interestingly, under our optimized catalytic conditions,11 procedures under mild and neutral conditions. An attrac-
benzoyl fluoride was found to react with ketene silyl ace- tive perspective of this organocatalytic transprotection of
tals 1p–r to afford b-keto esters 2p–r in 62–73% yields. alcohols is the resolution of chiral racemic silyl ethers 1
As previously demonstrated with other substrates 1a–n, by means of chiral DMAPs.17
the catalytic activity of DMAP was confirmed by con-
ducting a blank experiment where formation of the Clais-
en condensation product 2p was not observed (Table 5).
Acknowledgment
We thank the CNRS, the région Haute-Normandie and the
CRIHAN for financial and technical support. T.P. thanks the MRT
In summary, we have developed a simple, room tempera-
ture and metal-free catalytic transprotection of alcohols
for a grant.
from silyl ethers 1a–f to benzoates 2a–f. This interconver-
Synlett 2007, No. 3, 381–386 © Thieme Stuttgart · New York

LETTER
DMAP-Organocatalyzed O-Silyl–O-(or C-)-Benzoyl Interconversions
385
(3) Rawal, V. H.; Michoud, C.; Monestel, R. F. J. Am. Chem.
Soc. 1993, 115, 3030.
Table 4 DMAP-Catalyzed O-Benzoylation of Silyl Enol Ethers
1m–o with Benzoyl Fluoride
(4) (a) Roos, E. C.; Bernabé, P.; Hiemstra, H.; Speckamp, W. N.
J. Org. Chem. 1995, 60, 1733. (b) Furlán, R. L. E.; Mata, E.
G. ARKIVOC 2003, (x), 32–40. (c) Furlan, R. L. E.; Mata,
E. G. Tetrahedron Lett. 1998, 39, 6421.
OSiMe3
OCOPh
PhCOF (1 equiv),
r.t., MeCN, 12 h
DMAP (10 mol%)
(5) (a) Oriyama, T.; Oda, M.; Gono, J.; Koga, G. Tetrahedron
Lett. 1994, 35, 2027. (b) Chandra, K. L.; Saravanan, P.;
Singh, V. K. Tetrahedron Lett. 2001, 42, 5309. (c) Ganem,
B.; Small, V. R. Jr. J. Org. Chem. 1974, 39, 3728.
(d) Stamatov, D. S.; Kullberg, M.; Stawinski, J. Tetrahedron
Lett. 2005, 46, 6855. (e) Norsikian, S.; Holmes, I.; Lagasse,
F.; Kagan, H. Tetrahedron Lett. 2002, 43, 5715. (f)Harjani,
J. R.; Nara, S. J.; Salunkhe, M. M. Nucleosides, Nucleotides
Nucleic Acids 2005, 24, 819.
(6) (a) Höfle, G.; Steglich, W.; Vorbruggen, H. Angew. Chem.,
Int. Ed. Engl. 1978, 17, 569. (b) Hassner, A.; Krepski, L. R.;
Alexanian, V. Tetrahedron 1978, 34, 2069. (c) Murugan,
R.; Scriven, E. F. V. Aldrichimica Acta 2003, 36, 21.
(d) Scriven, E. F. V. Chem. Soc. Rev. 1983, 12, 129.
(7) (a) Spivey, A. C.; Arseniyadis, S. Angew. Chem. Int. Ed.
2004, 43, 5436. (b) Xu, S.; Held, H.; Kempf, B.; Mayr, H.;
Steglich, W.; Zipse, H. Chem. Eur. J. 2005, 11, 4751.
(8) (a) Chuit, C.; Corriu, R. J. P.; Reye, C. Chem. Rev. 1993, 93,
1371. (b) Rendler, S.; Oestreich, M. Synthesis 2005, 1727.
(9) General Procedure for the Preparation of Silyl Ethers
1a–f.
1m–o
2m–o
Entry Silyl enol ethers 1m–o Enol benzoates 2m–o Yield (%)
1
2
3
84
72
70
OSiMe3
OCOPh
2m
1m
OSiMe3
OCOPh
1n
1o
2n
2o
OCOPh
OCOPh
To a solution of 1-phenylethanol (122 mg, 1 mmol) and Et3N
(120 mg, 1.1 mmol) in CH2Cl2 (5 mL) was added TMSCl
(115 mg, 1.05 mmol). The reaction was stirred at r.t.
overnight. The solvent was removed under reduced pressure
and the resultant residue was diluted with pentane (10 mL).
Simple filtration through a short pad of Celite® provided
silyl ether 1a in 95% yield which could be used in the silyl
ether exchange reaction without further purification.
(10) One can notice that acylation of alcohols with DMAP/AC2O
is reported to be more effective in less polar solvents; see ref.
6. There is apparently no rational explanation for this
opposite solvent effect.
Table 5 DMAP-Catalyzed Claisen Condensation Between Ketene
Silyl Acetals 1p–r and Benzoyl Fluoride
PhCOF (1 equiv),
r.t., MeCN, 12 h
O
O
OTMS
O
Ph
O
DMAP (10 mol%)
2p–r
1p–r
62–73% yield
Entry
Ketene silyl acetals
Claisen condensation Yield (%)
1p–r
products 2p–r
(11) General Procedure for the Transprotection of Silyl
Ethers 1a–f, Silyl Enol Ethers 1m–o and Ketene Silyl
Acetals 1p–r.
OSiMe3
OMe
O
O
O
O
1
2
3
65
73
62
Ph
OMe
To a solution of silylated alcohol 1f (268 mg, 1 mmol) and
DMAP (12 mg, 0.1 mmol) in MeCN (2 mL) was added
benzoyl fluoride (130 mg, 1.05 mmol). The resulting
solution was stirred for 6 h at r.t., followed by addition of sat.
aq NaHCO3 (4 mL). After phase separation, the aqueous
phase was extracted with EtOAc (3 × 10 mL). The combined
organic layers were washed with sat. aq NH4Cl (10 mL) and
brine (10 mL). After drying (MgSO4) and evaporation of the
organic solvents under vacuum, the resulting residue was
chromatographed on silica gel (cyclohexane–EtOAc, 9:1)
affording benzoate 2f in 62% yield.
2p
1p
1q
OSiMe3
O
O
O
Ph
O
2q
Me3SiO
OMe
OMe
Ph
Selected Data for Benzoate 2f.
1H NMR (300 MHz, CDCl3): d = 8.01 (s, 1 H), 7.97 (d, 2 H,
J = 8.3 Hz), 7.46 (t, 1 H, J = 5.4 Hz), 7.27 (m, 3 H), 7.16 (m,
1 H), 6.85 (m, 2 H), 5.27 (s, 2 H). 13C NMR (75 MHz,
CDCl3): d = 168.56, 155.39, 133.52, 131.89, 130.91, 129.86,
129.28, 128.40, 121.88, 120.55, 117.38, 63.53. IR (KBr):
1r
2r
References and Notes
max = 1693, 3317 cm–1. Mp 64–66 °C.
Selected Data for 2q.
(1) Kociensky, P. Protecting Groups, 3rd ed.; Georg Thieme
Verlag: Stuttgart, 2004.
n
1H NMR (300 MHz, CDCl3): d = 7.95 (d, 2 H, J = 7.9 Hz),
7.52 (m, 1 H), 7.44 (t, 2 H, J = 7.9 Hz), 4.40 (t, 2 H, J = 6.9
Hz), 3.05 (m, 1 H), 2.22 (m, 1 H), 1.67 (s, 3 H). 13C NMR
(75 MHz, CDCl3): d = 196.65, 177.48, 134.88, 133.40,
(2) (a) Stork, G.; Zhao, K. Tetrahedron Lett. 1989, 30, 287.
(b) Nicolaou, K. C.; Baran, P. S.; Zhong, Y.-L.; Choi, H.-S.;
Yoon, W. H.; He, Y.; Fong, K. C. Angew. Chem. Int. Ed.
1999, 38, 1669. (c) Jones, T. K.; Reamer, R. A.; Desmond,
R.; Mills, S. G. J. Am. Chem. Soc. 1990, 112, 2998.
(d) Aoyagi, S.; Wang, T.-C.; Kibayashi, C. J. Am. Chem.
Soc. 1993, 115, 11393.
129.50, 128.87, 66.39, 55.30, 35.70, 22.01. IR (KBr): nmax
=
1766, 1678 cm–1.
Synlett 2007, No. 3, 381–386 © Thieme Stuttgart · New York

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T. Poisson et al.
LETTER
Selected Data for O-Benzoyl Cyanohydrine (2i).
Selected Data for 2r.
1H NMR (300 MHz, CDCl3): d = 7.78 (d, 2 H, J = 7.3 Hz),
7.49 (m, 1 H), 7.38 (m, 2 H), 3.63 (s, 3 H), 2.09 (t, 4 H,
J = 6.2 Hz), 1.53 (m, 6 H). 13C NMR (75 MHz, CDCl3): d =
198.56, 174.71, 136.50, 132.67, 128.73, 128.62, 58.35,
52.72, 32.68, 25.71, 22.58. IR (KBr): 1735, 1682 cm–1. All
other synthesized compounds are in accordance with the
literature data.
1H NMR (300 MHz, CDCl3): d = 8.05 (d, 2 H, J = 7.3 Hz),
7.56 (m, 3 H), 7.45 (m, 2 H), 6.97 (d, 2 H, J = 8.6 Hz), 6.62
(s, 1 H), 3.84 (s, 3 H). 13C NMR (75 MHz, CDCl3): d =
166.43, 164.85, 136.66, 134.70, 132.40, 130.89, 130.51,
129.14, 128.27, 128.15, 116.17, 63.19, 52.86. IR (KBr):
1732, 1729 cm–1. All other synthesized O-benzoyl
cyanohydrins are in accordance with the literature data.
(14) (a) Ito, H.; Ishizuka, T.; Tateiwa, J.-I.; Hosomi, A.
Tetrahedron Lett. 1998, 39, 6295. (b) Limat, D.; Schlosser,
M. Tetrahedron 1995, 51, 5799.
(12) Denmark, S. E.; Chung, W.-J. J. Org. Chem. 2006, 71, 4002.
(13) General ‘One-Pot’ Procedure for the Preparation of
O-Benzoyl Cyanohydrins 2g–l.
To a solution of methyl 4-formylbenzoate (164 mg, 1 mmol)
and DMAP (12 mg, 0.1 mmol) in MeCN (2 mL) was added
TMSCN (241 mg, 1.05 mol). The resulting solution was
stirred for 1 h at r.t., after which benzoyl fluoride (130 mg,
1.05 mmol) was added and the solution was stirred for a
further 12 h. The solution was treated with sat. aq NaHCO3
(4 mL). After dilution with H2O, the aqueous phase was
extracted with EtOAc (3 × 10 mL). The combined organic
phases were washed with sat. aq NH4Cl (10 mL), brine (10
mL) and dried (MgSO4). The solvents were evaporated
under vacuum and the residue was chromatographed on
silica gel (cyclohexane–EtOAc, 9:1).
(15) In contrast to this result, acylation of ketone silyl enol ethers
under similar conditions (acetyl fluoride/boron trifluoride in
nitromethane at –35 °C) was reported to afford mainly the C-
acylated product. See: Kopka, I.; Rathke, M. W. J. Org.
Chem. 1981, 46, 3772.
(16) Iida, A.; Takai, K.; Okabayashi, T.; Misaki, T.; Tanabe, Y.
Chem. Commun. 2005, 3173.
(17) (a) Mermerian, A. H.; Fu, G. C. J. Am. Chem. Soc. 2003,
125, 4050. (b) For a recent contribution from our group in
the chemistry of chiral DMAPs, see: Poisson, T.; Penhoat,
M.; Papamicaël, C.; Dupas, G.; Dalla, V.; Marsais, F.;
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Synlett 2007, No. 3, 381–386 © Thieme Stuttgart · New York