Toward improving the chemistry of N-acyliminium ions: Nucleophilic substitution reactions of pyrrolidinone derivatives with trialkylsilyl nucleophiles catalyzed by triisopropylsilyltrifluoromethane sulfonate (TIPSOTf)

Article abstract of DOI:10.1021/ol050576z

(Chemical Equation Presented) Nucleophilic substitution reactions of racemic and chiral 5-acetoxy-, 5-ethoxy-, and 5-methoxypyrrolidin-2-ones by silicon-based nucleophiles were efficiently catalyzed by TIPSOTf. This process was found to be general and acc

Full text of DOI:10.1021/ol050576z

ORGANIC
LETTERS
2005
Vol. 7, No. 14
2825-2828
Toward Improving the Chemistry of
N-Acyliminium Ions: Nucleophilic
Substitution Reactions of Pyrrolidinone
Derivatives with Trialkylsilyl
Nucleophiles Catalyzed by
Triisopropylsilyltrifluoromethane
Sulfonate (TIPSOTf)
Raja Ben Othman, Till Bousquet, Anthony Fousse, Mohamed Othman, and
Vincent Dalla*
Unite´ de Recherche en Chimie Organique et Macromole´culaire, Faculte´ des
Sciences et Techniques de l’UniVersite´ du HaVre, 25 rue Philippe Lebon, BP 540,
76058 Le HaVre Cedex, France
Vincent.dalla@uniV-lehaVre.fr
Received March 16, 2005
ABSTRACT
Nucleophilic substitution reactions of racemic and chiral 5-acetoxy-, 5-ethoxy-, and 5-methoxypyrrolidin-2-ones by silicon-based nucleophiles
were efficiently catalyzed by TIPSOTf. This process was found to be general and accommodates a broad range of substrate
−nucleophile
combinations.
Since the discovery by the Speckamp group in the middle
of the 1970s that cyclic imides can readily be converted into
endocyclic N-acyliminium ions,1 the latter have been in-
volved in an impressive number of synthetic applications,
mainly devoted to the synthesis of biologically important
nitrogen-containing cyclic compounds.2 Of particular interest
in this context are the intermolecular nucleophilic substitution
reactions of cyclic N-acyliminium ion precursors with
trialkylsilyl nucleophiles (R-amido alkylations) through
activation by a Lewis acid. When classical Lewis acids (BF3‚
Et2O, SnCl4, TiCl4, ZnBr2, ...) are used, stoichiometric
amounts and often more of these reagents are required to
achieve synthetically useful results.3 To improve further the
potential of these alkylation reactions, the development of
processes compatible with the use of reduced amounts of
the activator is highly desirable. Trimethylsilyltrifluo-
romethane sulfonate (TMSOTf) is a peculiar Lewis acid in
regard to these nucleophilic substitution reactions of cyclic
acyliminium precursors owing to regeneration during the
mechanism. Hence, a catalytic version can be envisioned with
this promoter. Surprisingly, there have been very few
reported examples of such catalytic reactions4 since the first
report by Barrett and co-workers in 1981.4a Moreover, the
(1) Hubert, J. C.; Wijnberg, J. B. P. A.; Speckamp, W. N. Tetrahedron
1975, 31, 1437.
(2) (a) Speckamp, W. N.; Moolenaar, M. J. Tetrahedron 2000, 56, 3817.
(b) Maryanoff, B. E.; Zhang, H.-C.; Cohen, J. H.; Turchi, I. J.; Maryanoff,
C. A. Chem ReV. 2004, 104, 1431.
(3) Acetoxy azetidinones are an exception to this rule and can be alkylated
by silicon-based nucleophiles under the catalytic activation of conventional
Lewis acids. For a representative example, see: Fuentes, L. M.; Shinkai,
I.; Salzmann, T. N. J. Am. Chem. Soc. 1986, 108, 4675.
10.1021/ol050576z CCC: $30.25
© 2005 American Chemical Society
Published on Web 06/04/2005

Table 1. Effect of Leaving Groupa
Table 2. Reactions with Various Nucleophiles
time
yield
(%)
run
NuSiR3 (equiv)
(min)
product (2)
1
2
CH2dC(Ph)(OSiMe3)(1.17)
CH2dC(t-Bu)(OSiMe3)(1.17)
15 2c(Nu ) CH2COPh)
15 2d(Nu ) CH2COt-Bu) 87
30 2e
15 2f(Nu ) CN)
150 2g(Nu ) H)
89
3a CH2dCHCH2SiMe3(1.2)
88
82
87
4
NCSiMe3(1.17)
run
time
TIPSOTf yield of 2 (%) yield of 3 (%)
5b HSiEt3(1.17)
1
1a 20 min 5 mol %
1a 20 min 5 mol %
1a 20 min 2 mol %
2a (75)
2a (75)
2a (75)
2a (75)
2a (45)
2b (76)
3a (15)b
3a (15)b
3a (17)b
3a (17)b
3a (0)
6b,c CH2dC(OMe)(OTBS)(1.4)b,c
40 2h(Nu ) CH2CO2Me) 45
2c
3
7b Me2CdC(OMe)(OSiMe3)(1.4) 720 no reaction
a 5 mol % of TIPSOTf was used. b The temperature was slightly raised
to room temperature. c Total consumption of 1a was achieved; a second
product (unidentified) was formed.
4
5d
6
1b 1 h
1c 12 h
5 mol %
16 mol %
1d 20 min 16 mol %
3b (13)e
a Reactions were carried out using 1a (0.8 mmol) and the silyl enol ether
(1.15 equiv), in dichloromethane at 0 °C, unless otherwise noted. b The
F-C adduct 3b was obtained as a mixture of either geometrical Z and E
stereoisomers (exclusive trans relationship for the substituents located at
the C2-C3 bond) or as a mixture of trans and cis stereoisomers (single
geometrical isomer for the double bond, E or Z) in a 4:1 ratio (stereochem-
istry not determined). c The reaction was performed on a 1 g scale. d Yield
estimated by 1H NMR spectroscopy on the crude mixture (55% 1c
recovered). e The FC adduct 3a was obtained as a mixture of either
geometrical Z and E stereoisomers (exclusive trans relationship for the
substituents located at the C2-C3 bond) or as a mixture of trans and cis
stereoisomers (single geometrical isomer for the double bond, E or Z) in a
4:1 ratio (stereochemistry not determined).
context of [4 + 2] cycloaddition reactions of dienes with
enoates. They demonstrated that N-triisopropylsilyltrifluo-
romethane sulfonimide Pr3SiNTf2 exhibits higher Lewis
i
acidity than its trimethylsilyl analogue.7 By analogy, we
anticipated that the high bulkiness of TIPSOTf would not
impede the formation of N-acyliminium ions and that low
catalyst loading (<10 mol %) may be envisioned regarding
its presumed high Lewis acid properties. We report herein
efficient nucleophilic substitution reactions of various race-
mic and optically pure five-membered ring N-acyliminium
ion precursors using TIPSOTf as a catalyst.8 The scope of
the process has been demonstrated by the evaluation of a
large array of donor-acceptor combinations.
number of truly catalytic reactions (e10 mol %) is few.4a,c,g-i,l
A landmark contribution has been recently made by the
Kobayashi group, who studied the effect of several metal
triflates (10 mol %) in such reactions using mainly 2-methoxy
and 2-acyloxy piperidine derivatives as acyliminium cation
precursors.5
On the other hand, TMSOTf is a very air- and moisture-
sensitive reagent that must be used freshly distilled. This
drawback of the TMSOTf reagent causes a severe limitation
from a practical point of view. As a result of its superior
stability, we realized that TIPSOTf could be advantageously
used in place of its TMS analogue in such catalytic
nucleophilic substitution reactions.6 This idea capitalizes on
recent observations made by Ghosez and co-workers in the
We selected lactam derivatives 1a-d (Tables 1 and 2),
4a-d (Table 3), and 7a-d (Table 4) as acyliminium cation
precursors. They were synthesized by conventional methods.
5-Acetoxylactams 1a and 4a-d were prepared using our one-
pot reduction-acetylation sequence.9
At the outset, we chose the 3,4-fused benzo N-allyl-5-
acetoxy pyrrolidinone 1a as one of the simplest acyliminium
cation precursors and the triisopropylsilyl enol ether derived
from butynone as a nucleophile to verify the competence of
TIPSOTf as a catalyst10 (Table 1).
We were pleased to observe that, in the presence of only
5 mol % of TIPSOTf, 1a was totally consumed within a
(5) (a) Okitsu, O.; Suzuki, R.; Kobayashi, S. Synlett 2000, 989. (b) Okitsu,
O.; Suzuki, R.; Kobayashi, S. J. Org. Chem. 2001, 66, 809.
(6) When carefully stored under argon at -20 °C, we observed that
TIPSOTf could be used with equal success over several months.
(7) Matthieu, B.; de Fays, L.; Ghosez, L. Tetrahedron Lett. 2000, 41,
9561.
(8) We are aware of one example of the use of TIPSOTf as catalyst (5
mol %) in a vinylogous Mannich reaction between a highly substituted
2-OTMS-furan derivative and N-BOC-2-methoxy proline methyl ester to
give the adduct in a poor yield of 32%. See: (a) Martin, S. K.; Barr, K. J.
J. Am. Chem. Soc. 1996, 118, 3299. (b) Martin, S. K.; Barr, K. J.; Smith,
D. W.; Bur, S. K. J. Am. Chem. Soc. 1999, 121, 6990.
(9) Szemes, F.; Fousse, A.; Ben Othman, R.; Bousquet, T.; Othman, M.;
Dalla, V. Unpublished results.
(10) Preliminary mechanistic experiments tend to suggest that TIPSOTf
does not dissociate and by the way is the real catalyst in the process; more
details are given in Supporting Information.
(4) (a) Barrett, A. G. M.; Quayle, P. J. Chem. Soc., Chem. Commun.
1981, 1076. (b) Bernardi, A.; Micheli, F.; Potenza, D.; Scolastico, C.; Villa,
R. Tetrahedron Lett. 1990, 31, 4949. (c) Pilli, R. A.; Dias, L. C. Synth.
Commun. 1991, 21, 2213. (d) Ahman, J.; Somfai, P. Tetrahedron 1992,
48, 9537. (e) Morimoto, Y.; Nishida, K.; Hayashi, Y.; Shirahama, H.
Tetrahedron Lett. 1993, 34, 5773. (f) Morimoto, Y.; Iwahashi, M. Synlett
1995, 1221. (g) Pilli, R. A.; Russowsky, D. J. Org. Chem. 1996, 61, 3187.
(h) Louwrier, S.; Ostendorf, M.; Boom, A.; Hiemstra, H.; Speckamp, W.
N. Tetrahedron 1996, 52, 2603. (i) Arndt, H. D.; Polborn, K.; Koert, U.
Tetrahedron Lett. 1997, 38, 3879. (j) D’Oca, M. G. M.; Pilli, R. A.; Vencato,
I. Tetrahedron Lett. 2000, 41, 9709. (k) Sugiura, M.; Kobayashi, S. Org.
Lett. 2001, 3, 477. (l) Sugiura, M.; Hagio, H.; Hirabayashi, R.; Kobayashi,
S. Synlett 2001, 1225. (m) Mikami, K.; Ohmura, H. Chem. Commun. 2002,
2626. (n) El-Nezhawy, A. O. H.; El Diwani, H. I.; Schmidt, R. R. Eur. J.
Org. Chem. 2002, 4137. (o) Maldaner, A. O.; Pilli, R. A. Synlett 2004,
1343.
2826
Org. Lett., Vol. 7, No. 14, 2005

Table 3. Reactions of Enantiopure Acyliminium Cation
Precursors. I. 5-Acetoxy Lactams Derived from L-Malic- and
L-Tartaric Acids
Table 4. Reactions of Lactams Bearing Chiral Auxiliaries
(Chiral Acyliminium Cation Precursors. II)
TIPSOTf time 8 yield %
runa
NuSiR′3 (equiv)
7
(mol %) (h)
(de)b
TIPS-
1
2
3c
4
5c
6
7c
8
9c
CH2dC(Ph)(OSiMe3) (1.4)
CH2dC(Ph)(OSiMe3) (1.4)
CH2dC(Ph)(OSiMe3) (1.4)
CH2dC(Ph)(OSiMe3) (1.4)
CH2dC(Ph)(OSiMe3) (1.4)
CH2dC(t-Bu)(OSiMe3) (1.4)
CH2dC(t-Bu)(OSiMe3) (1.4)
CH2dC(t-Bu)(OSiMe3) (1.4)
7a
7b
7b
7c
7c
7c
7c
7c
5
15
5
5
5
5
5
10d
5
5
5
5
5
5
10d
5
15 no reaction
15 8a 42 (50)
15 8a 78 (20)
15 8a 55 (20)
15 8a 74 (20)
15 8b 30 (26)
15 8b 32 (26)
24 no reaction
24 8c 56 (70)
24 no reaction
24 8d 93 (16)
24 8e 31 (33)
24 8e 34 (33)
24 8e 50 (33)
24 8e 39 (55)
24 8f 89 (20)
OTf
yield %
(t/ca)
run
NuSiR′3 (equiv)
4
(mol %) time
5
1
2
3
4
5
CH2dC(Ph)(OSiMe3)(1.4)
CH2dC(Ph)(OSiMe3)(1.4)b
CH2dC(t-Bu)(OSiMe3)(1.4)
4a
4a
4a
5
5
5
5
5
2 h 30 5a 77 (83/17)
2 h 30 5a 64 (90/10)
5b 74 (>97/3)
2 h 30 5c 50 (85/15)
4 h
CH2dC(C≡CH)(OSiiPr3)(1.5)c 4b
CH2dC(C≡CH)(OSiiPr3)
(1.5)d,c
4b
1 h
5c 50 (85/15)
Me2CdC(OMe)(OSiMe3) (1.4) 7c
10 CH2dCHCH2SiMe3 (1.4)
11 CH2dC(Ph)(OSiMe3) (1.4)
12 CH2dC(t-Bu)(OSiMe3) (1.4)
13 CH2dC(t-Bu)(OSiMe3) (2.5)
14c CH2dC(t-Bu)(OSiMe3) (1.4)
15c CH2dC(t-Bu)(OSiMe3) (1.4)
7c
7d
7d
7d
7d
7d
6
7
8
9
CH2dCHCH2SiMe3 (2)
CH2dCHCH2SiMe3 (2)e
NCSiMe3 (1.4)
4a
4a
4a
4a
5
5
5
5
5
5
5
3 h
3 h
5d 94 (74/26)
5d 77 (63/37)
3 h 30 5e 82 (43/57)
2 h 30 5e 94 (40/60)
5f 74 (>97/3)
4 h 30 5g 80 (87/13)
4 h 30 5h 67(30/70)
NCSiMe3 (1.4)b
10 Me2CdC(OMe)(OSiMe3)(1.4) 4a
5 h
11 CH2dC(Ph)(OSiMe3)(1.4)b
12 CH2dCHCH2SiMe3 (2)b
13 CH2dC(Ph)(OSiMe3)(1.4)
4c
4c
4d
16c Me2CdC(OMe)(OSiMe3) (1.4) 7d
a Unless otherwise noted, reactions were carried out as follows. To a
solution of the substrate (0.8 mmol) and the silyl enol ether (1.4 equiv) in
dichloromethane (0.8 mL) was added the Lewis acid at 0 °C under an argon
atmosphere, and the temperature was slowly raised to room temperature.
10 1 h
5i 73 (27/73)
14 CH2dC(C≡CH)(OSiiPr3)(1.2) 4d 100 0.25h 5j 55 (50/50)
a trans/cis ratio estimated by 1H NMR spectroscopy and GC-MS
analyses. b The reaction was carried out in toluene. c The F-C adduct 5c′
was also obtained in 13% yield as a mixture of either geometrical Z and E
stereoisomers (exclusive trans relationship for the substituents located at
the C2-C3 bond) or as a mixture of trans and cis stereoisomers (single
geometrical isomer for the double bond, E or Z) in a 4:1 ratio (stereochem-
istry not determined). d The reaction was carried out in ether. e The reaction
was carried out in acetonitrile.
b The stereoisomeric ratio was determined by H NMR spectroscopy and
1
GC-MS analyses. c The reaction was carried out in acetonitrile. d Sc(OTf)3
was used as the catalyst.
optimization revealed that the reaction was efficiently
catalyzed by 2 mol % of TIPSOTf (run 3). Below this amount
of catalyst, the reactivity was significantly lowered.13 The
ethoxy lactam 1b was also rapidly consumed using only 5
mol % of TIPSOTf, proving that the nature of the leaving
group is not of prime importance (run 4). On the other hand,
carrying out the reaction with the hydroxylactam 1c resulted
in an incomplete yet more selective transformation (run 5).
Gratifyingly, the present catalytic process also encompassed
the unprotected lactam 1d (run 6). Under the optimal
conditions for 1a, reactions with various silicon-based
nucleophiles were also investigated (Table 2). Other silyl
enolates, allyltrimethylsilane, trimethylsilyl cyanide, and
triethylsilyl hydride reacted smoothly to afford the desired
adducts 2c-h in good yields.
short reaction time to provide the â-aminoketone 2a in good
yield (run 1).11 The Friedel-Crafts (F-C) adduct 3 was also
isolated as a mixture of Z and E stereomers in 15% yield.12
The reaction could be scaled-up to 1 g of 1a without
alteration of reaction time and yield (run 2). Further
(11) The high reactivity of such a stabilized and bulky TIPS silyl enol
ether in catalytic R-amido alkylation reactions is unprecedented. Supporting
Information gives additional results for various combinations of TIPS enol
ethers and N-acyliminium ion precursors to demonstrate further the potential
of this new catalytic R-amido alkylation reaction.
(12) Products 3a and 3b referred to as F-C adducts by analogy with
the mechanism (and nomenclature) proposed by Mikami and co-workers
to explain formation of functionalized silyl enol ethers obtained during the
course of aldol reactions between simple silyl enol ethers and fluoral (see
ref 12b). From a mechanistic point of view, the TIPS silyl enol ether derived
from butynone initially condenses onto the iminium cation derived from 1
as a silyl ether nucleophile, which then as a result of the bulkiness of the
substituents on silicon fails to give cleavage of the Si-O bond and instead
loses a proton to regenerate the double bond. Selective iminium12a and
carbonyl12b F-C type reactions of silyl enolates have been developed. The
resulting adducts are highly valuable building blocks as a class of
functionalized silyl enol ethers readily accessible from simple ones. (a)
Wada, M.; Nishihara, Y.; Akiba, K. Y. Tetahedron Lett. 1984, 25, 5405.
(b) Ishii, A.; Kojima, J.; Mikami, K. Org. Lett. 1999, 1, 2013.
As could be expected, the reactions of trimethylsilyl enol
ethers were more selective than that of the less nucleophilic
triisopropylsilyl enol ether derived from butynone, with no
F-C products being detected (runs 1 and 2). To the best of
our knowledge, the formation of lactam 2g provides the first
example of a trialkylsilyltriflate-catalyzed hydroxylactam
(13) A conversion of 76% was observed using 1 mol % of the catalyst.
Org. Lett., Vol. 7, No. 14, 2005
2827

derivative-lactam conversion, this useful transformation being
in most cases effected in the presence of excesses of both a
Lewis (or Brønsted) acid and the hydride reagent.14
some silicon-based nucleophiles (Table 4). Steric effects
seem to be the major contributory factor in these reactions,
as demonstrated by the following results. The acetoxylactam
7a remained unchanged in the presence of the trimethylsilyl
enol ether derived from acetophenone under the standard
procedure (run 1). On the other hand, the desired reaction
took place when the less crowded, yet intrinsically less
electrophilic, alkoxylactams 7b,c were engaged (runs 2-5).
The reaction yield was significantly improved when the steric
demand of the substrate was diminished (run 4 versus run
2). The lower yields obtained with the sterically demanding
silyl enol ether derived from pivalone provides strong support
to the key role of sterics (runs 6-8). In light of these
considerations, the fair reactivity between 7c and the silyl
ketene acetal derived from 2-methyl propionic acid methyl
ester (run 9) has to be pointed out and suggests that the
present reactivity results from a subtle balance between
sterics and electronics. Although Meyers’s bicyclic lactam
7d (and related compounds) has previously been demon-
strated to be a convenient substrate for allylation,18,19
cyanation, or alkylation reactions with organocopper re-
agents,18 the results collected in runs 10-14 provide, as far
as we are aware, the first examples of their use in a Mannich-
type reaction. In general, the yields were able to be improved
by carrying out the reactions in CH3CN (runs 3 versus 2, 5
Since the relative bulkiness of TIPSOTf could be an
impediment to the reaction scope, we next examined enan-
tiopure pyrrolidone derivatives 4a-d (Table 3) and 7a-d
(Table 4). Unlike 1a, type 4 acetoxylactams required
prolonged reaction time for complete consumption in their
reactions with the screened nucleophiles. However, most of
the surveyed reactions proceeded well under similar condi-
tions as above (TIPSOTf 5 mol % and 1.4 equiv of
nucleophile), except allylation reactions that stopped at a
level of ∼90% conversion.
Complete reactions were achieved using 2 equiv (not
optimized) of allyltrimethylsilane (runs 6, 7, and 12). A key
feature of these reactions is provided by the observation that
they could also be carried out in toluene (runs 2, 9, 11, and
12) or, in some cases, in ether or acetonitrile (runs 5 and 7).
The compatibility of this catalytic reaction with highly
substituted substrates derived from tartaric acid like 4d when
conventional silyl enol ether is used illustrates further its
potential (run 13). On the other hand, 4d impeded the
addition of the less nucleophilic triisopropylsilyl enol ether
derived from butynone under catalytic conditions, and 1
equiv of TIPSOTf (not optimized) was used to secure a
synthetically useful yield for 5j (run 14).
versus 4, 14 versus 12).
Conversely, neither Sc(OTf)3
5a,b
(runs 8 and 15) nor an
The alkylations examined herein afforded a comparable
level of inherent stereocontrol as related reactions promoted
by conventional Lewis acids. Notably, 4,5-diacetoxylactams
4a,b gave moderate to virtually complete trans diastereo-
selectivity, depending on the size of the nucleophile (runs
1-9),4b,g,h,15,16 whereas 4d afforded adduct 5i in a trans/cis
ratio of 2:1 (runs 12 and 13).15 Allylation of the silyl ether
4c (run 12) gave a level of stereoselection comparable to
that obtained from reactions promoted by InCl3,15 TiCl4 and
BF3-OEt2.17 Conversely, the Mannich reaction of 4c with the
silyl enol ether derived from acetophenone gave an opposite
stereoselectivity (run 11 versus run 12), with the level of
stereoinduction paralleling that obtained in the reaction of
4a (runs 11 versus 1). These results suggest a minimization
of the iminium cation stabilizing neighboring group partici-
pation by the acetate commonly invoked in related chem-
istry4b,g,h and support the rationale recently proposed by
Kobayashi involving a strong dependence of the stereochem-
istry upon the nucleophile steric demand.16
excess of silyl enol ether (run 13) enhances the yield.
Unfortunately, the diastereocontrol remained low in these
reactions.
In summary, a broad range of nucleophilic substitution
reactions of various 5-alkoxy- and 5-acetoxypyrrolidin-2-
ones with silicon-based nucleophiles have been successfully
catalyzed using a small amount of TIPSOTf. This catalyst
produces rapid reactions and then provides an interesting
alternative to the usual TMSOTf reagent regarding the criteria
of stability and air and moisture sensitivity. Good to high
levels of trans-stereoselectivities, close to those obtained in
reactions carried out in the presence of stoichiometric
amounts of conVentional Lewis acids, were reached in the
reactions of 5-acyloxypyrrolidinones 4a-d.
Acknowledgment. The “Region Haute-Normandie” and
“the Reseau PUNCH Orga” are gratefully acknowledged for
a doctoral grant (T.B.) and financial support.
Supporting Information Available: Experimental pro-
cedure for the R-amidoalkylation reaction, analytical data,
TIPSOTf was also found to efficiently catalyze the
alkylation of 5-alkoxylactams bearing chiral auxiliaries with
1
and H NMR spectra for new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
(14) For a representative example, see: Hwang, D. J.; Kim, S. N.; Choi,
J. H.; Lee, Y. S. Biorg. Med. Chem. 2001, 9, 1429.
(15) Russowsky, D.; Zen Petersen, R.; Godoi, M. N.; Pilli, R. A.
Tetrahedron Lett. 2000, 41, 9939.
OL050576Z
(16) Recently, Kobayashi and co-workers rationalized the diastereose-
lectivities observed in nucleophilic substitution reactions of N-CBZ 2,3-
diacetoxy piperidines as a function of the nucleophile bulkiness. See ref
5b.
(18) (a) Meyers, A. I. Chem. Commun. 1997, 1. (b) Yamazaki, N.; Ito,
T.; Kobayashi, C. Tetrahedron Lett. 1999, 40, 739. (c) Yamazaki, N.; Ito,
T.; Kobayashi, C. Org. Lett. 2000, 2, 465.
(19) Baussane, I.; Chiaroni, A.; Royer, J. Tetrahedron: Asymmetry 2001,
12, 1219.
(17) Klitzke, C. F.; Pilli, R. A. Tetrahedron Lett. 2001, 42, 5605-5608.
2828
Org. Lett., Vol. 7, No. 14, 2005