Highly regio- and diastereoselective palladium-catalyzed allylic substitution. Synthesis of 3-(2-aminobutylidene)-4-arylazetidin-2-ones

Article abstract of DOI:10.1002/adsc.200404385

The palladium-catalyzed benzylamine attack to a particular allylic moiety, the 3-alkenyl-3-bromoazetidin-2-one is herein reported. This reaction shows interesting mechanistic aspects and allows us to introduce in one step and under high regio- and stereoc

Full text of DOI:10.1002/adsc.200404385

FULL PAPERS
Highly Regio- and Diastereoselective Palladium-Catalyzed
Allylic Substitution. Synthesis of 3-(2-Aminobutylidene)-
4-arylazetidin-2-ones
Giuliana Cardillo,* Serena Fabbroni, Luca Gentilucci, Rossana Perciaccante,
Alessandra Tolomelli
`
Dipartimento di Chimica “G. Ciamician”, Universita di Bologna, Via Selmi 2, 40126 Bologna, Italy
Fax: (þ39)-51-209-9456, e-mail: giuliana.cardillo@unibo.it
Received: December 17, 2004; Accepted: March 10, 2005
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The palladium-catalyzed benzylamine at- of non-conventional b-lactams, thus offering the op-
tack to a particular allylic moiety, the 3-alkenyl-3-bro- portunity for designing new potential glutamine syn-
moazetidin-2-one is herein reported. This reaction tethase inhibitors, such as Tabtoxin analogues.
shows interesting mechanistic aspects and allows us
to introduce in one step and under high regio- and Keywords: allylic substitution; azetidinone; diastereo-
stereocontrol the amino function in the C3 side chain selectivity; palladium; regioselectivity
Introduction
Results and Discussion
Palladium-catalyzed allylic substitution provides one of The starting compounds, 3-bromoazetidin-2-ones, were
the most efficient methods for the stereoselective con- efficiently prepared in one step from a-bromohexenoyl
struction of carbon-carbon or carbon-heteroatom chloride[7] and TEA with a Schiff base in CH2Cl2 at re-
bond. In this context, the substitution with soft nucleo- flux, under the conditions reported by Bose and Man-
philes, such as amines, is a very efficient method for has[8] for the preparation of a-vinyl-b-lactams. The 3-
the carbon-nitrogen bond formation.[1] One of the bromo-3-alkenyl-b-lactams were obtained in moderate
most interesting aspects of this type of chemistry is the yield, with a preferential 3,4-cis-configuration and ex-
possibility to control regio- and stereoselectivity on clusively with E-configuration of the unsaturated side
the allylic moiety.[2] In recent years, allylic displacements chain.
on symmetrically substituted substrates leading to p-al-
First of all we tested the reactivity of our substrates in
lylic intermediates have received growing interest. In the palladium allylic alkylation with dimethyl malo-
this case, the step of formation of the metal complex is nate,[9] which has become a standard nucleophile in allyl-
irrelevant for the regioselectivity of the overall reaction. ic chemistry (Scheme 1).
For substrates bearing different groups at C1 and C3 of Preliminary experiments on 1a showed that the reac-
the allylic system, interest increases when the regiose- tion was sluggish when Pd2(dba)3 ·CHCl3 was used in a
lectivity can be controlled.
5% amount at room temperature in CH2Cl2 for 48 hours,
Continuing our work on metal-catalyzed reactions[3]
leading to nitrogen-containing compounds of biological
interest,[4] we now report the palladium-catalyzed ben-
zylamine attack to a particular allylic moiety, the 3-al-
kenyl-3-bromoazetidin-2-one.[5] This reaction shows in-
teresting mechanistic aspects and allows us to introduce
in one step and under high regio- and stereocontrol the
amino function in the C3 side chain of the non-conven-
tional b-lactams, thus offering the opportunity for de-
signing new potential glutamine synthetase inhibitors,
such as Tabtoxin analogues.[6]
Scheme 1. Allylic alkylation of 1a–c with dimethyl malonate.
Adv. Synth. Catal. 2005, 347, 833–838
DOI: 10.1002/adsc.200404385
ꢀ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
833

Giuliana Cardillo et al.
FULL PAPERS
2a being isolated only in 11% yield and a 70:30 diaster-
eomeric ratio. The reaction performed with the same
catalyst in toluene at 908C gave 2a in 40% yield and a
45:55 d.r. Better results were attained when 1a was
treated with the sodium salt of dimethyl malonate, gen-
erated in situ from dimethyl malonate and sodium hy-
dride, in the presence of 5 mol % of Pd2(dba)3 in reflux-
ing CH2Cl2 for 48 hours. This simple methodology was
efficiently applied to substrates 1b–c (Scheme 1).
Scheme 2. Palladium-catalyzed benzylamine attack to 1a–d.
The yields of compounds 2a–c were determined on lized. Primary amines have also been efficiently ob-
isolated products after flash chromatography on silica tained via a palladium-catalyzed reaction with azide
gel, and are in the range of 60–65%. The reaction ion.[13] In this work we have used benzylamine, in the
showed a complete regioselectivity to give exclusively presence of Pd2(dba)3 as catalyst, as a soft nucleophile
compound 2, in accordance with the preferential nucle- that may be easily deprotected to the corresponding pri-
ophilic attack at the least hindered terminus of the p-all- mary amine (Scheme 2).
yl system.[10] No trace of the 3-malonyl-substituted iso-
A solution of 1a–d in CH2Cl2 in the presence of 5% of
mer was detected. The unsaturated b-lactams were ob- Pd2(dba)3 was stirred at reflux temperature under an in-
tained exclusively in the Z-configuration, unambiguous- ert atmosphere for 30 minutes. Then 2 equivalents of
ly assigned on the basis of the chemical shift of the vinyl benzylamine were added and the mixture was main-
proton resonating around 5.50 ppm, in accordance with tained at reflux, following the progress by TLC. Under
literature data.[11] The diastereomeric ratios were estab- these conditions, the reaction proceeded smoothly in
lished by evaluation of the integrated areas relative to good yield, although longer reaction times were re-
this proton in the 1H NMR spectra recorded in CDCl3. quired (about 72 hours). The results obtained are report-
The relative configuration of the major isomer, reported ed in Table 1.
in Scheme 1, was assigned on the basis of mechanistic
Compounds 3a–d showed exclusively the Z-configu-
ration of the double bond and good diastereomeric ra-
considerations.[1 h]
Having established optimal reaction conditions, we tios, ranging from 81:19 to 91:9, between the isomers
turned our attention to carbon-nitrogen bond forma- differing in the configuration at C2’. Diastereoselectivi-
tion. The palladium-catalyzed amination of allylic com- ty was determined on the basis of 1H NMR of the crude
pounds with secondary amines has been extensively mixture in C6D6 and mechanistic considerations allowed
studied.[12] For the synthesis of primary allylamines, the configuration of the major isomer to be deduced. In
preparation of the N-protected primary allylamine fol- order to confirm the stereochemical outcome of the pal-
lowed by removal of the protecting groups has been uti- ladium-catalyzed allylic amination, racemic 4-(p-nitro-
Table 1. Palladium-catalyzed allylic alkylation with benzylamine.
Entry
1
Conversion (yield [%])[a]
Major isomer
d.r.[b]
91: 9
1
94 (85)
2
3
95 (90)
90 (80)
90 (80)
85: 15
82: 18
81: 19
4
[a]
1
Conversion determined by H NMR on the crude reaction mixture, yield determined for isolated compounds, after flash
chromatography.
[b]
1
Determined by H NMR on the basis on the integrated area of the vinylic proton signal.
834
ꢀ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
Adv. Synth. Catal. 2005, 347, 833–838

Synthesis of 3-(2-Aminobutylidene)-4-arylazetidin-2-ones
FULL PAPERS
phenyl)-azetidin-2-one (1e) was treated under the
above reported conditions and the benzylamino-deriva-
tive 3e was obtained in 70% yield after flash chromatog-
raphy as a mixture of diastereomers (80:20 d.r.). Hydro-
genation of 3e with H2 on Pd/C for 4 h effected reduction
of the double bond and the nitro group as well as depro-
tection of the 2’-N-benzyl group, affording the 2’-amino-
b-lactam in 75% yield. Treatment of crude amine with
benzyl chloroformate led to diprotected cis-derivative
4e (Scheme 3). The 3,4-cis-stereochemistry of 4e was as-
signed on the basis of the H3,H4 (J¼5.2 Hz) coupling
constant. The spectroscopic properties of 4e were com-
pared to those of a racemic (2’S*,4R*)-b-lactam ob-
tained from hydrogenation and protection of azide
5,[14] already available in our laboratory, whose stereo-
chemistry was established on the basis of its X-ray anal-
ysis (Scheme 4).
The complete concordance of the analytical data of
the products obtained through two different pathways
allowed us to assign the (2’S*,4R*) configuration to
the major product 3e of the palladium-catalyzed amina-
tion. In a similar way compounds 3a and 3d were submit-
ted to hydrogenation affording, after protection of the
amino group, 4a and 4d in 75% and 80% yields, respec-
tively (Scheme 5).
Scheme 3. Synthesis of 3-(2’-CBz-amino)-azetidin-2-one 4e.
On the basis of the experimental evidence, we have
found that the Pd-catalyzed benzylamine addition to 1
affords compound 3 in good yield and exclusive Z-con-
figuration, with a good diastereomeric ratio. At present
no explanation can be given for the preference that was
found. The allylic substrate 1 reacts with the catalyst,
which enters the catalytic cycle at the Pd(0) oxidation
level anti to the bromide leaving group. Attack of the
soft nucleophile benzylamine must then occur on the all-
yl face opposite to the palladium complex. If capture of
the nucleophile is faster than palladium equilibration,
the olefin face complexation becomes the stereo-deter-
mining step, therefore giving exclusively the Z-configu-
ration of the final products and the (2’S*,4R*) relative
configuration. Finally, reduction of the reaction product
3 gives access to a new class of 3-(2’-amino)-substituted
Scheme 4. Synthesis of 3-(2’-CBz-amino)-azetidin-2-one 4e,
starting from azide 5.
Scheme 5. Synthesis of 3-(2’-CBz-amino)-azetidin-2-ones 4a – d.
Adv. Synth. Catal. 2005, 347, 833–838
asc.wiley-vch.de
ꢀ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
835

Giuliana Cardillo et al.
FULL PAPERS
nones 1a–e were prepared according to a previously reported
procedure[5] and isolated by flash chromatography on silica gel
and preparative HPLC (Zorbax Eclipse XDB C18 column, iso-
cratic program, eluent CH3CN/H2O, 75/25). Complete charac-
terizations for compounds 2a–c, 3a–e and 4a–e are reported
in the Supporting Information. Data for compound Z-5 can
be found in ref.[14] (CCDC 260701).
General Procedure for the Palladium-Catalyzed
Reaction of 1a–d with Dimethyl Malonate
NaH (1.5 mmol, 36 mg) was added to a solution of dimethyl
malonate (1.5 mmol, 0.17 mL) in CH2Cl2 (10 mL) under nitro-
gen atmosphere at room temperature. The mixture was stirred
for 30 minutes and then added via cannula to a solution of 1
(1 mmol) and Pd2(dba)3 (0.05 mmol, 45 mg.) in CH2Cl2
(10 mL) under a nitrogen atmosphere at room temperature.
The solution was refluxed for 48 h and then allowed to reach
room temperature. After quenching with 5 mL of water, the
mixture was filtered on a celite pad and washed twice with wa-
ter (10 mL). The organic layer was dried over Na2SO4 and sol-
vent was removed under reduced pressure. The products were
obtained after purification by flash chromatography on silica
gel (cyclohexane/EtOAc, 80/20). 2a: yield: 65%, d.r.: 70/30;
2b: yield: 65%, d.r.: 88/12; 2c: yield: 65%, d.r.: 82/18.
Figure 1. Mechanism suggested for the palladium-catalyzed
addition of benzylamine.
b-lactams, useful precursors for the preparation of pos-
sibly biologically active compounds.[15]
Conclusion
General Procedure for the Palladium-catalyzed
Reaction of 1a–d with Benzylamine
We have reported in this paper the palladium-catalyzed
attack of benzylamine to a particular allylic moiety, the
3-alkenyl-3-bromo-azetidin-2-one. This reaction affords
3-(2’-benzylamino)-azetidin-2-ones exclusivelyin the Z-
configuration of the double bond and good diastereo-
meric ratios between the isomers differing in the config-
uration at C2’. This methodology allows us to introduce
in one step and under high regio- and stereocontrol the
amino function in the C3 side chain of the non-conven-
tional b-lactams, thus offering the opportunity for de-
signing new potential enzyme inhibitors, such as Tabtox-
in analogues.
A solution of 1 (1 mmol) and Pd2(dba)3 (0.05 mmol, 45 mg.) in
CH2Cl2 (10 mL) under a nitrogen atmosphere was stirred at re-
fluxing temperature for 30 minutes. Benzylamine (2 mmol,
0.196 mL) was then added and the reaction was monitored
by TLC and quenched after 72 h with 5 mL of water. After
washing twice with water (10 mL), the organic layer was dried
over Na2SO4 and solvent was removed under reduced pressure.
The products were obtained, after purification by flash chro-
matography on silica gel (cyclohexane/EtOAc, 80/20). 3a:
yield: 85%, d.r.: 91/9; 3b: yield: 90%, d.r.: 85/15; 3c: yield:
80%, d.r.: 82/18; 3d: yield: 80%, d.r.: 81/19; 3e: yield: 70%,
d.r.: 80/20.
Experimental Section
General Procedure for the Reduction and Protection
of Azetidinone 3e
General Remarks
To a solution of 3e (1 mmol, 441 mg) in MeOH (10 mL), Pd/C
(30 mg/1 mmol) was added in one portion. The reaction mix-
ture was stirred vigorously at room temperature in a hydrogen
atmosphere (1 atm) for 3 hours. The solution was filtered and
evaporated. To a stirred solution of crude product in water/ace-
tone (5þ5 mL), NaHCO3 (2 mmol, 168 mg) and CbzCl
(2 mmol, 0.285 mL) were added. After 3 hours acetone was re-
moved under reduced pressure and the residue was diluted
with EtOAc (10 mL) and washed twice with 0.1 M HCl (2ꢀ
5 mL). The organic layer was separated, dried over Na2SO4
and solvent was removed under reduced pressure. Compound
4e was obtained in 75% yield after purification by flash chro-
matography on silica gel (cyclohexane/Et2O, 8/2).
Unless stated otherwise, solvents and chemicals were obtained
from commercial sources and used without further purifica-
tion. Flash chromatography was performed on silica gel
(230–400 mesh). NMR spectra were recorded with 300 or
600 MHz spectrometers. Chemical shifts were reported as d
values (ppm) relative to the solvent peak of CDCl3 set at d¼
7.27 (1H NMR) or d¼77.0 (13C NMR). Infrared spectra were
recorded with an FT-IR spectrometer. Melting points are un-
corrected. MS analyses were performed on a liquid chromato-
graph coupled with an electrospray ionization-mass spectrom-
eter (LC-ESI-MS), using H2O/CH3CN as solvent at 258C (pos-
itive scan 100–500 m/z, fragmentor 70 V). Starting azetidi-
836
ꢀ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
Adv. Synth. Catal. 2005, 347, 833–838

Synthesis of 3-(2-Aminobutylidene)-4-arylazetidin-2-ones
FULL PAPERS
R. Perciaccante, A. Tolomelli, Tetrahedron: Asymmetry
2002, 13, 1407–1410; h) G. Cardillo, S. Fabbroni, L. Gen-
tilucci, M. Gianotti, R. Perciaccante, S. Selva, A. Tolo-
melli, Tetrahedron: Asymmetry 2002, 13, 1411–1415.
[4] a) G. Cardillo, L. Gentilucci, C. Tomasini, M. P. Visa
Castejon-Bordas, Tetrahedron: Asymmetry 1996, 7,
755–762; b) G. Cardillo, S. Casolari, L. Gentilucci, C.
Tomasini, Angew. Chem. Int. Ed. Engl. 1996, 35, 1848–
1849; c) A. Bongini, G. Cardillo, L. Gentilucci, C. Toma-
sini, J. Org. Chem. 1997, 62, 9148–9153;d) G. Cardillo, L.
Gentilucci, A. Tolomelli, C. Tomasini, J. Org. Chem.
1998, 63, 2351–2353; e) G. Cardillo, L. Gentilucci, A.
Tolomelli, C. Tomasini, Synlett 1999, 1727–1730; f) G.
Cardillo, L. Gentilucci, A. Tolomelli, Chem. Commun.
1999, 167–168; g) G. Cardillo, L. Gentilucci, M. Gianotti,
A. Tolomelli, Eur. J. Org. Chem. 2000, 2489–2494.
[5] F. Benfatti, G. Cardillo, S. Fabbroni, L. Gentilucci, R.
Perciaccante, F. Piccinelli, A. Tolomelli, Synthesis 2005,
61–70.
[6] a) D. L. Lee, H. Rapoport, J. Org. Chem. 1975, 40, 3491–
3496; b) J. E. Baldwin, M. Otsuka, P. M. Wallace, Tetra-
hedron 1986, 42, 3097–3110; c) W. J. Greenlee, J. P.
Springer, A. A. Patchett, J. Med. Chem. 1989, 32, 165–
170; d) J. E. Baldwin, P. D. Bailey, G. Gallacher, M. Otsu-
ka, K. A. Singleton, P. M. Wallace, K. Prout, W. M. Wolf,
Tetrahedron 1984, 40, 3695–708; e) R. E. Dolle, C. S. Li,
R. Novelli, L. I. Kruse, D. Eggleston, J. Org. Chem. 1992,
57, 128–132; f) W. W. Stuart, Nature 1971, 229, 174–178.
[7] G. Cardillo, S. Fabbroni, L. Gentilucci, R. Perciaccante,
F. Piccinelli, A. Tolomelli, Tetrahedron 2004, 60, 5031–
5040; b) G. Cardillo, S. Fabbroni, L. Gentilucci, R. Per-
ciaccante, A. Tolomelli, Tetrahedron: Asymmetry 2004,
15, 593–601.
General Procedure for the Reduction and Protection
of Azetidinones 3a and 3d
To a solution of 3 (1 mmol) in MeOH (10 mL), Pd/C (30 mg/
1 mmol) was added in one portion. The reaction mixture was
stirred vigorously at room temperature in a hydrogen atmos-
phere (1 atm) for 3 hours. The solution was filtered and evapo-
rated. To a stirred solution of crude product in CH2Cl2, Et3N
(1.1 mmol, 1.1 equivs., 154 mL) and CbzCl (1.1 mmol, 1.1
equivs., 157 mL) were added. After 4 hours the solution was di-
luted with CH2Cl2 (10 mL), washed twice with 0.1 M HCl (2ꢀ
5 mL), the organic layer was separated, dried over Na2SO4 and
solvent was removed under reduced pressure. The products
were purified by flash chromatography on silica gel (cyclohex-
ane/Et2O, 8/2).
Acknowledgements
We thank MIUR (PRIN 2004 and FIRB 2001), CNR-ISOF and
University of Bologna (Funds for selected topics) for financial
support.
References and Notes
[1] a) B. M. Trost, Acc. Chem. Res. 1980, 13, 385–393;
b) B. M. Trost, T. R. Verhoeven, in: Comprehensive Or-
ganometallic Chemistry, (Eds.: G. Wilkinson, F. G. Stone,
E. W. Abel), Pergamon, Oxford, 1982, chapter 8; c) S. A.
Godleski, in: Comprehensive Organic Synthesis, Vol. 4,
(Eds.: B. M. Trost, I. Fleming, M. F. Semmelhack), Perga-
mon, Oxford, 1991, chapter 3.3; d) J. Tsuji, in: Palladium
Reagents and Catalyst, Innovations in Organic Synthesis,
Wiley, Chichester, 1995; e) A. Heumann, in: Transition
metals for organic synthesis, Vol. 1, (Eds.: M. Beller, C.
Bolm), Wiley-VCH, Weinheim, 1998, chap. 2.15; f) G.
Consiglio, R. M. Waymouth, Chem. Rev. 1989, 89, 257–
276; g) B. M. Trost, D. L. Van Vranken, Chem. Rev.
1996, 96, 395; h) A. Pfaltz, Acc. Chem. Res. 1993, 26,
339–345; i) A. Pfaltz, M. Lautens, in: Comprehensive
Asymmetric Catalysis, Vol. 2, (Eds.: E. N. Jacobsen,
A. Pfaltz, H. Yamamoto), Springer Verlag, Berlin,
1999, pp. 833–884.
[8] a) M. S. Manhas, B. K. Banik, A. Mathur, J. E. Vincent,
A. K. Bose, Tetrahedron 2000, 56, 5587–5601; b) M. S.
Manhas, M. Ghosh, A. K. Bose, J. Org. Chem. 1990, 55,
575–580; c) R. Zamboni, G. Just, Can. J. Chem. 1979,
57, 1945–1948; d) A. K. Bose, G. Spiegelmann, M. S.
Manhas, Tetrahedron Lett. 1971, 3167–3170.
[9] a) B. M. Trost, T. R. Verhoeven, J. Am. Chem. Soc. 1980,
102, 4730–4743; b) T. Hayashi, A. Okada, T. Suzuka, M.
Kawatsura, Org. Lett. 2003, 5, 1713–1715.
[10] a) B. M. Trost, T. J. Dietsche, T. J. Fullerton, J. Org.
Chem. 1974, 39, 737–738; b) B. M. Trost, L. Weber, J.
Org. Chem. 1975, 40, 3617–3619; c) B. M. Trost, L. Web-
er, J. Am. Chem. Soc. 1975, 97, 1611–1612; d) B. M.
Trost, L. Weber, P. E. Strege, T. J. Fullerton, T. J. Diet-
sche, J. Am. Chem. Soc. 1978, 100, 3416–3426; e) B. M.
Trost, L. Weber, P. E. Strege, T. J. Fullerton, T. J. Diet-
sche, J. Am. Chem. Soc. 1978, 100, 3426–3435; f) J. C.
Fiaud, J. Y. Legros, J. Org. Chem. 1990, 55, 4840–4846;
`
˜
[2] a) A. Aranyos, K. J. Szabo, A. M. Castano, J. E. Bꢁckvall,
Organometallics 1997, 16, 1058–1064; b) B. Akermark,
K. Zetterberg, S. Hansonn, B. Krakenberger, A. Vitaglia-
no, J. Organomet. Chem. 1987, 335, 133–142.
[3] a) R. Amoroso, G. Cardillo, P. Sabatino, C. Tomasini, A.
`
Trere, J. Org. Chem. 1993, 58, 5615–5619; b) G. Cardillo,
L. Gentilucci, A. Tolomelli, Tetrahedron Lett. 1999, 40,
8261–8264; c) G. Cardillo, L. Gentilucci, M. Gianotti,
A. Tolomelli, Tetrahedron: Asymmetry 2001, 12, 563–
569; d) G. Cardillo, L. Gentilucci, M. Gianotti, A. Tolo-
melli, Org. Lett. 2001, 3, 1165–1167; e) G. Cardillo, L.
Gentilucci, M. Gianotti, R. Perciaccante, A. Tolomelli,
J. Org. Chem. 2001, 66, 8657–8660; f) G. Cardillo, L.
Gentilucci, M. Gianotti, H. Kim, R. Perciaccante, A. Tol-
omelli, Tetrahedron: Asymmetry 2001, 12, 2395–2398;
g) G. Cardillo, S. Fabbroni, L. Gentilucci, M. Gianotti,
`
g) R. Pretot, A. Pfaltz, Angew. Chem. Int. Ed. Engl.
1998, 37, 323–325; h) B. M. Trost, J. W. Herndon, J.
Am. Chem. Soc. 1984, 106, 6835–6837; i) B. M. Trost,
J. W. Herndon, J. Am. Chem. Soc. 1984, 106, 6837–6839.
[11] a) S. Anklam, J. Liebscher, Tetrahedron 1998, 54, 6369–
6384; b) H. H. Otto, H. J. Bergmann, R. Mayrhofer,
Arch. Pharm. 1986, 319, 203–216.
Adv. Synth. Catal. 2005, 347, 833–838
asc.wiley-vch.de
ꢀ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
837

Giuliana Cardillo et al.
FULL PAPERS
[12] a) S.-I. Murahashi, Y. Taniguchi, Y. Imada, Y. Tanigawa,
J. Org. Chem. 1989, 54, 3292–3303; b) B. M. Trost, E.
Keinan, J. Org. Chem. 1979, 44, 3451–3457; c) S. E. By-
strom, R. Aslanian, J.-E Backvall, Tetrahedron Lett.
1985, 26, 1749–1752; d) Y. Inoue, M. Taguchi, M. Toyo-
fuku, H. Hashimoto, Bull. Chem. Soc. Jpn. 1984, 57,
3021–3022.
[13] S.-I. Murahashi, Y. Tanigawa, Y. Imada, Y. Taniguchi,
Tetrahedron Lett. 1986, 27, 227–230.
[14] G. Cardillo, S. Fabbroni, L. Gentilucci, R. Perciaccante,
F. Piccinelli, A. Tolomelli, Org. Lett. 2005, 7, 533–536.
[15] M. R. Linder, J. Podlech, Org. Lett. 1999, 1, 869–871.
838
ꢀ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
Adv. Synth. Catal. 2005, 347, 833–838