Carbon-Phosphorus Bond Formation by Enantioselective Palladium-Catalyzed Allylation of Diphenylphosphine Oxide

Article abstract of DOI:10.1002/ejoc.201403024

The enantioselective Pd-catalyzed allylation of diaryl-substituted allylic carbonates and diphenylphosphine oxide was investigated. This method gave allylic diphenylphosphine oxides in yields up to 95 % with 97 % ee. Pd-catalyzed allylation of (E)-methyl

Full text of DOI:10.1002/ejoc.201403024

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201403024
Carbon–Phosphorus Bond Formation by Enantioselective Palladium-Catalyzed
Allylation of Diphenylphosphine Oxide
Liang Zhang,[a] Wei Liu,[a] and Xiaoming Zhao*[a]
Keywords: Homogeneous catalysis / Palladium / Phosphorus / Allylation / Enantioselectivity / Regioselectivity
The enantioselective Pd-catalyzed allylation of diaryl-substi-
tuted allylic carbonates and diphenylphosphine oxide was
investigated. This method gave allylic diphenylphosphine
oxides in yields up to 95% with 97%ee. Pd-catalyzed allyl-
ation of (E)-methyl allyl carbonates with diisopropyl phos-
phonate was also examined.
substitutions of diphenylphosphine oxide, which allow the
synthesis of chiral allylic diphenylphosphine oxides.
Introduction
Pd-catalyzed asymmetric allylic substitution has become
a powerful method for carbon–carbon and carbon–hetero-
atom (O, N, and S) bond construction.[1] In this context,
such a reaction for carbon–phosphorus bond formation has
been less reported.[2] Organophosphorus compounds are of
great importance with regard to ligands,[3] functionalized
materials,[4] and bioactive molecules.[5] As such, an efficient
method for the synthesis of organophosphorus compounds
with a chiral C–P center is highly desirable. Several strate-
gies for C–P bond formation have emerged that employ P
nucleophiles such as phosphines,[2d,6] phenylphosphine bor-
ane,[2e,7] phosphites,[8] and phosphine oxides.[9] In pioneer-
ing work, phosphine derivatives were examined in Pd-cata-
lyzed asymmetric allylations;[2d] only diphenylphosphine
gave a good outcome and other phosphines offered unsatis-
factory results.[2d] In addition, both diphenylphosphine and
diphenylphosphine oxide were applied in the catalytic phos-
phorus-Michael addition reaction.[6,9] We assume that di-
phenylphosphine oxide may be directly utilized in Pd-cata-
lyzed allylation reactions for C–P bond formation. To the
best of our knowledge, there are no reports of such an allyl-
ation of diphenylphosphine oxide. In contrast with di-
phenylphosphine,[2d] diphenylphosphine oxide has some ad-
vantages: one, diphenylphosphine oxide is not capable of
coordinating with palladium; two, allylic phosphine oxides
are not air sensitive; three, allylic phosphine oxide can be
conveniently reduced with trichlorosilane (HSiCl3).[10]
Furthermore, the application of diphenylphosphine oxide
may inhibit the formation of byproducts such as bis(diphos-
phine oxide).[11] In this paper, we report Pd-catalyzed allylic
Results and Discussion
We started our investigation with the reaction between
(E)-1,3-diphenylallyl methyl carbonate (1a) and diphenyl-
phosphine oxide (2) under palladium catalysis. Upon con-
ducting this reaction in THF in the presence of tris(dibenz-
ylideneacetone)dipalladium [Pd2(dba)3] and 2,2Ј-bis(di-
phenylphosphino)-1,1Ј-binaphthyl (BINAP,[12] L1) at room
temperature, the formation of P-allylated product 3a was
observed in a poor yield but with 98%ee, and no bis(di-
phenylphosphine oxide) was obtained (Table 1, entry 1).
Screening solvents revealed that toluene afforded a better
yield than both THF and CH2Cl2 (Table 1, entries 2 and 3).
Significantly, an improvement in the yield was achieved if a
5:1 ration of 1a/2 was used (Table 1, entry 4). Notably, this
reaction is temperature sensitive in the 50–60 °C range
(Table 1, entries 5–7). We were pleased to observe that the
reaction could be performed at 55 °C and that it gave a
better result (see Table 1, entry 6). Analysis of 1a that was
recovered upon completion of the reaction by HPLC on a
chiral stationary phase revealed that it was racemic. At the
same temperature, a decrease in the ration of 1a/2 led to a
reduction in the yield, although high ee values were main-
tained (Table 1, entries 6, 8, and 9).
A range of chiral ligands such as L1, L2,[13] Josiphos
ligand L3,[14] Trost’s ligand L4,[15] and PHOX ligand L5[16]
were investigated (Figure 1). BINAP (L1) and L2 both gave
excellent enantioselectivities (Table 1, entries 6 and 10).
Ligand L3 gave good results (Table 1, entry 11). Note that
BINAP is an unsuitable ligand and that Josiphos L3 is,
however, the optimal ligand for the Pd-catalyzed asymmet-
ric allylation of diphenylphosphine.[2d] Both L4 and L5
failed to promote this reaction (see Table 1, entries 10, 12,
13). Palladium catalysts including Pd2(dba)3, [Pd(allyl)Cl]2,
and Pd(OAc)2 were then explored. Pd2(dba)3 led to the best
[a] Department of Chemistry, State Key Laboratory of Pollution,
Control and Resource Reuse, Tongji University,
1239 Siping Road, 200092 Shanghai, P.R. China
E-mail: xmzhao08@mail.tongji.edu.cn
http://www.tongji.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403024.
6846
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 6846–6849

C–P Bond Formation by Enantioselective Pd-Catalyzed Allylation
Table 1. Optimization of the reaction conditions for the enantio- substrates containing electron-donating groups; substrates
selective Pd-catalyzed allylation.[a]
1g–h with 4-Me and 3-Me groups on the phenyl ring led to
P-allylated products 3g–h in fair yields with good to high
enantioselectivities (see Table 2, entries 7 and 8). These re-
sults suggest that 2 is a weak nucleophile and that it favors
electrophilic substrates. (4) (E)-1,3-Di(naphthalen-1-yl)allyl
methyl carbonate (1i) resulted in P-allylated product 3i in
moderate yield with moderate enantioselectivity (Table 2,
entry 9). The ortho substituent effect was detrimental to the
enantioselectivity, and this is in agreement with transition-
metal-catalyzed allylations.[17] (5) In comparison to 1i, (E)-
1,3-di(naphthalen-2-yl)allyl methyl carbonate (1j) gave rise
to P-allylated product 3j in good yield with excellent
enantioselectivity (Table 2, entry 10).
Entry Pd
Ligand Solvent
T
[°C]
2/1a Yield[b]
ee[c]
[%]
[%]
1
2
3
4
5
6
7
8
Pd2(dba)3
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L1
L1
THF
25 1:1.2
25 1:1.2
25 1:1.2
15
25
15
98
97
90
95
93
97
90
95
93
91
87
–
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
[Pd(allyl)Cl]2
Pd(OAc)2
toluene
CH2Cl2
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
25
50
55
60
55
55
55
55
55
55
55
55
1:5
1:5
1:5
1:5
1:4
1:3
1:5
1:5
1:5
1:5
1:5
1:5
50
65
95
98
85
75
31
80
n.r.
n.r.
n.r.
n.r.
Table 2. Scope of (E)-allylic carbonates 1.[a]
9
10
11
12
13
14
15
–
–
–
Entry
R
Product
Yield[b] [%]
ee[c] [%]
[a] Reaction conditions: Pd (5 mol-%), ligand (10 mol-%), 1a (0.24–
1 mmol), 3a (0.20 mmol), solvent (2 mL), 25–60 °C. [b] Yield of iso-
lated product; n.r.: no reaction. [c] Determined by HPLC analysis
on a chiral stationary phase.
1
2
3
4
5
6
7
8
9
10
C6H5
3a
3b
3c
3d
3e
3f
95
75
92
62
93
72
45
53
65
60
97
94
90
94
86
95
70
88
–68
–95
4-FC6H4
4-ClC6H4
4-BrC6H4
3-FC6H4
3-ClC6H4
4-MeC6H4
3-MeC6H4
1-nathphyl
2-nathphyl
result, whereas [Pd(allyl)Cl]2 and Pd(OAc)2 were ineffective
in this reaction (Table 1, entries 6, 14, 15). Diisopropyl
phosphonate (5) instead of diphenylphosphine oxide (2)
was tested under the optimal conditions, and the corre-
sponding product was not observed.
3g
3h
3i
3j
[a] Reaction conditions: Pd2(dba)3 (5 mol-%), L1 (10 mol-%), 1a
(1 mmol), 3a (0.20 mmol), toluene (2 mL), 55 °C. [b] Yield of iso-
lated product. [c] Determined by HPLC analysis on a chiral sta-
tionary phase.
A possible reaction mechanism is proposed and is shown
in Scheme 1. The oxidation addition of the Pd complex to
1 in toluene at 55 °C gives (π–allyl)palladium intermediate
A, carbon dioxide (CO2), and methoxide (–OMe).[18] In the
presence of –OMe, HPOPh2 is converted into –POPh2;[9b,19]
the latter attacks A to form intermediate B. Reductive elimi-
nation of B occurs to give 3 with regeneration of the Pd
catalyst.
Figure 1. Ligands L1–L5 evaluated for the titled allylic substitu-
tion.
Having established the optimized reaction conditions
(Table 1, entry 6), the scope of allyl substrates 1 was exam-
ined. The results revealed the following factors: (1) (E)-1,3-
Diphenylallyl methyl carbonate (1a) gave 3a in excellent
yield with excellent ee (Table 2, entry 1). (2) The reaction
favored allyl substrates bearing electron-withdrawing
groups, for example, substrates 1b–f with 4-F, 4-Cl, 4-Br, 3-
F, and 3-Cl groups on the phenyl ring provided P-allylated
products 3b–f in moderate to high yields with high levels of
enantioselectivities, except for 1d, which gave a moderate
Scheme 1. Possible mechanism for the Pd-catalyzed allylic substitu-
tion of diphenylphosphine oxide (2).
We further extended the Pd-catalyzed allylic alkylation
yield (Table 2, entries 2–6). (3) The reaction disfavored allyl of P nucleophiles such as diisopropyl phosphonate to un-
Eur. J. Org. Chem. 2014, 6846–6849
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6847

L. Zhang, W. Liu, X. Zhao
SHORT COMMUNICATION
onate
1
(134 mg, 1 mmol) and diphenylphosphine oxide (2;
symmetrical allyl carbonates, which gave allylic phosphin-
ates.[5a] Upon using diisopropyl phosphonate (5) in such a
reaction in the presence of Pd2(dba)3 (1 mol-%) and
Xantphos[20] (2 mol-%) in THF at 85 °C, (E)-cinnamyl
methyl carbonate (4a) and allyl carbonates 4b–g with elec-
tron-donating groups (e.g., 4-Me and 3-MeO) or electron-
withdrawing groups (e.g., 4-F, 4-Cl, 3-F and 3-CF3) on the
phenyl ring gave linear products 6a–g[21] in moderate to ex-
cellent yields with high regioselectivities (Table 3, entries 1–
7). Additionally, naphthyl- and thienyl-substituted allyl
carbonates 4h and 4i also worked well (Table 3, entries 8
and 9). Unfortunately, (E)-methyl 5-phenylpent-2-enyl carb-
onate (4j) failed to undergo this reaction (Table 3, entry 10).
We found that diphenylphosphine oxide (2) was ineffective
in this reaction.
40.4 mg, 0.20 mmol) were added. The mixture was stirred at 55 °C.
Upon the completion of the reaction as monitored by TLC, the
mixture was filtered through Celite, and the solvent was removed
under reduced pressure. The crude residue was purified by flash
column chromatography (ethyl acetate/petroleum ether = 1:1) to
give product 3, and the enantioselectivity was determined by HPLC
analysis on a chiral stationary phase.
Supporting Information (see footnote on the first page of this arti-
cle): Optimization of the reaction conditions for the Pd-catalyzed
allylation reaction of 4 with 5; copies of the H NMR, 13C NMR,
1
31P NMR, and 19F NMR spectra for 3 and 6; and chiral HPLC
chromatograms for racemic and enantiopure allylic diphenylphos-
phine oxides.
Acknowledgments
Table 3. Pd-catalyzed allylation of (E)-methyl allyl carbonates 4
with diisopropyl phosphonate (5).[a]
The authors gratefully acknowledge the National Natural Science
Foundation of China (NSFC) (grant number 21272175) for gener-
ous financial support.
[1] For reviews and select papers, see: a) A. Saitoh, K. Achiwa, K.
Tanaka, T. Morimoto, J. Org. Chem. 2000, 65, 4227; b) Y. Tam-
aru, Eur. J. Org. Chem. 2005, 2647; c) B. M. Trost, D. L.
Van Vranken, Chem. Rev. 1996, 96, 395; d) M. Johannsen,
K. A. Jørgensen, Chem. Rev. 1998, 98, 1689; e) T. Hayashi, J.
Organomet. Chem. 1999, 576, 195; f) G. Helmchen, A. Pfaltz,
Acc. Chem. Res. 2000, 33, 336; g) L.-X. Dai, T. Tu, S.-L. You,
W.-P. Deng, X.-L. Hou, Acc. Chem. Res. 2003, 36, 659; h)
B. M. Trost, M. L. Crawley, Chem. Rev. 2003, 103, 2921; i)
B. M. Trost, J. Org. Chem. 2004, 69, 5813; j) B. M. Trost, M. R.
Machacek, A. Aponick, Acc. Chem. Res. 2006, 39, 747; k) Z.
Lu, S. Ma, Angew. Chem. Int. Ed. 2008, 47, 258; Angew. Chem.
2008, 120, 264; l) M. Dieguez, O. Pamies, Acc. Chem. Res.
2010, 43, 312; m) W. Liu, X. M. Zhao, Synthesis 2013, 45, 2052.
[2] a) The allylic diphenylphosphine product was obtained in 15%
yield as a byproduct, see: J.-C. Fiaud, J. Chem. Soc., Chem.
Commun. 1983, 1055; b) A. Galkina, A. Buff, E. Schulz, L.
Hennig, M. Findeisen, G. Reinhard, R. Oehme, P. Welzel, Eur.
J. Org. Chem. 2003, 4640; c) K. Bravo-Alta-mirano, Z. Huang,
J.-L. Montchamp, Tetrahedron 2005, 61, 6315; d) P. Butti, R.
Rochat, A. D. Sadow, A. Togni, Angew. Chem. Int. Ed. 2008,
47, 4878; Angew. Chem. 2008, 120, 4956; e) A. Duraud, O. Jac-
quet, J.-C. Fiaud, R. Guillot, M. Toffano, ChemCatChem 2011,
3, 883.
Entry
R
6/7[b]
Product, yield[c] [%]
1
2
3
4
5
6
7
8
9
C6H5
4-MeC6H4
4-FC6H4
4-ClC6H4
3-MeOC6H4
3-FC6H4
3-CF3C6H4
2-nathphyl
2-thienyl
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
–
6a, 92
6b, 95
6c, 71
6d, 65
6e, 93
6f, 87
6g, 65
6h, 75
6i, 65
6j, -
10
PhCH2CH2
[a] Reaction conditions: Pd2(dba)3 (1 mol-%), xantphos (2 mol-%),
4 (0.2 mmol), 5 (0.24 mmol), THF (2 mL), 85 °C. [b] Determined
by analysis of the crude product by 1H NMR spectroscopy.
[c] Yield of isolated product.
[3] a) A. Börner, Phosphorus Ligands in Asymmetric Catalysis:
Synthesis and Applications, Wiley-VCH, Weinheim, Germany,
2008, vols. 1–3; b) P. C. J. Kamer, P. W. N. M. van Leeuwen,
Phosphorus(III) Ligands in Homogeneous Catalysis, Wiley,
Chichester, UK, 2012; c) N. F. Blank, J. R. Moncarz, T. J.
Brunker, C. Scriban, B. J. Anderson, O. Amir, D. S. Glueck,
L. N. Zakharov, J. A. Golen, C. D. Incarvito, A. L. Rheingold,
J. Am. Chem. Soc. 2007, 129, 6847; d) for a recent review, see:
J. F. Teichert, B. L. Feringa, Angew. Chem. Int. Ed. 2010, 49,
2486; Angew. Chem. 2010, 122, 2538.
[4] a) Y. Iwase, K. Kamada, K. Ohta, K. Kondo, J. Mater. Chem.
2003, 13, 1575; b) J. F. Hulvat, M. Sofos, K. Tajima, S. I. Stupp,
J. Am. Chem. Soc. 2005, 127, 366; c) B. W. Messmore, J. F.
Hulvat, E. D. Sone, S. I. Stupp, J. Am. Chem. Soc. 2004, 126,
14452; d) C. W. Wu, H. C. Lin, Macromolecules 2006, 39, 7985;
e) X. H. Ma, F. Ma, Z. H. Zhao, N. H. Song, J. P. Zhang, J.
Mater. Chem. 2009, 19, 2975; f) S. Furukawa, S. Haga, J. Ko-
bayashi, T. Kawashima, Org. Lett. 2014, 16, 3228; g) M. Capo-
rali, Eur. J. Inorg. Chem. 2014, 1565.
Conclusions
In summary, we developed the enantioselective Pd-cata-
lyzed allylation of diphenylphosphine oxide to give allyl-
ation products in acceptable to excellent yields with moder-
ate to excellent enantioselectivities. These are the first exam-
ples in which diphenylphosphine oxide is employed as a
phosphorus source in Pd-catalyzed asymmetric allylic sub-
stitution reactions.
Experimental Section
General Procedure for the Pd-Catalyzed Allylation of Diphenylphos-
phine Oxide: A mixture of Pd2(dba)3 (9.2 mg, 0.01 mmol), the
ligand (12.5 mg, 0.02 mmol), and toluene (2.0 mL) was stirred in a
dry Schlenk tube filled with argon for 30 min; then, allylic carb-
[5] a) L. V. Kochian, Nature 2012, 488, 466; b) F. Palacios, C.
Alonso, J. M. de Los Santos, Chem. Rev. 2005, 105, 899; c) F.
Palacios, C. Alonso, J. M. de Los Santos, in: Enantioselective
6848
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 6846–6849

C–P Bond Formation by Enantioselective Pd-Catalyzed Allylation
Synthesis of β-Amino Acids (Eds.: E. Juaristi, V. A. So-
loshonok), 2nd ed., Wiley, Hoboken, NJ, 2005, p. 277.
[6] a) Y.-R. Chen, J.-J. Feng, W.-L. Duan, Tetrahedron Lett. 2014,
55, 595; b) J.-Z. Lu, J.-X. Ye, W.-L. Duan, Chem. Commun.
2014, 50, 698; c) Y. H. Huang, S. A. Pullarkat, Y. X. Li, P. H.
Leung, Inorg. Chem. 2012, 51, 2533; d) Y. H. Huang, R. J.
Chew, Y. X. Li, S. A. Pullarkat, P. H. Leung, Org. Lett. 2011,
13, 5862; e) Y.-R. Chen, J.-J. Feng, W.-L. Duan, Org. Lett.
2011, 13, 5824; f) J.-J. Feng, X.-F. Chen, M. Shi, W.-L. Duan,
J. Am. Chem. Soc. 2010, 132, 5562; g) A. Carlone, G. Bartoli,
M. Bosco, L. Sambri, P. Melchiorre, Angew. Chem. Int. Ed.
2007, 46, 4504; Angew. Chem. 2007, 119, 4588; h) I. Ibrahem,
R. Rios, J. Vesely, P. Hammar, L. Eriksson, F. Himo, A.
Córdova, Angew. Chem. Int. Ed. 2007, 46, 4507; Angew. Chem.
2007, 119, 4591
[12]
a) H. Takaya, K. Mashima, K. Koyano, M. Yagi, H. Kumo-
bayashi, T. Taketomi, S. Akutagawa, J. Org. Chem. 1986, 51,
629; b) R. Noyori, H. Takaya, Acc. Chem. Res. 1990, 23, 345;
c) S. Akutagawa, Appl. Catal. A 1995, 128, 171; d) M. McCar-
thy, P. J. Guiry, Tetrahedron 2001, 57, 3809; e) H. Shimizu, I.
Nagasaki, T. Saito, Tetrahedron 2005, 61, 5405.
C.-C. Pai, Y.-M. Li, Z.-Y. Zhou, A. S. C. Chan, Tetrahedron
Lett. 2002, 43, 2789.
A. Togni, C. Breutel, A. Schnyder, F. Spindler, H. Landert, A.
Tijani, J. Am. Chem. Soc. 1994, 116, 4062.
B. M. Trost, D. L. Van Vranken, Angew. Chem. Int. Ed. Engl.
1992, 31, 228; Angew. Chem. 1992, 104, 194.
a) P. von Matt, A. Pfaltz, Angew. Chem. Int. Ed. Engl. 1993, 32,
566; Angew. Chem. 1993, 105, 614; b) J. Sprinz, G. Helmchen,
Tetrahedron Lett. 1993, 34, 1769; c) G. J. Dawson, C. G. Frost,
J. M. J. Williams, Tetrahedron Lett. 1993, 34, 3149.
[13]
[14]
[15]
[16]
[7] B.-Q. Ding, Z.-F. Zhang, Y. Xu, Y.-G. Liu, M. Sugiya, T. Im-
amoto, W.-B. Zhang, Org. Lett. 2013, 15, 5476.
[8] a) L. Yin, Y. Bao, N. Kumagai, M. Shibasaki, J. Am. Chem. [17] F. López, T. Ohmura, J. F. Hartwig, J. Am. Chem. Soc. 2003,
Soc. 2013, 135, 10338; b) Y. Sohtome, N. Horitsugi, R. Takagi,
K. Nagasawa, Adv. Synth. Catal. 2011, 353, 2631; c) D. Uragu-
chi, T. Ito, T. Ooi, J. Am. Chem. Soc. 2009, 131, 3836; d) T.
Akiyama, H. Morita, P. Bachu, K. Mori, M. Yamanaka, T.
Hirata, Tetrahedron 2009, 65, 4950; e) J. L. Montchamp, Acc. [20] a) Y. Belabassi, K. Bravo-Altamirano, J.-L. Montchamp, J. Or-
Chem. Res. 2014, 47, 77.
125, 3426.
[18] J. Tsuji, I. Shimizu, I. Minami, Y. Ohashi, Tetrahedron Lett.
1982, 23, 4809.
[19] B. Springs, P. Haake, J. Org. Chem. 1977, 42, 472.
ganomet. Chem. 2011, 696, 106; b) S. Deprèle, J.-L. Montch-
amp, Org. Lett. 2004, 6, 3805; c) S. Deprèle, J.-L. Montchamp,
J. Am. Chem. Soc. 2002, 124, 9386.
[9] a) Z.-C. Geng, J.-X. Zhang, N. Li, J. Chen, X.-F. Huang, S.-Y.
Zhang, H.-Y. Li, J.-C. Tao, X.-W. Wang, Tetrahedron 2014, 70,
417; b) A. Russo, A. Lattanzi, Eur. J. Org. Chem. 2010, 6736.
[10] S. Demay, A. Kotschy, P. Knochel, F. Volant, M. Lotz, Patent
EP 1 182 205 A1, 2002.
[11] a) J. E. Nycz, R. Musiol, Heteroat. Chem. 2006, 17, 310; b) J. E.
Nycz, G. Malecki, S. Chikkali, I. Hajdok, P. Singh, Phosphorus
Sulfur Silicon Relat. Elem. 2012, 187, 564.
[21] C–P bond formation by Pd- and Ni-catalyzed allylation has
been described, see: a) R. Malet, M. Moreno-Mafias, R. Pleix-
ats, Synth. Commun. 1992, 22, 2219; b) X. Y. Lu, J. Y. Zhu,
Synthesis 1986, 563.
Received: August 1, 2014
Published Online: September 30, 2014
Eur. J. Org. Chem. 2014, 6846–6849
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6849