Microwave-promoted Beller's synthesis of substituted phthalates

Article abstract of DOI:10.1080/00397910500522082

A rapid and efficient synthesis of substituted phthalates via microwave-promoted Beller's reaction of aldehydes, amides, and dimethyl acetylenedicarboxylate is described. This one-pot, multicomponent reaction was performed under acetic anhydride-free and solvent-free conditions. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910500522082

Synthetic Communicationsw, 36: 1383–1390, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500522082
Microwave-Promoted Beller’s Synthesis
of Substituted Phthalates
Yun-Yun Yang, Wang-Ge Shou, and Yan-Guang Wang
Department of Chemistry, Zhejiang University, Hangzhou,
Zhejiang, China
Abstract: A rapid and efficient synthesis of substituted phthalates via microwave-
promoted Beller’s reaction of aldehydes, amides, and dimethyl acetylenedicarboxylate
is described. This one-pot, multicomponent reaction was performed under acetic
anhydride–free and solvent-free conditions.
Keywords: Beller’s reaction, microwave, multicomponent reactions, phthalates
In the past decade, multicomponent reactions (MCRs) have emerged as a
powerful tool for the creation of molecular complexity and diversity
through facile formation of several new covalent bonds in one-pot transform-
ations.[1] This methodology offers significant advantages over the stepwise
procedures and therefore possesses high efficiency in the synthesis of
complex organic building blocks from easily available starting materials.[2]
Recently, Beller and coworkers developed a multicomponent reaction of
amides, aldehyde, and dienophile (AAD reaction) for the straightforward
synthesis of highly substituted cyclohexenes[3] and anilines.[4] This AAD
reaction also provides an efficient strategy for the synthesis of alkyl
phthalates.[5] However, their synthesis of phthalates usually requires long
reaction times (24–64 h) and stoichiometric amounts of acetic anhydride.
Received in Japan September 13, 2005
Address correspondence to Yan-Guang Wang, Department of Chemistry, Zhejiang
University, Hangzhou, Zhejiang 310027, China. Fax: þ86-571-87951512; E-mail:
orgwyg@zju.edu.cn
1383

1384
Y.-Y. Yang, W.-G. Shou, and Y.-G. Wang
In connection with our research on microwave-promoted MCRs,[6]
herein we report a microwave-promoted Beller’s reaction of amides,
aldehydes, and electron-deficient alkynes, which led to a rapid and efficient
synthesis of alkyl phthalates under acetic anhydride–free and solvent-free
conditions.
Our work began with the investigation of Beller’s reaction using
microwave irradiation instead of conventional heating (Scheme 1). Initially,
equal equivalents of trans-octenal (1a), acetamide (2a), and dimethyl acetyle-
nedicarboxylate (3) and catalytic amounts of p-toluenesulfonic acid (TSA)
were used. The mixture was irradiated in a microwave at 300 W to result in
dimethyl phthalate 4a, and the yield was almost quantitative. We then
examined the possibility for using catalytic amounts of the amide that is elimi-
nated in the last step of the domino reaction as shown in Scheme 1. It was
found that the reaction of a,b-unsaturated aldehydes 1 and dimethyl
acetylene-dicarboxylate (3) with amides 2 (5 mol%) in the presence of TSA
(1.5 mol%) at 300 W for 10 min obtained phthalate derivatives 4 in
excellent yields (93–98%) (Table 1). The reactions were performed under
solvent-free and acetic anhydride–free conditions, which are necessary for
Beller’s procedure.[3–5]
We also employed the saturated aldehydes 5 instead of a,b-unsaturated
aldehydes 1 for the Beller’s reaction. The in situ generated a,b-unsaturated
aldehydes from 5 under the microwave reaction conditions could also afford
substituted phthalate derivatives 6 (Scheme 2). Eleven aliphatic aldehydes
(Table 2, entries 1–11, 13, and 14) and phenylacetaldehyde (Table 2,
entries 12 and 15) were tested. It was found that all of the reactions gave
good to excellent yields (80–98%). Additionally, acetamide (Table 2,
entries 3, 5, and 12) is almost as effective as benzamide (Table 2, entries
13–15).
Scheme 1.

Microwave-Promoted Beller’s Synthesis
1385
Table 1. Synthesis of substituted phthalates 4
Entry
R1
R2
R3
Product
Isolated yield (%)
1
2
3
4
5
H
H
H
n-C4H9
n-C4H9
H
CH3
Ph
4a
4a
4b
4c
4c
96
95
98
94
93
CH3
CH3
Ph
CH3
CH3
(CH3)2C55CHCH2
(CH3)2C55CHCH2
This domino reaction involves the intermediates of an acetamido
butadiene or benzamido butadiene species that undergoes Diels–Alder
addition to dimethyl acetylenedicarboxylate. The resultant acetamidodiene
or benzamidodiene is subjected to elimination of acetamide or benzamide
under microwave irradiation conditions to give substituted dimethyl
phthalates.
In summary, we have developed an efficient synthesis of substituted
phthalates via microwave-promoted Beller’s multicomponent reaction of
aldehydes, amides, and dimethyl acetylenedicarboxylate. As compared with
the Beller’s method,[5] this one-pot procedure is rapid and can be performed
in the absence of acetic anhydride and under solvent-free conditions. It is note-
worthy that the in situ generated a,b-unsaturated aldehydes under the reaction
conditions could also react to give the corresponding phthalate derivatives.
The domino condensation–cycloaddition–elimination sequence has been
applied to a variety of aldehydes.
EXPERIMENTAL
1
IR spectra were recorded on a Perkin-Elmer 983 FT-IR spectrometer. H
NMR and 13C NMR spectra were obtained on a Bruker Advance DMX 500
instrument. HRMS data were obtained on a Bruker FT-ICR-MS Apex III
apparatus. MS data were recorded on a Bruker Esquir 3000-plus instrument.
HPLC analysis was carried out on Agilent 1100 (250 ꢀ 4.6-mm C18
column, gradient elution 80% MeOH and 20% H2O, 0.8 mL/min, UV
detection at l 254 nm).
Scheme 2.

1386
Y.-Y. Yang, W.-G. Shou, and Y.-G. Wang
Table 2. Synthesis of substituted phthalates 6
Entry
R1
R2
CH3
Product
Isolated yield (%)
1
2
H
CH3
6a
6b
6c
6d
6e
6f
85
93
96
95
83
91
90
89
93
95
95
98
91
80
95
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
Ph
3
C2H5
4
n-C3H7
i-C3H7
n-C4H9
n-C5H11
n-C6H13
n-C7H15
n-C8H17
n-C10H21
Ph
5
6
7
6g
6h
6i
8
9
10
11
12
13
14
15
6j
6k
6l
C2H5
i-C3H7
Ph
6c
6e
6l
Ph
Ph
General Procedure for the Synthesis of Substituted Phthalates 4
from a,b-Unsaturated Aldehydes 1
Acetamide (13.2 mg, 5 mol%), a,b-unsaturated aldehydes (2.5 mmol),
dimethyl acetylenedicarboxylate (853 mg, 5 mmol), and p-toluenesulfonic
acid (22 mg, 1.5 mol%) were combined in a 5 mL glass bottle immersed in
a Pyrexw crystallization dish filled with neutral alumina (500 g, 100–200
mesh). This setup was placed at the cavity of microwave synthesizer and irra-
diated at 300 W for 10 min. The reaction mixture was cooled to room tempera-
ture and then dissolved in ethyl acetate (50 mL). The solution was washed with
water, dried over anhydrous Na2SO4, and evaporated in vacuum. The crude
products were purified by a flash-column chromatography on silica gel with
ethyl acetate–hexane (1 : 10, v/v) as eluent.
Compound 4a: Light yellow oil; IR (neat) 1732, 1592, 1460, 1433, 1283,
1
1208 cm21; H NMR (500 MHz, CDCl3) d 7.85 (d, J ¼ 7.6 Hz, 1 H), 7.43
(m, 2 H), 3.94 (s, 3 H), 3.89 (s, 3 H), 2.62 (t, J ¼ 7.80 Hz, 2 H), 1.59–1.57
(m, 2 H), 1.37–1.35 (m, 2 H), 0.94 (t, J ¼ 7.4 Hz, 3 H) ppm; 13C NMR
(125 MHz, CDCl3) d 170.1, 166.6, 140.7, 135.2, 134.0, 129.3, 128.0, 127.8,
52.7, 52.6, 33.6, 33.2, 22.7, 14.1 ppm; MS (ESI) m/z 273 ([M þ Na]þ);
HRMS (ESI) calcd. for C14H18O4Na (M þ Na)þ 273.1097, found 273.1098.
Compound 4b: Light yellow oil; IR (neat) 1733, 1593, 1460, 1433, 1285,
1
1253, 1208 cm21; H NMR (500 MHz, CDCl3): d 7.72–7.71 (d, J ¼ 7.5 Hz,
2 H), 7.53–7.52 (d, J ¼ 7.5 Hz, 2 H), 3.89 (s, 6 H) ppm; 13C NMR
(125 MHz, CDCl3): d 170.2, 132.1, 131.3, 129.1, 52.82 ppm; MS (ESI) m/z

Microwave-Promoted Beller’s Synthesis
1387
217 ([M þ Na]þ); HRMS (ESI): calcd. for C10H10O4Na (M þ Na)þ 217.0471,
found 217.0467.
Compound 4c: Light yellow oil; IR (neat) 1733, 1592, 1460, 1433, 1286,
1
1254, 1208 cm21; H NMR (500 MHz, CDCl3): d 7.69 (d, J ¼ 7.2 Hz, 1 H),
7.22 (d, J ¼ 7.2 Hz, 1 H), 5.18 (t, J ¼ 5.2 Hz, 1 H), 3.90 (s, 3 H), 3.88 (s,
3 H), 2.70 (d, J ¼ 4.9 Hz, 2 H), 2.35 (s, 3 H), 1.55 (s, 6 H) ppm; 13C NMR
(125 MHz, CDCl3): d 170.1, 167.8, 146.5, 136.5, 133.1, 132.8, 130.0, 128.9,
126.7, 118.5, 52.7, 52.6, 25.8, 20.2, 19.2, 17.8 ppm; MS (ESI) m/z 299
([M þ Na]þ). HRMS (ESI): calcd. for C16H20O4Na (M þ Na)þ 299.1254,
found 299.1242.
General Procedure for the Synthesis of Substituted Phthalates 6
from Aldehydes 5
Acetamide (13.2 mg, 5 mol%), aldehydes (5 mmol), dimethyl acetylenedicar-
boxylate (853 mg, 5 mmol), and p-toluenesulfonic acid (22 mg, 1.5 mol%)
were combined in a 5 mL glass bottle immersed in a Pyrexw crystallization
dish filled with neutral alumina (500 g, 100–200 mesh). This setup was
placed at the cavity of microwave synthesizer and irradiated at 300 W for
10 min. The reaction mixture was cooled to room temperature and then
dissolved in ethyl acetate 50 mL. The solution was washed with water,
dried over anhydrous Na2SO4, and evaporated in vacuum. The crude
products were purified by a flash-column chromatography on silica gel with
ethyl acetate–hexane (1 : 10, v/v) as eluent.
Compound 6a: Light yellow oil; IR (neat) 1733, 1593, 1460, 1433, 1285,
1
1253, 1208 cm21; H NMR (500 MHz, CDCl3): d 7.72–7.71 (d, J ¼ 7.5 Hz,
2 H), 7.53–7.52 (d, J ¼ 7.5 Hz, 2 H), 3.89 (s, 6 H) ppm; 13C NMR
(125 MHz, CDCl3): d 170.2, 132.1, 131.3, 129.1, 52.82 ppm; MS (ESI) m/z
217 ([M þ Na]þ); HRMS (ESI): cald. for C10H10O4Na (M þ Na)þ
217.0471, found 217.0467.
Compound 6b: Light yellow oil; IR (neat) 1733, 1592, 1460, 1434, 1283,
1253, 1208 cm21; 1H NMR (500 MHz, CDCl3): d 7.60 (s, 1 H), 7.18 (s, 1 H),
3.89 (s, 3 H), 3.85 (s, 3 H), 2.32 (s, 3 H), 2.29 (s, 3 H) ppm; 13C NMR
(125 MHz, CDCl3): d 170.1, 166.7, 139.3, 135.6, 135.2, 132.6, 128.0, 52.5,
21.1, 19.1 ppm; MS (ESI) m/z 245 ([M þ Na]þ); HRMS (ESI): calcd. for
C12H14O4Na (M þ Na)þ 245.0784, found 245.0785.
Compound 6c: Light yellow oil; IR (neat) 1732, 1607, 1459, 1435, 1287,
1253, 1208 cm21; 1H NMR (500 MHz, CDCl3): d 7.65 (s, 1 H), 7.25 (s, 1 H),
3.91 (s, 3 H), 3.87 (s, 3 H), 2.67–2.61 (m, 4 H), 1.27–1.25 (m, 6 H) ppm;
13C NMR (125 MHz, CDCl3): d 170.2, 166.8, 145.9, 142.0, 132.8, 132.4,
128.2, 127.2, 52.58, 52.58, 28.8, 26.6, 15.8, 15.5 ppm; MS (ESI): m/z 273

1388
Y.-Y. Yang, W.-G. Shou, and Y.-G. Wang
([M þ Na]þ); HRMS (ESI): calcd. for C14H18O4Na (M þ Na)þ 273.1097,
found 273.1097.
Compound 6d: Light yellow oil; IR (neat) 1730, 1594, 1460, 1433, 1283,
1
1254, 1208 cm21; H NMR (500 MHz, CDCl3): d 7.65 (s, 1 H), 7.27 (s, 1
H), 3.92 (s, 3 H), 3.87 (s, 3 H), 2.62–2.56 (m, 4 H), 1.67–1.62 (m, 4 H),
0.96–0.92 (m, 6 H) ppm; 13C NMR (125 MHZ, CDCl3): d 170.3, 166.9,
144.1, 140.4, 134.1, 132.7, 128.2, 127.9, 52.6, 37.8, 35.5, 24.7, 24.5, 14.2,
13.9 ppm; MS (ESI): m/z 301 ([M þ Na]þ); HRMS (ESI): calcd. for
C16H22O4Na (M þ Na)þ 301.1410, found 301.1403.
Compound 6e: Light yellow oil; IR (neat) 1731, 1592, 1460, 1433, 1284,
1
1253, 1208 cm21; H NMR (500 MHz, CDCl3): d 7.70 (s, 1 H), 7.37 (s, 1
H), 3.92 (s, 3 H), 3.87 (s, 3 H), 2.97–2.93 (m, 2 H), 1.26–1.24 (m, 12 H)
ppm; 13C NMR (125 MHz, CDCl3): d 170.4, 166.8, 150.5, 146.4, 132.2,
128.4, 127.8, 125.9, 52.6, 52.5, 34.3, 31.1, 24.2, 23.9 ppm; MS (ESI): m/z
301 ([M þ Na]þ); HRMS (ESI): calcd. for C16H22O4Na (M þ Na)þ
301.1410, found 301.1404.
Compound 6f: Light yellow oil; IR (neat) 1732, 1607, 1463, 1435, 1294,
1269, 1210 cm21; 1H NMR (500 MHz, CDCl3): d 7.63 (s, 1 H), 7.22 (s, 1 H),
3.91 (s, 3 H), 3.87 (s, 3 H), 2.64–2.58 (m, 4 H), 1.61–1.58 (m, 4 H), 1.37–
1.32 (m, 4 H), 0.94–0.90 (m, 6 H) ppm; 13C NMR (125 MHz, CDCl3): d
170.3, 166.8, 144.4, 140.7, 133.9, 132.5, 128.2, 127.8, 52.6, 35.5, 33.7,
33.5, 33.1, 22.8, 22.5, 14.1 ppm; MS (ESI): m/z 307 ([M þ H]þ). HRMS
(ESI): calcd. for C18H26O4Na (M þ Na)þ 329.1723, found 329.1715.
Compound 6g: Light yellow oil; IR (neat) 1733, 1607, 1464, 1435, 1279,
1
1208 cm21; H NMR (500 MHz, CDCl3): d 7.63 (s, 1 H), 7.22 (s, 1 H),
3.91 (s, 3 H), 3.87 (s, 3 H), 2.63–2.56 (m, 4 H), 1.61–1.59 (m, 4 H), 1.33–
1.29 (m, 8 H), 0.90–0.87 (m, 6 H) ppm; 13C NMR (125 MHz, CDCl3): d
170.2, 166.8, 144.3, 140.7, 133.9, 132.5, 128.2, 127.7, 52.5, 52.4, 35.7,
33.4, 31.8, 31.5, 31.2, 31.0, 22.7, 22.6, 14.2, 14.1 ppm; MS (ESI): m/z 357
([M þ Na]þ). HRMS (ESI): calcd. for C20H30O4Na (M þ Na)þ 357.2036,
found 357.2040.
Compound 6h: Light yellow oil; IR (neat) 1734, 1592, 1460, 1433, 1283,
1
1253, 1208 cm21; H NMR (500 MHz, CDCl3): d 7.63 (s, 1 H), 7.22 (s, 1
H), 3.91 (s, 3 H), 3.87 (s, 3 H), 2.63–2.56 (m, 4 H), 1.60–1.57 (m, 4 H),
1.30–1.26 (m, 12 H), 0.90–0.87 (m, 6 H) ppm; 13C NMR (125 MHz,
CDCl3): d 170.3, 166.9, 144.4, 140.7, 133.9, 132.5, 128.2, 127.7, 52.6, 52.5,
35.8, 33.5, 32.0, 31.97, 31.6, 31.4, 29.9, 29.3, 29.2, 22.9, 14.3 ppm; MS
(ESI): m/z 385 ([M þ Na]þ). HRMS (ESI): calcd. for C22H34O4Na
(M þ Na)þ 385.2339, found 385.2342.
Compound 6i: Light yellow oil; IR (neat) 1732, 1594, 1460, 1433, 1284,
1
1256, 1208 cm21; H NMR (500 MHz, CDCl3): d 7.63 (s, 1 H), 7.22 (s,
1 H), 3.91 (s, 3 H), 3.87 (s, 3 H), 2.63–2.56 (m, 4 H), 1.60–1.57 (m, 4 H),

Microwave-Promoted Beller’s Synthesis
1389
1.30–1.27 (m, 16 H), 0.89–0.87 (m, 6 H) ppm; 13C NMR (125 MHz, CDCl3):
d 170.3, 166.9, 144.4, 140.7, 133.9, 132.5, 128.2, 127.6, 52.5, 52.4, 35.7, 33.5,
32.0, 31.96, 31.6, 31.4, 29.6, 29.3, 29.2, 22.9, 14.3 ppm; MS (ESI): m/z 413
([M þ Na]þ). HRMS (ESI): calcd. for C24H38O4Na (M þ Na)þ 413.2662,
found 413.2646.
Compound 6j: Light yellow oil; IR (neat) 1733, 1592, 1460, 1433, 1283,
1
1253, 1208 cm21; H NMR (500 MHz, CDCl3): d 7.63 (s, 1 H), 7.22 (s, 1
H), 3.91 (s, 3 H), 3.87 (s, 3 H), 2.63–2.56 (m, 4 H), 1.60–1.57 (m, 4 H),
1.30–1.26 (m, 20 H), 0.90–0.87 (m, 6 H) ppm; 13C NMR (125 MHz,
CDCl3): d 170.3, 166.9, 144.3, 140.7, 133.9, 132.5, 128.2, 127.7, 52.6, 52.5,
35.8, 33.5, 32.1, 31.6, 31.4, 29.7, 29.6, 29.5, 29.4, 29.3, 22.9, 14.3 ppm; MS
(ESI): m/z 441 ([M þ Na]þ). HRMS (ESI): calcd. for C26H42O4Na
(M þ Na)þ 441.2962, found 441.2960.
Compound 6k: Light yellow oil; IR (neat) 1735, 1607, 1464, 1435, 1272,
1
1208 cm21; H NMR (500 MHz, CDCl3): d 7.63 (s, 1 H), 7.21 (s, 1 H),
3.91 (s, 3 H), 3.87 (s, 3 H), 2.64–2.57 (m, 4 H), 1.60–1.59 (m, 4 H), 1.30–
1.26 (m, 28 H), 0.89–0.87 (m, 6 H) ppm; 13C NMR (125 MHz, CDCl3): d
170.3, 166.9, 144.4, 140.7, 133.9, 132.5, 128.2, 127.7, 52.6, 52.5, 35.8,
33.5, 32.1, 31.5, 31.4, 29.9, 29.8, 29.7, 29.66, 29.62, 29.5, 29.4, 22.9,
14.3 ppm; MS (ESI): m/z 497 ([M þ Na]þ). HRMS (ESI): calcd. for
C30H50O4Na (M þ Na)þ 497.3585, found 497.3590.
Compound 6l: Light yellow oil; IR (neat) 1731, 1599, 1497, 1433, 1270,
1
1244, 1200 cm21; H NMR (500 MHz, CDCl3): d 8.28 (s, 1 H), 7.79 (s, 1
H), 7.67–7.65 (m, 2 H), 7.50–7.47 (m, 2 H), 7.44–7.40 (m, 6 H), 3.96 (s, 3
H), 3.70 (s, 3 H) ppm; 13C NMR (125 MHz, CDCl3): d 169.5, 166.5, 142.5,
141.5, 139.6, 139.3, 133.6, 132.9, 129.3, 129.2, 129.1, 128.9, 128.8, 128.6,
128.58, 128.54, 128.2, 127.7, 127.5, 52.9, 52.5 ppm; MS (ESI): m/z 369
([M þ Na]þ). HRMS (ESI): calcd. for C22H18O4Na(M þ Na)þ 369.1097,
found 369.1083.
REFERENCES
1. (a) Bhar, S. S.; Ramana, M. M. V. J. Org. Chem. 2004, 69, 8935–8937;
(b) Nordmann, G.; Bchwald, S. L. J. Am. Chem. Soc. 2003, 125, 4978–4979;
(c) Tietze, L. T. Chem. Rev. 1996, 96, 115–136; (d) Zeng, Y.; Reddy, D. S.;
Hirt, E.; Aube, J. Org. Lett. 2004, 6, 4993–4995; (e) Tejedor, D.;
Gonzalez, C. D.; Garcia, T. F.; Marrero, T. J. J.; Rodriguez, M. L. J. Am. Chem.
Soc. 2004, 126, 8390–8391; (f) Kraus, G. A.; Kim, J. Org. Lett. 2004, 6,
3115–3117; (g) Bear, B. R.; Shea, K. J. Org. Lett. 2001, 3, 723–726;
(h) Shindo, M.; Sato, Y.; Shishido, K. J. Am. Chem. Soc. 1999, 121,
6507–6508; (i) Eibracht, P.; Barfacker, L.; Buss, C.; Hollmann, C. L.;
Kitsos, R. B. E.; Krannemann, C. L.; Rische, T.; Roggenbuck, R.; Schmidt, A.
Chem. Rev. 1999, 99, 3329–3366.

1390
Y.-Y. Yang, W.-G. Shou, and Y.-G. Wang
¨
2. (a) Doming, A.; Ugi, I. Angew. Chem., Int. Ed. Engl. 2000, 39, 3168–3210;
(b) Domingo, L. R. Eur. J. Org. Chem. 2004, 4788–4793; (c) Appukkuttan, P.;
Dehaen, W.; Fokin, V. V.; Van der Eycken, E. Org. Lett. 2004, 6, 4223–4225;
(d) Hayes, J. F.; Shipman, M.; Twin, H. J. Org. Chem. 2002, 67, 935–942;
(e) Byk, G.; Kabahn, E. J. Comb. Chem. 2004, 6, 596–603; (f) Marcaccini, S.;
Miguel, D.; Torroba, T.; Garcia-Valverge, M. J. Org. Chem. 2003, 68, 3315–3318.
¨
3. (a) Neumann, H.; Wangelin, A. J. V.; Gordes, D.; Spannenberg, A.; Beller, M.
J. Am. Chem. Soc. 2001, 123, 8398–8399; (b) Wangelin, A. J. V.;
¨
Neumann, H.; Gordes, D.; Klaus, S.; Strubing, D.; Beller, M. Chem. Eur. J.
2003, 9, 4286–4294; (c) Wangelin, A. J. V.; Neumann, H.; Gordes, D.;
¨
¨
Spannenberg, A.; Beller, M. Org. Lett. 2001, 3, 2895–2898; (d) Neumann, H.;
¨
Wangelin, A. J. V.; Gordes, D.; Spannenberg, A.; Baumann, W.; Beller, M. Tetra-
hedron 2002, 58, 2381–2387.
¨
4. Neumann, H.; Wangelin, A. J. V.; Klaus, S.; Stru¨bing, D.; Gordes, D.; Beller, M.
Angew. Chem., Int. Ed. Engl. 2003, 42, 4503–4507.
¨
5. Wangelin, A. J. V.; Neumann, H.; Gordes, D.; Klaus, S.; Jiao, H. J.;
Spannenberg, A.; Kruger, T.; Wendler, C.; Thurow, K.; Stoll, N.; Beller, M.
Chem. Eur. J. 2003, 9, 2273–2281.
6. (a) Cui, S. L.; Lin, X. F.; Wang, Y. G. J. Org. Chem. 2005, 70, 2866–2869;
(b) Xia, M.; Wang, Y. G. Tetrahedron Lett. 2002, 43, 7703–7705.