Selective synthesis of 4-alkylidene-β-lactams and N,N′- diarylamidines from azides and aryloxyacetyl chlorides via a ketenimine-participating one-pot cascade process

Article abstract of DOI:10.1021/jo702733h

(Chemical Equation Presented) A one-pot cascade approach to 4-alkylidene-β-lactams and N,N-diarylamidines from aryl azides and aryloxyacetyl chlorides has been developed. The chemical outcome of the reaction can be controlled selectively by an appropriate choice of the stoichiometric ratio of different substrates and reagents. The products should find use in pharmaceutical discovery, especially in the development of new antimicrobial agents against multidrug-resistant pathogens.

Full text of DOI:10.1021/jo702733h

modern synthetic methods have also been reported.5 On the other
hand, the problem of microbial resistance, related to the use of
antibiotic agents over the last 50 years, is nowadays widely
recognized, and treatment options in clinical practice are limited
by multidrug-resistant bacteria. More recently, it was reported
that some 4-alkylidene-ꢀ-lactams exhibited antibiotic activity
against resistant bacteria.6 However, very few examples of this
new class of monocycle ꢀ-lactams have been reported in the
literature.6,7
Selective Synthesis of 4-Alkylidene-ꢀ-lactams and
N,N′-Diarylamidines from Azides and
Aryloxyacetyl Chlorides via a
Ketenimine-Participating One-Pot Cascade
Process
Yun-Yun Yang, Wang-Ge Shou, Deng Hong, and
Yan-Guang Wang*
Ketenimines are nitrogenated heterocumulenes, which can
account for the addition of a variety of nucleophiles8 and
radicals9 to their central carbon atom, and participate in
pericyclic reactions such as [2 + 2] and [2 + 4] cycloadditions10
as well as sigmatropic rearrangements.11 Previously, we devel-
oped an efficient synthesis of iminocoumarins, 5-arylidene-2-
imino-3-pyrrolines, 2-imino-1,2-dihydroquinolines, and 2-imi-
nothiochromenes, involving a ketenimine intermediate in situ
generated from copper-catalyzed cycloaddition of sulfonyl azides
and terminal alkynes.12 Encouraged by these works, we are
interested in the ketenimine-participating cascade reactions.
Herein, we describe a one-pot cascade synthesis of 4-alkylidene-
ꢀ-lactams and N,N′-diarylamidines from arylazides and aryl-
oxyacetyl chlorides through a ketenimine intermediate.
We began our investigations by looking into the reaction of
the triphenylphosphazene derived from (4-methyloxyphenyl)-
azide (1a) and the ketene in situ generated from phenoxyacetyl
chloride (2a) (Scheme 1). In a one-pot procedure, 1a reacted
with triphenylphosphine in 1,2-dichloroethane (DCE) to form
triphenylphosphazene 3 via the Staudinger-Meyer reaction,13a
which was immediately treated with 2 equiv of 2a and
triethylamine at -5 °C and then at room temperature to afford
4-phenoxymethylene-ꢀ-lactam 5a. We believe that this cascade
process involves an aza-Wittig reaction of triphenylphosp-
Department of Chemistry, Zhejiang UniVersity,
Hangzhou 310027, China
orgwyg@zju.edu.cn
ReceiVed December 21, 2007
A one-pot cascade approach to 4-alkylidene-ꢀ-lactams and
N,N′-diarylamidines from aryl azides and aryloxyacetyl
chlorides has been developed. The chemical outcome of the
reaction can be controlled selectively by an appropriate
choice of the stoichiometric ratio of different substrates and
reagents. The products should find use in pharmaceutical
discovery, especially in the development of new antimicrobial
agents against multidrug-resistant pathogens.
(5) (a) Zhao, L.; Li, C. J. Chem. Asian J. 2006, 1, 203–209. (b) Fustero, S.;
Fernández, B.; Bello, P.; del Pozo, C.; Arimitsu, S.; Hammond, G. B. Org. Lett.
2007, 9, 4251–4253.
(6) (a) Cainelli, G.; Giacomini, D.; Galletti, P.; Quintavalla, A. Eur. J. Org.
Chem. 2003, 9, 1765–1774. (b) Broccolo, F.; Cainelli, G.; Caltabiano, G.;
Cocuzza, C. E. A.; Fortuna, C. G.; Galletti, P.; Giacomini, D.; Musumarra, G.;
Musumeci, R.; Quintavalla, A. J. Med. Chem. 2006, 49, 2804–2811.
(7) (a) Bachi, M. D.; Goldberg, O.; Grass, A.; Vaya, J. J. Org. Chem. 1980,
45, 1481–1485. (b) Prasad, K.; Kneussel, P.; Schulz, G.; Stutz, P. Tetrahedron
Lett. 1982, 23, 1247–1250. (c) Greengrass, C. W.; Hoople, D. W. Tetrahedron
Lett. 1981, 22, 1161–1164. (d) Battaglia, A.; Cainelli, G.; Giacomini, D.; Martelli,
G.; Panunzio, M. Tetrahedron Lett. 1987, 28, 4347–4350.
The ꢀ-lactams are an important class of heterocyclic com-
pounds due to their antibiotic activity1 and their utility as
versatile building blocks in organic synthesis.2 The Staudinger
reaction3 and the Gilman-Speeter reaction4 are the classical
methods for the construction of ꢀ-lactam rings. A number of
(8) (a) Bae, I.; Han, H.; Chang, S. J. Am. Chem. Soc. 2005, 127, 2038–
2039. (b) Cho, S. H.; Yoo, E. J.; Bae, I.; Chang, S. J. Am. Chem. Soc. 2005,
127, 16046–16047. (c) Yoo, E. J.; Bae, I.; Cho, S. H.; Han, H.; Chang, S. Org.
Lett. 2006, 8, 1347–1349. (d) Whiting, M.; Fokin, V. V. Angew. Chem., Int. Ed.
2006, 45, 3157–3161. (e) Krow, D. R. Angew. Chem., Int. Ed. 1971, 10, 435–
449.
* To whom correspondence should be addressed. Tel: +86-571-87951512.
Fax: +86-571-87951512.
(1) (a) Magriotis, P. A. Angew. Chem., Int. Ed. 2001, 40, 4377–4379. (b)
Ojima, I.; Delaloge, F. Chem. Soc. ReV. 1997, 26, 377–386. (c) Rosenblum,
S. B.; Huynh, T.; Afonso, A.; Davis, H. R.; Yumibe, N.; Clader, J. W.; Burnett,
D. A. J. Med. Chem. 1998, 41, 973–980. (d) Veinberg, G.; Vorona, M.;
Shestakova, I.; Kanepe, I.; Lukevics, E. Curr. Med. Chem. 2003, 10, 1741–
1757. (e) Singh, G. S. Mini-ReV. Med. Chem. 2004, 4, 93–109.
(2) (a) Alcaide, B.; Almendros, P.; Aragoncillo, C. Chem. ReV. 2007, 107,
4437–4492. (b) Ojima, I. Acc. Chem. Res. 1995, 28, 383–389. (c) Walz, A. J.;
Miller, M. J. Tetrahedron Lett. 2007, 48, 5103–5105.
´
(9) (a) Alajarín, M.; Vidal, A.; Ortín, M.-M. Tetrahedron Lett. 2003, 44,
´
3027–3030. (b) Alajarín, M.; Vidal, A.; Ortín, M.-M. Org. Biomol. Chem. 2003,
1, 4282–4292. (c) Alajarín, M.; Vidal, Á.; Ortín, M.-M.; Bautista, D.; New,
J. Chem. 2004, 28, 570–577. (d) Alajarín, M.; Vidal, A.; Ortín, M.-M.; Bautista,
´
D. Synlett 2004, 99, 1–994.
´
(10) (a) Alajarín, M.; Molina, P.; Vidal, A.; Tovar, F. Tetrahedron 1997,
53, 13449–13472. (b) Alajarín, M.; Vidal, A.; Ortín, M.-M. Tetrahedron 2005,
´
61, 7613–7621. (c) Cassidy, M. P.; Raushel, J.; Fokin, V. V. Angew. Chem, Int.
Ed. 2006, 45, 3154–3157.
(3) (a) Singh, G. S. Tetrahedron 2003, 59, 7631–7649. (b) Palomo, C.;
Aizpurua, J. M.; Ganboa, I.; Oirabide, M. Eur. J. Org. Chem. 1999, 8, 3223. (c)
Jarrahpour, A.; Zarei, M. Tetrahedron Lett. 2007, 48, 8712–8714.
(4) (a) Hart, D. J.; Ha, D. C. Chem. ReV. 1989, 89, 1447–1465. (b) Chen,
L.; Zhao, G.; Ding, Y. Tetrahedron Lett. 2003, 44, 2611–2614. (c) Gilman, H.;
Speeter, M. J. Am. Chem. Soc. 1943, 65, 2255–2256. (d) Yuan, Q.; Jian, S. Z.;
Wang, Y. G. Synlett 2006, 1113, 1115.
´
(11) (a) Alajarín, M.; Ortín, M.-M.; Sánchez-Andrada, P.; Vidal, A.; Bautista,
D. Org. Lett. 2005, 7, 5281–5284. (b) Alajarín, M.; Ortín, M.-M.; Sánchez-
´
Andrada, P.; Vidal, A. J. Org. Chem. 2006, 71, 8126–8139.
(12) (a) Cui, S. L.; Lin, X. F.; Wang, Y. G, Org. Lett. 2006, 8, 4517–4520.
(b) Cui, S. L.; Wang, J.; Wang, Y. G. Org. Lett. 2007, 9, 5023–5025. (c) Cui,
S. L.; Wang, J.; Wang, Y. G. Tetrahedron 2008, 64, 487–492.
3574 J. Org. Chem. 2008, 73, 3574–3577
10.1021/jo702733h CCC: $40.75 2008 American Chemical Society
Published on Web 03/26/2008

SCHEME 1. Synthesis of 4-Alkylidene-ꢀ-lactam 5a from
4-Methyloxyphenyl Azide (1a) and Phenoxyacetyl Chloride
(2a)
TABLE 1. Synthesis of 4-Alkylidene-ꢀ-lactams 5 from Aryl Azides
1 and Aryloxyacetyl Chlorides 2a
FIGURE 1. X-ray structure of compound 5b.
SCHEME 2. Cascade Reaction Using Excess Azide 1f
entry
R1
R2
product yieldb (%)
1
2
3
4
5
6
7
8
4-MeOC6H4 (1a) C6H5 (2a)
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
5o
5p
5q
5r
5s
76
86
80
89
83
85
78
75
90
87
91
88
79
80
87
78
81
81
77
92
51
4-MeC6H4 (1b)
2-MeC6H4 (1c)
C6H5 (1d)
2a
2a
2a
3,4-Me2C6H3 (1e) 2a
3-MeC6H4 (1f)
4-EtOC6H4 (1g)
4-MeC6H4 (2b)
2a
variety of aryl azides 1 and aryloxyacetyl chlorides 2 were
investigated. As shown in Table 1, the electron-rich phenylazides
(Table 1, entries 1-20) gave better yield than the electron-
deficient phenyl azides (Table 1, entry 21). We also examined
the strong electron-withdrawing group substituted phenyl azides
such as 4-nitrophenyl azide, but did not isolate the desired
product due to the poor reactivity of the phosphazene intermedi-
ate derived from 4-nitrophenyl azide. According to the mech-
anism of aza-Wittig reaction,13 the electron-deficient phosp-
hazenes are less reactive than the electron-rich phosphazenes.
1g
1d
2b
2b
9
10
11
12
13
14
15
16
17
18
19
20
21
1b
1f
3-MeC6H4 (2c)
2c
1f
1a
2a
2-Me,4-ClC6H3 (2d)
1b
1c
1d
1f
1g
1a
1d
2d
2d
2d
2d
2d
2b
2c
2a
It is noteworthy that only Z-isomers were obtained in all cases.
The Z configuration of the CdC bond was established by the
X-ray diffraction analysis of compounds 5b (Figure 1) and 5m
(see the Supporting Information).
5t
5u
2-BrC6H4 (1 h)
a Reaction conditions: (1) 1 (1.25 mmol), PPh3 (1.25 mmol) in DCE
(8 mL), N2, 50 °C, 3-5 h; (2) Et3N (2.5 mmol), 2 (2.5 mmol), N2, –5
°C, 1 h, then rt, 0.5 h. b Isolated yields.
During screening of the reaction conditions, we surprisingly
found that excess of azide 1f resulted in a mixture of 4-alkyl-
idene-ꢀ-lactam 5l and amidine 6a (Scheme 2). When 2 equiv
of 1f was used, the reaction only gave 6a in 72% isolated yield.
We examined the scope of amidine formation reaction with
regard to the aryl azide (2 equiv) and the aryloxyacetyl chloride
components. As shown in Table 2, both the electron-rich and
the electron-deficient aryl azides gave good yields (70-85%).
hazenes 3 with ketene13 and a [2 + 2] cycloaddtion reaction of
ketenimine 4 with ketene. Although the formation of 3 could
take place at room temperature, the reaction required longer
reaction time (5 h) and gave 5a in lower yield (72%). Heating
could shorten the first step reaction time and increase the yield.
Under the optimum reaction conditions for the first step (50
°C, 4 h), we obtained 5a in 76% yield (Table 1, entry 1).
With this result in hand, we went on to study the scope of
the methodology. Using the optimized reaction conditions, a
Using two different azides to perform this reaction, we
obtained the amidines bearing two different substituted groups
at N and N′ atoms. For example, when 1 equiv of azide 1m
was used as starting material at the first step and then 1 equiv
of azide 1a or 1i was introduced at the second step of the
reaction sequence, we obtained a mixture of amidine 7 and its
tautomer 8 (Scheme 3).
(13) (a) Staudinger, H.; Meyer, J. HelV. Chim. Acta 1919, 2, 635–646. (b)
Palacios, F.; Alonso, C.; Aparicio, D.; Rubiales, G.; de los Santos, J. M.
Tetrahedron 2007, 63, 523–575. (c) Palacios, F.; Alonso, C.; Rubiales, G.;
Villegas, M. Tetrahedron 2005, 61, 2779–2794. (d) Cossio, F.; Alonso, C.; Lecea,
B.; Ayerbe, M.; Rubiales, G.; Palacios, F. J. Org. Chem. 2006, 71, 2839–2847.
J. Org. Chem. Vol. 73, No. 9, 2008 3575

TABLE 2. Synthesis of Amidines 6 from Azides 1 and Acyl
Chlorides 2a
SCHEME 4. Synthesis of the Deuterated Amidines [D]6
entry
R1
R2
product
yieldb (%)
1
2
3
4
5
6
7
8
1f
2a
2d
2b
2c
2b
2a
2c
2d
2a
2c
2c
2c
2c
6a
6b
6c
6d
6e
6f
6g
6h
6i
72
80
79
75
73
70
75
81
77
85
72
82
81
1d
1d
1d
1f
1d
1f
1f
9
4-ClC6H4 (1i)
3-ClC6H4 (1k)
3-BrC6H4 (1j)
4-BrC6H4 (1l)
2-ClC6H4 (1m)
10
11
12
13
6j
6k
6l
6m
SCHEME 5. Possible Mechanism for the Formation of
Amidines 6
a Reaction conditions: (1) 1 (2.0 mmol), PPh3 (1.0 mmol), DCE (8
mL), rt, 4 h, N2; (2) Et3N (1.0 mmol), 2 (1.0 mmol), -5 °C, 1 h, then
rt, 0.5 h, N. b Isolated yields.
SCHEME 3. Synthesis of Amidines 7 and 8 from Two Kind
of Azides19a
Amidines are important constituents of many biological active
compounds14 and highly useful in coordination chemistry and
materials science.15 In addition, amidines have long been
regarded as useful intermediates in the synthesis of heterocyclic
compounds.16 Consequently, a number of synthetic methods
have been developed for the preparation of amidines.17 Most
of these methods rely on the nucleophilic addition of amines
or ammonia equivalents to nitriles under forcing conditions or
to suitably activated carboxylate equivalents. As a complement
to these methods, our one-pot tandem reaction provides a rapid
procedure for the synthesis of N,N′-diarylamidines.
Having achieved an efficient process for the formation of
N,N′-diarylamidines, we performed deuterium-labeling experi-
ments to gain mechanistic insight into the reaction. When
treating the mixture with D2O or CD3OD after the reaction
completed, we obtained the deuterated amidines [D]6j and [D]6n
(Scheme 4).
On the basis of the above results, the possible reaction
mechanism is outlined as in Scheme 5. First, ketenimine 4 is
formed cleanly by an aza-Wittig reaction of triphenylphosp-
hazene 3 with ketene generated from phenoxyacetyl chloride.13
Intermediate 4 is then protonated to form A. In the next cascade
(14) (a) Collins, J. L.; Shearer, B. G.; Oplinger, J. A.; Lee, S.; Garvey, E. P.;
Salter, M.; Duffy, C.; Burnette, T. C.; Furfine, E. S. J. Med. Chem. 1998, 41,
2858–2871. (b) Meiering, S.; Inhoff, O.; Mies, J.; Vincek, A.; Garcia, G.; Kramer,
B.; Dormeyer, M.; Krauth-Siegel, R. L. J. Med. Chem. 2005, 48, 4793–4802.
(c) Black, S. L.; Jales, A. R.; Brandt, W.; Lewis, J. W.; Husbands, S. M. J. Med.
Chem. 2003, 46, 314–317.
(15) (a) Bambirra, S.; Bouwkamp, M. W.; Meetsma, A.; Hessen, B. J. Am.
Chem. Soc. 2004, 126, 9182–9183. (b) Jiang, X.; Bollinger, J. C.; Lee, D. J. Am.
Chem. Soc. 2005, 127, 15678–15679. (c) DeRosa, F.; Kibbe, M. R.; Najjar, S. F.;
Citro, M. L.; Keefer, L. K.; Hrabie, J. A. J. Am. Chem. Soc. 2007, 129, 3786–
3787.
(16) Ito, K.; Kizuka, Y.; Ihara, S. J. Heterocycl. Chem. 2006, 43, 1217–
1224.
3576 J. Org. Chem. Vol. 73, No. 9, 2008

cooled to -5 °C, Et3N (2.5 mmol) was added. Then the solution
of phenoxyacetyl chloride 2 (2.5 mmol) in DCE (5 mL) was added
dropwise at -5 °C over 1 h. The reaction mixture was stirred at
room temperature for 0.5 h. After removal of triethylamine
hydrochloride by filtration, the filtrate was evaporated in vacuum.
The crude product was purified by silica gel column chroma-
tography using hexane-EtOAc (20:1) as eluant to afford pure
ꢀ-lactam 5.
process, two possibilities remain. One is the intermolecular 1,2-
addition of azide 1 to A to form intermediate B, followed by
the nucleophilic attack of Cl- and extrusion of molecular
nitrogen (Scheme 4, pathway A). Another is the intermolecular
[3 + 2] cycloaddition of azide 1 to A to form tetrazolium
intermediate C and subsequent retro-[3 + 2] reaction with a
concomitant attack of nucleophile Cl- (Scheme 5, pathway B).
The use of azido group as a nucleophile in the 1,2-addition
reactions and as a 1,3-dipole in the [3 + 2] cycloaddition
reactions have been reported previously.17a,18,19 Thus, the
possibility of neither pathway A nor pathway B can be excluded.
The resulting chlorinating species D is scavenged with water
during workup process to give E, which easily transforms into
more stable amidine 6 by a [1,3] shift.
In conclusion, we have developed a ketenimine-participating
tandem reaction, which furnished a simple and efficient one-
pot synthesis of 4-alkyliden-ꢀ-lactams and N,N′-diarylamidines
from azides and aryloxyacetyl chlorides. The chemical outcome
of the reaction can be controlled selectively by an appropriate
choice of the stoichiometric ratio of different substrates and
reagents. The products will find their applications in pharma-
ceutical discovery, especially in the development of new
antimicrobial agents against multidrug-resistant pathogens.
Further investigations in the mechanism of the cascade process
are currently underway.
Compound 5a: colorless solid; mp 77–78 °C; IR (KBr) 1794,
1
1712, 1598, 1517, 1492, 1250, 1230, 1103 cm-1; H NMR (500
MHz, CDCl3) δ 7.50 (d, J ) 9.0 Hz, 2 H), 7.36–7.30 (m, 4 H),
7.16 (d, J ) 8.0 Hz, 2 H), 7.08–7.05 (m, 2 H), 7.00 (d, J ) 8.0 Hz,
2 H), 6.86 (d, J ) 8.0 Hz, 2 H), 6.38 (s, 1 H), 5.77 (s, 1 H), 3.78
(s, 3 H); 13C NMR (125 MHz, CDCl3) δ 164.4, 158.1, 157.4, 157.0,
130.0, 128.0, 124.1, 123.4, 123.3, 123.0, 119.8, 116.3, 116.0, 114.1,
81.9, 55.7; MS (ESI) m/z 374 (M + H); HRMS (ESI) m/z calcd
for (C23H19NO4 + Na) 396.1206, found 396.1209.
General Procedure for the Synthesis of N,N′-Diarylamidines
6. The solution of azide 1 (2.0 mmol) and triphenylphosphine (1.0
mmol) in DCE (8 mL) was stirred under nitrogen atmosphere at
room temperature for 4 h. After the solution was cooled to -5 °C,
Et3N (1.0 mmol) was added. Then the solution of phenyloxyacetyl
chloride 2 (1.0 mmol) in DCE (5 mL) was added dropwise at -5
°C over 1 h. The reaction mixture was stirred at room temperature
for 0.5 h. The mixture was filtered to remove triethylamine
hydrochloride, and the filtrate was evaporated in vacuum. The
productwaspurifiedthroughasilicagelcolumnusinghexane-EtOAc
(16:1).
Compound 6a: colorless solid; mp 94–95 °C; IR (KBr) 3418,
1652, 1589, 1548, 1496, 1240 cm-1; 1H NMR (500 MHz, CDCl3)
δ 7.55–7.52 (m, 2 H), 7.29–7.26 (m, 3 H), 7.20–7.17 (m, 2 H),
7.01–6.98 (m, 1 H), 6.89–6.84 (m, 4 H), 6.71–6.67 (m, 2 H), 4.64
(s, 2 H), 2.33 (s, 6 H); 13C NMR (125 MHz, CDCl3) δ 157.5, 150.4,
149.2, 139.6, 139.2, 138.9, 130.0, 129.2, 128.9, 123.9, 123.6, 122.5,
122.0, 120.2, 118.2, 116.8, 115.2, 64.8, 21.8, 21.7; MS m/z 331
(M + H); HRMS m/z calcd for (C22H22N2O + H) 331.1805, found
331.1803.
Experimental Section
General Procedure for the Synthesis of 4-Alkyliden-ꢀ-
lactams 5. The solution of aryl azide 1 (1.25 mmol) and triph-
enylphosphine (1.25 mmol) in DCE (8 mL) was stirred under
nitrogen atmosphere at 50 °C for 3-4 h. After the mixture was
(17) (a) Chang, S.; Lee, M.; Jung, D. Y.; Yoo, E. J.; Cho, S. H.; Han, S. K.
J. Am. Chem. Soc. 2006, 128, 12366–12367. (b) Saluste, C. G.; Whitby, R. J.;
Furber, M. Angew. Chem., Int. Ed. 2000, 39, 4156–4158. (c) Kusturin, C. L.;
Liebeskind, L. S.; Neumann, W. L. Org. Lett. 2002, 4, 983–985. (d) Kosugi,
M.; Ogata, T.; Tamura, H.; Sano, H.; Migita, T. Chem. Lett. 1986, 1197, 1200.
(e) Katritzky, A. R.; Huang, T.-B.; Voronkov, M. V. J. Org. Chem. 2001, 66,
1043–1045. (f) Charette, A. B.; Grenon, M. Tetrahedron Lett. 2000, 41, 1677–
1680. (g) Mandal, S.; Gauniyal, H. M.; Pramanik, K.; Mukhopadhyay, B. J.
Org. Chem. 2007, 72, 9753–9756. (h) Bae, I.; Han, H.; Chang, S. J. Am. Chem.
Soc. 2005, 127, 2038–2039. (i) Yoo, E. J.; Bae, I.; Cho, S. H.; Han, H.; Chang,
S. Org. Lett. 2006, 8, 1347–1350.
Acknowledgment. This work was supported by the National
Natural Science Foundation of China (No. 20672093) and the
Natural Science Foundation of Zhejiang Province (R404109).
Supporting Information Available: Experimental proce-
1
dures. Spectra data for other products. Copies of H and 13C
(18) For a review, see: Bräse, S.; Gil, C.; Knepper, K.; Zimmermann, V.
Angew. Chem., Int. Ed. 2005, 44, 5188–5240.
NMR spectra for all products. X-ray structure details for
compounds 5b and 5m (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
(19) (a) Kumagai, N.; Matsunaga, S.; Shibasaki, M. Angew. Chem., Int. Ed.
2004, 43, 478–482. (b) Lang, S.; Kennedy, A. R.; Murphy, J. A.; Payne, A. H.
Org. Lett. 2003, 5, 3655–3658. (c) Reddy, D. S.; Judd, W. R.; Aubé, J. Org.
Lett. 2003, 5, 3899–3902. (d) Reddy, P. G.; Vahghese, B.; Baskaran, S. Org.
Lett. 2003, 5, 583–585.
JO702733H
J. Org. Chem. Vol. 73, No. 9, 2008 3577