Pyrrole synthesis catalyzed by AgOTf or cationic Au(I) complexes

Article abstract of DOI:10.1021/jo060382c

Either silver trifluoromethanesulfonate or a mixture of gold(I) chloride, silver trifluoromethanesulfonate, and triphenylphosphine catalyze the formation of pyrroles from substituted β-alkynyl ketones and amines. The reactions proceed by using 5 mol % of catalyst with yields of isolated pyrroles ranging from 13% to 92%. Sixteen examples are used to compare the effectiveness of each catalyst.

Full text of DOI:10.1021/jo060382c

Pyrrole Synthesis Catalyzed by AgOTf or Cationic Au(I) Complexes
Tyler J. Harrison, Jennifer A. Kozak, Montserrat Corbella-Pane´, and Gregory R. Dake*
Department of Chemistry, 2036 Main Mall, UniVersity of British Columbia,
VancouVer, B.C., Canada, V6T 1Z1
gdake@chem.ubc.ca
ReceiVed February 23, 2006
Either silver trifluoromethanesulfonate or a mixture of gold(I) chloride, silver trifluoromethanesulfonate,
and triphenylphosphine catalyze the formation of pyrroles from substituted â-alkynyl ketones and amines.
The reactions proceed by using 5 mol % of catalyst with yields of isolated pyrroles ranging from 13%
to 92%. Sixteen examples are used to compare the effectiveness of each catalyst.
Introduction
Reactions catalyzed by silver and gold salts have recently
experienced a renaissance in the chemical literature.1 Classical
transformations that were previously impractical for the labora-
tory setting due to the severe conditions required are now made
synthetically useful with silver and gold catalysts.2 We recently
demonstrated that silver(I) trifluoromethanesulfonate (AgOTf)
smoothly catalyzes the addition of an enesulfonamide or an
enecarbamate onto a tethered alkyne to form five-membered
rings in high yield, as exemplified in the conversion of 1 to 2
(eq 1).3
in the crude product mixture. The addition of a catalytic amount
of gold(III) chloride enhanced the reaction to produce 4 in 59%
yield. Importantly, simple heating of 3 gave no reaction and
treatment of 3 with TFA led to decomposition. Interestingly,
the conversion of 3 to 4 does not proceed to a significant extent
(<10%) with gold(III) chloride in the absence of AgOTf.
The formation of pyrroles under these conditions is perhaps
not too surprising considering the work of Marshall forming
oxygen heterocycles using silver catalysis.4 Silver salts have
also been used to promote the intramolecular addition of nitrogen
nucleophiles to alkynes and allenes previously.5 The pyrrole
In our attempts to define the scope of this process, enecar-
bamate 3 was subjected to the conditions used for the cyclization
of 1 (eq 2). Unfortunately, none of the desired product was
obtained, but small traces of bicyclic pyrrole 4 could be observed
(4) (a) Marshall, J. A.; Wang, X. J. Org. Chem. 1991, 56, 960-969. (b)
Marshall, J. A.; Sehon, C. A. J. Org. Chem. 1995, 60, 5966-5968. (c)
Marshall, J. A.; Bennett, C. E. J. Org. Chem. 1994, 59, 6110-6113. (d)
Marshall, J. A.; Pinney, K. G. J. Org. Chem. 1993, 58, 7180-7184. (e)
Marshall, J. A.; Bartley, G. S. J. Org. Chem. 1994, 59, 7169-7171.
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Org. Lett. 2005, 7, 4553-4556. (c) Suhre, M. H.; Reif, M.; Kirsch, S. F.
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1795. (e) Morita, N.; Krause, N. Org. Lett. 2004, 6, 4121-4123. (f) Davies,
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Krause, N. Org. Biomol. Chem. 2005, 3, 387-391. (c) Dyker, G. Angew.
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10.1021/jo060382c CCC: $33.50 © 2006 American Chemical Society
Published on Web 05/06/2006
J. Org. Chem. 2006, 71, 4525-4529
4525

Harrison et al.
lecular alkylation reaction took place.13 Stoichiometric silver
salts have been used for pyrrole synthesis, as well.14 Inspired
by the serendipitous result outlined in eq 2, we decided to
examine the effectiveness of pyrrole formation using recently
popular metal catalysts such as AgOTf or “cationic” gold(I)
complexes.15
Results and Discussion
Our initial results are presented in eq 3 and Table 1. The test
FIGURE 1. Examples of pyrrole containing natural products.
ring is of particular interest to the synthetic organic chemist as
it is present in a large number of alkaloid natural products6 that
can exhibit biological activity7 and structural complexity, as
exemplified by the compounds in Figure 1.8 Many solutions
are available for the synthesis of pyrroles.9 The cyclization of
alkynylimines with 30 mol % of CuI in a triethylamine-
dimethylacetamide mixture at 130 °C generates pyrroles in 50-
93% yields.10 Cyclizations with 5 mol % of a gold(III) species,
NaAuCl4, in ethanol at 40 °C proceed in high yields.11 Platinum-
(II) chloride (20 mol % loading) at 60 °C for 100 h enabled the
cycloisomerization of an alkynyl imine in 77% yield.12 A recent
silver(I) promoted pyrrole synthesis suggested that metal-
promoted hydroamination of an alkyne followed by an intramo-
TABLE 1. Test of Amine Scopea
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van Soest, R. W. M. J. Nat. Prod. 2001, 64, 1576-1578. (b) Seki, M.;
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J. M. J. Nat. Prod. 1990, 53, 1611-1614.
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Summerville, K.; Price, C.; Hudson, S.; Kiakos, K.; Hartley, J. A.; Asao,
T.; Lee, M. J. Med. Chem. 2005, 48, 3903-3918. (b) Fu¨rstner, A. Angew.
Chem., Int. Ed. 2003, 42, 3582-3603. (c) Jacobi, P. A.; Coutts, L. D.;
Guo, J.; Hauck, S. I.; Leung, S. H. J. Org. Chem. 2000, 65, 205-213.
(8) (a) Schroder, F.; Franke, S.; Francke, W. Tetrahedron 1996, 52,
13539-13546. (b) Wang, Y.-F.; Lu, C.-H.; Lai, G. F.; Cao, J.-X.; Luo, S.
D. Planta Med. 2003, 69, 1066-1068. (c) Seki, M.; Mori, K. Eur. J. Org.
Chem. 2001, 503-506.
entry
amine
method
T (°C)
t (h)
yield (%)b
1
2
3
5a
5a
5a
A
B
C
50
50
50
3
2
12
76
79
87
4
5
5b
5b
A
C
50
50
12
18
66
80
6
7
5c
5c
A
C
50
50
12
18
77
70
8
9
10
5d
5d
5d
A
B
C
80
80
80
24
23
24
13
38
59
11
12
5e
5e
A
C
80
80
24
24
1
27
(9) For some recent syntheses of pyrroles: (a) Wurz, R. P.; Charette, A.
B. Org. Lett. 2005, 7, 2313-2316. (b) Banik, B. K.; Banik, I.; Renteria,
M.; Dasgupta, S. K. Tetrahedron Lett. 2005, 46, 2643-2645. (c) Kamijo,
S.; Kanazawa, C.; Yamamoto, Y. J. Am. Chem. Soc. 2005, 127, 9260-
9266. (d) Knight, D. W.; Sharland, C. M. Synlett 2003, 2258-2260. (e)
Ramanathan, B.; Keith, A. J.; Armstrong, D.; Odom, A. L. Org. Lett. 2004,
6, 2957-2960. (f) Gabriele, B.; Salerno, G.; Fazio, A. J. Org. Chem. 2003,
68, 7853-7861. (g) Quiclet-Sire, B.; Wendeborn, F.; Zard, S. Z. Chem.
Commun. 2002, 2214-2215. (h) Arcadi, A.; Rossi, E. Tetrahedron 1998,
54, 15253-15272. (i) Arcadi, A.; Rossi, E. Synlett 1997, 667-668. (j) Grigg,
R.; Savic, V. Chem. Commun. 2000, 873-874. (k) Ito, S.; Murashima, T.;
Ono, N. J. Chem. Soc., Perkin 1 1997, 3161-3165. (l) Braun, R. U.; Zeitler,
K.; Mu¨ller, T. J. J. Org. Lett. 2001, 3, 3297-3300. (m) Takaya, H.; Kojima,
S.; Murahashi, S.-I. Org. Lett. 2001, 3, 421-424. (n) Paulus, O.; Alcaraz,
G.; Vaultier, M. Eur. J. Org. Chem. 2002, 2565-2572. (o) Attanasi, O.
A.; De Crescentini, L.; Favi, G.; Filippone, P.; Mantellini, F.; Santeusanio,
S. J. Org. Chem. 2002, 67, 8178-8181. (p) Demir, A. S.; Akhemedov, I.
M.; Sesenoglu, O¨ . Tetrahedron 2002, 58, 9793-9799. (q) Smith, N. D.;
Huang, D.; Cosford, N. D. P. Org. Lett. 2002, 4, 3537-3539. (r) Ranu, B.
C.; Dey, S. S. Tetrahedron Lett. 2003, 44, 2865-2868. (s) Dhawan, R.;
Arndtsen, B. A. J. Am. Chem. Soc. 2004, 126, 468-469. (t) Siriwardana,
A. I.; Kathriarachchi, K. K. A. D. S.; Nakamura, I.; Gridnev, I. D.;
Yamamoto, Y. J. Am. Chem. Soc. 2004, 126, 13898-13899. (u) Masquelin,
T.; Obrecht, D. Synthesis 1995, 276-284.
13
14
5f
5f
A
C
80
80
24
24
37
35
15
16
5g
5g
A
C
50
50
16
12
74
68
17
18
5h
5h
A
C
80
80
24
24
41
60
a Reactions were carried out with 1 equiv of 5, 1 equiv of ketone, and
5 mol % of catalyst system. b Isolated yields.
reaction involved the condensation of 5-heptyn-2-one with a
monosubstituted amine derivative under either silver(I) (method
A) or gold(I) catalysis. Importantly, two sets of conditions for
(13) Robinson, R. S.; Dovey, M. C.; Gravestock, D. Tetrahedron Lett.
2004, 45, 6787-6789.
(14) (a) Agarwal, S.; Kno¨lker, H.-J. Org. Biomol. Chem. 2004, 2, 3060-
3062. (b) Kno¨lker, H.-J.; Agarwal, S. Synlett 2004, 1767-1768.
(15) (a) Shi, X.; Gorin, D. J.; Toste, F. D. J. Am. Chem. Soc. 2005, 127,
5802-5803. (b) Yang, C.; He, C. J. Am. Chem. Soc. 2005, 127, 6966-
6967. (c) Li, Z.; Shi, Z.; He. C. J. Organomet. Chem. 2005, 690, 5049-
5054. (d) Shi, Z.; He, C. J. Am. Chem. Soc. 2004, 126, 5964-5965. (e)
Shi, Z.; He, C. J. Org. Chem. 2004, 69, 3669-3671. (f) Shi, Z.; He, C. J.
Am. Chem. Soc. 2004, 126, 13596-13597. (g) Zhang, J.; Yang, C.; He, C.
J. Am. Chem. Soc. 2006, 128, 1798-1799. (h) Yang, C.; Reich, N. W.;
Shi, Z.; He, C. Org. Lett. 2005, 7, 4553-4556. (i) Cui, Y.; He, C. Angew.
Chem., Int. Ed. 2004, 43, 4210-4212.
(10) (a) Kel’in, A. V.; Sromek, A. W.; Gevorgyan, V. J. Am. Chem.
Soc. 2001, 123, 2074-2075. (b) Kim, J. T.; Kel’in, A. V.; Gevorgyan, V.
Angew. Chem., Int. Ed. 2003, 42, 98-101.
(11) Arcadi, A.; Di Giuseppe, S.; Marinelli, F.; Rossi, E. Tetrahedron:
Asymmetry 2001, 12, 2715-2720. (b) Arcadi, A.; Di Giuseppe, S.; Marinelli,
F.; Rossi, E. AdV. Synth. Catal. 2001, 343, 443-446.
(12) Nishibayashi, Y.; Yoshikawa, M.; Inada, Y.; Milton, M. D.; Hidai,
M.; Uemura, S. Angew. Chem., Int. Ed. 2003, 42, 2681-2684.
4526 J. Org. Chem., Vol. 71, No. 12, 2006

Ag and Au Catalyzed Pyrrole Synthesis
FIGURE 2. Examples of AgOTf or Au(I)-catalyzed pyrrole formation with benzylamine.
gold catalysis were attemptedsthe first with prepared (PPh3)-
AuCl16 and AgOTf (denoted as method B in Table 1) and a
second set in which AuCl, AgOTf, and PPh3 were mixed in
situ prior to the addition of substrates (denoted as method C).
Somewhat surprisingly, higher yielding results were obtained
with method C compared to method B (entries 2 and 3; 9 and
10). Consequently, method C was adopted as the “standard
conditions” for gold catalysis. The yields for these reactions
are generally good (entries 1-7, 15, and 16) except when
sterically demanding amines such as aniline, cyclohexylamine,
or tert-butylamine are used (entries 8-12). No product is formed
when tert-butylamine is used. Reactions between p-toluene-
sulfonamide (5f) and 5-heptyn-2-one proceed, but are not high
yielding (entries 13 and 14).
to a greater extent. On the other hand, the use of AgOTf results
in a qualitatively faster reaction rate and thus, for reactive
amines, these two counteracting features tend to result in
comparable yields with either catalyst.
The reaction also proceeds well in toluene, isopropyl alcohol,
and dichloromethane, but works poorly in methanol, acetonitrile,
and ethyl acetate, and fails in tetrahydrofuran. Comparable yields
can be obtained with other silver sources (silver(I) trifluoroac-
etate and silver(I) tetrafluoroborate), but only traces of pyrrole
products were observed with other metal salts such as platinum-
(II) chloride or iron(III) chloride. The addition of 5 mol % of
PPh3 to the AgOTf precatalyst system (ligand:metal ratio 1:1)
resulted in a significantly less reactive catalyst (35% yield of
6a after 24 h at 50 °C).
No pyrrole products were observed between 5a and 5-heptyn-
2-one when the reactions were run at 80 °C for 24 h in the
absence of a metal catalyst, or in the presence of 30 mol % of
camphorsulfonic acid or 30 mol % of BF3‚OEt2. The yields were
not affected when reactions between 5-heptyn-2-one and 5a with
AgOTf catalysis were run in the presence of 1.5 equiv of bases
such as magnesium oxide or calcium hydride. These experiments
suggest that adventitious acid catalysis is not responsible for
the observed reactivity. Yields suffer largely due to the lack of
conversion to product, a phenomenon that appears to be largely
dependent on the amine component. There was no change in
yield in the reaction between 5-heptyn-2-one and 5a with AgOTf
catalysis when run in the presence of 2 equiv of water or in the
presence of added 6a (1 equiv) at the start of the reaction,
implying that catalyst inhibition by the reaction products was
not a significant factor.17 The AuCl/AgOTf/ligand combination
appears to be more thermally stable compared to AgOTf and
consequently reactions that require higher temperatures progress
With these results in hand, a number of functionalized
4-pentynones were reacted with benzylamine in the presence
of either AgOTf or AuCl/AgOTf/PPh3 (Figure 2). As can be
seen in Figure 2, this reaction smoothly forms pyrroles within
bicycles (entries 1 and 2).18 In addition, the reaction is
reasonably tolerant of functional groups. Substrates 11, 13, 15,
and 17 were designed to examine the relative facility of
intramolecular pyrrole formation (cf. eq 2) compared to
intermolecular reactions with benzylamine. As can be seen, the
intermolecular reaction involving benzylamine typically is the
dominant process (entries 3-6). The reactions are usually
completed prior to the reaction times indicated in Figure 2. The
efficiency of the reaction did not significantly depend on the
catalyst.
Deprotection of the tert-butyl carbamate in 11 and 17 (TFA,
CH2Cl2) produced, as expected, imines 19 and 20. Acid-
promoted pyrrole formation was not observed. Treatment of
these imines with either AgOTf or AuCl/AgOTf/PPh3 generated
bicyclic pyrroles 21 and 22 in moderate yields (Scheme 1).
Our preliminary investigations of the gold species formed
upon mixing gold(I) chloride, AgOTf, and triphenylphosphine
(16) Bruce, M. I.; Nicholson, B. K.; Shawkataly, O. B. Inorg. Synth.
1989, 26, 324-325.
(17) There was also no change in yield in the reaction between 5-heptyn-
2-one and 5a with AgOTf catalysis when run open to air or in a foil-wrapped
flask.
(18) These compounds are known: see ref 9h.
J. Org. Chem, Vol. 71, No. 12, 2006 4527

Harrison et al.
SCHEME 1. Cyclization of Imines
TABLE 3. Evaluation of Plausible Gold Species for Catalysisa
entry amine
catalyst
T (°C) t (h) yield (%)b
1
2
3
4
5
6
5a
5a
5a
5a
5a
5a
AuCl, AgOTf, PPh3
PPh3AuCl, AgOTf
(PPh3)2AuCl, AgOTf
AuCl, AgOTf
50
50
50
0-25
50
12
2
19
10
21
12
87
79
80
39
21
40
AuCl3, AgOTf
[(PPh3)2AuCl + AgOTf)], 50
[AuCl + AgOTf]c
7
5a
[(PPh3)2AuCl + AgOTf],
[AuCl3 + AgOTf]c
50
22
69
TABLE 2. 31P NMR Data of Precatalyst Mixtures in CDCl3
8
9
10
5d
5d
5d
AuCl, AgOTf, PPh3
PPh3AuCl, AgOTf
(PPh3)2AuCl, AgOTf
80
80
80
24
23
24
59
38
48
entry
mixture
δa
δ (lit.)b
1
2
3
4
5
6
PPh3AuCl
(PPh3)2AuCl
AuCl + PPh3
PPh3AuCl + AgOTf
(PPh3)2AuCl + AgOTf
AuCl + PPh3 + AgOTf
33.8
29.7
33.8
28.8
45.6
45.6
32.9
29.7
a Reactions were carried out with 1 equiv of 5, 1 equiv of ketone, and
the designated catalyst system. b Isolated yields. c Only 2.5 mol % of each
precatalyst was used.
28.1
a Referenced to an external standard of 85% H3PO4. b Consult ref 19.
curious if an oxidation-reduction process takes place under the
reaction conditions, generating a Au(III) species and Ag(0). In
the event, the reaction with AuCl3 and AgOTf gave a poor result
(21% yield, entry 5). We then speculated that a mixed system
of (PPh3)2AuOTf in the presence of other “cationic” Au(I) or
Au(III) species might be catalytically viable. At 2.5 mol %
loading (mimicking the stoichiometry of the generation of the
catalytic species under the conditions in entry 1) of (PPh3)2-
AuOTf mixed with either “AuOTf” or “AuCl2OTf” the overall
yields of 6a were 40% and 69%, respectively. A single catalytic
species generated under these conditions is still elusive.
Although it appears that a (bis)phosphine gold(I) cationic species
is generated under the reaction conditions, its presence is not
responsible for all of the observed reactivity of this catalyst
system. Multiple entities capable of catalysis may be generated
under these reaction conditions, complicating the reaction
analysis. Even so, the pyrrole-forming process remains a useful
and effective transformation.
generated intriguing, but not definitive, results. 31P NMR
analysis of the mixture suggested that a single phosphorus-
containing species was generated (Table 2). The chemical shift
of this phosphorus-containing compound (δ 45.6) is consistent
with that found when (PPh3)2AuCl19 was treated with AgOTf
(δ 45.6). If all added phosphine was consumed to form the bis-
(phosphine)gold(I) cation, reaction stoichiometry would dictate
that three nonphosphine-containing byproducts, AuCl, AgOTf,
and AgCl, could also be present in the reaction mixture and
consequently be responsible for the observed catalytic activity.
We therefore performed the following catalyst screening experi-
ments (eq 4 and Table 3).
For comparison, the reaction between 5a and 5-heptyne-2-
one forming 6a with PPh3, AuCl, AgOTf (87%), PPh3AuCl and
AgOTf (79%) are given in entries 1 and 2. Somewhat satisfy-
ingly, the use of (PPh3)2AuCl and AgOTf (5 mol % each) gave
a 80% yield, lending credence to the suggestion that the bis-
(phosphine) gold cation was the active catalytic species (entry
3). This result was paralleled somewhat by using aniline (5d)
as the nucleophile rather than benzylamine (5a) (entries 8-10).
However, the efficiency of this process at 5 mol % loading still
did not compare favorably to that in entry 1, where the reaction
stoichiometry dictated that only 2.5 mol % of the bis(phosphine)
gold cation would be formed under a “best-case” scenario. We
then questioned whether a phosphine-containing species was
in fact serving as the active catalyst.
In summary, either silver or gold catalysts efficiently facilitate
the reactions between imines (formed in situ) and alkynes to
form functionalized pyrroles in good yields. These methods may
prove useful in the synthesis of pyrrole containing natural
products. An interesting observation in our studies was the
confirmation of increased thermal stability of the gold(I)
precatalyst relative to the silver salt. However, it appeared
qualitatively that reactions catalyzed by the silver salt proceed
faster. The lower cost of silver salts relative to gold salts
obviously may dictate the selection of catalyst for this trans-
formation. Efforts in applying this reaction to natural product
synthesis are ongoing in our laboratory.
Attempts to form a highly reactive “Au(I)OTf” species
necessitated low temperatures (0 °C). Although the species was
proficient at promoting the desired process, the reaction progress
stalled relatively quickly (39%), possibly because “Au(I)OTf”
is not thermally stable (entry 4). It does not appear that this
compound is the active catalytic species at 50 °C. We were
Experimental Section
Sample Procedures for Pyrrole Synthesis: AgOTf Catalysis.
To a solution of 3.7 mg of silver(I) trifluoromethanesulfonate (0.014
mmol) in 1.5 mL of 1,2-dichloroethane was added 31.3 mg of
5-heptyn-2-one (0.29 mmol) followed by 46 µL of benzylamine
(0.42 mmol) and the reaction was stirred at 50 °C for 3 h. The
reaction mixture was filtered through Celite and concentrated by
rotary evaporation in vacuo to afford a crude brown oil. Purification
by gradient column chromatography on triethylamine washed silica
(19) For the preparation of (PPh3)2AuCl, see: (a) McAuliffe, C. A.;
Parish, R. V.; Randall, P. D. J. Chem. Soc., Dalton Trans. 1979, 1730-
1735. For the 31P NMR data for PPh3AuCl and (PPh3)2AuCl, see: (b)
Woehrle, G. H.; Brown, L. O.; Hutchinson, J. E. J. Am. Chem. Soc. 2005,
127, 2172-2183. For the 31P NMR data of PPh3AuOTf, see: (c)
Preisenberger, M.; Schier, A.; Schmidbaur, H. J. Chem. Soc., Dalton Trans.
1999, 1645-1650.
4528 J. Org. Chem., Vol. 71, No. 12, 2006

Ag and Au Catalyzed Pyrrole Synthesis
gel (petroleum ether f 20:1 petroleum ether:diethyl ether) afforded
43 mg (76%) of 6a as a colorless oil.
3H), 1.29 (t, J ) 7.6 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ
138.6, 134.3, 128.6, 127.9, 126.9, 125.5, 105.4, 103.4, 46.4, 19.7,
12.8, 12.2. MS (ESI): 200 (M + H+), 230 (M + K+).
AuCl, AgOTf, and PPh3 Catalysis. To a flask charged with
4.7 mg of triphenylphosphine (0.018 mmol), 4.2 mg of gold(I)
chloride (0.018 mmol), and 4.8 mg of silver(I) trifluoromethane-
sulfonate (0.019 mmol) was added 2 mL of 1,2-dichloroethane and
the mixture was stirred at room temperature for 5 min. To the
resulting opaque solution was added 42 mg of 5-heptyn-2-one (0.38
mmol) followed by 40 µL of benzylamine (0.37 mmol) and the
reaction mixture was stirred at 50 °C for 12 h. The reaction mixture
was filtered through Celite and concentrated by rotary evaporation
in vacuo to afford a crude brown oil. Purification by gradient
column chromatography on triethylamine washed silica gel (pe-
troleum ether f 20:1 petroleum ether:diethyl ether) afforded 63
mg (87%) of 6a as a colorless oil.
Acknowledgment. The authors thank the University of
British Columbia, the Natural Science and Engineering Research
Council (NSERC) of Canada, the Canada Foundation for
Innovation (CFI), Glaxo SmithKline, and Merck-Frosst for the
financial support of our programs. T.J.H. thanks NSERC for
PGS A and PGS B graduate fellowships. M.C.-P. received a
grant from the Ministerio de Educacio´n y Ciencia.
Supporting Information Available: Experimental procedures
and characterization data for all previously unreported compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IR (film): 2070, 1419, 1300 cm-1 1H NMR (400 MHz,
.
CDCl3): δ 7.28-7.41 (m, 3H), 6.97 (d, J ) 7.3 Hz, 2H), 5.98-
6.04 (m, 2H), 5.11 (s, 2H), 2.57 (quartet, J ) 7.6 Hz, 2H), 2.24 (s,
JO060382C
J. Org. Chem, Vol. 71, No. 12, 2006 4529