Versatile and expeditious synthesis of aurones via AuI-catalyzed cyclization

Article abstract of DOI:10.1021/jo702197b

(Chemical Equation Presented) Aurones are conveniently formed in a three-step procedure including a gold^I-catalyzed cyclization of 2-(1-hydroxyprop-2-ynyl)phenols as a highly regio- and stereoselective key step. A wide diversity of derivatives can be obtained starting from substituted salicylaldehydes. Synthesis of natural 4,6,3′,4′-tetramethoxyaurone and structure revision of two natural products (dalmaisione D and 4′-chloroaurone) were achieved.

Full text of DOI:10.1021/jo702197b

SCHEME 1. General Retrosynthesis of Aurones and
Structure of a Few Natural Aurones
Versatile and Expeditious Synthesis of Aurones
via AuI-Catalyzed Cyclization
Hassina Harkat,† Aure´lien Blanc, Jean-Marc Weibel, and
Patrick Pale*
Laboratoire de Synthe`se et Re´actiVite´ Organique, associe´ au
CNRS, Institut de Chimie, UniVersite´ Louis Pasteur, 4 rue
Blaise Pascal, 67000 Strasbourg, France
ppale@chimie.u-strasbg.fr
ReceiVed October 10, 2007
aurone (Scheme 1), proved to be an inhibitor of iodothyronine-
deiodinase, an enzyme involved in hormone synthesis and
regulation.9 Non-natural aurones have been found to bind to
the nucleotide-binding domain of P-glycoprotein, which medi-
ates resistance of cancer cells to chemotherapy,2 inhibit cyclin-
dependent kinases in connection with antiproliferative proper-
ties,10 and act as anticancer agents.11
Numerous aurone syntheses were reported in the literature:
the Wheeler aurone synthesis from chalcone dihalides,12 oxida-
tive cyclization of 2′-hydroxychalcones,13 and ring closure of
o-hydroxyaryl phenylethynyl ketones.14 These methods give
generally good stereoselectivity, but they usually cannot com-
pletely prevent the formation of flavones. The most popular
preparation of aurones6-11 was developed by Varma15 and is
based on the condensation of benzofuran-3(2H)-ones with
benzaldehydes. However, this aldol-like coupling reaction gives
sometimes low yields and requires the synthesis of benzofuran-
3(2H)-ones from substituted 2-phenoxyacetic acids by an
intramolecular Friedel-Craft reaction. Such a reaction is usually
carried out under harsh conditions and yields are modest.
Aurones are conveniently formed in a three-step procedure
including a goldI-catalyzed cyclization of 2-(1-hydroxyprop-
2-ynyl)phenols as a highly regio- and stereoselective key
step. A wide diversity of derivatives can be obtained starting
from substituted salicylaldehydes. Synthesis of natural
4,6,3′,4′-tetramethoxyaurone and structure revision of two
natural products (dalmaisione D and 4′-chloroaurone) were
achieved.
Flavonoids represent a large class of plant natural products,
exhibiting multiple biological activities.1 Among them, aurones,2
i.e., (Z)-2-benzylidenebenzofuran-3(2H)-ones (Scheme 1), con-
stitute a subclass contributing to the pigmentation of flowers
and fruits,3 especially to the bright golden yellow color of
flowers.4 Aurones also exhibit a strong and broad variety of
biological activities.2 For example, they have been described
as antifungal agents,5 as insect antifeedant agents,6 as inhibitors
of tyrosinase,7 and as antioxidants.8 Aureusidin, a common
(9) Auf’mkolk, M.; Koerhle, J.; Hesch, R. D.; Cody, V. J. Biol. Chem.
1986, 261, 11623-11630.
† On leave from the chemistry department of the University of Batna, Batna,
Algeria.
(1) (a) lwashina, T. J. Plant Res. 2000, 113, 287-299. (b) Andersen, O.
M.; Markham, K. R. FlaVanoids: Chemistry, Biochemistry and Applications;
CRC Press: Boca Raton, 2006.
(2) For a review, see: Boumendjel, A. Curr. Med. Chem. 2003, 10,
2621-2330.
(3) Veitch, N. C.; Grayer, R. J. Chalcones, Dihydrochalcones and
Aurones in FlaVanoids: Chemistry, Biochemistry and Applications; CRC
Press: Boca Raton, 2006; pp 1003-1100.
(4) Ono, E.; Fukuchi-Mizutani, M.; Nakamura, N.; Fukui, Y; Yonekura-
Sakakibara, K.; Yamaguchi, M.; Nakayama, T.; Tanaka, T.; Kusumi, T.;
Tanaka, Y. Proc. Natl. Acd. Sci. U.S.A. 2006, 103, 11075-11080.
(5) Brooks, C. J. W.; Watson, D. G. Nat. Prod. Rep. 1985, 427-459.
(6) Morimoto, M.; Fukumoto, H.; Nozoe, T.; Hagiwara, A.; Komai, K.
J. Agric. Food Chem. 2007, 55, 700-705.
(7) Okombi, S.; Rival, D.; Bonnet, S.; Mariotte, A.-M.; Perrier, E.;
Boumendjel, A. J. Med. Chem. 2006, 49, 329-333.
(8) Venkateswarlu, S.; Panchagnula, G. K.; Subbaraju, G. V. Biosci.
Biotechnol. Biochem. 2004, 68, 2183-2185.
(10) Schoepfer, J.; Fretz, H.; Chaudhuri, B.; Muller, L.; Seeber, E.;
Meijer, L.; Lozach, O.; Vangrevelinghe, E.; Furet, P. J. Med. Chem. 2002,
45, 1741-1747.
(11) Lawrence, N. J.; Rennison, D.; McGown, A. T.; Hadfield, J. A.
Bioorg. Med. Chem. Lett. 2003, 13, 3759-3763.
(12) (a) Donnelly, J. A.; Fox, M. J.; Sharma, T. C. Tetrahedron 1979,
35, 875-879. (b) Bose, G.; Mondal, E.; Khan, A. T.; Bordoloi, M. J.
Tetrahedron Lett. 2001, 42, 8907-8909.
(13) (a) Le´vai, A.; To¨ke´s, A. L. Synth. Comm. 1982, 12, 701-707. (b)
Imafuku, K.; Honda, M.; McOmie, J. F. W. Synthesis 1987, 199-201. (c)
Sekizaki, H. Bull. Chem. Soc. Jpn. 1988, 61, 1407-1409. (d) Thakkar, K.;
Cushman, M. J. Org. Chem. 1995, 60, 6499-6510.
(14) Basic catalysis: (a) Garcia, H.; Iborra, S.; Primo, J.; Miranda, M.
A. J. Org. Chem. 1986, 51, 4432-4436. (b) Brennan, C. M.; Johnson, C.
D.; McDonnell, P. D. J. Chem. Soc., Perkin Trans. 2. 1989, 957-961. Pd
and Ag catalysis: (c) An, Z.-W.; Catellani, M.; Chiusoli, G. P. J. Organomet.
Chem. 1990, 397, 371-373. (d) Jong, T.-T.; Leu, S.-J. J. Chem. Soc., Perkin
Trans. 1 1990, 423-424.
(15) Varma, R. S.; Varma, M. Tetrahedron Lett. 1992, 33, 5937-5940.
10.1021/jo702197b CCC: $40.75 © 2008 American Chemical Society
Published on Web 01/15/2008
1620
J. Org. Chem. 2008, 73, 1620-1623

TABLE 1. Screening of Gold Catalysts for the Cyclization of 1a
TABLE 2. Comparison with the Cyclization of an Analogue
Ynone
catalyst
(mol %)
additive
(mol %)
time
(h)
aurone
flavone
entry
catalyst (mol %)
additive (mol %)
time (h)
yieldb (%)
entry
yielda (%)
yielda (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
none
K2CO3 (10)
none
24
0.5
2
0
c
1
2
3
none
AuCl (10)
AuCl (10)
K2CO3 (10)
none
K2CO3 (10)
1
24
24
25
traces
0
65
38
traces
AuCl (10)
AuCl (10)
AuCl (10)
AuCl (10)
AuCl (10)
AuCl (10)
PPh3AuCl (5)
PPh3AuCl (5)
PPh3AuCl (5)
AuCl3 (10)
AuCl3 (10)
AuCl (1)
K2CO3 (10)
NaHCO3 (10)
NEt3 (10)
pyridine (10)
NaH (100)
none
K2CO3 (10)
AgSbF6 (5)
none
78
67
50
0
24
24
24
24
24
15
15
0.1
24
30
a Yields of aurone and flavone were calculated by H NMR relative to
an internal standard (hexamethylbenzene).
1
42d
0
appeared to be the most effective reagent and smoothly furnished
aurone 3a in 90% yield. To better understand the cyclization
step, we then screened other bases associated with AuCl. Sodium
hydrogenocarbonate or triethylamine were also efficient in the
gold catalysis cyclization but gave lower yields and longer
reaction times (entries 4 and 5). Pyridine was completely
ineffective in this reaction, presumably due to its coordination
with gold chloride (entry 6).22 Preformed phenolate only
afforded a modest yield of 2a (entry 7). Other gold catalysts
were also less effective than AuCl. Surprisingly, the more
soluble triphenylphosphane gold chloride required longer reac-
tion time than gold chloride itself and gave only a modest yield
of the expected cyclized product (entry 9 vs 3). It is worth
mentioning that with this catalyst also, the presence of potassium
carbonate was required to get some transformation (entry 8).
More electrophilic catalysts either derived from triphenylphos-
phane gold chloride treated with silver salt or gold(III) trichloride
alone gave decomposition product (entries 10 and 11). Never-
theless, in the presence of potassium carbonate, gold trichloride
gave the cyclization product but in very low yield (entry 12).
Interestingly, with a low catalyst loading (1 mol %), the yield
was slightly increased without any loss in regioselectivity, but
the reaction time was, however, longer (entry 13 vs 3).
Knowing that arylated ynones could be cyclized into a
mixture of aurones and flavones,14 we were curious at this point
to compare our optimized conditions with these classical basic
conditions. The corresponding ynones were thus easily prepared
by MnO2 oxidation of 1a. This ynone was then submitted to
the known basic conditions and to gold catalysts in various
conditions (Table 2). The former led as expected to a mixture
of aurone and flavone (entry 1). Surprisingly, AuCl alone gave
almost exclusively the flavone, but in low yield (entry 2), and
K2CO3/AuCl failed to promote any cyclization.23 These results
and the experiment with the preformed phenolate (Table 1, entry
7) clearly evidenced the key role of the base in the Au-catalyzed
cyclization of 1-(2-hydroxyphenyl)-3-phenylpropynols.
55
c
c
22
86
K2CO3 (10)
K2CO3 (1)
a Reactions run under argon, c ) 0.1 mol/L. b Yields of 2a were
calculated by 1H NMR relative to an internal standard (hexamethylbenzene).
c Degradation products. d 14% of flavone and 35% of ynone19 product were
1
observed by H NMR of the crude mixture.
The aurone biological properties and the lack of regioselective
syntheses led us to develop an alternative route. We decided to
take advantage of our experience in oxygenated heterocycles
preparations,16 and we wish to report here a simple three-step
synthesis of aurones. Indeed, we reasoned that aurones 3 should
be available through metal-catalyzed cyclization of substituted
1-(2-hydroxyphenyl)-3-phenylpropynols 1 followed by oxida-
tion. The latter would be easily obtained by alkynylation of
salicylaldehyde derivatives (Scheme 1).
Various 2-(1-hydroxy-3-arylprop-2-ynyl)phenols 1a-h were
easily produced by addition of 2 equiv of lithium arylacetyl-
ides,17 substituted or not, at low temperature in THF to several
substituted salicylaldehydes. The yields were routinely higher
than 70%. In order to find more appropriate conditions for the
cyclization reaction of such substrates and based on previous
results,16f,18 we applied various conditions and gold catalysts
to the simplest aurone precursor 1a (Table 1).
Potassium carbonate alone was not able to promote any
cyclization, and the starting material 1a was recovered even
after prolonged contact time (entry 1). In sharp contrast, gold-
(I) chloride alone rapidly led to decomposition (entry 2). But
premixing the base (10 mol %) and the starting material in
acetonitrile before adding AuCl (10 mol %) gave the expected
cyclization product 2a in 78% yield as a single 5-exo-dig
regioisomer and Z stereoisomer (entry 3). The regio- and
stereochemistry were unambiguously established after compari-
son of the oxidation product 3a to the known aurone.20,21 Indeed,
after a rapid screening of oxidation conditions of 2a, MnO2
The above results showed that aurones could be obtained in
three steps from salicylaldehyde and phenylacetylene through
(19) Gold-catalyzed aerobic oxidation of alcohols: Miyamura, H.;
Matsubara, R.; Miyazaki, Y.; Kobayashi, S. Angew. Chem., Int. Ed. 2007,
46, 4151-4154.
(16) (a) Pale, P.; Chuche, J. Tetrahedron Lett. 1987, 51, 6447-6448.
(b) Dalla, V.; Pale, P. Tetrahedron Lett. 1994, 35, 3525-3528. (c) Dalla,
V.; Pale, P. New J. Chem. 1999, 803-805. (d) Pale, P.; Chuche, J. Eur. J.
Org. Chem. 2000, 1019-1025. (e) Harkat, H.; Weibel, J.-M.; Pale, P.
Tetrahedron Lett. 2006, 47, 6273-6276. (f) Harkat, H.; Weibel, J.-M.; Pale,
P. Tetrahedron Lett. 2007, 48, 1439-1442.
(17) Excess substituted phenylacetylenes used can be easily recovered.
(18) (a) Antoniotti, S.; Genin, E.; Michelet, V.; Geneˆt, J.-P. J. Am. Chem.
Soc. 2005, 127, 9976-9977. (b) Liu, Y.; Song, F.; Song, Z.; Liu, M.; Yan,
B. Org. Lett. 2005, 7, 5409-5412. (c) Barluenga, J.; Die´guez, A.; Ferna´ndez,
A.; Rodriguez, F.; Fanana´s, F. J. Angew. Chem., Int. Ed. 2006, 45, 2091-
2093. (d) Belting, V.; Krause, N. Org. Lett. 2006, 8, 4489-4492.
(20) 13C NMR allowed us to clearly distinguish between the aurone and
flavone structures; the C3 carbonyl signal of aurone 3a appeared at 184.8
ppm, while the one of the corresponding flavone was at 177.4 ppm.
(21) (a) Hastings, J. S.; Heller, H. G. J. Chem. Soc., Perkin Trans. 1
1972, 2128-2132. (b) Pelter, A.; Ward, R. S.; Heller, H. G. J. Chem. Soc.,
Perkin Trans. 1 1979, 328-329. (c) Gray, T. I.; Pelter, A.; Ward, R. S.
Tetrahedron 1979, 35, 2539-2543.
(22) Hashmi, A. S. K.; Weyrauch, J. P.; Rudolph, M.; Kurpejovic, E.
Angew. Chem., Int. Ed. 2004, 43, 6545-6547.
(23) See the Supporting Information.
J. Org. Chem, Vol. 73, No. 4, 2008 1621

TABLE 3. Scope of the Au-Catalyzed Aurone Synthesis
FIGURE 1. Structural revisions.
FIGURE 2. Structural revisions.
SCHEME 2. Synthesis of 5, Aglycon of Dalmaisione D
isolated from the marine brown alga Spatoglossum Variabile.25
However, spectroscopic data of the synthetic 3e we obtained
did not match with those reported. The vinylic proton resonated
at 6.84 ppm as a singlet, while it was reported at 6.91 ppm.
Comparison with related compounds led us think that the natural
product was misassigned and that it should correspond to an
isocoumarin. Indeed, Subbaraju26 et al. published during our
own attempts to elucidate this structure, a structural revision
and reassigned it as the known 3-(4′-chloroisocoumarin)
(Figure 1).
We also chose to prepare aurone 3g since it is the O-
methylated form of the aglycone of dalmaisione D, a natural
product isolated from roots of Polygala dalmaisiana,27 which
is composed of a disaccharide (â-glucopyranosyl-(â-1f6)-
glucopyranosyl) tethered to the (Z)-2′-hydroxyaurone 4 (Figure
2). But again, the spectral data reported for the compound
obtained after acid hydrolysis of dalmaisione D did not match
with the synthetic 4 we obtained after demethylation of 3g with
BBr3.23 Indeed, NMR shifts of carbonyl (C3 ) 177.1 ppm) and
olefinic proton (H10 ) 7.13 ppm) seem to correspond to the
flavone isomer 5. Unfortunately, reported NMR data of 2′-
hydroxyflavone28 were confusing, which forced us to synthesize
5 in three steps from phenol 1g (Scheme 2). We were pleased
a Cyclization yield obtained with 1 mol % of catalysts. b Trace of
E-aurone was observed (<5%). c The adduct is instable on silica gel. d Yield
1
was determined by H NMR on the crude mixture. e The natural product
isomerized in solution.24 f Yield of isolated E/Z products (ratio: 27/73),
determined over the two steps.
alkynylation, gold-catalyzed cyclization, and oxidation. We then
explored the scope of this new synthesis of aurones by applying
this sequence to variously substituted salicylaldehydes and
alkynes (Table 3).
The yields of cyclic products 2a-h were always good,
remaining between 65 and 86%. In all cases, no trace of other
regio- or stereoisomers was observed. The oxidation step gave
generally aurones 3a-h in high yields with sometimes trace of
E-aurones (entries 5 and 6), probably due to aurone instability
(entry 8).24
1
to find that H and 13C NMR data of synthetic 5 were now
consistent with the natural aglycon. These data led us to reassign
the natural product dalmaisione D as compound 6 (Figure 2).21
Some of the synthesized aurones in Table 3 need more
detailed comments. Indeed, we focused on the synthesis of
natural products to illustrate our strategy. We prepared (Z)-4′-
chloroaurone 3e since it was described as a natural product
(25) Atta-ur-Rahman; Choudhary, M. I.; Hayat, S.; Khan, A. M.; Ahmed,
A. Chem. Pharm. Bull. 2001, 49, 105-107.
(26) Venkateswarlu, S.; Panchagnula, G. K.; Gottumukkala, A. L.;
Subbaraju, G. V. Tetrahedron 2007, 63, 6909-6914.
(27) Kobayashi, S.; Miyase, T.; Noguchi, H. J. Nat. Prod. 2002, 65,
319-328.
(24) (a) Seabra, R. M.; Andrade, P. B.; Ferreres, F.; Moreira, M. M.
Phytochemistry 1997, 45, 839-840. (b) Bolek, D.; Gu¨tschow, M. J.
Heterocycl. Chem. 2005, 42, 1399-1403.
(28) (a) Blasko´, G.; Xun, L.; Cordell, G. A. J. Nat. Prod. 1988, 51, 60-
65. (b) Budzianowski, J.; Morozowska, M.; Wesolowska, M. Phytochemistry
2005, 66, 1033-1039.
1622 J. Org. Chem., Vol. 73, No. 4, 2008

General Procedure 2 for the Au-Catalyzed Cyclization. To
the alcohol (0.5 mmol, 1a-h) in dry acetonitrile (2.5 mL) was
added K2CO3 (0.05 mmol) at room temperature. The reaction
mixture was stirred for 5 min, and AuCl (0.05 mmol) was then
added in one portion. The reaction was monitored by TLC until
complete conversion of starting material. Acetonitrile was removed
in vacuo, and the residue was purified by flash chromatography.
(Z)-2-Benzylidene-2,3-dihydrobenzofuran-3-ol (2a). Following
the general procedure 2, alcohol 1a (116 mg, 0.5 mmol) gave 2a
(98 mg, 84%) as a white solid: TLC Rf 0.32 (cyclohexane/EtOAc
30%): mp 110-111 °C; IR (KBr) νmax 3305, 1684, 1613, 1600,
1478, 1466, 1448 cm-1; 1H NMR (300 MHz, CDCl3) δ 7.72 (d, J
) 7.9 Hz, 2 H), 7.50 (d, J ) 7.2 Hz, 1 H), 7.41-7.32 (m, 3 H),
7.27-7.22 (m, 1 H), 7.11-7.07 (m, 2 H), 6.01 (d, J ) 1.5 Hz, 1
H), 5.77 (d, J ) 7.5 Hz, 1 H), 2.20 (d, J ) 8.0 Hz, 1 H); 13C NMR
(75 MHz, CDCl3) 157.7, 157.0, 134.5, 130.6, 128.7, 128.5, 126.9,
126.8, 125.6, 122.9, 110.7, 106.0, 72.5; MS (ESI) m/z 223 (100,
M+• - H); HR-MS 223.0380 (C15H12O2 - H calcd 223.0354).
Finally, the synthesis of natural (Z)-4,6,3′,4′-tetramethoxyau-
rone24 3h isolated from Cyperus capitatus highlights this new
aurone access. It is noteworthy that although only one stereoi-
somer was formed during the cyclization step, a mixture of E/Z
3h was obtained after oxidation due to equilibrium of the two
isomers in the presence of light or on silica gel.24
In conclusion, we have reported for the first time an original
route toward aurones. This three-step approach based on a gold-
catalyzed cyclization led to an efficient and expeditious synthesis
of aurones. Moreover, a single regioisomer and stereoisomer is
produced in the cyclization step. We have accomplished the
synthesis of the natural 4,6,3′,4′-tetramethoxyaurone and reas-
signed the structures of dalmaisione D and another natural
product isolated from a marine brown alga.
Further work is now underway to understand the role of the
base involved in the gold-catalyzed reaction, to expand the use
of gold catalysts29 in organic synthesis, and to broaden the scope
of this reaction.
General Procedure 3 for the Oxidation. To the corresponding
benzofuranol (0.2 mmol, 2a-h) in dry CH2Cl2 (5 mL) was added
MnO2 (2 mmol) at room temperature. The reaction mixture was
stirred for 1 h at room temperature and was then filtered through
a pad of Celite. The organic phase was evaporated, and the crude
residue was purified by flash chromatography.
Experimental Section
General Procedure 1 for the Alkynylation. To an arylacetylene
(2.2 mmol) in dry THF (5 mL) was slowly added n-BuLi (2.1 mmol,
1.6 M in THF) at -78 °C. The reaction mixture was heated up to
-40 °C and then cooled down to -78 °C. A solution of the
corresponding salicylaldehyde (1 mmol in 5 mL of THF) was
dropwise added via cannula. The reaction mixture was stirred at
-78 °C for 4 h and was then quenched with saturated NH4Cl
solution. Excess of THF was removed in vacuo, and the aqueous
phase was extracted with Et2O. Combined organic layers were
washed with brine, dried over Na2SO4, and evaporated. Crude
product was purified by flash chromatography.
(Z)-Aurone (3a).6 Following the general procedure 3, benzo-
furanol 2a (100 mg, 0.45 mmol) gave 3a (90 mg, 90%) as a yellow
solid: TLC Rf 0.54 (Cyclohexane/EtOAc 30%); mp 99-100 °C;
IR (KBr) νmax 3030, 1714, 1652, 1594, 1472, 1462, 1445 cm-1
;
1H NMR (300 MHz, CDCl3) δ 7.93 (dd, J ) 7.0, 1.8 Hz, 2 H),
7.82 (ddd, J ) 7.7, 1.6, 0.7 Hz, 1 H), 7.65 (t, J ) 8.1 Hz, 1 H),
7.50-7.41 (m, 3 H), 7.34 (d, J ) 8.3, 1.2 Hz, 1 H), 7.22 (td, J )
7.5, 1.6 Hz, 1 H), 6.91 (s, 1 H); 13C NMR (75 MHz, CDCl3) 184.8,
166.8, 146.9, 137.0, 132.3, 131.9, 129.9, 128.9, 124.7, 123.5, 121.7,
113.1, 113.0; MS (ESI) m/z 245 (100, M+• + Na), 223 (28); HR-
MS 245.0548 (C15H10O2 + Na calcd 245.0573).
2-(1-Hydroxy-3-phenylprop-2-ynyl)phenol (1a).14b Following
the general procedure 1, salicylaldehyde (1.22 g, 10 mmol) and
phenylacetylene (2.04 g, 22 mmol) gave 1a (1.64 g, 73%) as a
pale white solid: TLC Rf 0.4 (cyclohexane/EtOAc 30%); mp 88 °C;
IR (CHCl3) νmax 2580, 3368, 3022, 2927, 2851, 2229, 1588, 1489,
Acknowledgment. We thank the CNRS and the French
Ministry of Research for financial support. H.H. thanks the
Algerian government for a Ph.D. fellowship.
1458, 1443, 1366, 1282, 1258, 1228, 1216, 1152, 1097, 1070 cm-1
;
1H NMR (300 MHz, CDCl3) δ 7.48-7.46 (m, 3 H), 7.35-7.24
(m, 5 H), 6.94 (m, 2 H), 5.91 (s, 1 H), 2.97 (br, 1 H); 13C NMR
(75 MHz, CDCl3) 155.2, 131.9, 131.8, 130.2, 130.2, 128.9, 128.4,
127.8, 124.6, 122.0, 120.3, 117.1, 88.1, 86.6, 64.3; MS (EI) m/z
223 (8, M+• - H), 206 (100).
Supporting Information Available: Spectral and characteriza-
tion data of each compound 1a-h, 2a-h, 3a-h, 4, and 5. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
(29) Hashmi, A. S. K. Chem. ReV. 2007, 107, 3180-3211.
JO702197B
J. Org. Chem, Vol. 73, No. 4, 2008 1623