1,2-Thio group migration in Rh(II) carbene reactions

Article abstract of DOI:10.1021/jo050109v

A series of β-thio group substituted α-diazo carbonyl compounds have been prepared by nucleophilic substitution reactions of thiophenol, thionaphthol, or benzyl mercaptan with β-acetoxy-α-diazo carbonyl compounds. The diazo decomposition of these diazo ca

Full text of DOI:10.1021/jo050109v

SCHEME 1
SCHEME 2
1,2-Thio Group Migration in Rh(II)
Carbene Reactions
Feng Xu, Weifeng Shi, and Jianbo Wang*
Key Laboratory of Bioorganic Chemistry and Molecular
Engineering of Ministry of Education, College of Chemistry,
Peking University, Beijing 100871, China
wangjb@pku.edu.cn
Received January 19, 2005
A series of â-thio group substituted R-diazo carbonyl com-
pounds have been prepared by nucleophilic substitution
reactions of thiophenol, thionaphthol, or benzyl mercaptan
with â-acetoxy-R-diazo carbonyl compounds. The diazo de-
composition of these diazo carbonyl compounds with various
transition metal catalysts, including Rh(II) carboxylates and
Cu(I) and Cu(II) complexes, has been investigated. It was
found that the diazo decomposition of these compounds gave
1,2-thio group migration products. No 1,2-hydride or 1,2-
aryl migration products were observed in all cases.
certain circumstances, 1,2-aryl migration can override
1,2-hydride migration. On the other hand, it has been
reported that the acetoxy group has higher 1,2-migratory
aptitude over hydride in the Rh2(OAc)4-mediated reaction
of â-acetoxy R-diazo carbonyl compounds, presumably due
to the interaction of the carbonyl oxygen lone pair with
the carbene carbon p orbital to form a favorable five-
membered-ring transition state.4 In this paper we report
1,2-thio group migration in the Rh2(OAc)4-catalyzed
reaction of the â-thio group-substituted R-diazo carbonyl
compounds. To the best of our knowledge, this type of
1,2-migration has not been reported in Rh(II) carbene
reactions.
Our recent study suggests that the â position of the
R-diazo carbonyl compound is liable to nucleophilic
substitution.5 This observation makes it possible to
prepare the diazo compounds with various â-substituents.
Consequently, the â-thio group-substituted R-diazo car-
bonyl compounds 1 that have been used in this investiga-
tion can be prepared by nucleophilic substitution of the
corresponding â-acetoxy-R-diazo carbonyl compounds 5
with the anion generated through the deprotonation of
a suitable thio compound,6 while the â-acetoxy-R-diazo
carbonyl compound 5 can be easily prepared from alde-
hyde and acyldiazomethane in two steps (Scheme 2).4c,7
Thus, â-acetoxy-R-diazo carbonyl compounds 5 were
prepared by the methods shown in Scheme 2 with the
yields ranging from 35% to 70%. Then R3SH was treated
1,2-Migration is one of the fundamental reactions in
transition metal-catalyzed reaction of R-diazo carbonyl
compounds.1,2 Although cyclopropanation, dimerization,
and X-H (X ) C, O, S, N, etc.) bond insertions are major
reaction pathways, the 1,2-migration may become com-
petitive in certain cases. Among the 1,2-migration reac-
tions, the 1,2-hydride migration is generally predomi-
nant, but 1,2-alkyl, 1,2-aryl, 1,2-vinyl, and 1,2-acetylenyl
migrations are also observed.3 Recent study in our group
has revealed that the migratory aptitude was dramati-
cally affected by the nonmigratory substituents.3e In
(1) For comprehensive reviews, see: (a) Doyle, M. P.; McKervey, M.
A.; Ye, T. Modern Catalytic Methods for Organic Synthesis with Diazo
Compounds; Wiley-Interscience: New York, 1998. (b) Ye, T.; McKervey,
M. A. Chem. Rev. 1994, 94, 1901.
(2) 1,2-Hydrogen migration: (a) Frazen, V. Liebigs Ann. Chem. 1957,
602, 199. (b) Pellicciari, R.; Fringuelli, R.; Ceccherelli, P.; Sisani, E. J.
Chem. Soc., Chem. Commun. 1979, 959. (c) Pellicciari, R.; Fringuelli,
R.; Sisani, E. J. Chem. Soc., Perkin Trans. 1 1981, 2566. (d) Hoffmann,
K.-L.; Regitz, M. Tetrahedron Lett. 1983, 24, 5355. (e) Hudlicky, T.;
Olivo, H. F.; Natchus, M. G.; Umpierrez, E. F.; Pandolfi, E.; Volonterio,
C. J. Org. Chem. 1990, 55, 4767. (f) Taber, D. F.; Hoerrner, R. S. J.
Org. Chem. 1992, 57, 441. (g) Taber, D. F.; Hennessy, M. J.; Louey, J.
P. J. Org. Chem. 1992, 57, 436. (h) Taber, D. F.; Herr, R. J.; Pack, S.
K.; Geremia, J. M. J. Org. Chem. 1996, 61, 2908. (i) Sarabia Garc´ıa,
F.; Pedraza Cebria´n, G. M.; Heras Lo´pez, A.; Lo´pez Herrera, F. J.
Tetrahedron, 1998, 54, 6867. (j) Ye, T.; McKervey, M. A. Tetrahedron
1992, 48, 8007. (k) Ohno, M.; Itoh, M.; Umeda, M.; Furuta, R.; Knodo,
K.; Eguchi, S. J. Am. Chem. Soc. 1996, 118, 7075.
(3) 1,2-Aryl and 1,2-alkyl migrations: (a) Nagao, K.; Chiba, M.; Kim,
S.-W. Synthesis 1983, 197. (b) Kanemasa, S.; Kanai, T.; Araki, T.;
Wada, E. Tetrahedron Lett. 1999, 40, 5055. (c) Jiang, N.; Qu, Z.; Wang,
J. Org. Lett. 2001, 3, 2989. (d) Shi, W.; Jiang, N.; Zhang, S.; Wu, W.;
Du, D.; Wang, J. Org. Lett. 2003, 5, 2243. (e) Jiang, N.; Ma, Z.; Qu, Z.;
Xing, X.; Xie, L.; Wang, J. J. Org Chem. 2003, 68, 893.(f) Aggarwal, V.
K.; Sheldon, C. G.; Macdonald, G. J.; Martin, W. P. J. Am. Chem. Soc.
2002, 124, 10300.
(4) 1,2-Acetoxy migration: (a) Ikota, N.; Takamura, N.; Young, S.
D.; Ganem, B. Tetrahedron Lett. 1981, 22, 4163. (b) Lopez-Herrera, F.
J.; Sarabia-Garcia F. Tetrahedron Lett. 1994, 35, 6705. (c) Lopez-
Herrera, F. J.; Sarabia-Garcia F. Tetrahedron 1997, 53, 3325.
(5) Shi, W.; Zhang, B.; Liu, B.; Xu, F.; Xiao, F.; Zhang, J.; Zhang,
S.; Wang, J. Tetrahedron Lett. 2004, 45, 4563.
(6) As suggested by one of the referees, an alcohol can be trans-
formed to a thioether under a mild condition with PhSSPh/Bu3P in
MeCN. See: (a) Wang, X.-Z.; Wu, Y.-L.; Jiang, S.; Singh, G. J. Org.
Chem. 2000, 65, 8146. (b) Hanessian, S.; Tyler, P. C.; Demailly, G.;
Chapleur, Y. J. Am. Chem. Soc. 1981, 103, 6243. We examined this
procedure with the â-hydroxy R-diazo carbonyl compounds 5a (R1
)
Et, R2 ) OEt) and 5j (R1 ) Ph, R2 ) OEt). The expected thioethers 1a
and 1j could be isolated after stirring the reaction mixture for 40 h.
However, the yields were less than 10%, and most of the starting
materials were recovered.
(7) Jiang, N.; Wang, J. Tetrahedron Lett. 2002, 43, 1285.
10.1021/jo050109v CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/12/2005
J. Org. Chem. 2005, 70, 4191-4194
4191

TABLE 1. Nucleophilic Substitution of 5 with R3SNa
TABLE 2. Rh2(OAc)4-Catalyzed Reaction of Diazo
Compounds 1a-m
entry
R1
R2
R3
product yield (%)a
substrate reaction
yield product ratio
1
2
Et
Et
OEt Ph
1a
1b
1c
1d
1e
1f
66
81
55
65
87
82
68
65
57
60
68
40
47
entry
1
time (h)
products
(%)a,b
(2Z:2E)
OEt PhCH2
OEt Ph
3
n-hexyl
n-hexyl
Et
1
2
1a
1b
1c
1d
1e
1f
10
12
10
19
2
11
24
24
3.5
10
13
6.5
2
2Za + 2Ea
2Zb + 2Eb
2Zc + 2Ec
2Zd + 2Ed
2Ze + 2Ee
2Zf + 2Ef
2Zg + 2Eg
2Zh + 2Eh
2Zi + 2Ei
2Zj + 2Ej
2Zk + 2Ek
2Zl + 2El
2Zm + 2Em
78
79
96
97
81
60
88
73
99
99
93
98
71
>20:1
2.4:1
6.3:1
1.6:1
8.0:1
9.3:1
3.7:1
1:1.7
1.1:1
1.7:1c
2.0:1c
2.0:1c
1.2:1c
4
OEt PhCH2
5
Me
Me
Ph
Ph
Ph
3
6
Et
PhCH2
Ph
4
7
Et
1g
1h
1i
5
8
Et
PhCH2
6
9
Et
OEt 2-naphthyl
OEt Ph
OEt PhCH2
7
1g
1h
1i
10
11
12
13
Ph
1j
8
Ph
1k
1l
9
p-PhC6H4 OEt Ph
10
11
12
13
1j
p-MeOPh
OEt Ph
1m
1k
1l
a Yields after column chromatographic purification with silica
gel.
1m
a Yields after column chromatographic purification with silica
gel (entries 1-9). b Crude product yields (entries 10-13). c Ratio
determined by 1H NMR (300 MHz) of the crude product. The ratio
could be either 2Z:2E or 2E:2Z for entries 10-13.
SCHEME 3
SCHEME 4
with NaH in anhydrous THF at 0 °C, followed by the
addition of a solution of â-acetoxy-R-diazo carbonyl
compound 5 in anhydrous THF. The substitution reaction
was completed within 8 h, and the â-thio group-
substituted R-diazo carbonyl compounds 1a-m were
obtained in yields ranging from 40% to 87% as shown in
Table 1. This reaction provides an effective method of
bringing the thio group to the R position of the diazo
group. It is demonstrated again that the adjacent position
of a diazo group is liable to nucleophilic attack.
It is worthwhile to note that the substitution reaction
can be quenched at 0 °C if R1 is an alkyl group and the
corresponding â-thio-substituted R-diazo carbonyl com-
pounds 1 can be obtained after column chromatography.
If R1 is aryl group, however, the reaction must be
quenched at -78 °C, otherwise the product will decom-
pose to give a mixture of 1 and other byproducts. It is
obvious that the diazo compounds with a thio group in
the benzylic position are unstable. Besides, when trying
to prepare the â-phenylthio R-diazo ester bearing a â-(p-
nitro)phenyl substituent, only R,â-unsaturated ester 7
and 8 were isolated (Scheme 3).
With â-thio group R-diazo carbonyl compounds 2a-m
in hand, we then proceeded to study their reaction with
Rh2(OAc)4 catalyst. The diazo decomposition occurred
smoothly at 0 °C in dichloromethane to give a stereoiso-
meric mixture of 1,2-thio group migration products 2Z
and 2E (Scheme 1 and Table 2). The reactions were
finished in 2 to 24 h and no 1,2-hydride migration
products were detected in all the cases. When other
commonly used catalysts, such as Cu(CH3CN)4PF6 and
Cu(acac)2, were employed in this reaction, similar 1,2-
thio group migration occurred, although with these
catalysts the reaction needed to be carried out in reflux-
ing benzene.
When R1 was the alkyl group, it was easy to identify
the products to be 1,2-thio group migration, rather than
1,2-hydrogen migration. In the 1H NMR spectrum of the
products, a triplet peak between δ 6.0 and 8.0 due to the
proton attached to the double bond is coupled with the
adjacent methylene protons.
When R1 was the aryl group, however, the structure
characterization was not straightforward. In these cases,
1,2-thio group, 1,2-aryl, or 1,2-hydrogen migrations all
give a singlet for the corresponding proton. The 1H NMR
spectra of the products show the absorption of this proton
appears in the aromatic region, thus excluding the 1,2-H
migration products because the R proton of an R,â-
unsaturated carbonyl compound is generally below δ 7.0.
The 1,2-thio and 1,2-aryl migration products can be
further differentiated by the reduction of the migration
products with NaBH4 8 and then comparing the reduction
products with known compounds (Scheme 4).9 Reduction
of the migration products from the Rh2(OAc)4-catalyzed
reaction of 1j-l with NaBH4 at 0 °C occurs smoothly to
give single product 8j-l in each case, thus unambigu-
ously confirming only 1,2-thio group migration occurs in
Rh2(OAc)4-catalyzed reactions.
It was noted when R1 was the alkyl group, in most
cases the Z and E isomers of 1,2-thio group migration
products could be separated by column chromatography
and it was confirmed that Z, E isomerization did not
(8) The NaBH4 reduction of a doubly activated unsaturated ester is
known. For an example, see: Salomon, R. G.; Sachinvala, N. D.;
Raychaudhuri, S. R.; Miller, D. B. J. Am. Chem. Soc. 1984, 106, 2211.
(9) Trost, B. M.; Salzmann T. N.; Hiroi, K. J. Am. Chem. Soc. 1975,
97, 4887.
4192 J. Org. Chem., Vol. 70, No. 10, 2005

2086, 1691, 1286, 1118, 743, 692 cm-1 1H NMR (300 MHz,
;
SCHEME 5
CDCl3) δ 1.05 (t, J ) 7.2 Hz, 3H), 1.18 (t, J ) 7.2 Hz, 3H), 1.67-
1.88 (m, 2H), 4.02-4.16 (m, 3H), 7.26-7.31 (m, 3H), 7.44-7.47
(m, 3H); 13C NMR (75 MHz, CDCl3) δ 11.8, 14.3, 26.5, 45.2, 60.7,
127.7, 128.8, 133.0, 133.3, 166.0; EI-MS (m/z, rel intensity) 264
(M+, 1), 236 [(M - 28)+, 11], 190 (16), 155 (33), 127 (31), 110
(62), 99 (60), 55 (33), 29 (100). Anal. Calcd for C13H16N2O2S: C,
59.07; H, 6.10; N, 10.60. Found: C, 59.26; H, 6.06; N, 10.53.
General Procedure for the Preparation of â-Thio-
Substituted r-Diazo Carbonyl Compounds 1j-m. To a
solution of mercaptan (1.2 mmol) in anhydrous THF (20 mL) at
0 °C was added sodium hydride (1.44 mmol) under N2. The
mixture was stirred for 5 min, and was then cooled to -78 °C
with dry ice-acetone. Then the solution of â-acetoxy-R-diazo
carbonyl compounds 5 (1.0 mmol) in anhydrous THF (5 mL) was
added dropwise during a period of 30 min. The mixture was then
stirred for 8 h until TLC analysis indicated the complete
disappearance of the starting material, during which the tem-
perature of the reaction mixture rose to 0 °C. The reaction
mixture was cooled to -78 °C again and quenched with dropwise
addition of the saturated aqueous NaCl (10 mL). The usual
workup gave a crude yellow product, which was purified by
column chromatography to yield â-thio R-diazo carbonyl com-
pounds 1j-m.
occur during the separation.10 The exceptions were the
reactions with 1g and 1h (entries 7 and 8), in which cases
Z, E isomerization occurred on the silica gel column. On
the other hand, when R1 was the aryl group, the Z and
E isomers of 1,2-thio group migration products could not
be separated by silica gel column. The ratios were thus
1
determined by the crude H NMR spectra (entries 10-
13). It was also noted in these cases that the Z, E
selectivities were poor, ranging from 1.2:1 to 2.0:1.
The facile 1,2-thio group migration is presumably due
to the easy overlap of the lone pair occupied sp3 orbital
of the sulfur with the p orbital of the Rh(II) carbene
carbon, which is positively polarized (A, Scheme 5). A
thiiranium intermediate (B) may be generated, followed
by the extrusion of Rh(II) catalyst to give 1,2-migration
product (C). This is comparable to the 1,2-acetoxy group
migration, in which case the interaction between the lone
pair of the carbonyl oxygen and the Rh(II) carbene carbon
generates a five-membered-ring transition state.4a
In conclusion, we have reported an unprecedented 1,2-
thio group migration in the Rh(II) carbene reaction. It
demonstrates that the thio group has a higher migratory
aptitude over hydrogen. The reaction sequence from
aldehydes represents a two-carbon homologation to the
R-thio unsaturated carbonyl compounds,11 which may
find application in organic synthesis.
Ethyl 2-diazo-3-phenyl-3-(phenylthio)propionate (1j):
60%; TLC petroleum ether:ethyl acetate ) 30:1 Rf 0.33; IR (film)
2094, 1676, 1368, 1296, 1097, 711 cm-1 1H NMR (200 MHz,
;
CDCl3) δ 1.13 (t, J ) 7.2 Hz, 3H), 4.11 (q, J ) 7.2 Hz, 2H), 5.38
(s, 1H), 7.21-7.46 (m, 10H); 13C NMR (50 MHz, CDCl3) δ 14.2,
47.0, 60.9, 127.1, 127.7, 128.0, 128.8, 128.9, 132.1, 133.5, 138.0,
165.4; EI-MS (m/z, rel intensity) 284 [(M - 28)+, 40], 211 (54),
203 (39), 175 (54), 147 (75), 129 (18), 109 (47), 103 (93), 91 (37),
77 (50). Anal. Calcd for C17H16N2O2S: C, 65.36; H, 5.16; N, 8.97.
Found: C, 65.49; H, 5.24; N, 8.61.
General Procedure for Rh2(OAc)4-Catalyzed Reaction
of â-Thio-Substituted r-Diazo Carbonyl Compounds 1a-
m. A solution of diazo compound 1a-m (0.5 mmol) in CH2Cl2
(20 mL) was stirred with Rh2(OAc)4 (1 mg) at 0 °C under N2
until TLC analysis indicated the complete disappearance of the
starting material (2-24 h). The solvent was then removed under
reduced pressure and the crude residue was purified by column
chromatography to give the corresponding 1,2-thio group migra-
tion products.
Experimental Section
Caution: Diazo compounds are potentially explosive. They
should be handled with care in a well-ventilated fumehood.
General Procedure for the Preparation of â-Thio-
Substituted r-Diazo Carbonyl Compounds 1a-i. To a
solution of thio alcohol (1.2 mmol) in anhydrous THF (20 mL)
at 0 °C was added sodium hydride (1.44 mmol) under N2. The
mixture was stirred for 5 min, then a solution of â-acetoxy-R-
diazo carbonyl compound 5 (1.0 mmol) in anhydrous THF (5 mL)
was added dropwise during a period of 30 min. The mixture was
then stirred until TLC analysis indicated the complete disap-
pearance of the starting material. The reaction mixture was then
quenched with saturated aqueous NaCl (10 mL). The mixture
was extracted with Et2O. The combined organic layers were
dried over anhydrous Na2SO4. The solvent was removed by
evaporation under reduced pressure, and the crude yellow
product was purified by column chromatography to yield â-thio
R-diazo carbonyl compounds 1a-i.
(Z)-Ethyl 2-(phenylthio)-2-pentenoate (2Za):12 10 h, 74%;
TLC petroleum ether:ethyl acetate ) 30:1; Rf 0.51; IR (film) 2927,
2361, 1713, 1236, 1044, 740, 690 cm-1 1H NMR (300 MHz,
;
CDCl3) δ 1.06-1.12 (m, 6H), 2.48-2.58 (m, 2H), 4.10 (q, J ) 7.2
Hz, 2H), 7.15-7.28 (m, 5H), 7.39 (t, J ) 7.5 Hz, 1H); 13C NMR
(75 MHz, CDCl3) δ 12.7, 13.7, 24.0, 61.3, 125.8, 126.0, 127.9,
128.7, 135.8, 154.3, 165.2; EI-MS (m/z, rel intensity) 236 (M+,
66), 190 (100), 161 (59), 147 (24), 129 (89), 109 (26), 105 (84), 91
(21), 85 (62), 65 (18), 45 (24), 29 (24). Anal. Calcd for
C13H16O2S: C, 66.07; H, 6.82. Found: C, 65.83; H, 6.82.
(E)-Ethyl 2-(naphthylthio)-2-pentenoate (2Ei): 3.5 h,
47%; TLC petroleum ether:ethyl acetate ) 30:1; Rf 0.50; IR (film)
1712, 1213, 1133, 1030, 813, 745 cm-1 1H NMR (300 MHz,
;
CDCl3) δ 1.05 (t, J ) 7.1 Hz, 3H), 1.10 (t, J ) 7.5 Hz, 3H), 2.51-
2.61 (m, 2H), 4.10 (q, J ) 7.5 Hz, 2H), 6.52 (t, J ) 7.5 Hz, 1H),
7.39-7.50 (m, 3H), 7.74-7.79 (m, 4H); 13C NMR (75 MHz,
CDCl3) δ 13.5, 13.9, 24.2, 61.1, 125.7, 125.9, 126.4, 127.2, 127.3,
127.6, 128.2, 128.5, 132.0, 132.4, 133.6, 151.0, 165.3; EI-MS (m/
z, rel intensity) 286 (M+, 68), 257 (3), 240 (64), 213 (19), 197
(12), 179 (29), 171 (11), 155 (100), 128 (54), 115 (42), 99 (5), 85
(12), 63 (3), 45 (6), 29 (6). Anal. Calcd for C17H18O2S: C, 71.30;
H, 6.34. Found: C, 71.14; H, 6.42.
Ethyl 2-diazo-3-(phenylthio)pentanoate (1a): TLC pe-
troleum ether:ethyl acetate ) 30:1; Rf 0.30; 8 h, 66%; IR (film)
(10) The major isomer is assigned to have the Z configuration by
comparing the 1H NMR spectral data with the corresponding simulated
spectra.
(11) For the examples of two-carbon homologation of an aldehyde
to the R-thio unsaturated ester or ketone, see: (a) Babin, D.; Demassey,
J.; Demoute, J. P.; Dutheil, P.; Terrie, I.; Tessier, J. J. Org. Chem.
1992, 57, 585. (b) Marcantoni, E.; Massaccesi, M.; Petrini, M.; Bartoli,
G.; Bellucci, M. C.; Bosco, M.; Sambri, L. J. Org. Chem. 1992, 57, 4553.
(c) Zhdankin, V. V.; Maydanovych, O.; Herschbach, J.; Bruno, J.;
Matveeva, E. D.; Zefirov, N. S. Tetrahedron Lett. 2002, 43, 2359. (d)
Deng, G. S.; Zhi Huang, Z.; Huang, X. Chin. Chem. Lett. 2003, 14, 3.
(e) Deng, G. Chin. J. Org. Chem. 2004, 24, 286.
General Procedure for Rh2(OAc)4-Catalyzed Reaction
of 1j-l Followed by Reduction with NaBH4. The crude
product obtained from Rh2(OAc)4-catalyzed reaction of 1i-j was
dissolved in anhydrous THF (10 mL), then NaBH4 (1.0 equiv)
was added at 0 °C under N2. The reaction mixture was stirred
until TLC analysis indicated the complete disappearance of the
starting material (24 h). The reaction mixture was then quenched
with saturated aqueous NaHCO3 (10 mL). The usual workup
(12) Miller, R. D.; Hassig, R. Tetrahedron Lett. 1985, 26, 2395.
J. Org. Chem, Vol. 70, No. 10, 2005 4193

gave a crude product, which was purified by column chroma-
tography to yield 8j-l.
MS (m/z, rel intensity) 286 (M+, 67), 195 (27), 177 (100), 149
(70), 135 (73), 121 (64), 109 (29), 103 (27), 91 (55).
Ethyl 3-phenyl-2-(phenylthio)propionate (8j):9 90%; TLC
petroleum ether:ethyl acetate ) 30:1; Rf 0.45; IR (film) 1687,
1592, 1478, 1356, 1224, 1209, 1024, 740, 690 cm-1; 1H NMR (300
MHz, CDCl3) δ 1.06 (t, J ) 7.2 Hz, 3H), 3.03-3.24 (AB part of
ABX, JAB ) 13.8 Hz, 2H), 3.88-3.93 (X part of ABX, JAX ) 6.3
Hz, JBX ) 9.3 Hz, 1H), 3.99-4.08 (m, 2H), 7.18-7.31 (m, 8H),
7.43-7.46 (m, 2H); 13C NMR (75 MHz, CDCl3) δ 13.8, 37.8, 52.1,
61.0, 126.7, 127.9, 128.3, 128.8, 128.9, 133.0, 137.6, 171.5; EI-
Acknowledgment. This project was generously
supported by the Natural Science Foundation of China
(Grant Nos. 20225205 and 20390050).
Supporting Information Available: Characterization
data for 1b-i, 1k-m, 1zb-i, 8k,l, and 7. This material is
available free of charge via the Internet at http://pubs.acs.org.
JO050109V
4194 J. Org. Chem., Vol. 70, No. 10, 2005