sponding diiodides 3A and 3B was not observed (Scheme
2). The structure of E-2a was unambiguously established by
the X-ray diffraction study.
as HX),5 bases were added to neutralize the in situ generated
HI. The result with K2CO3 was not satisfactory (entry 3,
Table 1). After trial and error, we found that LiOAc showed
the best result with the exclusive formation of E-2a in 69%
at rt, but 18% of starting material was recovered (entry 6,
Table 1). When the reaction was heated to 55 °C using 2.0
equiv of I2, E-2a was formed in 96% yield within 1 h (entry
8, Table 1). The reaction did not go to completion with 1.5
equiv of I2 even after 4 h at 55 °C (entry 4, Table 1). In
addition, the utilization of 4.0 equiv of water accelerated the
reaction but the yield was lower (entry 7, Table 1).
Scheme 2
Thus, we studied the iodohydroxylation of a series of
allenyl phenyl sulfoxides. Some typical results are sum-
marized in Table 2. The following points are noteworthy:
Table 2. The Reaction of Allenyl Phenyl Sulfoxides with
I2/H2O in Aqueous MeCNa
With the unique selectivity observed for the formation of
E-2a, we tried to optimize the conditions of this iodo-
hydroxylation reaction of 1a using iodine in MeCN-H2O
(7:1). The results are summarized in Table 1. When 2.0 equiv
Table 1. Reaction of (1,2-Propadienylsulfinyl)benzene (1a)
with I2/H2O in Aqueous MeCNa
1
c
entry
R1
R2
R3
I2
time (h) yield (%)d
1b
2
3
4
5
H
H
H
H
H
H (1a )
H (1b)
H (1c)
t-Bu (1d )
Me (1e)
2.4
1.6
1.6
1.2
1.2
1
96
79
82
87
87
74
88
Ph
n-Bu
H
H
H
2.2
1.5
3.2
0.5
0.3
1.3
b
Me
entry I2
additives time (h) temp (°C) yield (%)c 1a d (%)
6
7
(CH2)5 (1f) 1.2
Me (1g) 1.2
1
2
3
4
5
6
7e
8
1.0
2.0
5
4
4
4
2.5
5
0.2
1
rt
rt
rt
55
65
rt
55
55
49
66
3.9
62
64
69
60
96
34
0
84
29
0
18
0
0
n-Bu
Me
a The reaction was carried out using allenyl phenyl sulfoxide (0.5 mmol),
I2 (see Table 2), and LiOAc (1 mmol) in MeCN-H2O (1 mL, v/v ) 7:1)
at room temperature. b The reaction was carried out at 55 °C. c Equivakents
of iodine used. d Isolated yield of E-2 based on 1.
2.0 K2CO3
1.5 LiOAc
2.0 NaHCO3
2.0 LiOAc
2.0 LiOAc
2.4 LiOAc
(1) The iodohydroxylation reactions of C-aryl- or C-alkyl-
substituted allenyl phenyl sulfoxides were faster than that
of the unsubstituted 1a, and the reactions finished in from
0.3 to 3.2 h at room temperature (compare entries 2-7 with
1, Table 2). (2) The yields of the corresponding products 2
are high yields and the regio- and stereoselectivity of this
reaction are excellent. (3) When 1d with a de ratio of 93:7
was used the corresponding iodohydroxylation product E-2d
was formed in the same diastereomeric ratio as determined
a The reaction of allenic sulfoxide (1a, 1 mmol) with I2 was carried out
in MeCN-H2O (2 mL/0.3 mL). b Equivalents of iodine used. c Isolated yield
based on 1a. d Yield of recovered 1a. e Only 4.0 equiv of H2O was used.
of iodine was used, the reaction was complete in aqueous
MeCN within 4 h to afford the product E-2a in 66% yield
(entry 2, Table 1) together with some byproducts of low
polarity. Since the iodohydroxylation product (E-2a) and HI
were formed at the same time, and allenyl phenyl sulfoxides
are sensitive to even a small amount of strong acids (such
1
by H NMR spectra and HPLC (entry 4, Table 2).
Generally the iodohydroxylation of carbon-carbon double
bond directly using I2 and H2O is difficult.6 Reaction of
allenes or 2,3-allenic esters with halogens was known to form
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3; Pergamon Press: Oxford, 1979; p 130.
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Nun˜ez, M. E. Org. Lett. 1999, 1, 2125. (b) Masuda, H.; Takase, K.; Nishio,
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Org. Lett., Vol. 2, No. 24, 2000