4
362
A. Jee6anandam, Y.-C. Ling / Tetrahedron Letters 42 (2001) 4361–4362
Table 1.
Entry
R1
R2
Yield (%)a of 3 using indium
Yield (%)b of 3 and 4 using zinc
From nitrone
From aldimine
From nitrone
From aldimine
1
1
1
1
1
1
1
1
a, 2a, 3a, 4a
b, 2b, 3b, 4b
c, 2c, 3c, 4c
d, 2d, 3d
e, 2e, 3e, 4e
f, 2f, 3f
-H
-CH3
-CH CH
-CH(CH3)2
-Cl
-Br
-CH3
-CH CH
-H
-H
-H
-H
-H
-H
-Br
-Cl
88
72
70
67
18
12
52
45
61
58
59
53
12
6
49 (34)
41 (30)
44 (35)
–
42 (30)
38 (34)
23 (36)
–
2
3
10 (57)
8 (56)
–
–
–
–
–
–
g, 2g, 3g
h, 2h, 3h
38
34
2
3
a
Isolated yields.
Yields based on PMR analysis.
b
All nitrones and aldimines except 1e, 1f and 2e, 2f
afforded 3 in moderate to good yields; the formation of
vicinal diamines was not observed. It is known that the
chemistry of indium partly resembles that of zinc and
tin. However, the reaction of nitrones or aldimines with
zinc yielded a mixture of dihydrobenzofurans and sec-
ondary amines 4 (resulting from the reduction of
117.12, 120.28, 121.51, 124.76, 125.32, 125.79, 128.15,
128.90, 129.22, 130.06, 137.19, 143.07, 154.16, 159.26.
+
MS m/z 331 (M ), HRMS calcd for C H NO
22
21
2
331.1572; found 331.1574.
Acknowledgements
nitrones and aldimines). The close R values of 3 and 4
f
made their separation tedious by column chromatogra-
phy. The formation of 4 was not observed while using
indium. Our qualitative observations suggest the possi-
bility of the intermediacy of vicinal diamines. Halogen
substituted nitrones (1e and 1f) afforded 3 in lower
The authors are thankful to the National Tsing Hua
University and the National Science Council of the
ROC for their financial support.
12
yields, but dl vicinal diamines in 69–71% yields. This
could be attributed to the stability of the dl isomer by
1
3
strong hydrogen bonding. The higher yield of 3 and
the absence of vicinal diamines with other substituents
could be rationalized by subsequent conversion of
diamines to dihydrobenzofurans. Further mechanistic
studies, that is the sequence of coupling and deoxygena-
tion, oxidation of 3, and the extension of this protocol
to the synthesis of arylaminobenzothiopenes and
indoles are in progress.
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1
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3