C O M M U N I C A T I O N S
Table 3. Oxidation-Allylation of Amino Ketene Silyl Acetal 1a
Scheme 1. A Possible Mechanism of Nucleophilic Addition to the
Iminiumu Salt Prepared from Amino Ketene Silyl Acetal
entry
R1
allyl metal (equiv)
Lewis acid
R2
time (h)
6 (%)b
1
2
3
4
5
6
TMS (methallyl)4Sn(0.5) Et2AlCl
TBS (methallyl)4Sn(0.5) Et2AlCl
Me
Me
H
H
H
11.5
18.0
12.0
12.0
18.0
18.5
68
72
65
69
64
82
TMS (allyl)4Sn(0.5)
Et2AlCl
Et2AlCl
Et2AlCl
TMS allylSn(nBu)3(1.0)
TBS
TBS
allylSn(nBu)3(1.0)
allylSiMe3(2.0)
c
SnCl4
H
a See typical procedure. b Isolated yields. c In EtCN.
Scheme 3. First, DDQ reacts with the ketene silyl acetal 1b to give
the N,O-acetal 7, which collapses to the iminium salt 2. The
subsequent nucleophilic attack gives an addition product 3.10
In conclusion, we found that the iminium salt was easily prepared
using the oxidation of amino ketene silyl acetal with DDQ, and
that subsequent nucleophilic addition to this iminium species
proceeded efficiently to afford the amino esters in good yields.
yields (entries 11-13). Cyclopropylmagnesium bromide and tri-
methylsilylmethyl-magnesium bromide gave the addition products
5 in moderate to good yields (entries 14 and 15). Grignard reagents
bearing an olefin or an ether group were also employable (entries
16 and 17). It should be noted that products arising from the addition
of Grignard reagents to DDQ were not obtained in every case. For
further elaboration of these species, we next examined use of
allylation reagents. Table 3 summarizes the results.
Tetraallyl- and tetramethallyltin reagents effected allylation
reaction to give the adducts in moderate yields (entries 1-3), while
allyltributyltin could also be used with comparable efficiency under
the influence of diethylaluminum chloride as a Lewis acid (entries
4 and 5). Regarding the substituents at the silicon atom, TMS and
TBS derivatives recorded essentially the same range of product
yields. A better result was obtained using allyltrimethylsilane, and
in this case the reaction of the TBS derivative in propionitrile in
the presence of tin(IV) chloride was proved to be superior (entry
6).
Close examination of the 1H and 13C NMR spectra of the reaction
mixture revealed a possible formation of the iminium species 2.
Upon treatment of the ketene silyl acetal 1b with DDQ in CD3CN
at -40 °C and gradual warming of the mixture to room temperature
during 12 h, new signals appeared at 9.27 and 193.6 ppm in the 1H
and 13C NMR spectra, respectively, which actually indicated the
formation of the iminium species 2 (see Supporting Information).9
Furthermore, to check a possibility of the involvement of a radical
mechanism, the reaction was carried out in the presence of
galvinoxyl or 1,4-cyclohexadiene as a radical scavenger (eq 3).
However, the yields did not considerably decrease, indicating that
an ionic mechanism might be involved.
Acknowledgment. This work was supported by a Grant-in-Aid
for Scientific Research from the Ministry of Education, Science,
Sports, Culture, and Technology, Japan.
Supporting Information Available: Experimental procedures and
product characterization for new compounds. This material is available
References
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On the basis of these results, a possible mechanism of the present
iminium formation and nucleophilic addition reaction is shown in
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