Chemistry Reference
In-Depth Information
Fig. 8.6
Three methods for asymmetric synthesis of β-(trimethylsilyl)alanine
Fig. 8.7
Synthesis of
3-(dimethylsilyl)proline
The Schöllkopf reagent
22
can also be used to prepare silaproline
2
in enantio-
pure form. Deprotonation and alkylation with di(iodomethyl)dimethylsilane gives
28
, Fig.
8.7
. When this product is hydrolyzed and neutralized it cyclizes to form
the silaproline structure. Protection of the amine and hydrolysis of the ester yields
N
-protected silaproline
29
[
34
].
The Si-C bonds in these β-silyl amino acids are robust, however if the groups
attached to silicon are unsaturated they can suffer bond cleavage, Fig.
8.8
. Subject-
ing racemic
30
to standard ester hydrolysis conditions (refluxing 6 N HCl for 1 h)
led to cleavage of the silicon-phenyl bond. Isolation of the resulting product gave
disiloxane
32
as a mixture of diastereomers. When this amino acid dimer was dis-
solved in D
2
O only the monomeric
31
was detected, demonstrating reversibility of
the siloxane formation [
35
]. For an additional example of a silanol amino acid, see
Fig.
8.13
.
