Application of t-Butyldimethylsilyl Ethers of Serine, Threonine and Tyrosine in Peptide Synthesis

Article abstract of DOI:10.1016/S0040-4039(00)60836-5

The utility of Tbdms (t-butyldimethylsilyl) ethers, prepared conveniently in a one pot procedure from N^α-Fmoc (9-fluorenylmethoxycarbonyl) and N^α-Z (benzyloxycarbonyl) hydroxyamino acids, is demonstrated: peptide bond formation and esterification to 4-alkoxybenzylalcohol resin are achieved readily with these derivatives.The lability of the Tbdms ethers to various reagents enables selective deprotection of the hydroxyl side-chains assembly, desirable, e.g., for phosphorylation of glycosylation.

Full text of DOI:10.1016/S0040-4039(00)60836-5

Tetrahedron
Vol. 33. No. 49. pp.
in Great Britain
Application of t-Butyldimethylsilyl Ethers of
Synthesis
and
in
Peter M.
21, N-0371 Oslo, Norway
AS,
The utility of Tbdms
ethers, prepared conveniently in a one-pot procedure
and acids, is
from
demonstrate&
bond formation and
to
resin are achieved
readily with these derivatives. The lability of the Tbdms ethers to various reagents enables selective
deprotection of the hydroxyl side-chains after
or glycosilation.
chain assembly, desirable, e.g., for phosphorylation
Traditionally, the side-chain
functions in
Thr and Tyr have been protected as
protection and resin
linkage agents. When a more acid-labile hydroxyl protecting group is required, the corresponding trityl
ethers can be used’. Alternatively, side-chain unprotected and
OH’ can be incorporated directly into under certain conditions. The
ethers in solid-phase
synthesis (SPPS) based on
protecting
bond
group has been discussed’ but is not suitable since it removed under the conditions of
formation. The Tbdms ether is very important as a protecting group in general organic
but has
not found general application in
chemistry.
ii
i
R-Xaa-OR’
.
.
la;
lb; R=Fmoc,
lc;
R=Fmoc,
le;
Me/Et
2b;
2d;
Tbdms
R=Z,
i; a d:
or
e
then
a,
4
then
b, 2 %
d,
e
stands for Ser, Thr or Tyr residue).
Although the Fmoc-amino acid
ethers 3a are used widely in
and the commercial products are expensive. Temporary protection of both the amino and
functions in 1 is necessary during Usually this is achieved with the aid of
their preparation is
inconvenient’
esters (e.g. la or c) which can subsequently be removed by
stability of the Fmoc group in to hydrogenolytic
Because of the uncertain
Z group is usually preferred for
the
the
step before Pmoc-reprotection. Alternatively, carboxyl protection as an
ester” is
7605

e
f
f
e
c
t
i
v
e
a
n
d l
from 2b during subsequent base hydrolysis”. We find that carboxyl-protection as the phenacyl ester”
even more convenient because these esters can be prepared” easily from the Fmoc-amino acids le directly
and, unlike saponification of the alkyl esters selective deprotection with zinc in acetic acid is very
effective and there is no danger of racemisation Silylation of le or f yielded the silyl ether-esters
b
m
a
y
b
e
directly, provi
is
and f, which were transformed without isolation to the silyl ethers
and
f
Apparently the silyl esters
2e are more acid-sensitive than usual” although even
solvolysis can remove some Tbdms
esters”. Unlike the Bu’ ethers
the silyl ethers 3e can thus be obtained in a one-pot reaction sequence
from le directly.
Even when the hydroxysmino acid residues are incorporated into growing
side-chain protection, the conditions required for the esterification of the commonly used SPPS supports
with Ser. Thr Tyr derivatives obviously still necessitates masking of the hydroxyl function. For this
chains without
reason the utility of for resin esterification was studied (Table 1). Both the Ser and Thr derivatives
could be anchored successfully using the conventional dicyclohexylcarbodiimide
dimethylaminopyridine
but could be
method”. The Tyr derivative, on the other hand, failed to react with DCC
using a different
with the aid of Castro’s
The levels of
racemisation were found to be comparable to results obtained with
Table 1.
of
resin.”
D- or
isomer)
Derivative used for
F
%
04
98”
76”
were
1.2
.
of
0.1
by
internal Nle
Measured by
acid
of
samples
comparison with
amino acid resin
with
amide
before
detachment and chromatography of released
with 5 eq preformed symmetrical
DMF (2 x 30
in the presence of 0.5 DMAP
the presence of 5 eq BOP
5 eq N-methyl
morpholiae
with 5
acid derivative
hexatluorophosphate), 0.5
5
NMM DMF for 18 h.
Table 2 shows that the silyl ethers of
Thr are somewhat more labile to acidolysis by
the corresponding Bu’ ethers. For the Tyr derivatives the situation is reversed, the silyl
ether being almost resistant to dilute
Selective deprotection of in the presence of
chosen carefully. Under the hydrolytic conditions applied, the
the silyl ethers of Ser and Thr can be removed smoothly. The Tyr silyl ether is again more resistant. When
fluoride are used to effect Tbdms deprotection, the reactivities of the derivatives It can be
seen that for the relatively acid-stable aromatic silyl ether the fluoride treatment is very effective.
Therefore, if complete deprotection of Tyr(Tbdms)-containing is desired, it is advisable to remove
in
whereas the Bu’ ether is removed efficiently.
should be possible if the conditions are
ethers are completely stable, whereas

7607
the N-terminal Fmoc
and the aromatic Tbdms
ester simultaneously with
prior to
resin
The Tbdms
groups were found to be stable to piperidine,
as end
as well as to
basic reagents used in SPPS,
Table 2.
of
ethers.
%
or
Conditions’
Bu’
19
22
22
22
15
15
30
1%
25
35
2
3
43
42
14
46
80
100
100
99
100
95
100
100
9
27
96
80
2
27
53
99
100
47
72
91
3
18
17
0
0
0
0
0
100
0.1 M
6
8
0
36
100
0
0
0
20
DMF
1080
22
to
40 to 80 %
used for the
by integration of RP-HPLC
over 10
C,,,
0.1
nm); the
acid derivatives
hydrolysis
and the Z-amino
for the other
acid Tbdms ethers were
with one equivalent of
in
using
reactions were monitored by TLC and found to be
practically complete within minutes. The analytically pure dipeptides were obtained in 90 % isolated
yield after silica gel chromatography and no racemisation was detected upon RP-HPLC analysis of the
deprotected dipeptides.
Table 3.
Side-chain protecting groups
of
0.50’
0.50’
77 %
84 %
96 %
9 3 %
Peak
FAB-MS.
Determined
and
at 1
quantitative amino
usmg ammo
in 0.1%
gas-phase
and
with
analysis of the
in
quantities of
rosin.
cm, 0 to 30 %
cm, 2.5 to 12 %
30 min.
30 min.
A
rich in hydroxyamino acid residues was synthesised using two different approaches. In
one experiment commercial
experiment
KA resin and
used, in the other
KA prepared as described above and the
were used. In each case 4 eq of the amino acid derivatives were coupled for 1 h using the above coupling

conditions and
cleavage was achieved
effected for
%
in
for 2 h. In the
in The
Tbdms synthesis
last
min with 0.1 M
results in Table 3 show the efficacy of the Fmoc-amino acid Tbdms ethers in
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(Received in UK 19 August 1992)