Chemistry Reference
In-Depth Information
O
O
O
O
NaHSO
4
.
SiO
2
CH
2
Cl
2
/MeOH(9:1)
2.0 - 2.5 h
r.t.
HO
OTr
RO
O
RO
O
91 - 100%
R = H, Me, Bn, MOM, MEM, allyl, Bz, Ts
FIGURE 1.26
Chemoselective deprotection of trityl ethers using silica-supported sodium
hydrogen sulfate developed by Das et al. [44].
appropriate experiments, it is impossible to say whether similar selectivity is achiev-
able when, for example, one of the hydrogen atoms in a
t
-butyl group is replaced
with a large macromolecular unit.
Another accomplishment in deprotection of silyl ethers has been developed by
Chen, Le, Lin, and coworkers [46] who deprotect primary silyl ethers in the pres-
ence of the secondary ones and other protecting groups such as benzyl ethers. The
methodology requires that the silyl ethers are treated with a catalytic amount of CBr
4
in methanol under photochemical reaction conditions. Selected results are shown in
Figure 1.28.
Pale and coworkers [47] developed a new type of a protecting group-bis (4-
methoxy-phenyl) methyl group. The relevant ethers are formed (in acetonitrile) and
deprotected (in ethanol) in the presence of Cu(II) bromide. The yields are high.
Figure 1.29 shows the carbohydrate deprotection (decoupling) example. It is worth
noting that the size of the protecting group suggests that it may be equally effective
when large substituent is attached to one of the methoxyphenyl groups.
Sridhar and Chandrasekaran [48] developed a novel protecting group for amines
and alcohols, propargyloxycarbonyl (Poc) group. It can be easily removed under neu-
tral conditions using tetrathiomolybdate MoS
4
2
−
in acetonitrile at room temperature.
OTBS
OH
O
O
O
ZrCl
4
20 mole%
O
O
O
O
5h
O
91%
O
O
OTPS
OH
O
O
O
ZrCl
4
20 mole%
O
O
O
O
O
5h
0%
O
O
FIGURE 1.27
Sharma et al.'s deprotection of
t
-butyldimethylsilyl ethers in the presence of
t
-butyldiphenylsilyl ethers.
Search WWH ::
Custom Search