Page 292 - Advanced Organic Chemistry Part B - Reactions & Synthesis

264 4-Methoxyphenyl (PMP) ethers find occasional use as hydroxy protecting groups.
Unlike benzylic groups, they cannot be made directly from the alcohol. Instead,
CHAPTER 3 185
the phenoxy group must be introduced by a nucleophilic substitution. Mitsunobu
Functional Group conditions are frequently used. 186 The PMP group can be cleaved by oxidation
Interconversion
by Substitution, with CAN.
Including Protection and Allyl ethers can be removed by conversion to propenyl ethers, followed by acidic
Deprotection
hydrolysis of the resulting enol ether.
H 3 O +
ROCH CH CH 2 ROCH CHCH 3 ROH + CH CH CH O
2
3
2
The isomerization of an allyl ether to a propenyl ether can be achieved either by
treatment with potassium t-butoxide in dimethyl sulfoxide 187 or by catalysts such as
Rh PPh Cl 188 or RhH PPh . 189 Heating allyl ethers with Pd-C in acidic methanol
3 3
3 4
can also effect cleavage of allyl ethers. 190 This reaction, too, is believed to involve
isomerization to the 1-propenyl ether. Other very mild conditions for allyl group
cleavage include Wacker oxidation conditions 191 (see Section 8.2.1) and DiBAlH with
catalytic NiCl (dppp). 192
2
3.5.1.3. Silyl Ethers as Protective Groups. Silyl ethers play a very important role
as hydroxy-protecting groups. 193 Alcohols can be easily converted to trimethylsilyl
(TMS) ethers by reaction with trimethylsilyl chloride in the presence of an amine
or by heating with hexamethyldisilazane. Trimethylsilyl groups are easily removed
by hydrolysis or by exposure to fluoride ions. t-Butyldimethylsilyl (TBDMS) ethers
are also very useful. The increased steric bulk of the TBDMS group improves the
stability of the group toward such reactions as hydride reduction and Cr(VI) oxidation.
The TBDMS group is normally introduced using a tertiary amine as a catalyst in
the reaction of the alcohol with t-butyldimethylsilyl chloride or triflate. Cleavage
of the TBDMS group is slow under hydrolytic conditions, but anhydrous tetra-n-
butylammonium fluoride (TBAF), 194 methanolic NH F, 195 aqueous HF, 196 BF , 197 or
4
3
SiF 4 198 can be used for its removal. Other highly substituted silyl groups, such as
dimethyl(1,2,2-trimethylpropyl)silyl 199 and tris-isopropylsilyl, 200 (TIPS) are even more
185
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