The direct conversion of a silyl-protected group to a subsequent organic functionality without prior deprotection would be useful. A few selected examples of such direct conversions of silyl ethers are presented herein. The conversion of a TBS ether directly to the corresponding trichloroacetate ester has been reported. This is illustrated by the synthesis of 1-oleoyl-2-acetyl-sn-glycerol (Eq. 62).90
In one of several published methods to convert a silyl ether to the corresponding acetate this conversion was shown to occur with acetic anhydride under the influence of bismuth(III) catalysis (Eq. 63).91 THP ethers were also shown to undergo the corresponding conversion. Bismuth (III) chloride, triflate, and trifluoroacetate all produced similar high yields of the acetates. A similar reaction takes place to form benzoates with benzoic anhydride.
This same conversion can be accomplished with some selectivity employing Cu(II) triflate as the catalyst. Under these conditions it was shown that secondary silyl ethers react well, TBS ethers react only slightly more sluggishly than TMS ethers, but considerably more rapidly than TBDPS ethers and that phenolic TBS ethers react very poorly (Eqs. 64 – 66).92
The direct transformation of primary and secondary TBS ethers to their acetates is possible with acetic anhydride and HF•pyr (Eqs. 67 & 68).93
A high-yielding, one-step conversion of silyl ethers to the corresponding tosylate is accomplished by the reaction of the silyl ether with tosyl fluoride in the presence of DBU as a catalyst (Eq. 69).94 This direct methodology was demonstrated to be useful with TMS, TES, and TBS ethers in high yields as well. The presence of the fluoride aids in the promotion of the reaction as the use of tosyl chloride gave poor conversions to the tosylate.
A carbon-based solid acid was used as an alternative to the more common sulfuric acid for the conversion of trimethylsilyl ethers to their corresponding symmetrical ethers under mild conditions (Eq. 70).95
Iron(III) chloride was shown to catalyze the direct conversion of TES or TBS ethers to benzyl ethers with benzaldehyde employing triethylsilane as a reducing agent for the reduction of the benzaldehyde (Eq. 71).96 Aldehydes other than benzaldehyde such as propionaldehyde and n-pentanal provided the corresponding unsymmetrical dialkyl ethers.
The direct conversion of silyl ethers into their respective diphenylmethyl (DPM) ethers is readily brought about by the reaction with diphenylmethyl formate or acetate in the presence of a catalytic amount of TMSOTf (Eq. 72).97
Trimethylsilyl ethers are directly converted to alkyl azides in a rather straightforward manner. Primary silyl ethers are more reactive than secondary or tertiary silyl ethers in this transformation (Eqs. 73 & 74).98
In a similar fashion it was found that trimethylsilyl ethers can be directly converted to thiocyanates as shown in Eq. 75.99 Interestingly, the corresponding transformation of trimethylsilylcarboxylic acids serves to provide the acyl thiocyanate (Eq. 76).
In a slightly different approach triphenyldithiocyanatophosphorus, generated in-situ, directly converts silyl ethers to the thiocyanates in good yields (Eq. 77).100
The formation of a THP ether directly from a TBS ether is possible from inexpensive reagents. Catalysis by TBSOTf or TfOH provided the THP ethers in high yields (Eq. 78).101 The highly sterically crowded TIPS ethers are also converted to THP ethers under these conditions.
Under Vilsmeier-Haack conditions the trisilyl-protected D-glucal is selectively converted to the C-6 formate 18 (Eq. 79).102
TBS phenolic ethers can provide the starting point for the formation of aryl-alkyl and diaryl ethers in a fluoridecatalyzed substitution reaction (Eq. 80).103
The combination of triphenylphosphine, DDQ, and tetra-n-butylammonium cyanide serves to convert TMS ethers to the nitrile (Eqs. 81 & 82).104