Deprotection silyl groups

In this series of amides, hydrolysis or aminolysis of a simple ester, cleavage of a silyl groups a cis/trans isomerization, or reduction of a quinone to a hydro-quinone exposes an alcohol that then induces deprotection by intramolecular addition to the amide carbonyl. 

At this point, completion of the total synthesis required removal of the three acetonides and the two silyl protecting groups (Scheme 18). Removal of the silyl groups with TBAF and subsequent treatment to acidic deprotection conditions led to complete deprotection of 110, but failed to provide filipin III. It was sus-... 

The examples shown in Scheme 5 demonstrate the potential utility of these stereoselective alkylation technologies in synthesis. Thus, preparation of rac-14 through catalytic RCM of rac-13 (85% yield) and subsequent Zr-catalyzed resolution of the resulting racemic TBS-protected oxepin affords (S)-23 after silyl group deprotection. Diastereoselective alkylation with nBuMgBr affords (S)-24... 

It has been reported that (TMS)3SiCl can be used for the protection of primary and secondary alcohols [55]. Tris(trimethylsilyl)silyl ethers are stable to the usual conditions employed in organic synthesis for the deprotection of other silyl groups and can be deprotected using photolysis at 254 nm, in yields ranging from 62 to 95%. Combining this fact with the hydrosilylation of ketones and aldehydes, a radical pathway can be drawn, which is formally equivalent to the ionic reduction of carbonyl moieties to the corresponding alcohols. 

Enyne 7a having a silyloxy group on the alkyne gives cyclic compound 8a having a vinyl silyloxy moiety, which is converted into methyl ketone 9a by desilylation. In a similar manner, enyne 7b affords bicyclic methyl ketone 9b in 68% yield after deprotection of the silyl group. However, ynoate 7c and yne-phosphonate 7d do not give cyclized compounds. Ene-ynoate 12, which is obtained by treatment of enol ether 11 with BuLi affords cyclic enol ether 13 in good to moderate yields " (Scheme 4). 

All the reagents listed can be used to cleave trimethylsilyl groups from acetylenes fluoride, potassium carbonate under basic conditions in methanol, or silver nitrate/potassium cyanide.9 With the third method advantage can be taken of the fact that the later transition metals (e.g., copper or silvei) complex readily with acety-lides. Workup with concentrated potassium cyanide solution causes compound 32 to be cleaved tu alkyne 33. In this way silylated a -kynes can be deprotected in the presence of 0-silyl groups. 

Cyctization of piperazinediones. An efficient synthesis of the unusual ring system (3) of the antibiotic bicyclomycin is based on treatment of a protected piperazinedione (2) with 2.0equiv. of 1 at 25°. The reagent effects deprotection of the silyl group and removal of the pyridylthio residue prior to cyclization to give 3 in 90-99% yield in 2-3 minutes. The same overall transformation can be effected by treatment first with the HF-pyridine complex and then with AgC104 in yields of 60... 

Monosubstitution of acetylene itself to prepare terminal alkynes is not easy. Therefore, trimethylsilylacetylene (134) is used as a protected acetylene. After the coupling, the silyl group is removed by the treatment with fluoride anion. The hexasubstitution of hexabromobenzene (135) with 134 afforded hexaethynylbenzene (136) after desilylation in total yield of 28%. The product was converted to tris(benzocyclobutadieno)benzene (137) using a Co catalyst (see Section 7.2.1). Hexabutadiynylbenzene was prepared similarly [60], As another method, terminal alkynes 139 are prepared in excellent yields by the coupling of commercially available ethynyl Grignard (138) or ethynylzinc bromide with halides, without protection and deprotection [61]. 

Deprotection of silyl ethers can be accomplished by using a variety of reagents such as TBAF, BF3-OEt2, alkali metal tetrafluoroborate and HF. Here, PPTS is used to distinguish the TBS from the TPS group. Thus, the rearranged 27 cleaves selectively the least bulky silyl group to form mono-protected 8.12... 

The protection of amines with a silyl group is of wide use not only in organic synthesis but also in chromatographic analysis. Deprotection generally occurs in acidic medium. This has been applied to the chemistry of SMAs.122,177,179,180,186,187,189,191... 

The stabilities of silyl ethers are closely related to the electronic and steric effect of the substituents on the silicon atom and are generally proportional to the steric hindrance provided by the substituents. Moreover, electron-withdrawing substituents on the silicon atom increase the stability of the silyl groups toward acid but decrease their stability toward base. Consequently, their stability in acid follows the order TMSSelective deprotection among these groups can thus be achieved... 

The final step in a recent synthesis of the antifungal agent Papulacandin D involved deprotection of five O-silyl groups including a di-ferr-butylsilylene group, a triethylsilyl ether and two phenolic tri-isopropylsilyl ethers [Scheme 3.115],215 Acid conditions were precluded by the add lability of the side chain. Use of TBAF was complicated by problems in separating the product from tetrabuty-Lammonium salts. The desired global deprotection was accomplished with tris-(dime thy iamino)sulfoni urn difluorotrimethylsilicate (TAS-F). 

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