Protective groups silyl ethers

This strategy was followed in the example shown in Scheme 5.5, Scheme 5.6 and Scheme 5.7 using the tert-butyldimethylsilyl ether as the temporary protecting group. Silyl ethers, allyl, p-methoxyphenyl and 2-(trimethylsilyl)ethyl groups are all commonly used for temporary protection of the anomeric position. 

The hydrosilylation of carbonyl compounds by EtjSiH catalysed by the copper NHC complexes 65 and 66-67 constitutes a convenient method for the direct synthesis of silyl-protected alcohols (silyl ethers). The catalysts can be generated in situ from the corresponding imidazolium salts, base and CuCl or [Cu(MeCN) ]X", respectively. The catalytic reactions usually occur at room tanperature in THE with very good conversions and exhibit good functional group tolerance. Complex 66, which is more active than 65, allows the reactions to be run under lower silane loadings and is preferred for the hydrosilylation of hindered ketones. The wide scope of application of the copper catalyst [dialkyl-, arylalkyl-ketones, aldehydes (even enoUsable) and esters] is evident from some examples compiled in Table 2.3 [51-53],... 

Finally, alcohols can also be protected as silyl ethers. For example, the reaction of the alcohol with trimethylsilyl chloride in the presence of triethylamine (to react with the HC1 that is produced) produces the trimethylsilyl ether of the alcohol as shown in the following equation. (This reaction is a nucleophilic substitution by the oxygen on the silicon.) The silyl group can be removed in high yield by reaction with fluoride anion. 

The intramolecular 1,4-conjugate addition can be accomplished starting from a hydroxy group protected as silyl ether. The cyclization of the enone 13, however, performed by treatment with tetrabutylammonium fluoride in refluxing tetrahydrofuran, gives the 2,6-fra/t.s-tetrahydropy-ran 14 in only 11 % yield160. 

Poly (vinyl alcohol) macromonomers having the hydroxyl groups protected as silyl ethers were prepared using p-formylstyrene as initiator [218] (Scheme 67). 

Stereoselective aldol reactions of a-methyl aldehydesThe lithium enolatc of this ethyl ketone (1) reacts with various a-methyl aldehydes to form aldols, which after protection as silyl ethers and thermolysis (500 ) were isolated as the a, / -unsaturated ketones 2 and 3. The diastcrcoselectivity is highly dependent on the R group. Thus... 

Coordination polymerization of hydroxyl, carboxyl and amino monomers can also be effected by the judicious use of protecting groups. For example, amino and hydroxyl monomers have been protected by silyl ethers and aluminate salts for Ziegler Natta and/or metallocene polynrierization and carboxyl monomers have been protected via aluminate salts. Alternatively, organoborane synthons have been used to carry... 

Cleavage of Protecting Groups. THP ethers of primary, secondary, and phenolic alcohols can be conveniently deprotected at room temperature by treatment with 1.2 equiv of TMSOTf in methylene chloride (eq 96). Deprotection of /-butyldimethyl-silyl ethers in the presence of a /-butyldiphenylsilyl ether has been smoothly accomplished using TMSOTf at —78 °C, as one of the steps of the total synthesis of marine macrolide ulapualide... 

The ability to convert a protective group to another functional group directly without first performing a deprotection is a potentially valuable transformation. Silyl-protected alcohols have been converted directly to aldehydes, ketones, bro-mides, acetates, and ethers without first liberating the alcohol in a prior deprotection step. 

Silyl-derived protective groups are also used to mask the thiol function. A complete compilation is not given here since silyl derivatives are described in the section on alcohol protection. The formation and cleavage of silyl thioethers proceed analogously to simple alcohols. The Si—S bond is weaker than the Si—O bond, and therefore sulfur derivatives are more susceptible to hydrolysis. For the most part silyl ethers are rarely used to protect the thiol function because of their instability. Silyl ethers have been used for in situ protection of the — SH group during amide formation. ... 

Silyl ethers are among the most frequently used protective groups for the alcohol function. This stems largely from the fact that their reactivity (both formation and cleavage) can be modulated by a suitable choice of substituents on the silicon atom. Both steric and electronic effects are the basic controlling elements that regulate the ease of cleavage in multiply functionalized substrates. In plan-... 

---