Rearrangement to Silyl Enol Ethers

Geometrically defined a/ -epoxysilanes have been shown (6) to undergo a highly stereoselective rearrangement to silyl enol ethers (see also Chapter 15). This rearrangement is catalysed by boron trifluoride etherate, and seems to involved-opening of the epoxysilane, as shown ... 

Simple a-trimethylsilyl aldehydes are elusive compounds because of their ready protodesilylation, or rearrangement to silyl enol ethers. Hudrlik and... 

Regiospecific route to silyl enol ethers.11 Silyl enol ethers arc formed regio-specifically by reaction of lithium dialkyl cuprates or diaryl cuprates with 2-trimcthylsiloxyallyl halides. An allylic rearrangement occurs in reactions with hindered halides. 

Block has described a further use for a-haloalkanesulfonyl bromides radical addition to silyl enol ethers followed by simple Ramberg-BScklund rearrangement yields a,p-unsaturated ketones (Scheme 31) 94,98 Ethylene oxide, an acid scavenger, is used as solvent for the radical addition to prevent hydrolysis of the silyl enol ether. 

Bridged bicyclic ketones 7 are obtained by the Cope rearrangement of silyl enol ethers or lithium enolate anions derived from ent/o-vinylcyclopropanes 4. i 11 The corresponding exo-isomers of 5 require flash-vacuum pyrolysis at temperatures in the range of 250-500°C to... 

Ring expansion of the substituted bicyclo[4.1.0]heptanes 8, formed by addition of chloromethyl-carbene to silyl enol ethers, was accomplished almost equally effectively by refluxing in toluene, or by heating in methanol containing triethylamine. In each case, the crude mixture of cis- and /rani-7-chloro-7-methylbicyclo[4.1. Ojheptanes 8 was used, although it was observed that the traui-isomers, with the chlorine and trimethylsiloxy substituents tram to each other, rearranged more rapidly. 

Claisen rearrangement of ally silyl ethers (4,307-308). Katzenellenbogen and Christy have extended the rearrangement of silyl enol ether derivatives of allylic acetates to y,S-unsaturated acids to systems in which a trisubstituted double bond is generated. Thus 3-acetoxy-2-methyl-l-nonene (1) was treated with lithium N-isopropylcyclohexylamide (LilCA) in THF at -78° and then with f-butyldimethylchlorosilane to give the f-butyldimethylsiloxyvinyl ether... 

In contemporaneous studies by Kuwajima, the 1-silyloxy allyl anions were generated from 1-trimethylsilyl allylic alcohols such as 24. Treatment with a catalytic amount of base led to silyl enol ether 25 in excellent yield and (Z)-selectivity. The alkene geometry was proposed to arise from chelate 26, a structure consistent with Reich s results. Protonation of anionic intermediate 26 by alcohol 24, would provide product 25 and the alkoxide of 24, poised for Brook rearrangement. ... 

In addition to these enolate reactions, -silyl ketones can be thermally isomerized to silyl enol ethers by a regioselective rearrangement (eq 9), or treated with another Grignard reagent to provide alkenes through Peterson alkenation reactions. 

Silyl ketones are hydrolytically unstable and can be converted to the desilylated ketone by simple acid or base treatment, or used in a Peterson alkenation reaction to provide enones. They are also precursors to silyl enol ethers by rearrangement. Reaction of the /3-silyl ketone with a vinyl Grignard reagent provides a rapid entry to 2-substituted 1,3-dienes by a Peterson protocol. ... 

To overcome the limitation of the high stability of the aluminum enolates, the oxygen atom has been transformed to silyl enol ethers, enol acetates, and allyl enol carbonates. Silyl enol ethers and enol acetates are precursors to lithium enolates. Enol acetates and allyl enol carbonates are precursors of cx-allylated adducts via the Tsuji-Trost rearrangement [75-77]. The silylation of aluminum enolates using TMSOTf is well established [78], although in some cases the isolation is difficult [33]. Silyl enol ethers allow further modification to be performed as they behave as lithium enolates (Scheme 15). A recent application can be found in the silylation of the conjugate addition adduct (/ )-((3-(but-3-en-l-yl)-3-methylcyclopent-l-en-l-yl)oxy)triethylsilane which allows aldol condensation to form an intermediate in the synthesis of Clavirolide C [79], a diterpene with a trans-bicyclo[9.3.0] tetradecane structure (Scheme 16) [80]. 

The synthesis in Scheme 13.44 is also based on a carbohydrate-derived starting material. It controlled the stereochemistry at C(2) by means of the stereoselectivity of the Ireland-Claisen rearrangement in Step A (see Section 6.4.2.3). The ester enolate was formed under conditions in which the T -enolate is expected to predominate. Heating the resulting silyl enol ether gave a 9 1 preference for the expected stereoisomer. The... 

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