Silyl ynol ethers reactions

Since sUyl ynol ethers have an electron-rich triple bond, they are useful for Lewis acid catalyzed synthetic reactions. Lithium ynolates 175 are silylated by TIPSCl or TIPSOTf and TBSCl to afford the corresponding silyl ynol ethers 176 and 177, which are thermally stable and isolable, but sensitive toward acids (equation 71) . See also equations 9 and 10 in Section ll.C. An experimentally improved procedure for the purification of 176 derived from Kowalski s method is described. Lithium ynolate derived from Julia s method is also used for the preparation of 176. TMSCl and TESCl provide silyl ketenes 179, however, by C-silylation. These small silyl chlorides primarily gave the silyl ynol ethers 178, but, upon warming the reaction mixture, isomerization to the more stable silyl ketenes takes place. The soft electrophilic silyl chlorides like PhsSiCl afford silyl ketenes. Disi-lyl ynol ethers, prepared from ynolate dianions, are rearranged to disilylketenes mediated by salts . 

Both types of reactivity of ynolate anion have been reported in the literature. The O-attack is typical for the reactions of lithium ynolates with trialkylchlorosilanes b24,25 dialkyIchlorophosphates. Lithium ynolates, generated as shown in equations 5-10, react with sterically hindered trialkylchlorosilanes in THF affording silyl ynol ethers as primary products (equation 12). However, in some cases the silyl ynol ethers are unstable at room temperature and isomerization to the more stable ketenes, or decomposition, occurs The ketene rearrangement usually occurs in reactions of lithium alkynolates with methyl substituted silyl chlorides a typical example of such a rearrangement is represented by reaction 13 ". ... 

To date the most simple and general procedure for the preparation of silyl ynol ethers is the reaction of lithium alkynolates, generated in situ from ethyl esters, with silyl chlorides, according to equation 25 k However, this method (equation 25) does not work for the preparation of siloxyalkynes in which R is a lower alkyl group or hydrogen. Danheiser... 

Silyl ynol ethers have found some practical application in the synthesis of trisubstituted olefins and as partners in [2+2] cycloaddition reactions . Kowalski and coworker s reported that reaction of siloxyacetylenes with aldehydes in the presence of TiClq affords trisubstituted olefins with very high E/Z stereoselectivity under mild conditions (equation 29). 

The intramolecular [2+2+1] cycloaddition promoted by pentacarbonyliron can also be extended to silyl protected yne-ynamides (Scheme 4-7) and yne-ynol ethers (Scheme 4—8) ° to give after demetalation cyclopentadienones with fused N- or 0-heterocycles of different ring size. Subsequent Diels-Alder reaction with dimethyl acetylenedicarboxylate (DMAD) or ethyl propiolate transforms the cyclopentadienone moiety into the corresponding benzene derivative. 

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