Silyl ynol ethers

This homologation via lithium ynolates has been used to prepare silyl ynol ethers, as described in Section V. 

Ynol tosylates are synthesized from terminal alkynes via a unique sequence (equation 10). The hypervalent organoiodine compound 32, prepared by treatment of iodosoben-zene diacetate with /i-toluenesulfonic acid, reacts with the terminal alkynes 31 to give the iodonium tosylates 33, which are then treated with 10 mol% of CuOTf or AgOTf to afford the ynol tosylates 34. Finally, the ynol tosylates 34 are converted into lithium ynolates 35 by treatment with MeLi. The ynolates are trapped with r-butyldimethylsilyl chloride, triethylgermyl chloride and tributylstannyl chloride to give the silyl ynol ethers 36, the germyl ketenes 37 and the stannyl ketene 38. ... 

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 . 

Acetylenic ethers and esters represent an important class of functionalized acetylene derivatives of the hypothetical alkynols, RC=COH. The chemistry of acetylenic ethers has been well developed since their first preparation about 100 years ago Several exhaustive reviews covering the literature on acetylenic ethers and their analogs up to 1985 have been published in the last 30 years. In contrast to acetylenic ethers, esters of alkynols were unknown until the mid-1980s when the first preparation of alkynyl tosylates was reported. In the following years a wide variety of alkynyl carboxylate, phosphate and sulfonate esters has been prepared from alkynyl iodonium salts. The chemistry of these novel derivatives of alkynols has been summarized in a recent review. In the last 10 years considerable interest and research activity has arisen toward alkynols themselves and such derivatives as alkynolate salts and silyl ynol ethers. The present chapter will cover the chemistry of acetylenic ethers and esters as well as related derivatives of ynols with emphasis on new developments in this subject during the last 5-10 years. 

A recent approach to lithium alkynolates involves cleavage of some ynol ethers and esters with methyllithium Desilylation of silyl ynol ethers with methyllithium at room temperature in THF solution leads to the selective formation of the corresponding alkynolates (equation 9) k Similarly, alkynyl tosylates can be selectively cleaved with methyllithium at low temperatures (equation 10). ... 

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 ". ... 

The first reliable synthesis of silyl ynol ethers 37 was reported in 1985 by Maas and Briickmann ". This method employs a-diazocarbonyl compounds 35 and their silylated derivatives 36 as starting material (equation 24) and is based on a carbene rearrangement ". ... 

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 highly characteristic spectral properties. The triple bond stretching in the IR of siloxyalkynes is at 2260-2270 cm" which compares with that of ynol esters and alkoxyacetylenes (see Sections III and IV of the present review) ". Particularly noteworthy and characteristic are the acetylenic carbon signals in the NMR. The... 

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). 

Formation of Silyl Ynol Ethers. Esters can also be transformed into triisopropylsilyl 3mol ethers. The ester is first converted to the ynolate anion, followed by treatment with TIPSCl to... 

Ynolates cannot be prepared in a similar fashion to metal enolates, because the intermediates may be labile monosubstituted ketenes. Several preparative methods for ynolates have been reported, among which some have been used as a precursor of silyl ynol ethers, that is, silyl ynolates, in organic syntheses [42-45]. However, a general methodology for the preparation of ynolates has not yet been established. This is one of the reasons why ynolates have attracted much less attention than the corresponding enolates. 

Lithium ynolates are stable and keep their reactivity at 0 °C under inert gas for several days, but they decompose in 1 day at 20 Silyl ynolates (sUyl ynol ethers) are stable for a long period and they can be purified by distillation, but they are labile to acids (Section V). The stability of ynolates of metals other than hthium is unknown. 

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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