Ynolates formation

The ESI mass fragment ion spectra of the monomethyl glutarate anion (58) shows formation of ynolate anions (equation 23). ... 

In the same way, electrophilic reactions can be applied to the tandem reaction. The ynolate-initiated tandem [2 + 2] cycloaddition-Michael reaction followed by decarboxylation furnished the polysubstituted five-, six- and seven-membered cycloalkenes in good overall yield (equation 33) . The ester enolate intermediates 77 are nucleophilic, and further bond formation is possible. 

The reaction of (41) with electron-deficient imines yields 2-azetidinones resulting from a 1 2 addition (Scheme 45). The total stereoselectivity of the reaction has been explained on the basis of steric approach control and chelation in transition state (43). The intermediate p-lactam enolate (42) is more nucleophilic than the starting ynolate. This unfavorable nucleophilicity ratio is responsible for the formation of a 1 2 adduct. This represents a severe limitation of the reaction as a generd route toward P-lactams. 

Cycloaddition to ynolates as functional carbanions with formation of /1-lactones, N- and (9-heterocycles 03S2275. 

According to its structure, which can be represented by two resonance contributors (equation 11), an ynolate anion has two nucleophilic centers. In principle, attack of an electrophilic reagent on the oxygen atom of an ynolate anion leads to the formation of an ynol ether or ester, whereas C-attack gives a ketene molecule. 

Carbon electrophiles such as ketones and aldehydes react with ynolates, forming exclusively products of C-attack. Addition of phenylethynolate (15) to cyclohexanone affords spirolactone 23, the formation of which can be explained by the cyclization of the originally formed ketene 22 (equation 16) . Similarly, the reaction of lithium alkynolates with benzaldehyde can be rationalized by the intermediate formation of ketene 24 (equation 17), however, the final product in this case is the unsaturated acid 25 but not a lactone... 

Reactions of alkynyl esters with nucleophilic reagents usually involve the attack of the nucleophile on the electrophilic acyl moiety (i.e. C==0, P=0, SO2) but not the triple bond. A typical example of such a process is the formation of lithium ynolates in the reaction of alkynyl tosylates with methyl lithium (see equation 10 in Section II.B.l). Similarly, the base-catalyzed hydrolysis of alkynyl esters most likely proceeds via OH attack on the acyl moiety and the subsequent standard mechanistic steps. 

In an alternative pathway for dienylketene formation lithium ynolate addition to a vinylketene is proposed to proceed with initial formation of... 

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... 

Stereoselective synthesis of tetrasubstituted alkenes remains a challenging task. A unique solution to this problem is to use torquoselectivity-controlled oletina-tion of carbonyl compounds with ynolates, which is summarized in Chapter 1. In general, the stereoselective synthesis of Z-alkenes, which are thermodynamically less favorable, is more difficult than the synthesis of corresponding -isomers. In Chapter 2, various methods for stereoselective synthesis of Z-alkenes are reviewed. Finally, the C=C double bond formation through catalytic carbene transformation has recently emerged as a new approach toward olefin synthesis. Two chapters covering olefin synthesis based on catalytic carbene transformations are included (Chapter 5 and Chapter 8). 

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