Ammonium ketene enolate

Reactions of Chiral Ammonium Ketene Enolates as Nucleophiles with Different Electrophiles... 

Nucleophilic Addition of Ammonium Ketene Enolate to C O or C N Bonds... 

Cinchona alkaloids such as 121 possess a nucleophilic quinuclidine structure and can act as versatile Lewis bases to react with ketenes generated in situ from acyl halides in the presence of an add scavenger. By acting as nucleophiles, the resulting ketene enolates can react intermolecularly [53] or intramolecularly [54] with electrophilic C=0 or C=N bonds to deliver formal [2 + 2]-cycloadducts, such as chiral P-lactones or [1-lactams, via aldol (or Mannich)-i intramolecular cydization sequence reactions (Scheme 8.46). The nucleophilic ammonium enolate can also read with energetic... 

Silyi enol ethers can be dimerized to symmetrical 1,4-diketones by treatment with Ag20 in DMSO or certain other polar aprotic solvents." The reaction has been performed with R , R = hydrogen or alkyl, though best yields are obtained when r = r = H. In certain cases, unsymmetrical 1,4-diketones have been prepared by using a mixture of two silyi enol ethers. Other reagents that have been used to achieve either symmetrical or cross-coupled products are iodosobenzene-Bp3-Et20," ceric ammonium nitrate," and lead tetraacetate." If R =0R (in which case the substrate is a ketene silyi acetal), dimerization with TiCU leads to a dialkyl succinate (34, r =0R)." ... 

Dimerization of methylketene is catalyzed by an amine, trimethylsilylquinine, to give the P-lactone enantioselectively (Scheme 27) [129]. The catalyst amine attacks the ketene to form an ammonium enolate, an electron donating alkene. The donor is strong enough to react with a ketene across the C=0 bond. That is why the P-lactone is obtained instead of the 1,3-cyclobutandione, the uncatalyzed dimerization product of the monosubstituted ketene. 

Monocyclic 1,2,3-triazines react with ketene silyl acetal, or silyl enol ether, in the presence of 1-chloroethyl chloroformate to give 5-(substituted)-2-(l-chloroethoxycarbonyl)-2,5-dihydrotriazines which are readily oxidised, and hydrolysed, with ceric ammonium nitrate in aqueous acetonitrile to 5-(substituted)-triazines (Scheme 24) [95CPB881],... 

In 1982, Wynberg and coworkers discovered the cinchona alkaloid catalyzed enantioselective aldol lactonization of ketenes with chloral or trichloroacetone [35], in which the zwitterionic acyl ammonium enolate provides the carbon nucleophile. This work is probably one of the most important early contributions to enantioselective organocatalysis [36], One drawback associated with this process is the severe substrate limitations. The aldehydes should be highly reactive, presumably due to the relatively limited nudeophilicity of ammonium enolates. Nelson and coworkers first addressed the scope and reactivity problems associated with Wynberg s original protocol by combining a cinchona alkaloid derivative (O-trimethylsilylquinine (12) or O-trimethylsilylquinidine (13)) with a metal Lewis acid as a cocatalyst to... 

The 2,5-dihydro-l,2,3-triazines 20, obtained by reaction of 1,2,3-triazines 19 with ketene silyl acetals or silyl enol ethers in the presence of 1-chloroethyl chloroformate, can be readily oxidized and hydrolyzed with ammonium cerium(lV) nitrate (CAN) in acetonitrile/water to afford 5-substituted 1,2,3-triazines 21.3 64... 

The wide reaction scope was facilitated by an extensive investigation into the in situ synthesis of monosubstituted ketenes. A shuttle deprotonation strategy was identified, by which a thermodynamically strong but kinetically slow base such as proton sponge is used as a proton sink, in combination with a substoichiometric kinetically fast tertiary amine (Figure 3.1). Under these conditions the zwitterionic ammonium enolate may be formed via two distinct pathways in... 

In addition to the range of asymmetric routes toward 13-lactone and (3-lactam scaffolds, Lewis bases have been used to access a variety of other important four-membered heterocyclic classes via formal [2+2] cycloadditions. In this section, the use of tertiary amines or NHCs to generate ammonium or azolium enolates, respectively from ketenes and their subsequent application toward the formation of other synthetically interesting heterocycles, will be discussed. A series of reports concerning the use of sulfenes, the sulfonyl equivalents of ketenes, in Lewis base-catalyzed asymmetric [2+2] cycloadditions will also be detailed. 

The mechanism was posmlated to proceed via initial attack of the nucleophilic catalyst 18 to the ketene 15, generating ammonium enolate 140 that subsequently adds to the electron-deficient diazenedicarboxylate 135. Intramolecular ring-closure releases the catalyst and forms the aza-(3-lactam 136 (Figure 3.10). Interestingly, the configuration at the quaternary stereocenter is opposite to that produced in the Staudinger-type lactam process although both are catalyzed by planar-chiral pyridine derivative 18 and believed to proceed via a similar mechanism. 

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