Enolate acyl ammonium

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... 

Discussing the stereochemical outcome of the Claisen rearrangements, two aspects had to be considered. On one hand, the relative configuration of the new stereogenic centers was found to be exclusively syn in 308 and 309 suggesting a chair-like transition state c-a and c-fS, respectively, including a Z acyl ammonium enolate structure (complete simple diastereoselectivity/internal asymmetric induction, Scheme 10.64). 

Stereoselective [4 + 2] cycloadditions ofketenes and thioacylimines 509, both created in situ are catalyzed by an optically pure cinchona alkaloid (TMSQ) in the presence of LiC104/i-Pr2NEt (9 1 CH2Cl2/Et20, 78 °C) via the quinine-derived enolate 510 to afford the enantio-enriched ds-4,5-disubstituted 1,3-thiazin-6-one 512 via sulfur addition to the acyl ammonium ion 511, with 95 to >98% ee and cis/trans 95 5 to >97 3 (Scheme 167) (2007JA11690). 

The combination of fluoride ions and enol silyl ethers provides a useful method for the generation of enolate anions [9]. Watts, Haswell and coworkers applied a borosilicate glass microreactor, having channel dimensions of 100 pm x 50 pm and equipped with an electrosmotic flow (EOF) pumping system, to the C-acylation of enolate anions, which leads to 1,3-diketones [10,11]. A THF solution of tetrabutyl-ammonium fluoride (TBAF) was placed in reservoir A, a THF solution of benzoyl... 

The Nicholas reaction was used to synthesize the p-lactam precursor of thienamycin in the laboratory of P.A. Jacobi and thereby accomplish its formal total synthesis. The necessary p-amino acid was prepared by the condensation of a boron enolate (derived from an acylated oxazolidinone) with the cobalt complex of an enantiopure propargylic ether. The resulting adduct was oxidized with ceric ammonium nitrate (CAN) to remove the cobalt protecting group from the triple bond, and the product was obtained with a 17 1 anti.syn selectivity and in good yield. 

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... 

Another variation of this oxazole synthesis consists in the utilization of diesters of enediols (diacyl derivatives of the enol tautomer of a-hydroxy ketones). In the aliphatic series, enediol esters can directly be prepared from acyl chlorides and metallic sodium in ether containing a trace of water. When the esters react with ammonium acetate, symmetrical... 

To introduce additional diversity, the 3-nitrophenyl and 4-nitrophenyl derivatives in the R position were further functionalized. For example, the nitro group and enol ether of 3-nitrophenyl 39 were reduced, and the resulting aniline was acylated to give 41 (Scheme 7.9). Alternatively, selective reduction of the nitro group worked most effectively in the presence of ammonium formate to give compounds 42. 

Vinylic esters are able to act simultaneously as the enol precursor and acylating agent in stereoselective aldol reaction when catalysed by nucleophilic ammonium betaines, as illustrated in Scheme 17. ... 

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