Enolates generation in situ

A more effective control of both simple diastereoselectivity and induced stereoselectivity is provided by the titanium enolate generated in situ by transmetalation of deprotonated 2,6-dimethylphenyl propanoate with chloro(cyclopentadienyl)bis(l,2 5,6-di-0-isopropylidene-a-D-glucofuranos-3-0-yl)titanium. Reaction of this titanium enolate with aldehydes yields predominantly the. yyw-adducts (syn/anti 89 11 to 97 3). The chemical yields of the adducts are 24 87% while the n-u-products have 93 to 98% ee62. 

HI hi acetic acid allows the reduction of jS-peracetates of the higher sugar N-acetylneuraminic acid to the corresponding anomeric deoxy compounds.277 At room temperature this method gave exclusively the a-anomer, whereas at —20 °C a 4 1 a [i ratio resulted. This may be explained by thermodynamic and kinetic protonation of ester enolates generated in situ from anomeric iodide in a manner reminiscent of previous reductions of 2-iodo sugar lactones.278... 

Enantioselective Protonation of Enolates Generated in situ from Conjugated Unsaturated Carboxylates... 

The selenenylation of a lactone enolate generated in situ by copper-mediated Michael addition to an a-methylene lactone affords stereoselectively the (phenylseleno)lactone 1814. 

Treatment of 88 with /-BuOK afforded the corresponding palladacycles 89 in good overall yield, as a mixture of two diastereosiomers (Equation 13). The stereoselective event involved an intramolecular displacement of the iodide by a potassium ester enolate generated in situ <20030M2961>. 

Table 24. Palladium(0)-Catalyzed Allylations of Stereogenic Enolates Generated in situ from Allyl /t-Kelo...

Lithium or copper enolates, generated in situ by conjugate reduction of afi-unsaturated ketones, may be trapped by MeSSMe, PhSSPh, or PhSCl to give the /S-ketosulphide. /3-Keto-sulphides and -selenides are available on chloral-alumina oxidation of the corresponding /3-hydroxy compounds. The latter compounds may be prepared by alumina-promoted epoxide ring opening by thiols and selenols (Scheme 16). ... 

It has been indicated in Scheme 6 that the Type El Ac-Pd process involving trapping of the second acylpalladium intermediates with enolates generated in situ within the same molecules can take place in preference to the Type I Ac-Pd process under those reaction conditions that are otherwise suitable for the Type I Ac-Pd process. Although rare, formation of a mixture of both Types I and HI Ac-Pd products has also been observed, as exemplified by the results shown in Scheme... 

Pd complexes catalyze the arylation of ester enolates generated in situ in the presence of a base (Buchwald, Hartwig, Scheme 19.30b) [35]. Arylation of malonates proceeds under similar... 

The arylation of amide enolates generated in situ by a base is catalyzed by Pd but in harsher conditions because amides are less acidic (Scheme 19.30c, Hartwig [36a]). The intramolecular version with ArBr as R delivers oxindoles (L=PfBu3 or carbene precursor SIPr, base=NaOfBu). An enantioselective reaction takes place in the presence of a chiral ligand [36b]. Palladium catalyzes the arylation of zinc amide enolates (Hartwig, Cossy [36c, d]). 

The aldol reactions of titanium enolates generated in situ were reported by Harrison [27] to give excellent yield and selectivity for syn aldol products, as shown in Table 2.10. However, methyl ketones tended to eliminate under the reaction conditions and provided a,/i-unsaturated ketones. Reactions with propiophenone and benzaldehyde provided excellent yields of aldolates, with syn aldols being the major product (95 5 ratio). The stereochemical outcome was rationalized by Zimmerman-Traxler transition state model 67. 

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