Esters a-substitution

Trost and his co-workers succeeded in the allylic alkylation of prochiral carbon-centered nucleophiles in the presence of Trost s ligand 118 and obtained the corresponding allylated compounds with an excellent enantioselec-tivity. A variety of prochiral carbon-centered nucleophiles such as / -keto esters, a-substituted ketones, and 3-aryl oxindoles are available for this asymmetric reaction (Scheme jg) Il3,ll3a-ll3g Q jjg recently, highly enantioselective allylation of acyclic ketones such as acetophenone derivatives has been reported by Hou and his co-workers, Trost and and Stoltz and Behenna - (Scheme 18-1). On the other hand, Ito and Kuwano... 

Examples of cycloadditions of nitrile oxides with trifliioromethyl-substituted alkynes arc also known. Yields of isoxazoles are high when there is a second electron-withdrawing substituent, such as an ester. a substituted phenyl group. or a phosphate. 

Synthesis of Unsaturated Esters.—a-Substituted acrylic esters are readily prepared, in good yield, by a tandem reverse-Dieckmann-Michael procedure (Scheme 64). ... 

The racemization of the unreacted substrate can be accomplished either by spontaneous equilibration between the two enantiomers or by using a chemical or biological catalyst. The basic requirements for a successful DKR are that (i) the KR conditions must be compatible with the racemization process, (ii) the KR process must be very stereoselective, (iii) the catalyst for the racemization must not react with the product, and (iv) the racemization must be faster than the reaction of the slow reacting enantiomer k c 2> ksiow [la,b]. The concept of DKR has been applied to a variety of compounds such as a-amino acids, hemithioacetal esters, a-(hetero) arylcarboxylic acids, a-substituted nitriles, beta-keto esters, cyanohydrin esters, a-substituted thioesters, 4-substituted oxazolin-5-ones and thiazolin-5-ones [la]. 

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See also in sourсe #XX -- [ ]

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