Schiff bases, reaction with ester enolates

Potassium enolates derived from the chiral Schiff bases obtained by reaction of racemic a-amino esters with 2-hydroxypinan-3-one undergo diastereoselective protonation, as evidenced by release of optically active a-amino esters on subsequent cleavage of the imine (Scheme 5). ... 

Various steroids are fluorinated by conversion into the enolates, followed by fluorination with perchloryl fluoride to give the 16-fluorinated products 16.19 28 The Schiff base of 7-aminodeacetoxyeephalosporanic acid tart-butyl ester is converted into an anion with potassium tm-butoxide,29 while further reaction with perchloryl fluoride diluted with nitrogen at — 50 C gives 7-(benzylideneamino)-7-fluorodeacetoxycephalosporanicacid fer/-butyl ester (17) in high yield. An attempt to prepare 2-fluoropodophyllotoxin by reaction of the enolate with perchloryl fluoride resulted in a violent explosion.30... 

Reactions exhibiting diastereofacial selectivity, which occur when the imine or the enolate contains an endogenous stereocenter or a chiral auxiliary, have important applications for the synthesis of optically active 3-l ctams and 3-amino carboxylic acid derivatives. Early work by Furukawa et al. has demonstrated the viability of preparing optically active 3-amino acids from chiral imines. For example, the Schiff base derived from (5)-a-methylbenzylamine (110) reacts with Reformatsky reagent (111) to give, after hydrolysis and removal of the chiral auxiliary, 3-amino-2,2-dimethyl-3-phenylpropionic acid (112) in 33% ee (Scheme 21). Similar Reformatsky reactions have been performed using (-)-menthyl esters but the enantiomeric excess values are lower. ... 

Synthesis and Reaction.—No less than three different groups " have described various extensions to the general method for the homolagation of a-amino-acids by the reactions of electrophiles with enolates derived from Schiff s bases of a-amino-esters, originally reported by Stork s group last year. A full report has appeared on the electro-reductive coupling of Schiff s bases of a-amino-esters with alkyl halides yields of a-alkylated amino-acids are between 36 and 86%... 

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