Emil Popa - Profile on Academia.edu (original) (raw)
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Universidad de las Islas Baleares
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Papers by Emil Popa
Journal of Molecular Structure-theochem, 1994
A theoretical study of the A3 H A* isomerization of the cephalosporins is presented. Pairs of iso... more A theoretical study of the A3 H A* isomerization of the cephalosporins is presented. Pairs of isomers, and various possible intermediates resulting during the A3 double bond migration to the A* position are examined. Two model compounds, the 7-acetamido-desacetoxy cephalosporanic acid and 7-acetamido-3-chloro-3-cephem-4-carboxylic acids, having +I and -1 substituents at C-3 as well as their methyl esters were considered for this study. The AM1 and PM3 molecular orbital techniques, which included solvent (water) effects were employed. The study explains the resistance of the cephalosporanic acids toward isomerization as a result of the impossibility to deprotonate at C-2, due to the easier deprotonation of the carboxylic functionality. The ratio of isomers in equilibrium mixtures of the esters is determined by their thermodynamic stabilities. Structural factors have an important role in this process.
Journal of Molecular Structure-theochem, 1994
A theoretical study of the A3 H A* isomerization of the cephalosporins is presented. Pairs of iso... more A theoretical study of the A3 H A* isomerization of the cephalosporins is presented. Pairs of isomers, and various possible intermediates resulting during the A3 double bond migration to the A* position are examined. Two model compounds, the 7-acetamido-desacetoxy cephalosporanic acid and 7-acetamido-3-chloro-3-cephem-4-carboxylic acids, having +I and -1 substituents at C-3 as well as their methyl esters were considered for this study. The AM1 and PM3 molecular orbital techniques, which included solvent (water) effects were employed. The study explains the resistance of the cephalosporanic acids toward isomerization as a result of the impossibility to deprotonate at C-2, due to the easier deprotonation of the carboxylic functionality. The ratio of isomers in equilibrium mixtures of the esters is determined by their thermodynamic stabilities. Structural factors have an important role in this process.