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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 17, Number 7
BKCSDE 17(7)
July 20, 1996 

Semiempirical Calculations of Substituent Effects on the Reactions of Cephem-Like β-Lactam Molecules
Jung Chull Lee, Hun-Yeong Koh, Moon-Ho Chang*, Yoon Sup Lee
Semiempirical PM3 MO calculations are applied to estimate both 1-atom (X=S,O,C) and 3-substituent (Y=R, CH2R, SR, CH2SR) effects on the reactions of some 1-atom-replaced and 3-substituted cephem-like β-lactam compounds of thiacephems, oxacephems, and carbacephems. Stabilization energy (SE) of the reaction intermediate for the reaction with a hydroxyl ion can be used to evaluate the facility of a reaction and selected as a chemical reactivity index. With the 1-atom effect only, the SE values obtained imply that thiacephems are generally more reactive than the other two cephem-like molecules and the reactivity order is thiacephems>oxacephems>carbacephems. When it comes to the 3-substituent (Y=R, CH2R, SR, CH2SR) effect, chemical reactivity can be best realized by using a 3-substituted thiacephem molecule capable of giving a resonance-stabilized and electron-rich leaving group after the reaction with a nucleophile. SE values, however, decrease in most cases when an additional intervening ethylene group is present (Y=CH2R, CH2SR). The overall 3-substituent reactivity tendency is SR>CH2SR>R>CH2R.
604 - 607
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