Lecture21outline

Lecture21outline - I. Review Nucleophiles generally add to...

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I. Review • Nucleophiles generally add to anomeric carbocations from the axial face • This is a result of the anomeric effect that states that ionization is facilitated when a leaving group has a 180° dihedral angle with a lone pair • This allows the lone pair to interact with the C–LG σ * orbital, weakening the bond to the leaving group but stabilizing the molecule • Since leaving from the axial direction is the lowest energy pathway, attack from the axial trajectory must be favored (microscopic reversibility) • Equatorial approach can be achieved when an ester protecting group is adjacent to the carbocation • The ester carbonyl group reacts with the anomeric carbocation to form an outstanding leaving group that can react with nucleophiles in S N 2-like reactions • In principle disaccharides could be formed by mixing two monosaccharides in the presence of acid, but selectivity issues make this impractical • Protecting groups can be used to prevent hydroxy groups from acting as nucleophiles
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This note was uploaded on 05/17/2010 for the course CHEM 0740 taught by Professor Professor during the Spring '10 term at Pittsburgh.

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Lecture21outline - I. Review Nucleophiles generally add to...

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