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Diastereoselective Effects

MS Distinction of Stereoisomers in Fragmentation

! Retro-Diels-Alder of Stereoisomers

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EI

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Me syn

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Me O anti

EI gives highly stereospecific RDA fragmentation for isomers with cis-ring junction of cyclohexadiene

Suggestive that D-A mechanism is a concerted reaction that exhibits symmetry-conservation because a stepwise mechanism could not show such a difference between stereoisomers

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Methods for the MS Quantitation of Chiral Molecules

! Chiral Recognition Based on Ion/molecule Reactions

Diastereomeric adducts, generated using chiral reference compound, is investigated in a single-stage MS experiment.

One enantiomer of the analyte is isotopically labeled so that the corresponding mixture of diastereomeric adducts can be mass resolved.

! Chiral Recognition Based on Exchange Reactions

Diastereomeric aducts, generated from a chiral ligand and a chiral host such as !-cyclodextrin (CD), is mass-selected and allowed to exchange the chiral ligand in a reaction with a neutral gas (chiral or achiral) in an MS/MS experiment.

Chiral distinction is due to the exchange rate varying with the chirality of the analyte incorporated into the adduct ion.

! Chiral Recognition Based on CID of a Diastereomeric Adduct

CID of diastereomeric adducts formed from an analyte and a chiral reference in a tandem massspectrometry (MS/MS) experiment. Successful experiments are limited to particular molecules, most of which are stereorigid.

! Chiral Recognition Based on "The Kinetic Method"

The kinetic method (MS/MS) to quantify the chiral effects. Involves the CID of a trimeric transition metal ion- bound cluster ion which give rise to diastereomeric product ions.

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