Proton exchange stereochemistry

The formation of an intramolecular H-bond is supported by the slower rate of amide proton exchange in pyridine/10% CD3OD. The influence of the stereochemistry on turn formation and turn geometry has been investigated. Seebach and coworkers have demonstrated that dipeptide sequences consisting... 

The proposed mechanism for this catalytic asymmetric hydrophosphonylation is shown in Figure 35. The first step of this reaction is the deprotonation of dimethyl phosphite by LPB to generate potassium dimethyl phosphite. This potassium phosphite immediately coordinates to a lanthanoid to give I due to the strong oxophilicity of lanthanoid metals. The complex I then reacts (in the stereochemistry-determining step) with an imine to give the potassium salt of the a-aminophosphonate. A proton-exchange reaction affords the product... 

There is a correlation between this proton abstraction stereochemistry and exchange of the hydrogens at C-2 of tyramine or dopamine substrates. Thus the pro-S specific amine oxidases do not catalyse exchange at C-2 whereas the pro-R speeifie and non-specific amine oxidases catalyse this exchange. 

Perrin, C. L., Lollo, C, P, and Hahn, C.-S., Imidate anions Stereochemistry, equilibrium, nitrogen inversion, and comparison with proton exchange, J. Org. Chem. 50, 1405-1409 (1986). 

Since the pulse sequence is the same for EXSY and NOESY, NOESY (or ROESY) cross peaks might be mistaken for EXSY cross peaks. They can be distinguished in the phase-sensitive experiment, since EXSY and ROESY peaks have opposite phases, as do EXSY and NOESY peaks in the fast motion regime. For example, two resolved OH or NH resonances may exhibit EXSY cross peaks from slow proton exchange. These peaks could be mistakenly taken to be NOESY peaks and interpreted incorrectly in terms of stereochemistry. 

Muscarine, the poisonous component of the death cap mushroom, has the structure below. We give the proton NMR spectrum. Can you see any definite evidence for the stereochemistry Couplings are in Hz, m stands for multiplet, and means that the proton exchanges with D2O. 

The stereochemistry observed in proton exchange reactions of carbanions is dependent on the conditions under which the anion is formed and trapped by proton transfer. The dependence on solvent, counterion, and base is the result of the importance of ion pairing effects. The base-catalyzed cleavage of 1 is illustrative. The anion of 1 is cleaved at elevated temperatures to 2-butanone and 2-phenyl-2-butyl anion, which under the conditions of the reaction is protonated by the solvent. Use of resolved... 

Clearly, besides proton transfer, pyridoxal phosphate is also implicated in mechanisms where carbanion chemistry is involved. By leaving a negative charge on the a-carbon of an amino acid substrate, a new problem is introduced one of stereochemistry. On the enzyme, does this carbanion (negative charge) finally get protonated by proton exchange with the medium or by a tautomeric form of the coenzyme What kind of models can one select to best imitate such processes ... 

Investigations of the stereochemistry of proton exchange at the a-carbon of sulphoxides continue to produce conflicting results. For a detailed discussion, readers are referred to the chapter in this Report discussing sulphoxides. Suffice it to mention here that there seems to be general agreement that protonation of a-lithiosulphoxides occurs with retention of configuration." " However, selectivity in removal of the pro-R or the... 

The stereochemistry observed in hydrogen-exchange reactions of carbanions is very dependent on the conditions under which the anion is formed and trapped by proton... 

The stereochemistry of hydrogen-deuterium exchange at the chiral carbon in 2-phenylbutane shows a similar trend. When potassium t-butoxide is used as the base, the exchange occurs with retention of configuration in r-butanol, but racemization occurs in DMSO. The retention of configuration is visualized as occurring through an ion pair in which a solvent molecule coordinated to the metal ion acts as the proton donor... 

We now propose that the proton transfer process could be occurring via an intramolecular 4-centered transition state between the newly formed silyl ketal (Figure 10). A similar 4-centered transition state has been previously proposed by Sommer and Fujimoto to explain retention of the stereochemistry at an asymmetric silicon center during the exchange of alkoxide groups (Scheme 37).34 Brook and co-workers also proposed a 4-centered transition state for the thermal rearrangement of (3-ketosilanes (Scheme 38).35... 

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