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Trans-hydrogenation

Fig. 6.6 Schematics of hydrogen bonds between purine and pyrimidine bases with indicated trans-hydrogen bond scalar interactions and related coupling constants, which can be measured using NMR. In addition to correlations between exchangeable protons and nitrogens, also a relayed transfer to nonexchangeable aromatic protons, shown by a dashed arrow, can be employed. The... Fig. 6.6 Schematics of hydrogen bonds between purine and pyrimidine bases with indicated trans-hydrogen bond scalar interactions and related coupling constants, which can be measured using NMR. In addition to correlations between exchangeable protons and nitrogens, also a relayed transfer to nonexchangeable aromatic protons, shown by a dashed arrow, can be employed. The...
Here, we present the example of the trans hydrogen bond coupling between the C of the acceptor and the N of the donor h/(N, C ) that is measured by excitation of double-quantum and zero-quantum coherence between the HN and the C nuclei [12] in a protein. Thus, the double-quantum coherence is split by h /(N, C )+ /(N, H) while the zero-quantum coher-... [Pg.151]

Tab. 9.1 Observed trans hydrogen bond couplings in nucleic acids... Tab. 9.1 Observed trans hydrogen bond couplings in nucleic acids...
The geometry of the cis-alkylcyclohexanol is favorable for trans elimination since the hydroxyl and the neighboring trans hydrogen are coplanar, but this is not true for the l,i-trans isomer hence the molecular conformation has to flip over, to set the hydroxyl group in the axial position for the trans elimination to occur. This would require a few kilocalories of energy and for frans-lert-butylcyclohexanol it would be more difficult to achieve than for IroMs-methylcyclohexanol. It is, therefore, possible that the trans elimination from a boat conformation, or possibly even an epimerization from the trans to the cis isomer which then undergoes a trans elimination reaction. Such an epimerization was found to occur under conditions of dehydration of certain alcohols over alumina, as will be seen under 1,4-cyclohexanediol. The more facile elimination of the cis-i-tert-butylcyclohexanol system as compared with the trans system in solution was also reported in the literature 63). [Pg.63]

The C-Br group formed can be converted into an ester group via organometal-lic reactions [54]. For one of the PAE described later (No. 16 in Table 2), trans-hydrogenation of the -C C- bond has also been reported [41]. [Pg.184]

Campos, Sampedro and Rodriguez studied theoretically in 1998 the competition of the concerted and stepwise mechanism of hydrogen migration and lithium iodide a-eUmination in the trihydrate 1-iodo-l-lithioethene 28. Since the lithiated derivative of 1-iodoethane readily decomposes to ethylene, experimental techniques are found to be of less use. Their ab initio theoretical calculations indicate that the preferred mechanism is the concerted rather than stepwise pathway as illustrated in Eigure 33. Their studies also suggest that 1-iodo-l-thioethene prefers to be a monomer in THE and the activation barriers for the cis and trans hydrogen migration are nearly the same. [Pg.41]

The known preference for transoid elimination of the elements of water from alcohols such as (6-3) controls the stereochemistry of the product. The arrangement in the starting material of the groups about the incipient olefin actually determines the steric identity of the product. The two rotamers of alcohol (6-3) that have the trans hydrogen and hydroxyl shown as their Newman projections (6-3a) and (6-3b) are equally probable since they differ only in the placement of the remote basic ether. The dehydration in fact gives a mixmre of the trans isomer (7-2) and the cis isomer (7-3) presumably from rotamers (6-3a) and (6-3b), respectively. Reaction... [Pg.195]


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See also in sourсe #XX -- [ Pg.809 ]




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