Glycosidation reactions with conformationally

From the above discussion, it is easy to identify those acetals that will react with ozone and also those acetals that either do not react or react, but with great difficulty. Note the three different conformers of 74. The conformers 74a and 74b meet the requirement for reaction with ozone for having two electron pair orbitals antiperiplanar to the crc H bond. The conformer 74c does not meet this requirement, as it has only one such electron pair orbital. To test whether 74c indeed does not react or reacts with ozone but slowly in comparison to the conformers 74a and 74b, one needs to freeze the conformer 74c as in 75. The species 75 was indeed discovered to be inert to ozone. Likewise, the reactive conformer 74b could be frozen as in 76 and 77. Note that the key structural features of the three conformers of 74 resemble that of a p-glycoside. Also note further the stereo-functional similarity between 75 and 78 the [Pg.16]

High-resolution H n.m.r. spectroscopy has been used to probe the conformations of a number of o-ribofuranosylamine derivatives and such rigid molecules as 2,2 -cyclonucleosides and nucleoside 3, 5 -phosphates in aqueous solution. H N.m.r. spectroscopy has also been used to study details of the intramolecular association and conformations of a- and j8-linked pyridine ribo-nucleosides and their 5 -phosphates. The results were analysed in terms of base-D-ribose, o-ribose-side-chain, and base-side-chain interactions and the conformational restraints imposed by the cis HO-2-HO-3 interaction in jS-nucleo-tides and the additional cis HO-2 -base interaction in a-nucleotides. H N.m.r. measurements - including measurements of nuclear Overhauser effects and paramagnetic relaxations effected by Mn + cations - have been used to investigate the preferred conformation about the jV-glycosidic bond of 8-amino-, 8-methyl-amino-, and 8-dimethylamino-adenylic acid, all of which competitively inhibit the coenzyme NADH in the reaction with chicken-muscle lactate dehydrogenase. The primary and secondary amines were shown to prefer anti conformations, whereas the tertiary amine prefers a syn conformation. 

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