Cyclic model compounds, conformational energy

From the methodological point of view, a different F factor derived from the correlation of energies of relevant acyclic compounds should have been used. The anomeric carbon atom in II is secondary, whereas in 2-substituted tetrahydropyrans it is tertiary. The introduction of a third substituent at the anomeric carbon may result in a change of preferred conformation (58, 59), appropriate angles (60), and the magnitude of the anomeric effect (61), thus making cyclic models inadequate for the calculation presented above. 

Two groups have reported the synthesis of compounds of type (139) as potential models for ring-opening reactions of cyclic AMP. These studies included cases where the dioxaphosphorinane ring was both apical-equatorial and diequatorial, and conformations were studied by n.m.r.i98,i99 xhe predominant formation of 3 -monophosphates in the base-catalysed hydrolysis of nucleoside 3, 5 -cyclic phosphates has been interpreted in terms of the lone pair orientation effect, which may decrease the energy of the transition state for P-O bond-breaking.200 Conformational studies have been carried out on the diastereomers of adenosine cyclic 3 ,5 -phosphorothioate, where chair conformations predominate, and for the / p-isomer of deoxyadenosine cyclic phosphoranilidate, where a chair-twist equilibrium... 

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