Acyclic model compounds, conformational

The high cis selectivity has been rationalized on the basis of acyclic conformational control27. A model for the attack of an electrophile to an alkene containing an internal nucleophile under kinetic conditions has been developed33 based on the relative affinity of the diastereotopic face of the double bond towards a proton. When an alkoxy group is present, preferential attack of an electrophile on the OR-in-plane-conformer occurs from the face of the double bound syn to the allylic hydrogen. When R = H, the m-diastcrcomcr is formed as the major compound. On the other hand, substrates with a Z double bond, which destabilizes the OR-in-plane-conformer, react on the opposite face, leading to the trans-isomer. 

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. 

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