Conformational Study of Chiral Alkenes: The Influence of Protective Groups on the Relative Stability of Ground-State Rotational Isomers

Article abstract of DOI:10.1021/jo00077a023

A variable temperature NMR study shows that a protective group on the hydroxy function of a chiral allylic alcohol can either enhance or counter the influence of the vinyl substituent on the ground-state (GS) conformations.If the allylic hydroxy is protected as a methyl ether, the CH-eclipsed form I becomes favored to a greater degree for normal chiral alkenes.Furthermore, conformer I becomes preferred even for the γ-hydroxy-α,β-unsaturated esters, which normally favor the CO-eclipsed form (II).On the other hand, the tert-butyldimethylsilyl (TBDMS) ether enhances the preference for conformer II for the γ-hydroxy-α,β-unsaturated esters and diminishes the preference for the CH-eclipsed form of normal chiral alkenes.These facts are explained by the size of the allylic oxygen lone pairs.