CONFORMATIONAL STUDY OF CHIRAL ALKENES - THE INFLUENCE OF PROTECTIVE GROUPS ON THE RELATIVE STABILITY OF GROUND-STATE ROTATIONAL ISOMERS

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 gamma-hydroxy-alpha,beta-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 gamma-hydroxy-alpha,beta-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.