PROFOUND PREFERENCE FOR THE CO-ECLIPSED CONFORMATION IN 1,5-DIENYL-3,4-DIOLS - THE ORIGIN OF PI-FACIAL SELECTIVITY

A variable-temperature MMR study shows that the derivatives of 1,5-die nyl-3,4-diols do not have rigid conformation at room temperature as su ggested previously. These compounds favor the CO-eclipsed conformer to a greater degree than their mono-allylic counterparts. The influence of the substituents on conformation in these compounds follows the sam e trends observed for monochiral-centered allylic ethers. The silyl et hers prefer the CO-eclipsed form while the methyl and benzyl ethers fa vor the OH-eclipsed conformation. The MOM ether group mimics the methy l ether and promotes the CH-eclipsed form.