An allyl group was attached to 3-keto function of ketolides in the presence of allyl bromide and KOtBu. Consequently, the Heck reaction of the resulting 2, 3-dehydro-3-O-allyl-10, 11-anhydroclarithromycin derivatives, in the presence of palladium (II) acetate and tri(o-tolyl)phosphine, afforded a 3-O-(3-aryl-E-prop-2-enyl) sidechain, not the previously reported 3-O-(3-aryl-Z-prop-1-enyl) sidechain. The results suggested that some steric factors in β-hydrogen elimination might regulate the isomerization. The activity of 2, 3-dehydro-3-O-(3-aryl-E-prop-2-enyl)-10, 11-anhydroclarithromycin derivatives was low.