kyl, allyl, cycloalkyl(C1-3 alkyl), adamantyl(C1-3 alkyl) or substituted phenyl(C1-3 alkyl) each substituent of which is selected from the group consisting of halo, C1-4 alkoxy, C1-4 alkyl and C1-4 haloalkyl (the halogen atoms including fluoro and chloro and cycloalkyl including cyclohexyl); ##STR15## wherein the two Ro b groups together form a radical of the formula ##STR16## wherein R2 b is hydr