Abstract: Compounds are disclosed of the general formula I ##STR1## wherein R.sub.1 is a group of the formula IIR.sub.3 --X-- IIwherein R.sub.3 is alkyl; phenyl; phenyl substituted with halogen, lower alkyl, hydroxy, lower alkoxy, or phenoxy; phenyl-lower alkyl in which the phenyl group may be substituted with halogen, lower alkyl, hydroxy, lower alkoxy, or phenoxy; phenyl-lower alkenyl in which the phenyl group may be substituted with halogen, lower alkyl, hydroxy, lower alkoxy or phenoxy; and X is oxygen or sulfur; or R.sub.1 is a group of the formula III ##STR2## wherein R.sub.4 has the same meaning as R.sub.3 as defined above; or R.sub.4 is a group of the formula IV ##STR3## wherein R.sub.7 has the same meaning as R.sub.3 as defined above; or R.sub.7 is an aromatic 5- or 6-membered heterocyclic ring containing one or two heteroatoms selected from the group consisting of nitrogen, oxygen, and sulfur; and wherein R.sub.5 and R.sub.6 are the same or different and each represent hydrogen or have the same meaning as R.