Abstract: The present invention provides novel compounds represented by the general formula I: ##STR1## wherein R.sub.1 is a straight or branched lower alkyl, a cycloalkyl-lower alkyl, an aryl-lower alkyl, a cycloalkyl, an aryl an aryl-cycloalkyl, lower alkoxy, an aryloxy, or a heteroaryl;R.sub.2 is hydrogen, a straight or branched lower alkyl, a cycloalkyl, or a cycloalkyl-lower alkyl;R.sub.3 and R.sub.3 ' are each the same or different and each is hydrogen atom, a straight or branched lower alkyl, a cycloalkyl, an aryl-lower alkyl, an aryl, or a heteroaryl; orR.sub.3 and R.sub.3 ' together form a ring structure;R.sub.3 " is hydrogen, lower alkyl or an aryl; orR.sub.2 and R.sub.3 " together form a lower alkylene group --(CH.sub.2).sub.n -- wherein n is an integer of 1, 2 or 3; orR.sub.2 and R.sub.3 " together form a group represented by the formula: --(CH.sub.2).sub.p --Ar-- or --Ar--(CH.sub.2).sub.p --, respectively, wherein p is zero or an integer of 1 or 2, and Ar is an arylene or heteroarylene;C(.dbd.X) is C(.dbd.