Compounds are disclosed of general formula: ##STR1## wherein R.sup.1 is H, C.sub.1-6 alkyl or C.sub.3-6 alkenyl; R.sup.2 is H, C.sub.1-3 alkyl, C.sub.3-6 alkenyl, aryl, ar(C.sub.1-4)-alkylene, or C.sub.5-7 cycloalkyl; R.sup.3 is H or C.sub.1-3 alkyl; R.sup.4 and R.sup.5 each represents H, C.sub.1-3 alkyl or 2-propenyl, or R.sup.4 and R.sup.5 together form an aralkylidene group; R.sup.6 represents --CO.sub.2 R.sup.7, COR.sup.7, --COCO.sub.2 R.sup.7, or --CONHR.sup.7, where R.sup.7 represents H, C...