This invention relates to compounds of the formula:
##STR1##
wherein
R.sub.1 is H or C.sub.1 -C.sub.6 alkyl and the C.sub.1 -C.sub.6 alkyl
moiety is straight or branched;
R.sub.2 and R.sub.3 are each independently C.sub.1 -C.sub.6 alkyl and the
C.sub.1 -C.sub.6 alkyl moiety is straight or branched; or stereoisomers
thereof.
