A compound of the general formula, ##STR1## wherein R.sub.1 and R.sub.2 independently are --H, --CH.sub.3, or --CH.sub.2 CH.sub.3 ; and R.sub.3 and R.sub.4 are independently alkyl or alkenyl of from 1 to 10 carbons, or aryl of from 6 to 10 carbons, or R.sub.4 =hydrogen.