Preferably, R.sub.1 is a substituted or unsubstituted, branched or unbranched alkyl group having from 1 to 6 carbon atoms, and most preferably, it is a substituted or unsubstituted methyl group.R.sub.2 can be a substituted or unsubstituted alkyl group (as defined above, and additionally a cyanoalkyl group, a hydroxyalkyl group or alkoxyalkyl group), substituted or unsubstituted alkoxy having 1 to