Preferably, R1 to R4 are independently selected from hydrogen; a substituted or unsubstituted alkyl or cycloalkyl group having from 1 to 20 carbon atoms; an aralkyl group having from 6 to 20 carbon atoms such as benzyl or p-dodecylbenzyl; a carboalkoxyalkyl group with the alkoxy group having from 1 to 20 carbon atoms such as carboethoxymethyl or carbooctyloxymethyl; and a carboaryloxyalkyl group w