Abstract: A compound having the formula wherein R1 is COOR8 wherein R8 is alkyl having from 1 to 10 carbon atoms or aralkyl, the aralkyl having from one to four carbon atoms in the alkyl portion and a carbocyclic or heterocyclic group in the aryl portion; R2 is H, alkyl having from 1 to 10 carbon atoms, or COOR7 wherein R7 is alkyl having from 1 to 10 carbon atoms; R3 is H, alkyl having from 1 to 10 carbon atoms, or COOR8; R4 is H, alkyl having from 1 to 10 carbon atoms, or COOR8; R5 is H, alkyl having from 1 to 10 carbon atoms, aralkyl having from 1 to 4 carbon atoms in the alkyl portion, aryl having from 1 to 10 carbon atoms, acyl having from 1 to 10 carbon atoms, or COOCH3; and R6 is H or COOCH3; and thereof.