Preferably X is different from Y. [0062] In some embodiments, when X is Ar???CH???CH???CH???CH???CO2??? where Ar is as defined above, Y is ???O???(CH2)n???CHO or ???O???(CH2)n???CH(ZR5)2 where n is a number of 1-6, Z is O or S, and R5 is H or an alkyl group; when X is ???O???(CH2)n???CH(ZR5)2 where n is a number of 1-6, Z is O or S, and R5 is H or an alkyl group, Y is preferably ???O2CCH2CH2CO2R6