The present invention is directed to compounds of the structural formula (I), and pharmaceutically acceptable salts, solvates and hydrates thereof: Formula I (a) R1, R2 and R6 are each independently selected from the group consisting of hydrogen, C1-C8 alkyl substituted C1-C8 alkyl, aryl-C0-4-alkyl, substituted aryl-C0-4-alkyl, heteroaryl-C0-4-alkyl, substituted heteroaryl-C0-4-alkyl, C3-C6 cycloalkylaryl-C0-2-alkyl, and substituted C3-C6cycloalkylaryl-C0-2-alkyl; (b) X is an optionally substituted C1-C5 alkylene linker wherein one carbon atom of the linker may be replaced with O, NH or S; (c) Y is C, O, S, NH or a single bond; and (d) E is selected from the group consisting of hydrogen, C(R3)(R4)A, A, (CH2)n COOR19 and substituted (CH2)n COOR19.