A compound of the formula:or a pharmaceutically acceptable salt or solvate thereof whereinY1 is NP2 or OP1 and P2 are independently hydrogen, or a substituent selected from Substituent Group S1 and amino protecting groupR11 is hydrogen, or a substituent selected from Substituent Group S1 and amino protecting group, with the proviso that R11 is not -CO(CH2)3-CO2H, -Ph, -CH2Ph, and -(CH2)3CH(4-F-Ph)2 wherein Ph is phenylSubstituent Group S1 consists of optionally substituted lower alkyl, optionally substituted lower alkenyl, optionally substituted cycloalkyl, optionally substituted formyl, optionally substituted (lower alkyl) carbonyl, optionally substituted cycloalkylcarbonyl, optionally substituted (lower alkyl) oxycarbonyl, optionally substituted (lower alkyl) oxythiocarbonyl, optionally substituted arylcarbonyl, optionally substituted aryloxycarbonyl, optionally substituted heterocyclecarbonyl, optionally substituted heterocycleoxycarbonyl, optionally substituted carbamoyl, optionally substituted carbamoylcarbonyl, optionally substituted thiocarbamoyl, optionally substituted (lower alkyl) thiocarbonyl, optionally substituted cycloalkylthiocarbonyl, optionally substituted arylthiocarbonyl, optionally substituted heterocyclic thiocarbonyl, optionally substituted (lower alkyl) sulfonyl, optionally substituted phenylsulfonyl, optionally substituted aromatic heterocyclesulfonyl, optionally substituted aminosulfonyl, optionally substituted aryl, optionally substituted heterocyclic group, cyano, optionally substituted thioformyl, optionally substituted (lower alkenyl) carbonyl, and optionally substituted heterocycleoxycarbonylwith the proviso that the compound is not the following compound:wherein Ph is phenyl.
