A compound of formula Ia: ** (See formula) ** or a pharmaceutically acceptable salt thereof, wherein: R1 is selected from the group consisting of aryl and (C1-C6) alkyl, each optionally substituted with one or more Rh； each Rh is independently selected from the group consisting of halo, cyano, nitro and -OH； A1 is N (nitrogen) or CR2； A2 and A3 are each independently O (oxygen) or N (nitrogen), with the proviso that when A2 is O, A3 is N and when A2 is N, A3 is O； R2 is H (hydrogen), (C1-C6) alkyl, (C1-C6) arylalkyl, (C2-C6) alkenyl, (C2-C6) alkynyl or aryl optionally substituted with one or more halo； X is -C (>； = O) -； each n is independently an integer selected from 0, 1 and 2； each z is independently an integer selected from 0, 1 and 2； Y is R4, -N (R4) 2, -OR4, -SR4, or -C (R4) 3, each optionally substituted with one or more Rd； each R4 is independently selected from the group consisting of hydrogen, -OH, (C1-C6) alkyl, (C2-C6) alkenyl, (C2-C6) alkynyl, (C1-C6) alkanoyl, (C1-C6) alkoxycarbonyl , cycloalkyl (C3-C8), - (CH2) n -cycloalkyl (C3-C8), heteroaryl, aryl, arylalkyl (C1-C6), heterocycle, heterocycloalkyl (C1-C6), heterocycloalkanoyl (C1-C6) and NRaRb； or when Y is -N (R4) 2, then two R4 groups are optionally taken together with the nitrogen to which they are attached to form a 3-8 membered monocyclic or 7-12 membered bicyclic ring system, each optionally comprising one or more groups of additional heteroatoms selected from O (oxygen), S (O) z, and NRc, wherein each ring system is optionally substituted with one or more Rd； each Ra and Rb is independently hydrogen or (C1-C6) alkyl, or Ra and Rb are optionally taken together with the nitrogen to which they are attached to form a 3-8 membered monocyclic or 7-12 membered bicyclic ring system, each optionally substituted with one or more C1-C6 alkyl groups； each Rc is independently selected from the group consisting of hydrogen, (C1-C6) alkyl, aryl, heteroaryl, (C1-C6) alkylsulfonyl, alkylsulfonyl, (C1-C6)
