A compound represented by the formula (I) or a salt thereof:wherein a ring Z is a 5 to 6-membered heteroaromatic ring having one or two heteroatoms in the ring; X1 is a hydrogen atom, a hydroxy group, a hydroxy C1-6 alkyl group, -B(OH)2, a boronate ester group, a cyclic boronate ester group, a boranyl group, a cyclic boranyl group, -BF3Mn1, -Sn(R12)(R13)(R14), a leaving group, a carboxy group, a formyl group, or -NR16R17; and X2 is a hydrogen atom or -CO2R18.