(15) the compound according to (14) wherein R1 is a group represented by the formula: ???Y1???Y2???Y3???Ra wherein Y1, Y3, Ra are as defined in (14); Y2 is ???O???, ???NRb???C(???O)??? or ???C(???O)???NRb???; Rb is hydrogen or optionally substituted alkyl; a prodrug, a pharmaceutically acceptable salt or solvate thereof, (16) the compound according to (14) or (15) wherein R3 and R4 are not taken t