Compound of formula (I) ** Formula ** in which R1 is (C1-C7) alkyl, R4-O- or R9-C (O) -O-, in which R4 is optionally (C1-C7) alkyl substituted by one to three halos and R9 is mono (C1-C7) alkylamino； R2 is (C1-C7) alkyl optionally substituted by (C1-C7) alkyl -O-C (O) -, di-C1-C7 alkylamino or hydroxyl； or R2 is aryl (C1-C7) alkyl -, heteroaryl (C1-C7) alkyl -, heterocyclyl (C1-C7) alkyl, wherein said heterocyclyl is optionally substituted by (C1-C7) alkyl； and R3 is (C1-C7) alkyl； wherein heteroaryl refers to a condensed 5-14 membered monocyclic or bicyclic or polycyclic ring system, having 1 to 8 heteroatoms selected from N, O or S； and heterocyclyl refers to a monocyclic unsaturated monocyclic ring or system of 4, 5, 6 or 7 members, 7, 8, 9 10, 11 or 12 member bicyclic or 10, 11, 12, 13, 14 tricyclic or 15 members, containing at least one heteroatom selected from O, S and N, where N and S may also be optionally oxidized to various oxidation states； and aryl refers to a monocyclic or bicyclic aromatic hydrocarbon group having 6-10 carbon atoms in the ring part； or a pharmaceutically acceptable salt thereof； or an optical isomer thereof； or a mixture of optical isomers.Compuesto de fórmula (I)**Fórmula** en la que R1 es alquilo (C1-C7), R4-O- o R9-C(O)-O-, en el que R4 es alquilo (C1-C7) opcionalmente sustituido por de uno a tres halos y R9 es mono-alquilamino (C1-C7)； R2 es alquilo (C1-C7) opcionalmente sustituido por alquil (C1-C7)-O-C(O)-, di-alquilamino (C1-C7) o hidroxilo； o R2 es aril-alquil (C1-C7)-, heteroaril-alquil (C1-C7)-, heterociclil-alquilo (C1-C7), en el que dicho heterociclilo está opcionalmente sustituido por alquilo (C1-C7)； y R3 es alquilo (C1-C7)； en el que heteroarilo se refiere a un sistema de anillos monocíclico o bicíclico o policíclico condensado de 5-14 miembros, que tiene de 1 a 8 heteroátomos seleccionados de N, O o S； y heterociclilo se refiere a un anillo o sistema de anillos no aromático insaturado monocíclico de 4, 5, 6 ó 7 miembros
