The present invention relates to compounds of the formula (I), or pharmaceutically acceptable salts, enantiomer or diastereomer thereof, wherein R1 to R4 are as described above. The compounds may be useful for the treatment or prophylaxis of hepatitis B virus infection.本發明係關於式(I)化合物,或其醫藥上可接受之鹽、對映異構體或非對映異構體,其中R1至R4係如上文所闡述。該等化合物可用於治療或預防B型肝炎病毒感染。