Compounds of formula
 (wherein R, R₁, R₂ and X have the meanings reported in the specification),
the processes for their preparation and the pharmaceutical compositions containing them as active ingredient are described.
The compounds of formula I are useful in the treatment of Parkinson's disease, renal failure, cardiac decompensation and hypertension.  
                            式化合物
 (其中 R、R₁、R₂ 和 X 的含义见说明书)、
描述了它们的制备工艺和含有它们作为活性成分的药物组合物。
式 I 化合物可用于治疗帕
金森病、肾衰竭、心脏失代偿和高血压。