Compounds of formula (I) wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4, X are as defined in the specification and are prepared by elimination of one of the groups X.sub.1 and X.sub.2 from a compound of formula (II) wherein X.sub.1 and X.sub.2 each independently represent a carboxylic acid group or salt thereof, or a protected carboxylic acid group, the non-eliminated group X.sub.1 or X.sub.2 corresponding to the group X in the desired compound of formula (I). ##STR1##
式(I)化合物的
化学式中,R.sub.1、R.sub.2、R.sub.3、R.sub.4和X的定义如规范中所述,并通过从式(II)化合物中消除X.sub.1和X.sub.2中的一个群组来制备,其中X.sub.1和X.sub.2各自独立地表示
羧酸基或其盐,或保护的
羧酸基,未被消除的群组X.sub.1或X.sub.2对应于所需的式(I)化合物中的群组X。 ##STR1##