particularly preferred in the sense of the invention consists of the compounds of formula i in which a is a direct bond ar 1 is a heteroaryl ar 2 is a heterocycloalkyl r 3 represents a hydrogen atom r 4 represents an oh or u2014 conh 2 group r 8 represents an alkyl group alkyl si alkyl 3 u2014 so 2 alkyl si alkyl 3 phenyl alkoxy imino heterocycloalkyl substituted with one or more halogen atoms one or more hydroxyl or hydroxy alkyl groups or else r 8 and r 9 together with the carbon atom to which they are bound form a cycloalkyl group optionally substituted with one or more halogen atoms or one or more carboxyl groups r 9 represents a hydrogen atom or an alkyl group provided that when r 8 is an alkyl group it is attached to the silicon atom of ar 2 advantageously r 8 represents an alkyl group alkyl si alkyl 3 u2014 so 2 alkyl si alkyl 3 phenyl alkoxy imino pyrrolidinyl substituted with one or more halogen atoms a hydroxyl or hydroxy alkyl group thiomorpholinyl or else r 8 and r 9 together with the carbon atom to which they are bound form a cycloalkyl group optionally substituted with one or more halogen atoms or one or more carboxyl groups r 9 represents a.