alkyl c 1 c 5 alkoxy u2014 o u2014 ch 2 1 5 u2014 och 3 or r 2 and r 3 in positions 6 and 7 together form the group if a is thieno r 1 r 2 and r 3 denote hydrogen together with the central atom to which they are bonded form a c 3 c 7 ring r 6 denotes c 1 c 12 alkyl or a phenyl or benzyl group substituted by the groups r 7 and r 8 wherein r 7 and r 8 independently of one another denote hydrogen halogen f cl br i c 1 c 5 alkyl c 1 c 5 alkoxy or c 1 c 5 fluoroalkyl group z denotes o s or an nhco u2014 group or if at least one of the groups r 4 and r 5 is other than hydrogen z may also denote a single bond the invention further relates to the compounds of formula i above with the proviso that the compound of the following formula preferred compounds are the compounds of formula ia and their use as described above wherein r 1 r 2 and r 3 preferably independently of one another denote hydrogen methyl or methoxy particularly wherein r 2 and r 3 denote methoxy and r 1 is methoxy or hydrogen also preferred are compounds of general formula i and their use as specified above wherein r 4.