prov:value
| - The compounds of formula (I) in which A is sulphur may be prepared by treating a compound of formula (II) ##STR4## in which R1, R2, R6, R7, W and Z are as defined in formula (I), and Y is a leaving group, with a compound of formula (III) ##STR5## in which R3 and X are as defined in formula (I), and Ra is an alkyl group, preferably C1-6 alkyl, for example methyl or ethyl, or an ammonium or alkali m
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