| 摘要 |
The invention relates to compounds of formula (I) wherein R1, R2, R3 and R4 independently represent OR3 or NR6R7; R5, R6 and R7 independently represent hydrogen, substituted or unsubstituted alkyl, especially C1-C4 alkyl, or substituted or unsubstituted aryl, especially C6-C10 aryl; R6, R7 can also form an aliphatic or aromatic ring, together with the N atom to which they are bonded and optionally other heteroatoms; M represents hydrogen, an equivalent of a monovalent or bivalent metal ion, especially from the group of alkali metals or alkaline earth metals, or of an optionally organically substituted ammonium ion; and B1 represents a bivalent binding link.
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