ON THE ALPHA,ALPHA'-OXIDATIVE COUPLING OF BETA-ALKOXY-SUBSTITUTED 2-PYRROLECARBOXYLATES

The electrochemical and chemical oxidative alpha,alpha'-coupling of 3- methoxy-4-methyl-2-pyrrolecarboxylic acid (2) and its ethyl ester 4 is compared with that of the corresponding 4-methoxy-2-pyrrolecarboxylat es (1 and 3 respectively). The beta-alkoxy substituent exerts a strong directing influence on its vicinal alpha-position, which in the case of compounds 2 and 4 determines the 2,5'-coupling and allows the isola tion of the corresponding 1H,2'H-2,2'-bipyrrole 6 in the case of 4. Fo r 2 the polypyrrole can be obtained by exhaustive electrochemical or c hemical coupling, whereas in the case of 1 only the corresponding tetr amers are formed. The structures of these tetrapyrroles, already descr ibed in the literature, here obtained by oxidation of 4-alkoxy-2-pyrro lecarboxylic acid (1), have been reexamined: they are mixtures of tetr amers with zwitterionic structure, have at least two substitution patt erns and are partially oxidized to the corresponding bipyrrolylidenes.