Orientation effects in the nitration of the dithieno[b,d]pyridine N-oxides. Its dependence on acidity of the reaction medium

The nitration of three dithienopyridine-N-oxides was investigated. The regiochemistry of the reaction was dependent on the reaction conditions used. Under strongly acidic conditions the positional preference is similar for the N-oxides and free bases. However, under mildly acidic or neutral conditions a completely different substitution pattern was obtained. In the latter case those ring positions were substituted which are expected to be unfavored or forbidden in electrophilic substitution of the free bases. The structures of the nitro derivatives were proven by extensive use of ¹H and ¹³C nmr spectroscopy.