The synthesis of ethyl 4‐oxo‐1, 4‐dihydro‐1, 8‐phenanthroline‐3‐carboxylate (2a) and some other 1, 8‐phenanthrolines is reported. The ethylation of the above ester yields a thermochromic product 2b and some ethyl 4‐ethoxy‐1, 8‐phenanthroline‐3‐carboxylate (3b). Reexamination of similar ethylations of analogous esters derived from 1, 7‐phenanthroline and 1, 10‐phenanthroline indicated formation of the O ‐ethyl derivative of the former and the N ‐ethyl derivative of the latter.
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D. G. Markees (1983) studied this question.