EXPERIMENTAL AND THEORETICAL-STUDY OF TAUTOMERISM IN 1,4-BIS[METHOXYAMINO]ANTHRACENE-9,10-DIONES AND THEIR REDUCED FORMS

Treatment of leucoquinizarin 4a with methoxylamine hydrochloride (O-me thylhydroxylamine hydrochloride) in pyridine led to tautomer 4e. Oxida tion of 4e in refluxing nitrobenzene led to 1,4-diaminoanthracene-9,10 -dione 1e while oxidation with manganese dioxide yielded; bis(methoxyi mino)-2,3-dihydroanthracene-9,10-dione 6. Attempted diplacement of the fluorides from 1,4-difluoroanthracene-9,10-dione by methoxylamine in dimethyl sulfoxide led to 9,10-dihydroxy-1,4-bis(methoxyimino)anthrace ne 2h instead of the anticipated bis(methoxyamino) tautomer Ih. Calcul ations have been carried out on some of these derivatives using the 3- 21G basis set and the results for 2h compared with X-ray data from a s ingle crystal, The calculated results show that that Ih is marginally preferred over 2h by 0.68 kcal mol(-1), The reduced forms show the opp osite trend with 4e preferred by 11.3 kcal mol(-1) over 3e in line wit h NMR data recorded in chloroform, The presence of an oxygen atom adja cent to the nitrogen atom in the methoxyimine substituent forces a tet rahedral conformation at nitrogen resulting in a destabilization of th e anthracene-9,10-dione tautomers 1h and 3e.