Keiji Kobayashi

Novel rearrangements in the reactions directed toward preparation of spiro-N,N-ketals: reactions of naphthalene-1,8-diamine with ninhydrin and isatin.

Spiro-N,N-ketal 5, consisting of a phthaloperine heterocyclic ring and a naphtha[1,8-ef][1,4]diazepine ring, was obtained along with spiro-N,N-ketal 2 via 2,2-condensation in the reaction of ninhydrin with naphthalene-1,8-diamine. Their molecular structures were elucidated by X-ray crystal structural analysis. Aside from these spiro compounds, the diazapleiadiene compound 3 formed by 1,2-condensation and the 1,4-isoquinolinedione compound 4 arising from ring expansion were isolated. When isatin was reacted with naphthalene-1,8-diamine, spiro-N,N-ketal 6 and the two 1H-perimidine-based compounds 7 and 8 were isolated. Compound 8 was revealed to undergo a fast dynamic prototropic tautomerization in solution. Plausible mechanisms of the formation of the products are proposed.

REACTIONS OF NINHYDRIN WITH BENZO[b]THIOPHENES

The reaction of ninhydrin with benzo[b]thiophene in acetic acid in the presence of a small amount of sulfuric acid afforded a novel fluorenone compound fused to benzo[b]thiophene rings. In the reaction using 2,2-bibenzo[b]thiophene, phthalide conjoined in a spiro framework was unexpectedly isolated along with an isocoumarin derivative which is fused to benzo[b]thiophene and benzene rings. The product related to the latter was obtained in the reaction of 3,3-bithiophene with ninhydrin. The structures of these novel heterocycles were supported by spectroscopic studies and X-ray crystallography.

The oxidation of benzo〔b〕thiophene derivatives with DDQ: preparation of π-extended hemithioindigoid compounds

Hemithioindigoid compounds, e. g., 2-(6H-cydopenta[1,2-b:4,3-b]-bis[ 1 ]benzothiophen-6-ylidene)benzo[b]thiophene-3(2H)-one (1), were synthesized in good yields by the oxidation of benzo[b]thiophene derivatives such as 9-(2-benzo[b]thienyl)fluorene with DDQ in the presence of acid and under atmospheric oxygen. Under nitrogen, on the other hand, the reaction of 3 with DDQ resulted in an oxidative dimerization, the product of which was revealed to be equilibrated with radical species.