Abstract
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.
Highlights
Among modifications of spiroketals, the aza-aza spiro systems, designated as spiro-N,N-ketals or spiroaminals [1], have received much less attention than their oxygenated analogues
The reactions of ninhydrin with primary diamines generally afford 1,2-condensed compounds based on the indenopyrazine framework [8,9,10]
We report the formation of new spiro-N,N-ketal along with some unexpected rearranged compounds in the reaction of ninhydrin with naphthalene-1,8-diamine in the presence of an acid catalyst
Summary
The aza-aza spiro systems, designated as spiro-N,N-ketals or spiroaminals [1], have received much less attention than their oxygenated analogues. Despite the apparent absence of extended -conjugation, the occurrence of such long-wavelength absorption is attributed to Molecules 2012, 17 spiro conjugation [5,6] through intramolecular charge-transfer interactions between the 1,3-indandione moiety as the acceptor unit and aromatic diamine as the donor unit [7]. With this background in mind, we prepared spiro-N,N-ketals by the reactions of ninhydrin (2,2'-dihydroxy-1H-indene-1,3(2H)-dione) and isatin (1H-indole-2,3-dione) with diamines. We report the outcome of these reactions and the characteristic structural and spectroscopic properties of the products
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