Synthesis, Reactivity, and Properties of Benz[a]azulenes via the [8 + 2] Cycloaddition of 2H-Cyclohepta[b]furan-2-ones with an Enamine

Abstract

Starting with the reaction of 2H-cyclohepta­[b]­furan-2-ones with an enamine, which was prepared from 4-tert-butylcyclohexanone and pyrrolidine, benz­[a]­azulenes having both formyl and tert-butyl groups were obtained in the three-step sequence. Subsequently, both the formyl and tert-butyl groups were eliminated by heating the benz­[a]­azulene derivatives in 100% H3PO4 to give benz­[a]­azulenes without these substituents in high yields. In terms of product yield, this method is the best one ever reported for the synthesis of the parent benz­[a]­azulene so far. The conversion of the benz­[a]­azulene derivatives with a formyl group into cyclohept­[a]­acenaphthylen-3-one derivatives was also investigated via Knoevenagel condensation with dimethyl malonate, followed by Brønsted acid-mediated intramolecular cyclization. The structural features including the bond alternation in the benz­[a]­azulene derivatives were revealed by NMR studies, NICS calculations, and a single-crystal X-ray structural analysis. The optical and electrochemical properties of a series of benz­[a]­azulene derivatives were evaluated by UV/Vis, fluorescence spectroscopy, and voltammetry experiments. As a result, we found that some benz­[a]­azulene derivatives showed remarkable luminescence in acidic media. In addition, the benz­[a]­azulene derivatives with the electron-withdrawing group and cyclohept­[a]­acenaphthylen-3-one derivative displayed good reversibility in the spectral changes under the electrochemical redox conditions.

The road to bis‐periazulene (cyclohepta[def]fluorene): Realizing one of the long‐standing dreams in nonalternant hydrocarbons

“…Shoji et al found that the reaction of 2 H ‐cyclohepta[ b ]furan‐2‐one with an enamine gives the benzo[ a ]azulene derivative 12 having formyl and tert ‐butyl groups and subsequent removal of the formyl and tert ‐butyl groups afforded 3 in high yield as a green solid (Figure 3A). [ 19 ] The synthesis of benzo[ f ]azulene 4 has also been established. [ 20–22 ] Jutz and Schweiger found that the condensation of 6‐methylazulene and trimethinium perchlorate generated the azulene dienamine 13 that readily isomerized upon heating to form 13′ .…”

“…Examples of the reported synthetic routes for (A) benzo[ a ]azulene 3 , [ 19 ] (B) benzo[ f ]azulene 4 , [ 14 ] and (C) benzo[ e ]azulene 5 . [ 27 ] …”

Synthesis, Structural, and Optical Properties of Azuleno[1,2-c]pyran-1-ones: Bro̷nsted Acid-Mediated Cyclization of 2-Azulenylalkynes

“…Therefore, the structure of 3a was verified through X-ray structural analysis (Figure ). Unlike the ring-fused azulenes, the X-ray structural analysis of 3a does not observe any obvious bond alternation in the azulene moiety. The analysis also revealed the planar structure in the azuleno[1,2- c ]pyran-1-one moiety of 3a , while there is a slight twist with a dihedral angle of 10.23° between azuleno[1,2- c ]pyran-1-one moiety and the phenyl group substituted.…”

Selective 6-endo-dig and ring-expansion cycloisomerizations of ortho-disubstituted thiophenes bearing 1-en-3-yne moieties

“…After the protection of the active bare site on the cyclopentadiene ring, such as with an aldehyde group, its methyl analogue could be converted to a S-fused [5]helicene by stirring with 2,3-dichloro-5,6dicyano-p-benzoquinone (DDQ) in 1,4-dioxane at 100 °C for 12 hours. 36,37 Furthermore, it was not mandatory to involve the CvC double bond of the 1-en-3-yne moiety in a cyclic ring. When a phenyl group was substituted on the vinyl group in the precursor, product 4f could be obtained in 86% yield.…”