Barrelene

First synthesized and described by Howard Zimmerman in 1960, the name derives from the resemblance to a barrel, with the staves being three ethylene units attached to two methine groups.

The original Zimmerman synthesis modified in 1969[2] starts from coumalic acid:[note 1] Many alternative routes have been devised since then, one of them starting from benzene oxide:[3][4] An alternate route that allows synthesis of the parent barrelene system and a variety of substituted barrelenes has also been reported.

Bromination with bromine in tetrachloromethane gives a di-bromo adduct because a coupling reaction intervenes: Epoxidation of barrelene with oxone gives the trioxatrishomobarrelene[6] which on rearrangement with boron trifluoride (driving force:relief of strain energy) converts into the trioxatrishomocubane:[7] This compound can be envisioned as a cubane with three oxygen atoms inserted into three opposite edges or as 9-crown-3 capped by two methine units.

Oxidation of the polymer with DDQ affords the naphthalene pendant of poly(p-phenylene vinylene).

Isopentane solutions of barrelene undergo photolytic isomerisation when acetone is added as a photosensitizer to produce semibullvalene.

The synthesis of barrelene as reported by Zimmermann in 1969.