Dendralenes

A dendralene is a discrete acyclic cross-conjugated polyene. The simplest dendralene is buta-1,3-diene (1) or [2]dendralene followed by [3]dendralene (2), [4]dendralene (3) and [5]dendralene (4) and so forth. [2]dendralene (butadiene) is the only one not cross-conjugated.

The name dendralene is pulled together from the words dendrimer, linear and alkene. The higher dendralenes are of scientific interest because they open up a large array of new organic compounds from a relatively simple precursor especially by Diels-Alder chemistry. Their cyclic counterparts are aptly called radialenes.

Vinylbutadiene ([3]dendralene) was first prepared in 1955 by pyrolysis of a triacetate:

This compound reacts with two equivalents of maleic anhydride in a tandem DA reaction:

With benzoquinone the reaction product was a linear polymer.

Several syntheses of substituted [3]dendralenes have been reported, one via an allene, one via a Horner–Wadsworth–Emmons reaction, one via a cross-coupling reaction and one from an allylic carbonate.

One synthetic route to [4]dendralene starts from chloroprene. This compound is converted to a Grignard reagent by action of magnesium metal which is then reacted with copper(I) chloride to an organocopper intermediate which is in turn dimerized using copper(II) chloride in an oxidative coupling reaction to give the butadiene dimer called [4]dendralene.

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