Hexadiynes, rearrangement

Hexadiyne rearranges at elevated temperatures (220-400 °C) to give 3,4-bismethylenecyclobutene (315) in nearly quantitative yields - The reaction involves a slow [3,3] sigmatropic rearrangement to the bisallene 314, followed by... 

A similar synthesis starts from commercially available 1,5-hexadiyne and 2-methyl-cyclopent-2-enone. The benzocyclobutene is obtained from a bis-acetylene in a cobalt-catalyzed reaction. It rearranges regio- and stereoselectively to a 3-deoxy steroid derivative. The overall yield from the cyclopentenone was 40% (R.L. Funk, 1977). 

Substituted bismethylenecyclobutenes 317 are obtained by rearrangement of derivatives 316 in which the acetylenic hydrogens of 1,5-hexadiyne are replaced by... 

Rearrangement of l,5-hexadiyn-3-ol gives the aldehyde 323 and phenol. With the diol 324, however, the intermediate enol 325 tautomerizes instead of cyclizing and the acyclic diketone 326 is obtained. 

Wolovsky describes the oxidative coupling of 1,5-hexadiyne, followed by prototropic rearrangement by heating with potassium /-butoxide in f-butanol-benzene at 100°. The resulting mixture of unsaturated hydrocarbons was easily separated by elution chromatography on a column of alumina impregnated with 20% silver nitrate into the tetradehydro[18]annulene (111) and the two isomeric tridehydro[18]annulenes (1) and (11). 

A Cope-type rearrangement in a radical cation has been reported, when 1,5-hexadiyne (90) is irradiated in a Freon matrix. The intermediate (91) can also be generated by treatment of the diallene under similar conditions. 

The four structures are possible because the two cis/trans butadiene units can each be incorporated into the ring in two ways. This degeneracy is inherent in the preparation by oxidative coupling of 1,5-hexadiyne and subsequent pro-totropic rearrangement... 

The PAs of 1,5-hexadiyne and 2,4-hexadiyne are also anomalous when compared to lower-level theoretical calculations and probably also protonate with ultimate rearrangement to allyl cation structures. ... 

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