OF 1,2,4,5-HEXATETRAENE

Diels—Alder reaction of 1,2,4,5-hexatetraene tetramethyl[2.2]paracydophane-4,5,12,13-tetra-carboxylate. Org. Synth. 1990, Coll. Vol. VII, 485—490. 

DIELS-ALDER REACTION OF 1,2,4,5-HEXATETRAENE TETRAMETHYL[2.2]PARACYCLOPHANE-4,5,12,13-TETR AC ARBOX Y LATE... 

Note 10). A few crystals (0.2-0.5 g) of hydroquinone are added to stabilize the reddish solution, which is then dried with anhydrous potassium carbonate. The drying agent is filtered, and the filtrate is concentrated to a volume of ca. 400 mL by distillation under nitrogen at atmospheric pressure with a 40-cm Vigreux column and a heating bath kept at 40-45°C. The concentrate is purified by vacuum transfer (Note 11), and the now colorless solution, which contains ca. 25-30 g (30-36%) of 1,2,4,5-hexatetraene, is stabilized by adding another 0.1-0.5 g of hydroquinone (Notes 12 and 13). 

The crude solution of 1,2,4,5-hexatetraene may also be employed in part B. However, yields are lower, and the purification of the product becomes tedious owing to the presence of insoluble by-products. 

The amount of 1,2,4,5-hexatetraene in solution was estimated by the submitters from H NMR spectra and GC analyses. The major product of the dimerization reaction is l,2-hexadien-5-yne. The submitters have shown that this hydrocarbon does not interfere with the cycloaddition in part B. 

CIS- and /rfl j-l,3-Hexadien-5-yne (277 and 278) arc formed as major products, along with small amounts of 2,4-hexadiyne (283), when 275 is heated with potassium /-butoxide in /-butyl alcohol or ethanolic sodium ethoxide. In addition a small amount of 1,2,4,5-hexatetraene (280) was present in the reaction mixture involving sodium ethoxide. l,2-Hexadien-5-yne (276) the product expected from the first isomerization step was not detected, but this is understandable because it was demonstrated in separate experiments that 276 isomerizes much faster than the starting diyne. When 276 is treated with /-BuOK//-BuOH at room temperature, reaction is immediate, producing a mixture of the dienynes 277 and 278 on the other hand, when NaOEt is used, 280 is the exclusive product . ... 

In addition, [2.2]paracyclophane was transformed into reactive bis-dienes. By reaction of the furan cycloadducts 49 with tetraphenylcyclopentadienone and thermal retro-Diels-Alder reaction [2.2](4,7)isobenzofuranophane 51 was obtained [38]. The highly reactive molecule was trapped in situ with para-ben-zoquinone. The more stable tetraphenylisobenzofuranophane 57 was synthesized via the classical procedure to [2.2]paracyclophanes of Hopf [39] the reaction of 1,2,4,5-hexatetraene 53 with dibenzoylacetylene 54 gave 4,5,12,13-tetra-benzoyl[2.2]paracyclophane (56), that was reduced and cyclized to the target molecule 57 [38]. 

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