1.2.3.5- Hexatetraene

Another impressive example is the synthesis of paracyclophanes as 4-9 by Hopf [3], starting from a 1,2,4,5-hexatetraene 4-6 and an electron-deficient alkyne 4-7 to give 4-9 via the intermediate 4-8 (Scheme 4.2). 

In principle, the direct cydopropanation of vinylallenes and related substrates also allows the preparation of cydopropylallenes. However, because of low or no discrimination between the differrent double bonds, complex product mixtures are to be expected. A case in point is provided by 1,2,4,5-hexatetraene (12), which yields the cyclopropylallene 149 on diazomethane/CuX treatment - but only in a (difficult to separate) mixture with all other cydopropanation products of 12 (Scheme 5.21) [61]. 

Depending on the reaction conditions, 1,2,4,5-hexatetraene (12) and its alkyl derivatives can be transformed into four-, five- or six-membered carbo- or heterocyclic ring compounds, as illustrated by the transformations shown in Scheme 5.48. 

The best known example is the thermal isomerization of 1,5-hexadiyne, which involves a [3.3]-sigmatropic shift to 1,2,4,5-hexatetraene (12) followed by an electrocydic ring closure to 3,4-bismethylencydobutene (324) for reviews, see (a) W. D. Huntsman, in The Chemistry of Ketenes, AUenes and Related Compounds, S. Patai (ed.), Wiley Chichester, 1980, Vol. 2, Chapter 15, pp. 521-667 (b) A. Viola, J. J. Collins, N. Fillip, Tetrahedron 1981, 37, 3765—3811 for quantum chemical calculations on this and related isomerizations, see K. A. Bleck,... 

Bisallene (1,2,4,5-hexatetraene) reacts with reactive dienophiles to afford the corresponding [4 + 2]-adducts [182],... 

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. 

The conditions and isolation procedure given are those used by the checkers. The submitters heated the solution first to 45°C, after which the temperature was gradually increased to 70°C over several hours. The solution was then heated at 70°C overnight, and the solvents were removed by rotary evaporation. The semisolid, yellow-orange residue was recrystallized from benzene or methanol. Concentration of the mother liquor afforded additional crops of crystalline product. The total yield of the tetraester was 25-30 g (30-36% based on 1,2,4,5-hexatetraene), mp 206-207°C. 

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. 

Pure 1,2,4,5-hexatetraene polymerizes readily when exposed to air at room temperature. However, solutions of the purified compound are stable for months at 0°C, especially if protected by an inert gas. Contact with air should be minimized to avoid inducing polymerization. 

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 . ... 

Hexatetraene (280) is stable to ethanolic NaOEt at room temperature, but reacts at 65 °C to give 277, 278 and 283 in the ratio 21 22 31. Thus 280 occupies a key position in the scheme relating the various CoHo isomers, but it is argued that the rearrangement of 276 may lead directly to 277 and 278 and does not have to... 

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