Eneyne-allene

Another example of a zwitterionic product of a cycloaromatization reaction was given by Kerwin and coworkers. Their skipped (aza)enediynes rearranged to yield (aza)eneyne-allenes that subsequently cyclized under addition of methanol (in a byproduct), which is consistent with a partitioning between a diradical and a zwitterionic reaction pathway (Scheme 15).66... 

Scheme 15 Formation of a methanol adduct in cyclization of eneyne-allenes, formed from skipped enediynes.

Nagata, R. Yamanaka, H. Okazaki, E. Saito, I. Biradical formation from acyclic conjugated eneyne-allene system related to neocarzinostatin and esperamicin-calichemicin, Tetrahedron Lett. 1989, 30,4995-4998. 

Myers, A.G. Kuo, E.Y. Finney, N.S. Thermal generation of alpha,3-dehydrotoluene from (Z)-l,2,4-heptatricn-6-ync. J. Am. Chem. Soc. 1989, 111, 8057-8058 Nagata, R. Yamanaka, H. Okazaki, E. Saito, I. Biradical formation from acyclic conjugated eneyne-allene system related to neocarzinostatin and esperamicin-calichemicin. Tetrahedron Lett. 1989, 30, 4995 998. 

Related to the Bergman cyclization is the cyclization of an eneyne allene system 34 to provide access to an aromatic system 36. This variant on... 

Lipton et al. noticed that the switching of cyclization pathways observed by Schmittel takes place due to a combination of effects. The calculations of the energies of diradicals formed by Myers-Saito and Schmittel cyclization indicate that benzannulation of the eneyne-allene as in 15 plays a significant role in promoting C2-C6 cyclization. The energy of diradicals for the benzannulated system is 10.5 kcal/mol less as compared to those for the parent monocyclic diradicals (16 vs. 13, and 17 vs. 14). ... 

It is interesting that when the enyne-allene moiety is incorporated into a bicyclic system, such as [7.3.2]-eneyne-allene 30, cyclization does not proceed. In contrast, the Bergmann cycloaromatization has been observed for a related substrate, [7.4.1]-enediyne (not illustrated). ... 

Scheme 3.87 Activation through eneyne-allene formation at the decarboxylation of methoxymalonate [26, 341],...

The third approach to activation involves pyridoxal-mediated conversion of the endiyne to an eneyne-allene [26, 344]. Acyclic enediynyl amine 3.756, upon treatment with pyridoxal in the presence of a base (EtsN), undergoes isomerization to the eneyne-allene 3.757 following imine formation. The allene then reacts at room temperature to generate the products 3.759 and 3.760 (Scheme 3.89) via diradical intermediate 3.758 A or B depending on the mode of hydrogen abstraction [26, 344]. 

Scheme 3.88 Activation through eneyne-allene conversion by an intramolecular Sn reaction (Shibuya) [26, 342]. TFA.

Scheme 3.89 Activation through pyridoxal-mediated eneyne-allene conversion [26, 344],...

Nagata, R., Yamanaka, H., Murahashi, E. and Saito, 1. (1990) DNA cleavage by acyclic eneyne-allene systems related to neocarzinostatin and esperamicin-calicheamicin. Tetrahedron Letters, 31(20), 2907-2910. 

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

Joint catalysis by tin(IV) chloride and zinc chloride induces coupling of the acetal 114 with the alkyne 115 to give the propargyl ether 116. Allyl halides condense with terminal alkynes in DMF in the presence of copper(I) iodide, potassium carbonate and tetrabutylammonium chloride under phase-transfer conditions to afford eneynes, e.g. equation 19. Palladium-catalysed coupling reactions of 1-bromoallenes with terminal acetylenes have been reported for the first time thus acetylene and the allenes 117 (R R = Me or Et) in triethylamine in the presence of tetrakis(triphenylphosphine)palladium and... 

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