1,3,5-Heptatrienes thermal reactions

The acetylene 27 became of interest since it undergoes a novel type of thermal cyclization reaction, later to be called the Myers or Myers-Saito cyclization (see Chapter 20 for a discussion of its relevance). The Z-diastereomer of 1,2,4-heptatrien-6-yne (27) was prepared by the sequence summarized in Scheme 5.33 [85]. 

The ability of (Z)-l,2,4-heptatrien-6-ynes (enyne-allenes) and the benzannulated derivatives to undergo cyclization reactions under mild thermal conditions to produce biradicals has been the main focus of their chemical reactivities [1-5]. With the development of many synthetic methods for these highly conjugated allenes, a variety of biradicals are readily accessible for subsequent chemical transformations. Cyclization of the enyne-allene 1 could occur either via the C2-C7 pathway (Myers-Saito cyclization) leading to the a,3-didehydrotoluene/naphthalene biradical 2 [6-10] or via the C2-C6 pathway (Schmittel cyclization) producing the fulvene/benzofulvene biradical 3 [11] (Scheme 20.1). 

The Myers-Saito cyclization was first described independently in 1989 by Isao Saito (Kyoto University, Japan) and Andrew G. Myers (California Institute of Technology, Pasadena) whose findings were submitted for publication on June 7 and June 12, respectively. As a parallel transformation to the Moore cyclization (Chapter 4.12), in which an allenic fragment replaces the ketene moiety in the substrate, the Myers-Saito reaction provides a path to carbon diradicals. In its pioneering version, the reaction involved the cyclization of (Z)-l,2,4-heptatrien-6-yne (enyne-allene) 3, or its phosphine oxide derivative 5, to substituted a,3-dehydrotoluene diradicals, and subsequently to toluene derivatives 4 and 6. The reaction proceeds under thermal neutral conditions in 1,4-cyclohexadiene or other organic solvents such as methanol or carbon tetrachloride. 

From a mechanistic point of view, in the initial step, the (Z)-l,2,4-heptatrien-6-yne, or compounds containing an equivalently unsaturated core, undergoes a mild, thermal electrocyclization reaction to form an a,3-alkylbenzenediyl, a diradical intermediate with substantial polar character. Dehydroaromatic intermediate 7, when trapped by the solvent or compounds (e.g., 1,4-cyclohexadiene) present in the reaction medium, forms than aromatic products of type 8, 9, and 10. In methanol, a mixture of hydrogen atom abstraction and polar addition product are obtained. ... 

Oxoheptafulvene (197), prepared by in situ dehydrochlorination of cyclo-heptatriene-l-carboxylic acid chloride, apparently undergoes [8 + 2]-cycloaddition with benzophenone to give (198), but this possibly arises via the [2 -t- 2]-adduct (199). On reaction of the heptafulvene with cyclopentadiene the [2 -h 2]-adduct is observed, which rearranges thermally to (2(X)), perhaps via (201) or two consecutive [1,5]-shifts. ... 

Concerted thermochemical [1,7] hydrogen shifts are unknown in cyclo-heptatrienes or similar cyclic molecules. The best known example is given by the equilibration of vitamin Dj and precaliferol (Equation 6,98). Presumably the reaction centres adopt a gentle spiral conformation, as indicated, to facilitate the transfer of the H-atom from the top face at one terminus to the bottom face at the other terminus. The necessity for this type of conformation makes the rarity of thermal [1,7] H-shifts rather more understandable. The [1,5] shifts are usually available as competing processes. Thus O-deuteriated... 

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