Cascade electrocyclic ring closures

Recently, further examples for dehydro Diels-Alder reactions were published, which are believed to proceed via intermediates of the type 205. To explain the formation of products besides those of the type 206, the authors [132] proposed a remarkable reaction cascade and supported it by quantum-chemical calculations. Accordingly, an isonaphthalene of the type 205 undergoes an electrocyclic ring expansion to give a l,2-dehydro[10]annulene derivative, in which a configurational isomerization occurs followed by an electrocyclic ring closure, yielding a further isonaphthalene of the type 205, and aromatization. 

Scheme 3 Cascade [l,5]-hydride transfer/dn-electrocyclic ring closure 2.2 Reactivity of Different Hydrogen Donors...

Most probably, the rapid electrocyclic ring closure of allenyl azides was responsible for the fact that the chemistry of these azides was discovered so lately. This cyclization of azidoallenes allowed to create cascade reactions for the synthesis of functionalized NH-l,2,3-tiiazoles. 

The above are in class of cascade of reactions, that involve two successive transformations Ru-catalyzed coupling of 1,6-diynes with acyclic alkenes forming bicyclic cyclohexadiene through electrocyclic ring closure and the migration of the double bond. Under the same reaction conditions, the [2-b2-b2] cycloaddition with cyclic olefins results in tricyclic cyclohexadienes with another arrangement of conjugated double bonds. 

The cis relationship between methyl and ring junction methine hydrogen atom in 7 indicates a disrotatory process. Spirotricyclic product 8 was also isolated in 17% yield from this reaction, presumably as a consequence of a cascade of two pericyclic reactions. A thermally allowed [1,7]-hydrogen shift leads from 6 to triene 9 that undergoes 6ti-disrotatory ring closure to 8. In both photochemical as well as thermal electrocyclizations, there may be multiple... 

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