Bicyclo heptatrienes

Similiar to the case of bicyclo[4.1.0]heptatriene (2a), the proposed31 bi-cyclic intermediate (2b) in the ring expansion of lb has never been directly observed. However, the analogous azirines, formed by photolysis of 1- and 2-naphthyl azides, have been characterized by IR (Scheme 13)47 and both trapping48 3,6,0 and IR spectroscopic characterization484 of substituted derivatives... 

As an example, the suprafacial [1,7] sigmatropic H-shift will be considered that occurs on irradiation of 1,3,5-CHT with formation of bicyclo[3.2.0]hepta-2,6-diene (BHD) as a minor product. In 1-substituted CHT this [1,7]-H shift is regiose-lective in 1-cyano-cycloheptatriene (CN-CHT) the hydrogen atom moves exclusively to the unsubstituted terminal carbon of the heptatriene moiety, whereas in 1-methyl-cyclohep-tatriene (Me-CHT) only 2% of the product exhibits this regio-chemistry and 98% corresponds to a hydrogen shift toward the substituted carbon, as shown in Scheme 6.1. Different models have been proposed to explain this regioselectivity. ... 

There are numerous examples of interconversion of 1,3,5-trienes and 1,3-cyclohexadiene systems by the electrocyclic mechanism. An especially interesting case of hexatriene-cyclohexadiene interconversion is the equilibrium between cyclo-heptatrienes and bicyclo[4.1.0]hepta-2,4-dienes. ... 

Bicyclo[2.1.0]pentane-5-spirocyclopropane, Bicyclo[3.2.0]-hept-1-ene, 5-Methylenebicyclo[2.1.1]hexane, 1,2,6-Heptatriene, 3-Methylene-1,5-hexadiene and Tricyclo[4.1.0.0 ]heptane... 

Flash thermolysis of a series of methylated bicyclo[3,2,0]hept-2-ene-7-ones gave dihydrotolualdehydes and tolualdehydes as the major products. The suggested reaction path involved recyclization of a heptatrienal formed from the p,y-unsaturated bicyclic ketones by sequential [2 + 2]cycloreversion to the acyclic ketene and 1,5-hydride shift. 

Heptatriene 18 having electron-withdrawing substituents, undergoes rapid conversion into bicyclo[4.1.0]-hepta-2,4-diene 19 and both the isomers remain in equilibrium because of low activation energy Ea < 10 kcal/mol. This transformation is known as valence tautomerism [18]. 

Carbene-carbene rearrangements are believed to proceed via bicyclo-[4.1.0]heptatrienes (Scheme 6). 

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