Electrocyclic reactions component analysis

Electrocyclic reactions, 163, 165 butadienes to cyclobutenes, 164-165 component analysis, 168 stereochemistry, 165 Electron... 

As discussed in Chapter 12, electrocyclic reactions may proceed in a conrotatory or disrotatory fashion, that is, the n system cyclizes in an antarafacial or suprafacial manner, respectively. Since there is only a single component, it should be counted according to the general component analysis for a thermally allowed reaction, that is, Ti rn = conrotatory and = disrotatory. Thus, the cyclization of butadienes... 

The reaction of carbenoids with aromatic systems was first reported by Buchner and coworkers in the 1890s.6 The reaction offers a direct entry to cycloheptatrienes and has been used to synthesize tropones, tropolones and azulenes.6 Neither the thermal nor copper-catalyzed reactions, however, proceed in good yield. The problems associated with these transformations were clearly demonstrated in a recent reexamination of die thermal decomposition of ethyl diazoacetate in excess anisole (137).129 A careful analysis of the reaction mixture revealed the presence of seven components (138-144) in 34% overall yield (Scheme 29). The cycloheptatrienes (138M142) were considered to be formed by cyclopropanation followed by electrocyclic ring opening of the resulting norcaradienes. A mixture of products arose because the cyclopropanation was not regioselective and, also, the initially formed cycloheptatrienes were labile under the reaction conditions. 

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