Azulene, total synthesis

A key step in the total synthesis of the hydrocarbon azulene follows. Outline a suitable mechanism to account for the reaction. 

The approach to cycloheptadienes via thermal Cope rearrangement of an appropriate 1,2-divinylcyclopropane is an attractive one, limited only by the availability of the latter. A number of groups of workers have now described efficient routes to divinylcyclopropanes based on addition of 2-vinylcyclopropyl-lithium or -cuprates to 3-alkoxy- or 3-halogeno-cycloalk-2-enones (Scheme 30), and extensions of these studies to the total synthesis of the biologically important natural hydro-azulene systems are anticipated. 

Hafner s synthesis of azulene,216 the cyclopentadienyl anion can also be used to introduce the five-membered fragment. This anion reacts with heterocyclic quaternary salts in a complex manner the pseudoazulene, however, is obtained in one step. The total yields are only about 10%, but it is possible to use starting materials that are easily available in large quantities. This variant was applied to prepare 2H-cyclopenta[thiadiazolium salts145 and cyclopenta[c]thiopyranes (31) using N-methyl thiazoliumbromide.90... 

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