Synthesis of Heptafulvenes

2//-Cyclohepta[b]furan-2-one 472 was prepared by the reaction of 2-tosyloxytropone 473 and dimethyl malonate in the presence of sodium methoxide in ethanol. The heptafulvene 472 is a versatile synthetic building block for azulene 

A disadvantage of the enamine method is that the condensation reaction does not work well with alkylated tropolones. Polyalkylatedcyclohepta[fe]furan-2(2//)-ones 475 can easily be prepared from polyalkylphenyl propiolates 476 by dynamic gas phase thermoisomerization. The resulting heptafulvenes react thermally with 

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In addition, the same group reported on a synthesis of heptafulvenes and related seven-membered ring systems (e.g., 363) from triafulvene 357 (Scheme 7.80) [91]. A series of donor-substituted 1,3-dienes, when reacted with tria-fulvenes by a Diels-Alder addition, yielded bicyclic methylene cyclopropanes, which were subsequently transformed to 1,6-diphenyl-substituted heptafulvenes or seven-membered ring derivatives. 

Although pentafulvenes are the by far most common fulvenes, there are some publications describing the synthesis of heptafulvene complexes. The reaction of (cycloheptatrienyl-7-methyl) phenylsulfonate (92) with sodium [cyclopentadi-enyldicarbonylferrate(II)] afforded complex 93 in 45% yield, from which hydride was abstracted quantitatively by treatment with triphenylcarbenium ions. The heptafulvene complex formed was described by resonance formulas 94 and 95, the latter with an intact aromatic cycloheptatrienylium substructure (Scheme 10.32) [84, 85]. 

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