Fulvenes anthracene adduct

Dimethyl anthracene and diphenyl isobenzofuran form remarkably stable233 cyclopropanone derivatives (353/354), whilst with other diene components (butadiene, tetracyclone, and fulvene) the primarily formed Diels-Alder adducts either suffer ketalizing attack of the solvent (356 - 357, 359 - 358/360) or undergo irreversible changes such as decarbonylation to 362 or rearrangement to 355. 

As the focus of this chapter is on the synthetic utility of the rDA reaction, an overview of mechanism is beyond the scope of this review however, the subject has beoi reviewed previously. Structural and medium effects on the rate of the rDA reaction are of prime importance to their synthetic utility, and therefore warrant discussion here. A study of steric effects cm the rate of cycloreversicHi was the focus of early work by Bachmann and later by Vaughan. The effect of both diene and dioiophile substituticHi on Ae rate of the rDA reaction in anthracene cycloadducts has been reported in a study employing 45 different adducts. If both cycloaddition and cycloreversion processes are fast on the time scde of a given experiment, reversibility in the DA reaction is observed. Reversible cycloaddition reactions involving anthracenes, furans, fulvenes and cyclopentadienes are known. Herndon has shown that the well-known exception to the endo rule in tiie DA reaction of furan with maleic anhydride (equation 2) occurs not because exo addition is faster than endo addition (it is not), but because cycloreversion of the endo adduct is about 10 000 times faster than that of the exo adduct. ... 

Cumulenes, generally regarded as difficult to prepare, can be obtained readily via rDA methods. Al-lenes, the most basic cumulenes, with a wide range of substituents on the allenic carlxm, can be obtained by FVP of anthracenic DA adducts as shown in equation (114). Butatriene (258), the next in the series of cumulenes, has been produced by FVP of a variety of adducts that includes 9,10-ethano-9,10-dihy-droanthracene, " cyclohexyne and fulvene, furan and benzene cycloadducts. Pentatetraene (260), a compound that is stable enough to be purified by GLC at room temperature, has also been obtained by the FVP of vinylallene (259). ... 

Also monocyclic dienes having exocyclic double bonds, like fulvenes, fall in this class in their adducts the "diene portion presents ir-electron systems on both sides of the plane containing the four atoms involved in the addition, as for naphthalene (attacked in 1.4-positions), anthracene (attacked in positions 9, 10). etc. 

The reversibility of the Diels-Alder reaction has been put to good use in the protection of olefins. Ardis [69] used the butadiene adduct to protect the olefin in his synthesis of vinylidene cyanide. Alder [70] was able to monoepoxidise various benzoquinones as their cyclopentadiene adducts, and Chapman [71] used the same method to obtain half-reduction of benzoquinone. Regeneration of the olefin is achieved thermally and Alder [70] noted that this was easier with the fulvene adduct than with the cyclopentadiene adduct. Sauer [72] observed that the retro-Diels-Alder reaction was easier with adducts involving furan, fulvene, and anthracene as the diene component, and he made the anhydride (8) using this method. 

---