Cyclopentadienones, Diels-Alder addition reactions

Convergent synthesis (Fig. 4.18) also uses cyclopentadienone for construction of the dendrons. However, in this case two molecules of the diene are added via Diels-Alder reaction to 4,4 -diethynylbenzil. The resulting second-generation den-dron is reacted with dibenzylacetone in a twofold Knoevenagel reaction. The newly functionalised cyclopentadienone dendron obtained again undergoes (fourfold) Diels-Alder addition to the tetraethynylated tetraphenylmethane core unit [34a]. 

The main side reactions are cyclotrimerization (of the alkyne), co-cyclotrimer-ization (alkyne/alkyne/alkene), and formation of cyclopentadienones 5 and 6. In addition, spirofuranones such as 7 and 8 can form, with the latter being the Diels-Alder product of 7 with ethylene. It is generally observed that high ethylene and CO pressures disfavor the formation of side products. Further details on the side-product formation were given in [4d]. 

Kinetic data on addition reactions, and their implications as to the reaction mechanism, have been reviewed in the recent past this is particularly true for the Diels-Alder reaction - - and its reverse reaction , and for 1,3-cyclo-additions . As well, kinetics and mechanism have been reviewed for particular cases, like Diels-Alder reactions of cyanogen and nitroso derivatives as dienophiles, and of cyclopentadienones as dienes. Other discussions of mechanism, of the Diels-Alder reaction and of 1,2-cycloadditions - , are also worth noting. Another particular case is that of 1,3-cycloadditions of nitrile oxides ", ... 

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