Diels-Alder reactions pincer/domino

SCHEME 8. Competition between pincer and domino Diels-Alder reactions in the synthesis of pagodane precursurs92... 

Dailey and colleagues109 employed a domino Diels-Alder reaction to synthesize the complex hexacycle 146. The intermolecular reaction of tetracycle 143 with maleic anhydride 144 afforded a single adduct (145) which immediately underwent an intramolecular Diels-Alder reaction to give 146 (equation 42). This reaction is similar to a reaction performed previously by Prinzbach and colleagues110. Prinzbach observed that when alkynes were used as dienophiles, either domino or pincer Diels-Alder reactions occurred. In the latter type, the triple bond reacts with both diene units. 

Slee and LeGoff performed further investigations on the reaction of dimethyl acetylenedicarboxylate 4-20 with an excess of furan 4-21, as first described by Diels and Alder (Scheme 4.5) [la]. At 100 °C, 4-24 and 4-25 were not produced (as proposed), but rather 4-22 and 4-23, since at elevated temperature an equilibrium takes place and the primarily formed 4-24 and 4-25 isomerize to give a 6 1-mixture of the exo-endo and the exo-exo products 4-22 and 4-23, respectively. However, at lower temperature, in the primarily formed [4+2] cycloadduct the double bond substituted with the two carbomethoxy group acts as the dienophile to give the two products 4-24 and 4-25 in a 3 1 ratio with 96% yield within five weeks, as has been shown by Diels and Olsen [la,lc]. For a differentiation of these two types of adducts, Paquette and coworkers [7] used a domino and pincer product . The Cram group [8] described one of the first examples of a reaction of a tethered bisfuran 4-26 with dimethyl acetylenedicarboxylate 4-20a to give 4-27. 

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