Scavengers dienophiles

Scheme 24 Use of PS-anthracene to scavenge excess of dienophiles after a hetero Diels-Alder reaction...

Another interesting scavenger is polymer-supported anthracene, developed by Porco for the scavenging of dienophiles [109]. An example of its application to the synthesis of a complex 5,8-dihydro-(l,2,4)triazolo[l,2-a]pyridazine-l,3-diones via hetero-Diels-Alder reaction followed by removal of the excess of triazole-3,5-dione under microwave irradiation is depicted in Scheme 24. For this particular example, moving from thermal heating (toluene, 100 °C) to a microwave-assisted protocol (DCE, 150 °C) reduced scavenging time from 3 h to just 15 min. 

An interesting approach involving the use of a fluorous dienophile as a diene scavenger under microwave conditions has been investigated by Werner and Curran [101]. The classical Diels-Alder-type cydoaddition of diphenylbutadiene with maleic anhydride as dienophile was accelerated under microwave heating, such that it... 

Scheme 7.88 Diene scavenging utilizing a fluorous dienophile.

Scheme 7.130 Preparation and application of a novel polystyrene-bound anthracene as a dienophile scavenger.

Maleimide is a well-known Michael acceptor, dienophile and dipolarophile and hence is another versatile functional moiety that has found multiple uses in combinatorial chemistry as both a scavenger as well as a template in library synthesis [15]. Barrett [16] and Porco [17] have both reported the synthesis of a polystyrene resin-supported maleimide but did not report its use in the scavenging of nucleophiles. Hall and coworkers have described the synthesis of a supported-maleimide... 

In 2004, Porco described the application of a polymer-supported anthracene for the scavenging of excess dienophile from Diels-Alder reactions [48]. This scavenger was not only demonstrated with a range of dienophiles, but also its application was related in the synthesis of flavonoid dienes and other classes of cyclic molecules (Scheme 8.35). 

Scheme 8.35 Scavenging of excess dienophile from Diel-Alder reactions using a polymer-supported anthracene.

Scheme 8.54 Application of a fluorous-supported dienophile as a scavenger for dienes.

Another interesting development is the use of fluorous-based scavengers in conjunction with microwave synthesis and fluorous solid-phase extraction (F-SPE) for purification. This was recently illustrated by Werner and Curran [74] in their investigation of the Diels-Alder cycloaddition of maleic anhydride to diphenylbutadiene (Scheme 11.23). After performing microwave-assisted cycloaddition (160 °C, 10 min) with a 50% excess of the diene, the excess diene reagent was scavenged by a structurally related maleimide fluorous dienophile under the same reaction conditions. Elution of the product mixture from an F-SPE column with Me0H-H20 provided the desired cycloadduct 89 in 79% yield and 90% purity. Subsequent elution with diethyl ether furnished the fluorous Diels-Alder cycloadduct. 

---