Scaffold structures Diels-Alder reactions

Abstract An overview on the microwave-enhanced synthesis and decoration of the 2(lH)-pyrazinone system is presented. Scaffold decoration using microwave-enhanced transition-metal-catalyzed reactions for generating structural diversity, as well as the conversion of the 2(lH)-pyrazinone skeleton applying Diels-Alder reactions to generate novel heterocyclic moieties are discussed. The transfer of the solution phase to polymer-supported chemistry (SPOS) is also described in detail. 

The structural diversity (and complexity) of the products obtained by the MCR between tertiary isocyano amides, aldehydes, and amines could be increased to various heterocyclic scaffolds by combining the initial 2,4,5-tiisubstituted oxazole MCR with in situ intramolecular tandem processes (Fig. 17). Most tandem processes reported are based on the reactivity of the oxazole ring toward C=C or C=C bonds in hetero Diels-Alder reactions followed by ring opening reactions generating the rather complex heterocyclic products with high degrees of variation. 

Far more attempts have been undertaken to covalently assemble multicomponent structures with at least one Pc moiety, which will be briefly described thereafter. In particular the C6o fullerene has been connected to the Pc by using different methodologies. Besides the Pc-C6o dyads presented above, the Pc scaffold has also been covalently connected through employment of Diels-Alder reaction onto the... 

The [4+2] cycloaddition is certainly one of the most important reactions in organic chemistry, and many books and reviews are dedicated to this topic [54]. In particular, Diels-Alder reaction of (hetero)dienes with (hetero)dienophiles is extensively used at the early stages of numerous syntheses to establish a structural scaffold which is then usually further elaborated toward more complex target structures. The [4+2] cycloaddition is usually an efficient method with predictably high regio and stereoselectivity as such it can enable the synthesis of highly functionalized polycyclic systems. 

Liao s group [15] used the masked benzoquinone to trigger intramolecular Diels-Alder reaction and completed a series of total synthesis (Fig. 2.14). In the total synthesis of bilosespenes A and bilosespenes B, they used oxidative dearomatization to construct masked benzoquinone. With the allyl alcohol, they triggered oxidative dearomatization of the phenol substrate using iodosobenzene diacetate. At the same time, they completed the allyl alcohol protection to benzoquinone. And the intramolecular Diels-Alder reaction happened between the double bond of side chain and the diene in bone structure, and [2.2.2] caged scaffold system was constructed. 

Andreana and co-workers [188] developed a one-pot method for generating molecular diversity via a multicomponent coupling reaction under microwave irradiation. The initially formed Ugi four-component coupling products gave rise to three structurally distinct scaffolds depending on the solvent effects and sterics the 2,5-diketopiperazines 137, the 2-azaspiro[4.5]deca-6,9-diene-3,8-diones 138, and the thiophene-derived Diels-Alder tricyclic lactams 139 (Scheme 107). 

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