Diels maleimide moiety

Several highly enantioselective Diels-Alder reactions are known for which the di-enophile does not fit any of the above classes. Corey and coworkers applied the chiral aluminum reagent 36 with a C2-symmetric stilbenediamine moiety (videsu-pra) to the Diels-Alder reaction of maleimides as dienophiles [54] (Scheme 1.68). In most asymmetric Diels-Alder reactions the reactants are usually relatively simple dienes such as cyclopentadiene or monosubstituted butadienes, and unsym-... 

An example of the second type of modification is the application of the Diels-Alder cycloaddition reaction to polders and copol ers containing pendant or backbone furan moieties. The use of bis-dienophiles such as propiolic acid and its esters or bis-maleimides provides a means of crosslinking based on multiple bridging by the double interchain lycloadditions. The thermal reversibility of these reactions allows the return to the original linear structure (thermoplastic material) by simply heating the gel. 

The technique is also used to introduce diene moieties, which enable the polymer for a highly efficient modification with dienophile components via Diels-Alder reaction [354], Therefore, methyl octa-4,6-dienoate is converted to octa-4,6-dienoic acid hydrazide, which is bound to dextran via reaction with BrCN. The maleimide-modified protein (albumin) is easily attached to the polymer backbone by Diels-Alder reaction (Fig. 50). 

Azo compounds like esters or imides of azo dicarboxylic acid act as reactive dienophiles in normal electron demand hetero Diels-Alder reactions due to the strong activation caused by two electron-withdrawing moieties. In the last years, considerable attention has focused on alkyl and phenyl derivatives of 1,2,4-tria-zoline-3,5-diones since their cycloadditions to chiral dienes proceed with often excellent facial selectivities. Thus, when reacting an oxapropellane derived diene with N -methyltriazolinedione, Paquette et al. obtained the cycloadduct as single diastereomer, but both maleic anhydride and N-phenyl maleimide were distinctly less reactive and turned out to undergo cycloadditions with poor selectivities [289]. 

Interestingly, activation of the 2-methylene unit with the ester moiety was not required for the successful outcome of the above Diels-Alder reactions. Thus, the indole analogs 343a-e participated in [4+2] cycloaddition reactions with maleimides 171,220, and 344 to produce the cycloadducts 345a-e in 40-75% yields (Scheme 73). 

Boydston and co-workers recently revisited the mechanochemical chain scission of star-shaped polymers in acoustic fields using a fluorogenic tum-on mechanophore (Fig. 30a) [232]. They synthesized PMA-L 61.6 and PMA-S3 28.6/ 85.9 with Mspan = 2Maim = 57.2 kDa (Fig. 30b). In both samples they linked one of the PMA arms to the core by an anthracene-maleimide Diels-Alder adduct. This mechanophore was previously studied by Bielawski and co-workers [57, 58]. Upon cycloreversion by mechanical force, it produces an anthracene moiety which displays strong UV-Vis and photoluminescence signals. 

Fig. 30 (a) Anthracene-maleimide Diels-Alder adduct that can generally fluoresce an anthracene moiety upon mechanical force, (b) Mechanophore-containing star and linear polymers bearing similar (c) Photoluminescence spectra in DMF of PMA-S3 28.6/85.9 upon increasing... 

The feasibihty of the Diels-Alder coupling was shown on the example of biodegradable polyester by Ninh and Bettinger [47]. In this case, hyperbranched poly(glycerol-co-sebacate) with pendant furan groups was coupled with a bifunctional maleimide cross-linker to produce an elastomeric material. POSS moieties may serve as cross-linking sites, as well. Ishida et al. [48] have shown how a... 

Gandini and co-workers described a unique double click strategy related to the preparation of monomers based on vegetable-oil derivatives bearing furan heterocycles appended through thiol-ene click chemistry, and their subsequent polymerisation via the Diels-Alder (DA) polycondensation between furan and maleimide complementary moieties (i.e., a second type of click chemistry). Details about the DA reaction, its mechanism, applications and the reason why it is classified as a click reaction can be found in Chapter 7. 

Recently, Diels-Alder reaction between maleimide-containing NLO chro-mophores and pendant anthracenyl moieties of the polymer chain has been used in the context of post-functionalization approach [69], This is particularly interesting in view of the mild reaction conditions, quantitative yield and absence of ionic species and catalyst. 

Ethylene-co-allene polymers, as prepared with (iso-Bu)3Al-VCl3 catalyst, have side-chain methylene, side-chain vinyl, in-chain cis, and in-chain trans unsaturation.The side-chain methylene moiety appears to predominate in the copolymer. The copolymer is easily grafted with MA via the Diels-Alder reaction. Films of the grafted material, in comparison to ungrafted copolymer, exhibited greatly enhanced printability, due to the increased polarity. Polyal-lene has also been treated with MA and maleimides to obtain Diels-Alder type grafting and new polymers for possible use in coatings. Little is known about the properties of these modified polymers and copolymers. 

Furan or maleimide group was introduced to polyMeOZO as a diene and a dienophile moiety, respectively. Intermolecular Diels-Alder reaction between these two groups caused the gelation at room temperature (Scheme 8). The obtained gel absorbed water to form a non-ionic hydrogel [15]. 

In 2008, the first asymmetric Diels-Alder reaction of 3-vinylindoles 91 and maleimides 80 with a bifunctional thiourea catalyst (92) was realized by Bernardi and coworkers (Scheme 38.25) [39]. The resultant highly-enantioenriched fused heterocycles are very useful for the synthesis of biologically important natural or unnatural alkaloids. As shown in the proposed transition state N, catalyst 92 probably activates the diene with the basic moiety and activates the dienophile with the thiourea moiety through hydrogen bonding interactions. 

The reported organic bifunctional catalysts possessing both hydrogen bond donor and acceptor moieties are often derived from the cinchona alkaloids. Tan and coworkers developed the simple and readily available amino-indanol 95 as an efficient bifunctional organocatalyst for the enantioselective Diels-Alder reaction of 3-hydroxy-2-pyridones 94 (Scheme 38.26) [40]. Besides maleimides 80, alkyl vinyl ketones were also suitable dienophiles for this catalytic system. 

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