Via Diels-Alder Cycloaddition

Stable cA-1-phenyl-1-cyclohexene 24 photodimeiizes via Diels Alder cycloaddition to trans adduct 25 (Equation 1.33) [66] and the photoexcitation of dihydrobenzofuran-fused cyclohexenone 26 in net furan gives the trans fused Diels-Alder adduct 27 (Equation 1.34) [67]. 

Synthesis of functionalized aryloxy 1,3-butadienes and their transformation to dienyl ethers via Diels-Alder cycloaddition reactions [136]... 

Posner G. H. Stereocontrolled Synthesis of Functionalized Cyclohexenes Via Diels-Alder Cycloadditions of 2-Pyrones and 2-Pyridones-Applications to Synthesis of Physiologically Active Compounds in Stereocontrolled Org. Synth. 1994 177, Ed. Trost B. M., Pb. Blackwell Oxford... 

SCHEME 8.10 Engler s synthesis of taxodione 1 via Diels-Alder cycloaddition. 

SCHEME 8.11 Konopelski s synthesis of taxodone 5 via Diels Alder cycloaddition. 

Fig. 38 Synthesis of imino sugars via Diels-Alder cycloaddition on sulfinyl pentadiene. Reagents and conditions (a) CH2Cl2, -78 °C to 0 °C (b) 0s04, NMO (c) DMP, PTSA (d) Pd/C, H2 (e) ClC02Bn.

Nonstoichiometric mixtures were also concocted from BCB and K-353. All were completely compatible, as evidenced by a single initial Tg, except the BCB/dicyanate blends, which showed small exothermic transitions attributable to crystallization phenomenon. The results from the thermal analyses of these blend systems, in particular the K353/BCB system, lend credence to our belief that curing via Diels-Alder cycloaddition may predominate. As such, the blend systems were more stable toward thermo-oxidative degradation than their pure bisdienophile components. 

Scheme 25 Synthesis of ROMP monomers via Diels-Alder cycloaddition of furan with different dienophiles [26]...

A facile synthesis of enantiopure tricyclic furyl derivatives employing 4-vinyl-2,3-dihydrofuran via Diels-Alder cycloaddition reaction was reported <02TL7983>. A new capture-ROMP-release procedure for chromatography-free purification of N-hydroxysuccinimide Mitsunobu reactions was reported by Hanson, who used a Mitsunobu reaction to capture a variety of alcohols onto a norbomenyl A-hydroxysuccinirmde monomer. Treatment of this monomer under ROM-polymerization then generated a water-soluble polymer that was readily separable from other by-products. Subsequent reaction with hydrazine was utilized to release the O-alkylhydroxylamines in good purity from the water-soluble polymer <020L1007>. 

Nucleophilic ring opening of the lactone ring in bicycloadducts of type 5 leads directly to tetrasubstituted cyclohexenes in which the relative stereochemistry of all four contiguous stereocenters is established. Thus, pyrones provide attractive synthetic equivalents to acyclic dienes of type 6 which may be difficult to prepare as pure geometrical isomers and which in many cases do not lead via Diels-Alder cycloaddition to the desired stereochemical relationships. The application of [4+2] cycloaddition reactions of 2-pyrones to synthesizing functionalized cyclohexenes was the partial subject of a 1994 review. ... 

There are two conceivable mechanisms which can accommodate these results, v/z., (i) there are two competitive reactions which give a cyclic product (6) and an acyclic product (7) independently, where the cyclic product is formed via Diels-Alder cycloaddition while the acyclic product is yielded by a simple nucleophilic y-addition of VKSA (1), and (ii) there is a common intermediate complex for both the cyclic and acyclic products and 6 is formed through an addition-cyclization process. 

Stumpe, K., Komber, H., and Voit, B.L. (2006) Novel branched polyphenylenes based on A(2)/B-3 and AB(2)/AB monomers via Diels-Alder cycloaddition. Macromolecular Chemistry and Physics, 207,1825. 

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