Radicals from Oxygenated Furans

There has been an ESR study of the persistent annelated radical (13), obtained by thermolysis of 2,2 -dioxo-3,3 -diphenyl-2,2, 3,3 -tetrahydro- 

Interestingly, electrochemical reductions of alkylidenephthalides (14), which might conceivably give radicals isomeric in the heterocyclic moiety with 13, resulted either in the addition of two electrons or in ring scission, although phthalide radicals such as 15, which is not heteroaromatic, may be prepared.  

Furan-2,5-semidione has been incorporated as a spin-label into a wide variety of structures, both conformationally labile and rigid (e.g., 17-19) 

Karaflloglou, J.-P. Catteau, A. Lablache-Combier, and H. Ofenberg, J. C. S. Perkin II, 

Hrnciar, and M. Lacova, Collect. Czech. Chem. Comtmm. 37, 3295 (1972). 

4-dimethylfuran-2,5-semidione which exhibits methyl splittings of 6.1 G. The ready reduction of maleic anhydride had been noted by Peover and by Takahashi and Elving. The latter compared the polarographic reduction of maleic anhydride and maleic esters in pyridine. They noted that the reduction wave for the anhydride occurred at significantly more positive potentials than that for the diester, a fact they correlated with the conjugated cyclic (aromatic) nature of the radical produced from the anhydride.  

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In the above condensation resist designs, the phenolic resin offers a reaction site as well as base solubility. Self-condensation of polymeric furan derivatives has been utilized as an alternative crosslinking mechanism for aqueous base development (Fig. 126) [375]. The copolymer resist is based on poly[4-hydroxy-styrene-co-4-(3-furyl-3-hydroxypropyl)styrene], which was prepared by radical copolymerization of the acetyl-protected furan monomer with BOCST followed by base hydrolysis. The furan methanol residue, highly reactive toward electrophiles due to a mesomeric electron release from oxygen that facilitates the attack on the ring carbons, readily yields a stable carbocation upon acid treatment. Thus, the pendant furfuryl groups serve as both the latent electrophile and the nucleophile. Model reactions indicated that the furfuryl carbocation reacts more preferentially with the furan nucleus than the phenolic functionality. 

Studies of lignoceUulosic biofuel model compounds have likewise increased significantly in the past decade. Reaction pathways with implications for soot, aldehydic, and emissions from oxygenated and otherwise-functionalized fuels have been examined. Several key intermediates in pyrolytic pathways have been identified, including furanic carbenes, furanylmethyl radicals, dihydrofurans, and unsaturated ketones and aldehydes. Low-temperature combustion reactions available to these compounds are largely unexplored and, where modeled, involve highly functionalized per-oxy radicals with the capacity for novel reactions. As cycHc oxygenated species break down into smaller acyclic species, some potential arises for overlap with extant mechanisms for HC combustion. Composite ab initio and DFT calculations have been proved to be particularly useful in recent computational explorations of these compounds and their reaction pathways. 

The synthesis of oxygen heterocycles in which cyclization onto a pendant alkyne is a key step has also been achieved. Reaction (7.36) shows an example of iodoacetal 29 cyclization at low temperature that afforded the expected furanic derivative in moderate Z selectivity [47]. A nice example of Lewis acid complexation which assists the radical cyclization is given by aluminium tris(2,6-diphenyl phenoxide) (ATPH) [48]. The (3-iodoether 30 can be com-plexed by 2 equiv of ATPH, which has a very important template effect, facilitating the subsequent radical intramolecular addition and orienting the (TMS)3SiH approach from one face. The result is the formation of cyclization products with Z selectivity and in quantitative yield (Reaction 7.37). 

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