SUBJECTS quinonoid

The synthesis of natural products containing the quinonoid stmcture has led to intensive and extensive study of the classic diene synthesis (77). The Diels-Alder cycloaddition of quinonoid dienophiles has been reported for a wide range of dienes (78—80). Reaction of (2) with cyclopentadiene yields (79) [1200-89-1] and (80) [5439-22-5]. The analogous 1,3-cyclohexadiene adducts have been the subject of C-nmr and x-ray studies, which indicate the endo—anti—endo stereostmcture (81). 

It was subsequently shown that the polymers contain semi-quinonoid structures 47 proposed to arise from a-p coupling of radicals 46 as shown in Scheme 9.15., (M It was also suggested that 47 could be subject to radical-... 

The third chapter of this volume is concerned with mono- and diazaquinones. S. R dl (Prague, Czech Republic) reviews compounds that contain one nitrogen or two nitrogen atoms directly within the quinonoid ring, a subject that has not been previously summarized in a comprehensive manner. This is in distinction to the several specialized reviews of quinones themselves. Heterocyclic quinones, in the sense of compounds in which the quinonoid ring is fused to a heterocyclic ring, were covered in Volume 45 of our series in 1989. 

The p-quinonediimines are the parent substances of the indamine dyes and of the tricyclic quinonoid salts of the phenazine, phenthiazine, and phenoxazine groups which are related to them. (Detailed information on this subject is to be found in special works, e.g. of Nietzki-Mayer and of Bucherer.)... 

Absorption of light. Observation of the absorption of light in different vibration directions may also be useful. Not very much work has yet been done on this subject, but it seems that for molecules containing chromophoric groups such as a polyene chain (—CH=CH—)n, or quinonoid... 

Of the three aromatic amino acid hydroxylases, the reaction catalyzed by L-phenylalanine hydroxylase has been subjected to mechanistic scrutiny most often (B-71MI11003, B-74MH1005, B-76MI11006). Of a number of isomeric dihydrobiopterins that are possible, it is the p-quinonoid dihydrobiopterin (20) that is the coenzyme-derived product in the reaction catalyzed by this enzyme (Scheme 7) (B-71MIH003). (20) is reduced back to (19) by an... 

Anthraquinone derivatives substituted in the 1,5-positions should be subject to steric strain because of the close approach of the substituent groups to the quinonoid oxygen atoms. Several anthraquinone derivatives have been examined crystallographically. The structure of the parent molecule (62) (Sen, 1948) has been refined by Murty (1960) and is planar to within 0-01 A. 

Although not a subject of this chapter, Toney and coworkers have quantitated the reaction coordinate of a PLP-dependent L-alanrne racemase [15]. Despite the expectation that the cofactor provides resonance stabilization of the carbanion/enolate anion (quinonoid) intermediate derived by abstraction of the a-proton, the spectroscopic and kinetic analyses for the wild type racemase at steady-state provided no evidence for the intermediate in the reaction catalyzed by the wild type enzyme. Indeed, Toney had previously demonstrated that a kinetically competent quinonoid intermediate accumulates in the impaired R219E mutant [16] Arg 219 is hydrogen-bonded to the pyridine nitrogen of the cofactor. For the wild type racemase, the derived transition state energies for conversion of the bound enantiomers of alanine,... 

Copolymers of isothianaphthene and thiophene have been the subject of calculations, one of the ideas being that the isothianaphthene would prefer the quinonoid structure whereas the thiophene would prefer the aromatic structure and this could give rise to a lower band gap situation. The calculated bandgaps for 32, 33, and 34 [using the Longuet-Higgins-Salem (LHS) Hamilton-... 

---