Quinonoid compounds, isomerization

The azaindolenines have been discussed by Ficken and Kendall, who have pointed out that protonation, as well as methylation, of the 7 isomer (31. Scheme 7) occurs at N(7>, which can be seen from the similarity of the acid spectrum of the 2,3,3-trimethyl compound (31) and the alcoholic spectrum of the 7-methiodide (117). The behavior of the 7-methiodide in alkaline media is distinctly different from that of 1,2,3,3-tetramethyl indolenine iodide, which with alkali gives rise to the 2-methylene indoline, the spectrum of which was determined independently and is in agreement with this transformation, although the band appearing at ca. 300 m/i has not been reported in all cases. The bands indicated in Table VII for the MeOH/HCl spectrum of this compound should probably be stepped to the left. The large red shift shown by the 7-methiodide (117) is due to isomerization to the orfAo-quinonoid compound (118). This does not take place in the hydrocarbons. 

Between 1951 and 1953 investigations by three English groups clearly demonstrated the preponderance of the oxo forms of pyrimidin-2- and -4-ones by comparing the ultraviolet spectra of these compounds with those of the N and 0-alkylated derivatives, The o-quinonoid form 91 (R = H) is favored by the evidence that A -methylation of the 6-methyl derivative of 89a does not cause a bathochromic shift in the ultraviolet spectrum (A -methylation of pyrid-4-one causes a bathochromic shift, but this is not observed for pyrid-2-one) The isomeric A -methyl derivatives of pyrimidin-4-ones [e.g., 91 (R = Me) and 90 (R = Me)] form similar cations (e.g., 92 and 93), and hence the equilibrium constant between... 

Quinonoid (6.R)-dihydrobiopterin (45) was expected to be a labile compound which readily isomerizes to 7,8-dihydrobiopterin (44). When a so-... 

The UV spectra of isomeric pairs of pyrazolo[3,4-d]pyrimidine-4,6-diones and pyrazolo[4,3-<7]pyrimidine-5,7 diones show that the isomers with a [4,3-d] ring fusion absorb at longer wavelengths (red shift) compared to the [3,4-d] isomers <82IJC(B)585>. The />-quinonoid structure of the pyrimidine moiety in compounds (74) and (75) causes a bathochromic shift of 20-25 nm, relative to compounds (53) and (76). The close values of 2max for 7-methyl (77 R = Me) and 2,7-dimethyl allopurinol (74) suggest that the former is present in aqueous solutions as the A(2)-H tautomer <79JCS(Pl)2795>. 

New 7-substituted biopterin (L-primapterin) (361) and neopterin (anapterin) (360) analogues were discovered in 1988 as excretion products in the urine of patients suffering from a new form of atypical phenylketonuria <88BBR(153)715,88MI 718-03). This interconversion proceeds most probably on the quinonoid dihydrohydropterin level and is due to a 4a-carbinol-amine dehydratase defect allowing isomerization via an intermediary spiro-compound <90BBR(172)1060>. 

J With enamines. l-Oxa-2-azulenones (as tethered heptafulvenes) with enamines undergo [8+2] cycloaddition as well (85HOU(5/2c)127, p. 218 98SL950). This way, in the case of heterocyclic enamines (such as 40), azuleno[l,2]-fused azoles and azines are formed (Schemes 13 and 14) The reactions of oxaazulenone 37 with the isomer mixtures of enamines 40a or 40b yield mixtures of isomeric dihydrothiophenes 41 and 42 that are dehydrogenated by 2,3-dichloro-5,6-dicyano-p-benzoqui-none (DDQ) to yield thiophenes 43 and 44, respectively (83CL1721). Whereas the latter [l,2-c]-fused compound is unstable because of its o-quinonoid structure, thiophenes 43a and b are demethoxycarbonylated to yield substances 45a and b. 

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