Tautomerism aromaticity

Benzo[b]furan, 2-ethyl-3-(4-hydroxy-3,5-diiodobenzyl)-applications, 4, 708 Benzo[b]furan, halo-reactions, 4, 650 Benzo[b]furan, 2-hydroxy-tautomerism, 4, 36 aromaticity and, 4, 595 Benzo[6]furan, 3-hydroxy-tautomerism aromaticity and, 4, 595 Benzo[b]furan, 5-hydroxy-synthesis, 4, 127... 

Furan, 2,3-dihydro-5-methyl-polymers, 1, 276 Furan, 2,3-dihydro-3-methylene- H NMR, 4, 577 Furan, 2,5-dihydro-2-methylene- H NMR, 4, 577 tautomerism aromaticity and, 4, 595 Furan, 2,5-dihydro-2-nitro-structure, 4, 550 Furan, 2,3-dihydroxy-tautomerism, 4, 37 Furan, 2,4-dihydroxy-tautomerism, 4, 37 Furan, 3,4-dihydroxy-tautomerism, 4, 37 Furan, 2,5-diiodo-nitration, 4, 602 synthesis, 4, 712 Furan, 3,4-diiodo-reactions, 4, 650 Furan, 2,3-dimethoxy-synthesis, 4, 625, 648 Furan, 2,5-dimethoxy-synthesis, 4, 648 Furan, 3,4-dimethoxy-cycloaddition reactions, 4, 64, 625 lithiation, 4, 651 reactions... 

Only those compounds which do not have tautomeric aromatic triazole structures will be considered here, the others having been treated as triazoles. The triazolines are unstable and have been subjected to little study. Compounds which are disubstituted at the C(3) or C(5) atom are more stable than the mono- or unsubstituted analogues. The equilibrium has been observed by NMR spectroscopy between the six-membered tetrazine (75) and the triazolinethione (76) via the open-chain form, thus mirroring monosaccharide equilibria (Scheme 12) <90TL3927>. 

Again, as with the triazolines, only those compounds which do not have tautomeric aromatic triazole structures would be treated here and these systems are unstable and have been even less studied than their dihydrocounterparts. 4-Phenyltriazolidininedione is a precursor of PTAD (see Section 4.02.6.3). 

Although 4-phenyltriazolinedione (PTAD) has a tautomeric aromatic triazole structure it has been treated separately here because of its distinct and extensive chemistry. 

Leszczynski et al. have shown interest in comparing experimental (matrix isolation) and calculated (B3LYP) IR data (frequency and intensity) to discuss the tautomerism of benzo-annelated pyridonone, pyrazinone, and pyrimidinone (144-148). These equilibria were well reproduced by theoretical calculations carried out at the QCISD and QCISD(T) levels. The combined experimental and theoretical results reveal links between aromaticity and tautomerism. Moreover, a UV-induced phototautomeric reaction transforming the oxo forms into the hydroxy tautomers was observed for all (except 3-hydroxyisoquinoline) studied compounds [144], The interest of Leszczynski in problems related to tautomerism, aromaticity, and proton transfer is also apparent in a study of (lH-aza-hetero-2-ylidene)-acetaldehyde and 2-azahetero-2-yl-ethanol tautomeric pairs [145],... 

Nitrenium ions may occur during bioactivation of aromatic amines and amides, which are usuaUy A-oxidized into A-hydroxylated derivatives. By sulfation or esteriUca-tion foUowed by elimination of the newly formed leaving group, the latter may be transformed into highly reactive nitrenium ions. In the case of aromatic nitrenium ions they are in equiUbrium with their tautomeric aromatic carbocations, which react with ceUular nucleophiUc macromolecules (nucleic acids, etc.). 

Dihydro-1,2,4-triazines with two double bonds in the ring and no tautomeric aromatic structure are well-known compounds <78HC(33)189>. The x-ray crystal structure of2-methyl-3-methylthio-5,6-diphenyl-2,5-dihydro-1,2,4-triazine has been published <8iBCJ4l, 84CHEC-i(3)369>. However, only a few studies on the reactivity of dihydro-1,2,4-triazines have been reported during the last decade. On the other hand, many papers on the reactions of 1,2,4-triazinones with a tautomeric aromatic structure can be found in the literature of the same period. 

Furan, 2-(p-hydroxyphenyI)-2,5-dihydro-synthesis, 4, 677 Furan, 3-hydroxytriphenyl-tautomerism, aromaticity and, 4, 595 Furan, iodo-synthesis, 4, 712 Furan, 2-iodo-Grignard reagents, 4, 79 Furan, 5-iodo-2-nitro-synthesis, 4, 711 Furan, 2-isopropyl-cycloaddition reactions with ff, ,a, ff -tetrabromoacetone, 1, 418 Furan, 2-lithio-5-methyl-synthesis, 1, 419 Furan, 2-mercapto-tautomerism, 4, 38 Furan, 3-mercapto-tautomerism, 4, 38 Furan, methoxy-planarity, 4, 544 synthesis, 4, 664 Furan, 2-methoxy-synthesis, 4, 648 Furan, 3-methoxy-Diels-Alder reaction, 1, 416 synthesis, 4, 649... 

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