A-tautomer

Aminopyridothiadiazine 2,2-dioxides 126 (X = CR, r = H) were found to exist in water (from UV spectra) and in DMSO-iie (from NMR spectra) exclusively as N(8)H tautomers 126c [86CS607 88AHC(44)81]. This was explained by the strongly acidifying effect of the SO2 function, which makes the formation of a tautomer with the more distant acidic proton energetically more favorable. 

The occurrence of a 5a-C-centered tocopherol-derived radical 10, often called chromanol methide radical or chromanol methyl radical, had been postulated in literature dating back to the early days of vitamin E research,12 19 which have been cited or supposedly reconfirmed later (Fig. 6.5).8,20-22 In some accounts, radical structure 10 has been described in the literature as being a resonance form (canonic structure) of the tocopheroxyl radical, which of course is inaccurate. If indeed existing, radical 10 represents a tautomer of tocopheroxyl radical 2, being formed by achemical reaction, namely, a 1,4-shift of one 5a-proton to the 6-oxygen, but not just by a shift of electrons as in the case of resonance structures (Fig. 6.5). In all accounts mentioning... 

FIGURE 6.5 The hypothetical chromanol methide radical 10 as a tautomer of a-tocopher-oxyl radical 2. 

The proton transfer may occur rapidly after the excitation and form a tautomer, when either acidic or basic moieties of the same molecule become stronger acids or bases in the excited state. The majority of reactions of this type involve the proton transfer from an oxygen donor to an oxygen or nitrogen acceptor, although a few other cases are known, where a nitrogen atom can function as a donor and a carbon atom as the acceptor. Usually an intramolecular hydrogen bond between the two moieties of a molecule facilitates the proton transfer. 

In 5 (6) -substituted benzofuroxans, there are a number of reasons for the preference for a tautomer at equilibrium. Nevertheless, the presence of an electron-withdrawing group in 5(6)-substituted benzofuroxans frequently favors the 6- tautomer over the 5- tautomer. In 4(7)-substituted benzofuroxans the steric interactions between the substituent and the N+-CT moiety play important roles, and thus the 4-isomer is frequently favored at equilibrium. 

Later even more complexity was demonstrated (Makino et al., 1992) in the reaction between DMPO and Fe111 in water. The HO-DMPO" formed was transformed into a hydroxamic acid [24] which is a tautomer of 2-hydroxy-DMPO [25] in a Fenton system transfer of a hydroxyl (cf. p. 133) from the ligand-FeOOH complex to either of these species leads to additional epr-active nitroxyls [26] and [27] in reaction (72). 

The spirocyclic zwitterionic A5S7-silicates 4740,41 and 4842 contain two cis-l,2-diphenylethene-l,2-diolato(2-) ligands. This particular type of ligand formally derives from the diol c/5 -HO(Ph)C = C(Ph)OH, a tautomer of benzoin. 

The ether bond between glucosyl and phenyl was cleaved and glucosyl leaves as a tautomer of 2-hydroxyglucal. 

This observation led to the suggestion that the reacting nucleophile is a tautomer of the anion that is initially formed rather than its decarboxylation product ... 

E 7SB-E occurs, with a tautomer ratio identical to that for the isomers initially dissolved. 

C = compound T = tautomer a) tautomer 1 with shortwave Sq-Si transition tautomer 2 with longwave Sq-Si transition b) values of fluorescence quantum yield were measured for tautomers 1, which were the predominating component in solution at 77 K c) fluorescence of compound 1 is strongly quenched in dimer at 77 K due to the irreversible energy transfer to compound 2. 

The synthesis of cis-1,4 polymers was also tried by e use of monomers with an s-cis conformation. The solid-state photopolymerization of pyridone derivatives, which is a six-membered cyclic diene amide and is a tautomer of 2-hydroxypyridine, was attempted [100]. Pyridones make hydrogen-bonded cocrystals with a carboxylic acid in the crystalline state. Because the cyclic structure fixes its s-cis conformation, if the polymerization proceeds, a cis-2,5 polymer would be obtained. Actually, however, the photopolymerization did not occur, contrary to our expectation, but [4-1-4] photodimerization proceeded when the carbon-to-carbon distance for the dimerization was small (less than 4 A) [101]. A closer stacking distance of the 2-pyridone moieties might be required for the topochemical polymerization of cychc diene monomers. 

During the dehydration of pentoses in acidic solution, the cis and trans 2-enes (homologs of 44 and 46 see p. 177) react rapidly to form the 3-enes, 94a and 94b. Anet8 has presented evidence for the existence of cis and trans forms of the 6-carbon 2-enes. The relative proportion of the geometrical isomers 94a and 94b is controlled by the ratio of the cis and trans forms of the reacting 2-enes and by the rate of interconversion through the tautomer, 72. The trans form (94a) is unable to cyclize, whereas the cis isomer (94b) could readily cyclize to 28 and this could be dehydrated to 2-furaldehyde (27). Formed by extended enolization of 94, the diene 72 has the electronic arrangement needed for cyclization to afford 75a, a tautomer of re-... 

In 1901 Anschutz suggested the denotation tetramic acid for 1,5-dihydro-4-hydroxy-2-pyrrololone 2a, a tautomer of 2,4-pyrrolidinedione la (09LA55). By analogy to the term tetronic acid, denoting the lactone of 4-hydroxy-3-oxo-butanoic acid (60QR292), tetramic acid is used to name the lactam of 4-amino-3-oxo-butanoic acid. (See Fig. 1.)... 

The reaction between DAP and conjugated ynones is reported to give two products (17 and 18), one being a tautomer of the anticipated second product (Scheme 7).49... 

Pyridine reacts with lithium alkyls and aryls under rather vigorous conditions (e.g. xylene at 100°C) to afford 2-alkyl- and 2-aryl-pyridines. The reaction proceeds by way of the corresponding dihydropyridines (e.g. 275 or a tautomer), and these may be isolated at lower temperatures. The less reactive Grignard reagents give poorer yields of the same products. 

Pyridines are more susceptible to reduction than benzenes. Sodium in ethanol or in liquid ammonia evidently reduces pyridine to 1,4-dihydropyridine (or a tautomer) because hydrolysis of the reaction mixture affords glutaric dialdehyde (318 — 317 — 316). Reduction of pyridines with sodium and ethanol can proceed past the dihydro stages to A3-tetrahydropyridines and piperidines (318 — 319 and 320). 

The process of mutation by tautomerization is similar to the excited-state process described here. If a misprint induced by a tautomer takes place during replication, then an error is recorded. Because reaction path calculations of DNA base pairs show similar potential-energy characteristics to those discussed here, we anticipate being able to explore the relevance of tautomerization dynamics to mutagenesis. In this area, we are currently examining these and other systems, also in solutions. 

Adenosine is similar to cytosine in its acid-base chemistry N-l, adjacent to the -NH2 group, is the principal site of protonation. A tautomer of the cation protonated at N-3 is formed in smaller amounts. Guanosine is electronically more complex, being protonated mainly at N-7 and to a lesser extent at N-329. This can be understood in terms of electronic interaction with the adjacent oxygen as indicated in the resonance structure to the right in the following diagram ... 

From the reaction of aminoguanidine with 2-bromoisobutyraldehyde dimethylacetal (522) a product was isolated which was assigned structure (523) or a tautomer thereof (70BSF1606). Semicarbazide (459a) reacts with the glyoxal-bisulfite addition product (524) to give a product formulated as structure (525) (28JA2731). 

CB2093) however, Stolle proposed the alternative structure (277) <07JPR(75)416) and this, or a tautomer thereof, is likely to be correct. 

Carbonic acid derivatives were used in early work to put a hydroxy group on C-5.40,41 Ethyl chloroformate reacts with amidoximes to give a carbonate ester that loses a molecule of ethanol in the cyclization. The expected oxadiazolone (8) is a tautomer of the 5-hydroxy form (9) (see... 

The molecule spiropyran and its derivatives are one of the most widely used photochromic systems. The starting material is colourless, hence near UV light is needed for the photochromic action. The coloured species is a tautomer which reverts to the starting material thermally in the dark (Figure 6.23). 

A second postulated intermediate in the catalyzed reduction of CO by H2 is coordinated formaldehyde which is a tautomer of hydroxycarbene. Early in 1979 Roper (88f) reported the formation and structural characterization of a stable formaldehyde complex, thus providing support for the proposed intermediacy of this species in homogeneously catalyzed CO reduction schemes. The CH20 complex has the specific structure (17d), and eliminates H2 on heating to reform the starting Os(CO)3L2 complex. 

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