Synthesis and Tautomerism

Erbium(III) triflate is an efficient catalyst in the synthesis of aldimines, ketoimines, and enaminones.15 For aromatic imines, the problem of Michael addition found with the CeCE/Nal-catalysed addition to unsaturated aldehydes is avoided. 

An A-phosphinoylhemiaminal (3) has been used as a precursor to trifluoromethyl-ketimines (4) in situ alkylation with a dialkylzinc in the presence of a diphosphine monoxide auxiliary gives chiral , ,o -trifluoromc(.hyl amines (5) in high yield and ee up to 99%.16 

The Skraup-Doebner-Von Miller synthesis of quinolines - involving condensation of an aniline with an ,/i-unsalurated ketone - has been investigated using 13C-labelled ketones in cross-over experiments a complex fragmentation-recombination mechanism involving imine intermediates is indicated.17 

In another synthesis of quinolines involving imine intermediates, o-oxazoline-substi-tuted anilines (6) react with ketones in dry butanol reflux to give 4-amino-substituted quinolines [e.g. (7)], or 4-quinolones, using tosic acid as catalyst.18 A mechanism involving ketoimine formation with subsequent tautomerization to give an enamine which attacks the oxazoline ring is discussed. 

A related one-pot, three-component synthesis of fi-amino carbonyl compounds has been achieved using a cascade reaction of anilines with aromatic aldehydes and car- (g) bonyl compounds, catalysed by zinc triflate.19 

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Dolzhenko AV, Pastorin G, Chui WK (2009) An aqueous medium synthesis and tautomerism study of 3(5)-amino-1,2,4-triazoles. Tetrahedron Lett 50 2124-2128... 

Fromm, K., Nikolova, R., Shivachev, B., and Nikiforova, M. (2012) Switching azonaphthols containing a side chain with limited flexibility. Part 1. Synthesis and tautomeric properties. Dyes Pigments, 52, 3—14. 

No data on tautomerism of dihydropyiimidines were available at the time of the early summary (76AHCS1), but much has been done since then. The results of tautomeric studies carried out during the period between 1976 and 1984 were reviewed comprehensively in [85AHC(38)l,pp. 63-77]. Later, Weis and vanderPlas published an excellent review on the synthesis, structure, and tautomerism of dihydropyrimidines [86H(24)1433], where the tautomeric interconversions of these compounds were discussed in detail. In a more recent review on dihydropyrimidines (94MI1), the question of tautomerism in partially hydrogenated pyrimidines was also included. 

Triazenes obtained from arenediazonium ions with secondary aliphatic or aromatic amines are not subject to tautomerism. They are used as stable sources of arenediazonium ion in organic synthesis and in technological applications. 

The structure of the modified flavocoenzymes was elucidated by chemical synthesis and comparative physical studies 126, iso) (Scheme 2, (7), (S)). The compounds possess some unusual properties some of which are collected in Table 3. The most prominent difference between (7) and (S) is the fluorescence behaviour (7) is fluorescent, (5) does not fluoresce. Moreover, at pH > 10 the fluorescence quantum yield of (7) increases by a factor of about 2, in contrast to normal flavin the fluorescence of which is quenched. By this property (7) and (5) are easily distinguished (Table 3). From the visible absorption properties of analogs of (7) and (5) it was concluded that both compounds can exist in two tautomeric forms (proton on N(l) or C(8)O, C(6)O), leading to quinoid structures. 

Employing a staked plate microreactor (channel dimensions = 100 pm, total volume = 2 ml), Acke and Stevens (2007) reported the continuous flow synthesis of a series of chromenones via a multicomponent route consisting of a sequential Strecker reaction-intramolecular nucleophilic addition and tautomerization, as depicted in Scheme 2. 

Synthesis and ring-chain tautomerism of 2-amino-l,3,4-thiadiazolines 93KGS991. 

Several papers have dealt with 1,2,4-triazolo[3,4-fc][ 1,3,4]thiadiazines including synthesis of new derivatives and tautomeric studies <02SC3455 02ZN(B)552 02PS487> and a one-pot synthesis starting from 4-amino-5-substituted-l,2,4-triazole-3-thiones involving a solid acid-induced cyclocondensation <02PS2403>. 

A simple procedure for the synthesis of 3-ethoxycarbonylbenzofurans from salicylaldehydes and ethyl diazoacetate has been developed. The reaction is believed to occur via a semipinacol rearrangement and tautomerization to a /3-hydroxy acrylate which is trapped by the adjacent phenoxy group (Equation 118) <2006S1711>. 

An important matter connected with Mannich synthesis is the possibility of obtaining stereoi-someric products and/or derivatives exhibiting tautomerism. The preparation of enantiomeric products as well as their racemizaiion, along with some uses of optically active Mannich bases in research, are here described. The main results of the. studies of conformational and tautomeric equilibria in Mannich bases arc also highlighted. 

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