Tautomeric isomers

Recently, Nesmeyanov and co-workers have published definitive evidence that dwaZ reactivity (the formation of derivatives of two different structural formulas) extends beyond tautomerism (isomers in equilibrium or reversible isomeric transformation). A single molecular species can form two series of derivatives, in one of which a transfer of the reaction center occurs in the reaction. 

Interestingly, the tautomeric isomer 44 was predicted to be a strong to moderate sensitizer with a probability of 90%, which was greater than that obtained for thiadiazole 21 (70%) <2003CRT1226, 2004JCI688>. 

Formally, protonation of 6 might lead to five different acids according to Scheme 1. The most stable acid is formaldoxime. All tautomeric isomers can formally be derived from either 1,3- or 1,2-hydrogen shifts (pathway A or B, Scheme 2). ... 

Acids of DNCM. The most stable form of DNCM acids is the oxime (Figure 28), while all other species, protonated either at the central C atom (dinitrosocyanomethane dinitrosoacetonitrile) or at the nitrogen of the cyano group, represent high-lying tautomeric isomers. Neither of the acids of DNCM is experimentally known. 

When dihydro-1,3-benzothiazine 203 was allowed to react with KF in DMF (or NaOMe in MeOH), an open ring tautomeric isomer 204 was formed. When 2-bromoacetophenone 169 (R = H) is present in the reaction medium, a mixture of two diastereoisomers of 206 was obtained (75%) after enolate/imine addition of intermediate 205 with a slight excess of the m-isomer 206a over the trans-isomer 206b (Scheme 35). The same result was obtained by a synthesis from [6+1] fragments, reported in the next section <1995TL753>. 

Enol The tautomeric isomer of a ketone or aldehyde, e g. the enol of propanone is CH2=C(OH)... 

A short time after the discovery of pyrazolin-5-ones Knorr809 proposed the 2-pyrazolin-5-one and the 3-pyrazolin-5-one structures on the basis of analyses, methods of preparation and reactions. These structures in the main approximate the correct ones and in some cases are correct. However, a large number of structures are theoretically possible for most pyrazolin-5-ones and in many cases no one structure can be said to fit completely. The tautomeric isomers (VII)-(XIII) are possible for unsubstituted pyrazolin-5-ones ... 

In addition a number of ionic tautomeric isomers can be envisaged which could make very important contributions to the over-all structure. These are (XIV)-(XVIII). Substitutions at N-l, N-2 and disubstitution at 0-4 substantially alter the possibilities for various tautomeric... 

By thermolysis or photolysis of appropriate diazo compounds, various substituted cy-anocarbenes can be generated. The addition of these carbenes to benzene gave 7-substituted bicyclo[4.1,0]hepta-2,4-diene-7-carbonitrile/cyclohepta-l, 3,5-triene-7-earbonitrile 9/10, valence-tautomeric isomers. ... 

In some cases nonaromatized adducts are isolated. The reaction of isoquinoline IV-imine17,190 and phenanthridine iV-imine17,33 with dimethyl acetylenedicarboxylate gives tautomeric isomers of the primary adducts, e.g., 61. These IV-imines also react with iV-phenylmaleimide to give the primary 1 1 cycloadducts, e.g., 62.33,190 Of particular interest is the reaction of isoquinoline IV-imine with mesityl oxide which produces 63. This reaction involves disproportionation and elimination of methane from the primary adduct.17... 

As is apparent from Figure 1, SB shows two absorption bands with maxima at 400 and 318 nm, which are assigned to two tautomeric isomers, C and D in Scheme 2, respectively [7]. Since the relative intensities of these bands sensitively vary depending on the medium polarity, the microenvironmental polarity provided by the octopus cyclophane can be estimated. Figure 2 shows electronic absorption spectra of the SB species formed with PLP and octylamine in the presence and absence of 1, While SB is present exclusively in form C in an aqueous phase, form D is remarkably favored in the presence of 1 the microenvironmental polarity in the cavity of 1 is roughly equivalent to that provided by 2-propanol. Accordingly, it is apparent that PLP bound to the hydrophilic site of the octopus cyclophane moves into a more hydrophobic domain of the host molecule via formation of SB with the hydrophobic alkylammonium substrate as schematically illustrated in Figure 3. 

Abstract. Very successful stereo-, regio-, and enantio-controls of the photoreaction of a guest compound were achieved by irradiation of the host-guest complex in the sohd state. In order to discover the reason for the successful control, the X-ray crystal structure of the complex was studied. The complexation method was also effective to freeze the equilibrium in solution and isolate the labile tautomeric isomer as an inclusion complex. 

Azirines and Aziridines.— The photolysis and thermolysis of the azido ester (143) to the azirine (144) was described by Hassner and Fowler in 1968. A study of isomeric compounds (145) has now been described, leading to the tautomeric isomer (146) of the antibiotic azirinomycin (147). 3,5-Dimethylaminoisoxazoles (148) have been prepared by the addition of hydroxylamine to phosgene iminium salts. The photoisomerization of the 4-substituted isoxazoles (148) to the azirines (149) is already known, and a paper has now described the near-quantitative thermal... 

Similar irradiation of 1,3.5,7,9- and 1,3,6,8,10-pentamethylcyclooctapyrimidine-2,4-diones (2fi, 2fii) gave the corresponding tautomeric isomers (7fi, 7fii). Upon being kept in the dark at room temperature, the former (7fi) (half Kfe, 30 min) was transformed into cyclobutaquinazohne (llfi) with competitive reversion into 7fi, while the latter (7fii) reverted to 2fii with half-life of 8 h. 

These results substantiate the mechanism that pentacydic compounds (12x) are derived from the tautomeric isomer (7x) by way of the semibuUvalene intermediate (15x) followed by rearrangement to pentalenopyrimidine (16x). Thus, the present work demonstrates that pentacyclo[6.4.0.0. > 0. 0 ]dodecanes (3x)... 

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