Open-chain structure

Thus the following problems are unsolved (1) Does the threefold tautomerism assumed by Gadamer really exist for heterocyclic pseudo bases and which factors determine the stability of the various forms (2) Does ring-chain tautomerism occur in the derivatives of these bases when this is formally possible, and if not, do these derivatives possess the cyclic or open-chain structure, and by what path have they been formed ... 

Aliphatic Hydrocarbons. These are acyclic hydrocarbons with an open-chain structure, which can be either straight (i.e., linear) or branched. The former type are called normal (or n-) aliphatic compounds. Unsaturation is manifested in the form of double or triple bonds. 

Reaction of hydrazine 165 with a number of monosaccharides give hydrazones 166 (92BCJ546). Their acetylations gave peracetyl derivatives 167. The same reactions were extended to the /V-methyl and the 7-methyl derivatives of 165 (93 JPROO). That compounds 166 exist as open-chain structures was based on a 2D NMR study (93MI1) (Scheme 35). 

Ab initio MO calculations at a high level of theory [28] support the open-chain structure for the ground state of S2O while density functional calculations predicted a cyclic ground-state structure [29]. 

A molecule of composition S4O has not been observed yet. Ab initio MO calculations indicate that a cyclic and an open-chain structure have almost the same energy but no details have been released yet [58]. 

From a structural point of view, the carbohydrate template can have either furan or pyran rings although in some cases open chain structures can be formed. A large variety of aldopentoses (e.g. d- and L-arabinose, D-ribose, D-xylose), aldohexoses (e.g. D-glucose, D-mannose, D-galactose) as well as ketohexoses (e.g. D-fructose, L-sorbose) can be used as scaffolds. 

By very careful hydrogenation (with stannous chloride in ethereal hydrogen chloride) phenyl azide has been converted into the exceedingly sensitive phenyltriazene (Dimroth), which, as has been shown, can be reconverted into the former by dehydrogenation. As in the case of the aliphatic diazo-compounds, an open chain structural formula has lately also been assigned to hydrazoic acid and its esters, so that the changes just mentioned may be formulated as follows ... 

Condensation of [3- or "y-amino alcohols with aldehydes or ketones RR CO gives the product 27. In solution the position of the equilibrium varies with R and R, and with the solvent (73). When the carbonyl reactant is a substituted benzaldehyde, the solid is found (IR, KBr) to comprise molecules of the open-chain structure 27a, whereas aliphatic aldehydes and ketones give crystals of dihydro- 1,3-benzoxazines, 27b. An interesting case is that of the condensation product of o-hydroxybenzylamine with cyclopentanone, for which McDonagh and Smith (73) suggest that ring and chain tautomers coexist in the solid. 

The essential role of our concept of ligand in the proper functioning of the Sequence Rule becomes apparent on examining an example taken from the paper by Cahn et al. (4). The authors state that C(3) of their anhydride 25 (15) is symmetric, as in the free acid, and hence receives no label. Both molecules lack symmetry beyond the trivial and ubiquitous one of C,. The center of C(3) of the anhydride is symmetric only in the sense that it is not linked to four different ligands and therefore is not an asymmetric carbon atom as defined by van t Hoff. However, this observation can be made only if the ring ligands are viewed as open-chain structures, as we are defining them, because only these... 

Pyridylcarbonyl)- and 2-(2-quinolylcarbonyl)benzoic acids possess the open-chain structure in the solid state (84KGS1231). In dioxane solution, the ring-chain equilibrium is observed. Protonation of the pyridine or quinoline ring nitrogen atom leads to the formation of the protonated cyclic forms 11. Evidently, protonation stabilizes the tautomer that is the stronger base, i.e., the cyclic form. 

Benzoylpyridine-4-carboxylic (13) and 4-benzoylpyridine-3-carboxylic (14) acids possess the open-chain structure both in the solid state and in dioxane solution. Their hydrochlorides retain the open-chain structure. The IR spectrum of the hydrochloride of acid 14 displays a weak C=0 band at 1805 cm indicating the presence of a small admixture of the protonated cyclic form (86KGS80). 

N-Benzylamides 22 and 23 (R = PhCH2) are isolated as stable cyclic isomers 22B and 23B, whereas N-r-butylamides possess open-chain structures 22A and 23A (R = f-Bu) in the solid state (86KGS80). The hydrochloride of the amide 22A (R = r-Bu) retains the open-chain structure in the... 

Although cyclic structures are unhkely for the sulfur-rich anions, S4N and SsN, because of ring strain, it should also be noted that the tc and 7t levels would be fully occupied in planar rings. In fact, the anions adopt open chain structures in which each terminal sulfur atom accommodates two lone pairs of electrons and the anti-bonding levels of the delocalised 7t-system are only partially occupied. 

Enolates may be derived from a,/l-unsaturated ketones 16 by base-catalyzed proton abstraction. Under kinetic control the a -proton is abstracted and a cross-conjugated metal dienolate is formed, whereas under thermodynamic conditions the extended dienolate is the major product3,, l. Successful alkylations of dienolates derived from cyclic a,/l-unsaturated ketones have been performed (see Section 1.1.1.3.1.1.2.1.). The related a,/ -unsaturated ester systems have also been investigated22-24. Open-chain structures 16 pose a rather complicated... 

The photoaddition of aldehydes to o-quinones is a general reaction leading to the formation of products which are to be regarded as derivatives of methylene ethers of o-dihydroxy compounds (A).198 194 247 Such compounds exhibit ring-chain tautomerism and are generally assigned open-chain structures (B).188 242... 

It is easy to picture the formation of a ring from an open-chain structure if it is remembered that the carbon chain is curving into the plane of the... 

The most well-known tautomeric systems of this type involve a-azidoazoles. Unlike their azine counterparts normally existing as condensed tetrazoles (Sec. 2.2.5.4), they exist as a rule in an open-chain structure, e.g. (255). However, in alkaline media N-anion (256) has a strong tendency to cyclize into tetrazole (257) which often becomes predominant in an equilibrium mixture. 

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