Configuration compounds, proof

Both methods of approach moreover proved that a series of methylated derivatives of D-glucosamine described by Cutler, Haworth and Peat,14 and most useful as reference compounds in this field, were all of the pyranose configuration. Further proof of the pyranose structure of methyl D-glucosaminide and methyl N-acetyl-D-glucosaminide was pro-... 

This reaction is proof of the structure of the dye, easily obtained from the dihydro compound (3). Yields are low as a result of numerous side reactions. The stereochemical configuration of these quinoid dyes is trans. 

Chemical proof for the configuration of the three-membered ring is obtained by conversion of compounds (18) and (19), the known la- and Ifi-methyltestosterone acetates. 

In a study of the infrared spectra of acetylated and benzoylated gly-copyranosyl fluorides, it was found that, with few exceptions, such compounds exhibit an absorption band at 802-748 cm-1 if the fluorine atom is axial, whereas no such absorption band is observed if the fluorine atom is equatorial therefore, it is necessary to know the conformation of the molecule before the anomeric configuration can thus be determined.45 It is possible that reinterpretation of some of these observed absorptions may be necessary, because some of the conformations of some of the glycopyranosyl fluorides are now known to exist in the conformation opposite to that originally anticipated without proof, because of the strong anomeric effect of a fluorine substituent (see Section III,2e). 

Additional proof of these conclusions is obtained from the chemical shifts of the tertiary acetamido resonances in compounds (78) and (13), of which the configuration of the latter has been established chemically (Sect. 2.2). They appear at 7.95 and 8.09 t, respectively (cf. table), clearly indicating an axial orientation. On the basis of these results, it seems... 

Since Dickinson s first determinations, crystal structures of many other complexes of various coordination numbers have been determined. All these investigations and others have provided a complete and direct confirmation of Werner s views to support his indirect configurational proofs obtained during the previous decades by preparation of isomers and resolution of optically active compounds (see Section 1.1.4), and today the terminology and concepts of coordination theory are routinely used in crystallography. 

In 1893 a third important regularity was observed by Nikolai Semenovich Kumakov (1860—1941).107 While investigating the substitution of ligands by thiourea and thioacetamide, Kumakov found that replacement occurs with all the ligands of the cis compound but only with the acid radicals of the trans compound (Scheme 2). Since the two isomers yield different products, this reaction, known as Kumakov s reaction or Kumakov s test, may be used to differentiate cis from trans isomers of dipositive platinum or palladium. Kumakov s classic reaction played a crucial role in Werner s proof of the square planar configuration of Pt11 and in Chemyaev s formulation of the trans effect. 

Of these, 11 is achiral (meso), whereas 10 is chiral. Therefore, by simply determining which oxidation product is optically active, and hence chiral, we can assign the configurations of 8 and 9. Direct comparison of these synthetic aldopentoses with the naturally occurring compounds then could be used as proof of the structure of natural aldopentoses. By this reasoning 8 turns out to be D-arabinose and 9 is D-ribose. 

Nevertheless, when this article is continued after two decades, it should not remain restricted to stereochemical problems. During that time, other very powerful methods will have been developed which are mostly easier and often also more reliable. Particularly, the recent development in X-ray analysis gives a completely reliable proof of configuration valid even for the isolated molecule in the case of conformation it may appear necessary to prove that it is unchanged in solution (see for instance Reference 3). While X-ray is the most reliable method, NMR spectroscopy is the fastest. It still uses some empirical rules and comparison with model compounds, but in a modem version (2-D NMR, NOE) it is also completely trustworthy. Therefore, many recent dipole-moment studies investigated compounds whose steric arrangement was already known, and attention was focused on the electron distribution on the individual bonds, or in conjugated systems. The difference in the point of view may be explained as follows. 

Note added in proof Very recent work by Schrock has extended this tantalum ylide chemistry very considerably and some cyclopentadienyl tantalum methylenes and benzylidenes could be isolated and fully characterized (82b). Among the new compounds the complex (C5H4CH3)2Ta(CH3)(CH2) is particularly noteworthy. Its crystal structure has been determined and the temperature dependence of its nmr spectra has been carefully investigated (17b). The ylidic carbon is in a trigonal planar configuration ... 

Synthesis and Proof of Configuration of Biologically Important, Non-carbohydrate Compounds.205... 

Configurational proofs for 86,90,88, and 91 were obtained by n.m.r. analysis of 92 and 93, and 94 and 95. Compounds 92 and 93 were obtained from 86 and 90, respectively, by successive treatment as follows partial hydrolysis with acid, periodate oxidation, acid hydrolysis, acetonation, and acetylation. Compounds 94 and 95 were similarly prepared from 88 and 91, respectively. The main argument in favor of the structures assigned was that, in 94 and 95, the acetoxy-methyl group is shielded by the phenyl substituent and resonates... 

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