Configuration about

The R, S convention is a scheme which has largely superseded the D, i. system to denote configuration about a chiral centre in a molecule. The convention allows unequivocal designation of the absolute configuration in a description of the positions in space of ligands attached to a chiral centre, in relation to an agreed standard of chirality like a right-hand helix. 

Molecules with an inversion vibration which is qualitatively similar to that of NH3 are formamide (NH2CHO) and aniline (C6H5NH2) in which the vibration involves primarily the hydrogen atoms of the NH2 group. Both molecules are non-planar, having a pyramidal configuration about the nitrogen atom, with barriers to planarity of 370 cm (4.43 kJ moP ) and 547 cm (6.55 kJ mon ), respectively. 

Initial attempts to synthesize the compounds (22) were hampered by the failure to obtain the correct stereochemical configuration about the vindoline—catharanthine linkage, a most difficult problem eventually solved by insight and hard work (83). 

Maleic and fiimaric acids have physical properties that differ due to the cis and trans configurations about the double bond. Aqueous dissociation constants and solubiUties of the two acids show variations attributable to geometric isomer effects. X-ray diffraction results for maleic acid (16) reveal an intramolecular hydrogen bond that accounts for both the ease of removal of the first carboxyl proton and the smaller dissociation constant for maleic acid compared to fumaric acid. Maleic acid isomerizes to fumaric acid with a derived heat of isomerization of —22.7 kJ/mol (—5.43 kcal/mol) (10). The activation energy for the conversion of maleic to fumaric acid is 66.1 kJ/mol (15.8 kcal/mol) (24). 

Phosphoric acids and the phosphates maybe defined as derivatives of phosphoms oxides where the phosphoms atom is in the +5 oxidation state. These are compounds formed in the M2O—P20 system, where M represents one cation equivalent, eg, H", Na", 0.5 Ca ", etc. The molecular formula of the phosphoms(V) oxide [1314-56-3] is actually P O q, but this oxide is commonly referred to in terms of its empirical formula, P2O5. StmcturaHy, four phosphoms—oxygen (P—O) linkages are arranged in an approximate tetrahedral configuration about the phosphoms atom in the phosphate anion. Compounds containing discrete, monomeric PO ions are known as orthophosphates or simply as phosphates. 

The stereochemistry of pyrazolidines has briefly been discussed in Section 4.04.1.4.3. The pyrazolidine ring in (429) in the crystal state is not planar and the configuration about the amide bond is E as represented. 

Aldoses with at least three carbons and ketoses with at least four carbons contain chiral centers (Chapter 4). The nomenclature for such molecules must specify the configuration about each asymmetric center, and drawings of these molecules must be based on a system that clearly specifies these configurations. 

When 10 (R configuration about phosphorus) was treated with ribo-nuclease in aqueous methanol, nucleophilic attack by methanol in the enzyme-catalyzed process led to formation of a methyl ester, 11, which has been shown by X-ray analysis to be the isomer with the R configuration about the phosphorus (60) ... 

Selectivity is often conferred by on-board functionality, as in the a-sulfenyl-directed ringopening of l-phenylthio-2,3-epoxyalkanes (e.g., 74) with trimethylaluminum, a reaction which gives exclusively C-2 alkylat products with complete retention of configuration about C-2. 

The synthesis of the first optically active compound is illustrated in Eq. (16) it was obtained with retention (>85%) of configuration about silicon 223). 

Ribulose 5-phosphate is the substrate for two enzymes. Ribulose 5-phosphate 3-epimerase alters the configuration about carbon 3, forming another ketopentose, xylulose 5-phosphate. Ribose 5-phosphate ketoisom-erase converts ribulose 5-phosphate to the corresponding aldopentose, ribose 5-phosphate, which is the precursor of the ribose required for nucleotide and nucleic acid synthesis. Transketolase transfers the two-carbon... 

The complex IrL3, HL = 8-quinolinethiol (212), has been prepared from an aqueous solution of (NH4)3[IrCl6] and NaL. An X-ray diffraction study confirms the mer configuration about the Ir... 

The first letter represents the conformation about the carbon-carbon single bond and the second letter represents the configuration about the carbon-carbon double bond. 

These differences reflect the conformations of (+)- and meso-isomers as they sit at the air-water interface. What is much harder to elucidate is the effect of stereochemistry on intermolecular interactions. How does changing the stereochemistry at one chiral center affect interactions between diastereomers Ab initio molecular orbital calculations have been used to address the problem of separating stereochemically dependent inter- and intra-molecular interactions in diastereomeric compounds (Craig et al., 1971). For example, diastereomeric compounds such as 2,3-dicyanobutane exhibit significant energetic dependence on intramolecular configuration about their chiral centers. So far, however, little experimental attention has been focused on this problem. 

Compound (6) has the structure shown in Fig. 4(d). It formally has 14-electron configurations about each metal atom and all fourteen electrons are involved in the bonds represented by lines in the structural drawing. Either the U2-CO group makes no appreciable demand for tt electron density or there may be an indirect feeding of electrons from the lone-pair orbitals of the alkoxide groups to y2-C0. Alkoxide groups are known to be 7r-electron donors. 

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