| Autor: |
Ganit Levi-Ruso, Robert Glaser, Dror Shiftan, Artem Novoselsky, Shimona Geresh, Itzhak Ergaz |
| Rok vydání: |
2005 |
| Předmět: |
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| DOI: |
10.1016/s0066-4103(05)56003-4 |
| Popis: |
Solution-state 13C and 1H nuclear magnetic resonance (NMR) spectroscopy (including nuclear overhauser effect studies); density functional theory quantum mechanical conformational modeling (and resultant calculations of 13C shielding constants and JHH spin–spin coupling constants) were combined with solid-state cross-polarization/magic angle spinning 13C-NMR spectroscopy of samples whose structures are known by X-ray crystallography to help unravel the extra complications present in cis-cyclooctene and cis-cyclononene medium ring stereochemical investigations. The stereochemistry of medium rings is considerably more complicated than that of the common rings (five to seven members) due to the presence of chiral conformations. An achiral cycloalkane conformation has only two sub-types: a reference conformation and a ring-inverted (inverso) conformation in which corresponding torsion angles exhibit sign-inverted values. The reference and its ring-invertomer lose their structural degeneracy and become diastereomeric when axial and equatorial substituents on a stereogenic ring atom in one conformation exchange their orientations in the other. The reference and ring-inverted structures of chiral conformations (e.g. those for cis-cyclooctene) represent a pair of reference/inverso enantiomers. Chiral conformations of heteroatom substituted cis-cyclooctenes result in the formation of two pairs of enantiomers differing in the directionality (tropicity) of the heteroatom vis-a-vis the double bond (i.e. a reference/inverso pair and a reversed tropicity diastereomeric retro/retro-inverso pair). Labeling of a ring atom in a chiral conformation cis-cyclooctene with a single substituent makes that ring atom stereogenic. This causes all four of these isomers to be diastereomeric, and each now has an enantiomeric partner. The theoretical possibility of stereochemically labeled chiral medium rings having diastereomeric conformational subtypes, differing in the stereogenic elements of ring chirality and/or substituent tropicity, was shown to exist in practice for some of the 2,5-benzoxazocine and 2,6-benzoxazonine skeletons reported in this study. These were characterized by both solution- and solid-state NMR spectroscopy together with X-ray crystallography. |
| Databáze: |
OpenAIRE |
| Externí odkaz: |
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