Non‐Chair Conformations of Six‐Membered Rings

Correlations among experimental and theoretical NMR data to determine the absolute stereochemistry of darcyribeirine, a pentacyclic indole alkaloid isolated from Rauvolfia grandiflora

“…A significant increase in energy is observed for geometries of 1a/1b, having ring D in a boat conformation and ring E in a planar one. Isomers 1g/1h have the highest energy due to the boat conformation of the ring D and envelope conformation of the ring E. These energy values (DE) are in agreement with the expected difference in energy between conformations of the cyclohexane ring: difference in energy between chair and boat conformations is 4.8e6.0 kcal/mol [36]; twist conformation is 1.5 kcal/mol more stable than boat one [37]; chair conformation is 5 kcal/mol more stable than twist one [38,39]. Isomers 1g/1h also have the highest dipole moment (m ¼ 2.928 D), however, there is no similar relation between energy and dipole moment to the other sets of enantiomers.…”

Computational evaluation concerning the deviation of the atoms in 1- and 4-postions on the six-member ring and the effects on 1HNMR chemical shift

“…In addition, the boat conformation is further destabilized by van der Waals repulsion between the "flagpole" hydrogens. Both van der Waals repulsion and the torsional strain are somewhat reduced in the twist conformation [19]. However, the data of IR spectroscopy confirmed that the chair conformation is more stable than the twist conformation [20].…”

“…The most stable conformation of cyclohexane is the chair [12]. However, two nonchair conformations of cyclohexane that have normal bond angles and bond lengths are the twist and the boat both of which are less stable than the chair [19]; the chair conformation was determined to be 5.5 kcal/mol lower in energy than the twist, and the twist and the boat conformations are more flexible than the chair, but are destabilized by torsional strain, as the bonds along the "sides" of the boat are eclipsed [19,20]. In addition, the boat conformation is further destabilized by van der Waals repulsion between the "flagpole" hydrogens.…”

Probing the Influence of a 4,6-O-Acetal on the Reactivity of Galactopyranosyl Donors: Verification of the Disarming Influence of thetrans–gaucheConformation of C5–C6 Bonds

“…In the Bols systems ( 2, 3, 5, 6, 9, 11 ) and in their extension to the galactose series presented here ( 23 RS ) an oxygen atom in a 1,3- dioxane is replaced by a methylene group and numerous new potential torsional interactions are brought into play in both the trans - and cis -fused systems (Fig 5b). The reduced potential for torsional interactions in 1,3-dioxanes as compared to cyclohexanes on distortion from the ideal chair form is borne out by the lower barrier for conformational inversion in 1,3-dioxane (9.0–9.9 kcal.mol −1 ) than in cyclohexane (10.2 kcal.mol −1 ), 72–73 and in tetrahydropyran (10.3 kcal.mol-1), 74 even if the twist boat conformer is less highly populated in 1,3-dioxane than in cyclohexane owing to the shorter C-O bonds in 1,3-dioxane which give rise to greater steric interactions across the ring in its twist boat conformers. 72 - 73 Thus, while the Bols-type systems have value in estimating the influence of the O5-C5-C6-O6 torsion angle on the reactivity of glycosyl donors, the replacement of an oxygen atom by a methylene group in the 4,6- O -alkylidene ring system brings additional C-H bonds into play that are capable of suffering torsional interactions on formation of the oxocarbenium ion.…”