Conformational analysis.

Abstract

The conformations of cyclopentanol and cis-and trans-cyclopentane-1,2-diol have been studied by ab initio and molecular mechanics (MM) calculations and by the LIS technique, using Yb(fod) 3 to obtain the induced shifts of all 1 H and 13 C nuclei in the molecule, together with complexation shifts obtained by the use of La(fod) 3 . The MM calculations gave two optimised geometries for cyclopentanol. These were envelope conformations with the hydroxyl group equatorial (1A) and axial (1B) at the flap of the envelope. In contrast Gaussian 98 at the B3LYP level with the 6-31G** basis set gave an optimised geometry (1C) which was an envelope conformation with the hydroxyl group in an axial position at the fold of the envelope. ∆E(1A Ϫ 1B) = 0.47 kcal mol Ϫ1 (MM) and 0.93 kcal mol Ϫ1 (ab initio) and ∆E (1B Ϫ 1C) = 0.15 kcal mol Ϫ1 (ab initio). The MM and ab initio calculations for cis-1,2-cyclopentanediol gave different envelope conformations (2A) and ( 2B), both with one equatorial and one axial hydroxyl group. For trans-1,2-cyclopentanediol both calculations gave the same geometries, an envelope conformation with two axial hydroxyls (3A) and a half chair conformer with diequatorial hydroxyls (3B). ∆E (3A Ϫ 3B) = 2.9 kcal mol Ϫ1 ( MM) and 0.70 kcal mol Ϫ1 (ab initio). The LIRAS4 model involving an sp 3 hybridised oxygen atom with two symmetric lone pairs was used for these compounds. The calculated LIS for cyclopentanol gave poor agreement with the observed data for 1A, moderate agreement for 1B but good agreement for 1C. A LIS analysis combining 1B and 1C suggests that the population of 1C was >80% in CHCl 3 solution. The ab initio calculations and the LIS analysis agree that the unsymmetric conformer 1C is the major form in solution. The similarity between this conformer of cyclopentanol and that of the furanose sugars suggests that the anomeric effect may be more fundamental than hitherto realised. In cis-cyclopentane-1,2-diol the observed data were in good agreement with the calculated LIS for both 2A and 2B. In trans-cyclopentane-1,2-diol the observed data were in good agreement with the calculated LIS for 3B but in poor agreement for 3A. The LIS allowed the assignment of the proton chemical shifts of the individual methylene protons in these molecules which had not been given previously.

Conformational analysis. Part 40: a theoretical and NMR investigation of the conformations of cis‐ and trans‐cyclopentane‐1,3‐diol

“…The spectrum 26,27 but in cyclopentanol and the 1,2 diols the OH substituent chemical shift (SCS) always gives the vicinal proton cis to the OH to lower frequency than the trans proton. 1 On this basis, this proton was assigned as H-2cis and this was confirmed subsequently.…”

“…However, a recent theoretical and LIS investigation of cyclopentanol and cis-and transcyclopentane-1,2-diol 1 showed clearly that cyclopentanol in solution exists in two conformers (Scheme 1, A and B) in which the major form (B) is a non-symmetric conformation with the OH group in an axial position at the fold of the conformers, the cis and trans isomers may have different conformations. 1,3-Disubstituted cyclopentanes have been studied using NMR spectroscopy, dipole moments, 11 the relative stabilities of stereoisomers and chemical behaviour.…”

Is the ring conformation the most critical parameter in lipase-catalysed acylation of cycloalkanols?

“…The most stable conformer of trans-cyclopentane-1,2-diol is shown in Figure 4A (X= OH). 22 Normally, five-membered ring derivatives react enzymatically faster than cyclohexanes. 3 Accelerative factors could be, first, the smaller size of the cycle, which would produce less steric hindrance in the active site, and secondly, the greater ease of cyclopentanes to convert from conformation into another, by this increasing the possibility of finding an acceptable disposition in the active site.…”

“…22 The phosphonate intermediate shown in Figure 5 was chosen as the best model for the acetylation of R-3. As with the trans isomer, the ring binds perfectly to the medium-sized pocket, with the hydroxyl group in axial position at the fold of the envelope and the ester located in a less hindered equatorial position at the flap of the envelope ( Figure 4B, X= CO 2 Et).…”

Novel Liquid Crystalline Compounds Containing Bicyclo[3.1.0]hexane Core Units

“…This must be attributable to the concerted action of two energetic factors namely the general preference (by ca. 0.5Ϫ1.0 kcal·mol Ϫ1 ) [34] of alkyl substituents on The packing of the studied molecules in the crystals differs greatly from one to the other, but in all cases the most important short intermolecular interactions are of the type CϪH···OϭC/NϵC, where the CH groups are either part of an aromatic or a cyclopropyl ring. Interestingly, both types of interaction exist simultaneously in all of the studied structures, which probably indicates similar strengths for these attractive interactions.…”