Ring flexibility

We will deal with these N-acyl derivatives by gathering them as a function of ring dimension, the conformational characteristics of the acyl group in these derivatives also being dependent on ring flexibility. 

Sadowski, J. (1997) A hybrid approach for addressing ring flexibility in 3D database searching. J Comput Aided Mol Des 11, 53-60. 

The absence of Bohlmann bands in the infrared (IR) spectrum indicates that the nitrogen lone pairs of the di-iV-nitroso-l,4-diazepines 23 are delocalized <1995JOC7461>. While the absence of an amide moiety in the ring confers torsional freedom, the m-2,7-di-Ph groups reduce ring flexibility and the 111 NMR data are consistent with major and minor families of four twist chair conformers that project the C-7 Ph and C-6 R -substituents equatorially These interconvert by a pseudorotation that allows the C-2 phenyl and C-3 R substituents to adopt a pseudoaxial orientation in the preferred conformer family. 

CScore CaluilStfan jijj None v Parallel v Serial CScoreDetails Assign Formal Charges, j Place Particles J Treat Rings Flexibly Stereochemistry Mode Modify H.3 E/Z. .i R/S FlexX Details-, ... 

There is no reason to confine these ideas to six-membered rings. Flexibility, which allows antiperiplanar configuration to be achieved in the transition state, is a crucial factor governing the reactivity of any ketone. To check this idea, we have studied the addition of cyanide ion to cyclopentanone and 3-pentanone.78 Cyclopentanone can be... 

Jain AN (2007) Surflex-Dock 2.1 robust performance from ligand energetic modeling, ring flexibility, and knowledge-based search. J Comput Aided Mol Des 21 281-306... 

Small differences between anomeric effects of a and p anomers and inherent ring flexibilities have complicated the development of a general method for stereoselective introduction of 1,2-m-furanosides. By examining possible conformers of the arabinofuranosyl oxocarbenium ion, factors that favor attack from the p face (a 1,2-cis attack) were identified. Oxocarbenium ions of furanosides can adopt 3E or E3 as two possible low-energy conformations (Scheme 4.15). The Newman projection of the E3 conformer of D-arabinofuranose showed that nucleophilic attack from the a face is disfavored because an eclipsed 2-H will be encountered. On the other hand, an approach from the p face is expected to be more favorable because it experiences only staggered substituents. 

Michael reaction of macrocyclic ketene silyl acetals or a-enones occurs smoothly under electron-transfer conditions whereas the reaction of 6-membered analogs is more sluggish, indicating the importance of ring flexibility to enable the a,/8-carbon-carbon bonds of both reaction components to rotate when the radical species are generated [72c]. 

When the Co-C bond is broken in the adenosyl coenzyme, the adenosyl group must move and transfer the free radical to the substrate site that is probably not directly on the cobalt atom. This analysis of corrin ring flexibility also included information on the accessibility of various portions of the molecule. Such steric accessibility indicates the availability of that part of the molecule to attack by other molecules. Random points (50 points per A , for example) were generated around each atom at its van der Waals radius. If any of these points around an atom was within the van der Waals radius of another atom in the structure, the... 

It is noteworthy that in the tribenzcycloheptatriene series, the replacement of a —CH2— group with an > SiMe2 group does not result in a decrease in the barrier to ring inversion in direct contrast to the previously discussed silacyclohexane system (77). There now exist two extremes in ring flexibility for silicon compounds. It is necessary to study other heterocyclic derivatives before it will be possible to evaluate the effect on ring flexibility with heteroatom replacement. 

Although the pyramidal inversion barrier for As in the arsanthracene system has not yet been reported, the AG for pyramidal inversion for a related sulfur derivative, 9,9-dimethyl-10-phenylthioxanthylium perchlorate, was calculated to be 25.4 kcal/mole at 200 5° (5). Since considerable evidence demonstrating ring flexibility in 9,10-dihydro-anthracene derivatives has accumulated, the isolation of optically active arsanthracene derivatives most probably is due to configurational stability of arsenic. 

Low acrylonitrile) Gaskets, grommets, and O-rings (flexible at a very low temperature), adhesives. Fuel binder in solid rocket propellants. 

Less favorable for intercyclic cycloadditions are systems with a six-membered central ring (e.g. 14). Because of ring flexibility, the reaction partners are not properly aligned and intracyclic [2[Pg.2205]

The H NMR spectrum of 1,4-oxaselenane at 155 K consists of ABCD multiplets, but these coalesce as the temperature of the experiment is raised, eventually leading to an AA BB system, when ring inversion is faster than the timescale of the NMR experiment AG = 42.6 kJ mol ). However, even at 155 K the spectrum of 1,4-oxatellurane appears as AA BB multiplets, slightly broadened. In this molecule the greater ring flexibility is considered to be a reflection of the increased bond lengths (C—Te vs. C—Se) <75JCS(P2)1354>. 

Casarotto MG, Craik DJ. Ring flexibility within tricyclic antidepressant drugs. J Pharm Sci 2001 90 713-721. 

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