Preferential conformer stabilization

Subsequently, several studies over the years have been carried out to determine whether 1,6-dlene systems undergo cycliza-tlon in an Intramolecular mode by forming a 5-membered ring or 6-membered ring (9). Since polymer systems are structurally difficult to evaluate some very interesting studies on monomer substrates were carried out by Brace (10), Julia (11), Solomon (12), Lancaster (13), and Ottenbrite (.9). These studies have shown radical addition to a 1,6-diene system occurs readily in an intramolecular mode to preferentially form a 5-membered ring. There are several factors that influence this cyclization process (a) the relative conformational stability of a 6-member-... 

Preferential conformations of a molecule are studied in conformational analysis. This can be done by microwave, uv, ir, nmr, and Raman spectroscopy, X-ray crystallography, kinetic, equilibrium and dipole moment measurements, etc. The existence and stability of conformers can also be determined by energy calculations. 

The anion is assumed to exist preferentially in the anti conformation A, which offers a stabilizing 7rc -> minimizes steric interactions69,71 11. The nonplanar nature of the carhanionic center and coordination of the lithium to the oxygen atom could be demonstrated for a related compound by an X-ray analysis72. Usually, the electrophile attacks the carhanionic center with stereoinversion. 

III. Preferential Stabilization of Different Conformers of the Rare Gas-Dihalogen Complexes... 

Stable conformation for CHjSOH and CH3SOCH3 may be illustrated by the Newman projections of 32 and 33, while those of the carbanions are given by 34 and 35. This means that the a-sulfinyl carbanion is nonplanar and the Hj proton on the opposite side of the S—O bond is removed preferentially. The same calculation also indicated that conformer 32 is 16.5kcalmol and 17.2kcalmol" more stable than conformers 29 and 31, respectively . This new MO calculation thus contradicts the earlier suggestion that the most stable carbanionic conformer is 29 but emphasizes the importance of the contribution of 3d-orbitals for the stabilization of a-sulfinyl carbanions. 

The preferential stabilization of conformation [96] by the cation is annulled upon binding of the cation with a cryptand. Similar effects have been observed with crown ethers. The sodium salt of acac in methanol at —58°C consists of 35% [104] and 65% [105]. The addition of 18-crown-6 [3] shifts the... 

HR agonists stimulate the active state in the receptor and inverse agonists, the inactive one. An agonist with a preferential affinity for the active state of the receptor stabilizes the receptor in its active conformation leading to a continuous activation signal. An inverse agonist with a preferential affinity for the inactive state stabilizes the receptor in this conformation and consequently induces an inactive state, which is characterized... 

No large conformational changes occur in the enzyme during catalysis, but many small movements take place. The structural basis for the catalytic power of ribonuclease thus resides in several different features tight, specihc binding of a strained conformation of the substrate, general acid-base catalysis by His-12 and His-119, and preferential stabilization of the transition state by ionic interactions with Lys-41. 

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