Rings, flexible

Percentage of structures with an RM S deviation of the atoms in flexible rings less than 0.3 A. 

Anomeric effects are cumulative, and can cause a potentially flexible ring to adjust to a more rigid conformation in order to maximize the overlap of suitable lone pair and a orbitals. It has been particularly instructive in explaining anomalous preferences for substituent orientations in tetrahydropyrans and related compounds. In the case of 2-methoxytetrahydropyran, for example, the axial conformer is three times more populated than the equatorial form (Scheme 1.2). 

Conformation. The arrangement about single bonds and of flexible rings. 

This isomer was also formed with larger rings containing both cis- and trani-alkenes. In these cases with more flexible ring systems, the diastereochemical outcome is explained... 

Most small molecules in pharmaceutical research have at least a few rotatable bonds or even flexible ring systems. Seventy percent of drug-like... 

Discussion of the conformation and relative stabilities of bicyclic diacetals with cis ring junctions requires consideration of the possibilities of H-inside and 0-inside conformations of the flexible ring system (XVIII and XIX) as well as of axial and equatorial positions of residues. One possibility that may be excluded is an inside axial position for a substituent of the type XVII shown for cis-decalin, as this would be extremely unstable compared to other conformations. 

Recently Freudenberg and Cramer have proposed that flexible ring forms intermediate between B1 and 3B are involved in varying degree in the Schardinger dextrins and starch. 

Diaphragm—(1) A flexible ring covered with rubber or other plastic material, fitted over the cervix of the uterus to prevent pregnancy. (2) Muscular membrane separating the abdominal and thoracic cavities, used for respiration. 

In the case of 5-membered rhodopsin, only a long-lived excited state (r = 85 ps) was formed without any ground-state photoproduct (Fig. 4.5D), giving direct evidence that the CTI is the primary event in vision [39]. Excitation of 7-membered rhodopsin, on the other hand, yielded a ground-state photoproduct with a spectrum similar to photorhodopsin (Fig. 4.5C). These different results were interpreted in terms of the rotational flexibility along the C11-C12 double bond [39]. This hypothesis was further supported by the results with an 8-membered rhodopsin that possesses a more flexible ring. Upon excitation of 8-membered rhodopsin with a 21 ps pulse, two photoproducts - photorhodopsin-like and bathorhodopsin-like products - were observed (Fig. 4.5B) [40], Photorhodopsin is a precursor of bathorhodopsin found by picosecond transient absorption spectroscopy [41]. Thus, the picosecond absorption studies directly elucidated the correlation between the primary processes of rhodopsin and the flexibility of the Cl 1-02 double bond of the chromophore, and we eventually concluded that the respective potential surfaces were as shown in Fig. 4.5 [10,40]. 

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