Steric rigidity

Synthesis of sterically rigid macrocycles by the use of pressure-induced repetitive Diels-Alder reactions [84]... 

A similar consideration is very likely also to apply to the reaction illustrated by [2.5] involving the sterically rigid precursors, 2,9-dichloro-1,10-phenanthroline and 2,9-diamino-l,10-phenanthroline (Ogawa, Yamaguchi Gotoh, 1974). The product macrocycle (57) was obtained... 

Decomposition of highly reactive disulfonium dications in sufficiently nucleophilic medium is accompanied by formation of disulfides. Even hydrolysis of disulfonium dications generated from sterically rigid bis-sulfides often affords some disulfide as a by-product in addition to the monosulfoxide main product. For example, hydrolysis of dication 72 yields sulfoxide 74 and trace amounts of heterocycle 73 (Scheme 28).56... 

A characteristic feature of the dianhydrohexopyranoses is their steric rigidity. Consequently, they usually react with high and predictable regio-and stereoselectivity, mainly by diaxial cleavage of the oxirane ring. Thus, they became versatile starting materials for synthesis of various carbohydrates as well as noncarbohydrate structures. The most important compounds are l,6 3,4-dianhydro-2-0-tosyl-/l-D-galactopyranose (146), the manno epoxide 143, the altro epoxide 159, and the 2,3- and 3,4-anhydro-a//o epoxides 138 and 139. The tosyl epoxide 146 (for its crystal structure,... 

Application of the model-free methods to axial bimetallic lanthanide complexes with sterically rigid ligands... 

The use of sterically rigid ligands ensures the formation of well-defined coordination spheres which strongly limit dynamic behaviours occurring in solution on the NMR time scale. 

Strongly chelating ligands provide a sterically rigid ligand frame, a prerequisite for induction of asymmetry at the lanthanide center. Complexes derived from (S)-( — )-BINOL were thoroughly studied in the nitroaldol reaction (Henry reaction, Scheme 27) [250]. 

Torsional rigidity Bonds of even order are sterically rigid because of a barrier to rotation, representing the energy needed to generate the o-a-m that occurs in the twisted system, and which can be calculated directly. 

In solution, the intensity of the emission plotted against the concentration of emitting species exhibits a maximum. This phenomenon is known as the concentration effect, leading to self-quenching and/or radiationless deactivation. It can be observed in the gas phase when emitting species are present under physically or chemically stressed environments (i.e., at high pressures or for sterically rigid states, respectively). 

Hetero[ll]annulenes are known either in sterically rigid bridged forms (25) (72AG(E)217) and (26) (72TL4129) or as the more flexible triannulated varieties (27) (70JA5284), (28) and (29) (71CC1434). Both types of system are strictly atropic by NMR which is not unexpected... 

The high electronic density associated with the dinuclear centre was suggested to produce electric multipolar interactions and this combined with the sterically rigid core leads to the high transition temperatures in the mesogens. 

A characteristic feature of these compounds is their steric rigidity in the crystalline state,4 as well as in solution,5 the l,6-anhydro-/3-D-hexopyranoses adopt exclusively the 4C4(d) conformation (la), whereas the corresponding D-hexoses and their glycosides generally occur in the 4Ci(d) conformation. These different conformations determine the chemical and physical properties of the l,6-anhydro-/3-D-hexopyra-noses. A similar situation exists with the 1,6-anhydroaldohexofuranoses. 

Centropolyindanes constitute a complete family of arylaliphatic polycyclic hydrocarbons containing several indane units. Mutual fusion of the five-membered rings leads to three-dimensional, carbon-rich molecular frameworks bearing a central carbon atom, such as benzo-annelated [3.3.3]propellanes, triquinacenes, and [5.5.5.6]- and [5.5.5.5]fenestranes. In this review, the structural concept of centropolyindanes is contrasted to other fused indane hydrocarbons. Besides the syntheses of the parent centropolyindanes and recently described related indane hydrocarbons, the preparation of a large variety of bridgehead and arene substituted centropolyindanes is presented including strained, heterocyclic, and centrohexacyclic derivatives. In appropriate cases, the particular reactivity and some structural features of these unusual, sterically rigid polycyclic compounds are pointed out. 

Structural features of cellulosic materials that determine their susceptibility to enzymatic degradation include (1) the moisture content of the fiber (2) the size and diffusibility of the enzyme molecules involved in relation to the size and surface properties of the gross capillaries, and the spaces between microfibrils and the cellulose molecules in the amorphous regions (3) the degree of crystallinity of the cellulose (4) its unit-cell dimensions (5) the conformation and steric rigidity of the anhydroglucose units (6) the degree of polymerization of the cellulose ... 

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