Steric hindrance Strain

Steric hindrance Strain in a molecule caused by two atoms in that same molecule that would (under normal bonding conditions for each) approach closer than normal. To alleviate this closeness of approach the atoms readjust their positions slightly with resultant bond angle and torsion angle changes. 

In this case the relationship between stability and stereochemistry is easily explained on the basis of van der Waals strain The methyl groups on the same side of the ring m cis 1 2 dimethylcyclopropane crowd each other and increase the potential energy of this stereoisomer Steric hindrance between methyl groups is absent m trans 1 2 dimethylcyclopropane... 

Several generalities can be formulated regarding selective reduction of polyolefins. Usually the least hindered double bond is hydrogenated pre ferentially (123), and, with steric hindrance about equal, the most strained bond will be reduced first. Exocyclic olefins are reduced more easily than those in the ring (R)-(+ )-Limonene, 190 g, was shaken with W-4 Raney nickel (12 g) under hydrogen at atmospheric pressure. After 31.9 1 of hydrogen had been absorbed, the solution was filtered. Essentially, pure (R)-( -i- )-carvomenthene was obtained in 96% yield (58). 

Diels-Alder reaction and. 494-495 El reaction and, 392 E2 reaction and, 387-388 R.S configuration and, 297-300 S 1 reaction and, 374-375 S -2 reactions and, 363-364 Stereogenic center, 292 Stereoisomers, 111 kinds of, 310-311 number of, 302 properties of, 306 Stereospecilic, 228, 494 Stereospecific numbering, sn-glycerol 3-phosphate and, 1132 Steric hindrance, Sjvj2 reaction and, 365-366 Steric strain, 96... 

Fig. 3.31 Steric control in alternating ROMP Tendencies of norbomene and cyclooctene to give productive olefin metathesis upon coordination are illustrated by a thick arrow (preferred monomer) or a thin arrow (less favoured monomer) (a) only minor steric hindrance SlMes greatly favours the polymerisation of the strained norbomene (b) the rotating phenylethyl-group induces a steiically more congested active site, leading to preferred incorporation of the smaller cyclooctene (c) the flexible and small cyclooctene derived polymer fragment permits the incorporation of the bulky norbomene...

The complete degradation of isoquinoline has not been reported, which indicates that this molecule is not as susceptible to degradation as quinoline. Aislabie et al. [336] found that using an Acinetobacter strain, the degradation of isoquinoline was accompanied by the accumulation of 1-hydroxyisoquinoline. This strain did not metabolize 1,5-dihydroxyisoquinoline nor 1-methylisoquinoline. So, its inability for full degradation was explained in terms of steric hindrance. The enrichment technique leads to a metabolic improvement but results in complete degradation. 

A Lactobacillus strain was recently shown to inhibit competitively adhesion of enteropathogenic E. coli to pig ileum and interfered with bacterial attachment to the mucosal layer of ileal conducts (Blomberg et al., 1993). Although L. acidophilus inhibits the adhesion of several enteric pathogens to human intestinal cells in culture, when pathogen attachment preceded L. acidophilus treatment, no inhibitory interference occurred indicating that steric hindrance of site occupation is important in the inhibition of adhesion. Thus, therapeutic use is likely to be limited to preventive application of probiotics. 

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