Steric problems

A number of structures have been prepared which do not have the steric problems associated with the cyclodeca-l,3,5,7,9-pentaenes. In compound 1, the steric problem is avoided with only a slight loss of planarity in the n system ... 

A 14-electron n system can be generated in circumstances in which the steric problem associated with the internal hydrogens of [14]aimulene are avoided. This can be achieved in 10b,10c-dihydropyrene systems in which the annulene ring is built aroimd a saturated... 

We similarly attempted to remove the hydride in the sandwich series and room-temperature experiments faced the same steric problem. Reactions occurred by an ET pathway followed by cleavage of the C-C bond in the 17e cation [39]. 

There will be many times in the second half of this course when you will be trying to determine which way a reaction will proceed from two possible outcomes. Many times, you will choose one outcome, because the other outcome has steric problems to overcome (the geometry of the molecules does not permit the reactive sites to get close together). In fact, you will learn to make decisions like this as soon as you learn your first reactions Sn2 versus SnI reactions. Now that we know why geometry is so important, we need to brush up on some basic concepts. 

Recently, Muha (83) has found that the concentration of cation radicals is a rather complex function of the half-wave potential the concentration goes through a maximum at a half-wave potential of about 0.7 V. The results were obtained for an amorphous silica-alumina catalyst where the steric problem would not be significant. To explain the observed dependence, the presence of dipositive ions and carbonium ions along with a distribution in the oxidizing strengths of the surface electrophilic sites must be taken into account. The interaction between the different species present is explained by assuming that a chemical equilibrium exists on the surface. 

Since sialic acid is a frequent terminal sugar constituent of the polysaccharide trees on glycoproteins, this method selectively forms reactive aldehydes on the most accessible parts for subsequent modifications. The carbohydrate polymer of a protein provides a long spacer arm that can be used to conjugate another large macromolecule, such as a second protein, with little steric problems. 

The addition of a radioactive iodine atom to a protein molecule typically has little effect on the resultant protein activity, unless the active center is modified in the process. The size of an iodine atom is relatively small and does not result in many steric problems with large molecules. The sites of potential protein modification are tyrosine and histidine side chains. Tyrosine may be modified with a total of two iodine atoms per phenolate group, whereas histidine can incorporate one iodine. Sulfhydryl modification at cysteine residues is typically unstable. 

LC-SPDP (Chapter 5, Section 1.1) is an analog of SPDP containing a hexanoate spacer arm within its internal cross-bridge. The increased length of the extended crosslinker is important in some conjugations to avoid steric problems associated with closely linked macromolecules. However, for the preparation of immunotoxins, no advantages were observed for LC-SPDP over SPDP (Singh et al., 1993). 

Figure 10.5 Schematic diagram to show the steric problems encountered when 0 tends to unity. The incoming molecule of adsorbate must approach from within the confines of the cone shape if adsorption is to succeed...

The reaction of pyridine (py) with bare metal ions (except Fe+) has not been studied widely. The reaction of Fe+ produced by electron ionization of Fe(CO)5 with a mixture of two pyridines (108) was used to compare the proton affinities with the Fe+ affinity. A good correlation was observed. The absolute Fe+ affinity of py was determined to be 49 3 kcal mol-1, which is higher than the value of 44 3 kcal mol 1 for the Fe+-NH3 bond dissociation energy (46). Steric problems with ortho substituted pyridines gave lower than expected affinities. The reaction of py and substituted pyridines showed a maximum addition of four pyridines, similar to the GIB experiments with ammonia (46). 

One could plunge into the steric problems posed by the mechanism of protein synthesis on the ribosome 25 26)> or consider the steric fit of the hormone insulin to its acceptor in the cell membrane 27>. Or one could delve into the beautiful intricacy of terpenoid, squalene and steroid metabolism, or get lost in double bond formation, or in the steric problems posed by the branched chain fatty acids and their derivatives 28-34). 

Allhough rtol shown, the nitrogen atom of the sulfonic acid is probably protonated, or even sLilfonated. under the reaction conditions, exaggerating the steric problems experienced by the substituent at C-8. The mechanism of the desuifonation ol aromatic sulfonic acids occurs via the reverse of the sulfonation process. 

Other X-ray analyses (78JOC3513) together with H and 13C NMR spectroscopic data have given a firm basis for the stereochemistry of this class of ring compounds. The information has helped to clarify steric problems of other phosphorus heterocycles, e.g. (117)-(120), where the difficulties of interpretation of NMR or other physical data are greater. 

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