Reducing bulky

Considering first Table 5, it can be seen that Tg values for the reported poly(organophosphazenes) spanned from very low (-105 °C in the case of poly[bis(n-butoxy)phosphazene]) up to very high (-1-220 °C for poly[tris(2,2 -dioxy-l,T-binaphthyl)phosphazene]), covering almost all the intermediate temperatures between these two limits. Low TgS are indicative of very high torsional freedom of the polyphosphazene chain, which is manifested clearly when flexible substituents of reduced bulkiness are used in the substitution... 

Footnotes are adopted and appear in the appendix, to reduce bulky text in the guideline and to be more user-friendly. This is consistent with the ICH guideline. 

With a standard image intensifier, characterized by a gain of more than x10,000, the quantum well effect is clearly avoided. Nevertheless this very high gain reduces the image dynamics unless strong attenuation is introduced at the tube output (iris or neutral filter). Also a standard intensifier is bulky, affected by pincushion distortion and magnetic fields which can be a serious limitation in some applications. 

Selective Toluene Disproportionation. Toluene disproportionates over ZSM-5 to benzene and a mixture of xylenes. Unlike this reaction over amorphous sihca—alumina catalyst, ZSM-5 produces a xylene mixture with increased -isomer content compared with the thermodynamic equihbtium. Chemical modification of the zeohte causing the pore diameter to be reduced produces catalysts that achieve almost 100% selectivity to -xylene. This favorable result is explained by the greatly reduced diffusivity of 0- and / -xylene compared with that of the less bulky -isomer. For the same reason, large crystals (3 llm) of ZSM-5 produce a higher ratio of -xyleneitotal xylenes than smaller crystahites (28,57). 

L-Menthol [2216-51-5] (75) and D-menthol [15356-70-4] have been used as chiral auxiharies in the synthesis of optically active mandehc acids. Reduction of (-)-menthol ben2oylfomiate (76) with a stericaHy bulky reducing agent, ie, sodium bis(2-methylethoxy)aluminum hydride (RED-Al), followed by saponification, yields (R)-mandelic acid (32) of 90% ee. 

The aromatic ring of alkylphenols imparts an acidic character to the hydroxyl group the piC of unhindered alkylphenols is 10—11 (2). Alkylphenols unsubstituted in the ortho position dissolve in aqueous caustic. As the carbon number of the alkyl chain increases, the solubihty of the alkah phenolate salt in water decreases, but aqueous caustic extractions of alkylphenols from an organic solution can be accomphshed at elevated temperatures. Bulky ortho substituents reduce the solubihty of the alkah phenolate in water. The term cryptophenol has been used to describe this phenomenon. A 35% solution of potassium hydroxide in methanol (Qaisen s alkah) dissolves such hindered phenols (3). 

Further variations in the polycarbonate system may be achieved by copolymerisation. The reduced regularity of copolymers compared with the parent homopolymers would normally lead to amorphous materials. Since, however, the common diphenylol alkanes are identical in length they can be interchanged with each other in the unit cell, providing the side groups do not differ greatly in their bulkiness. 

The stereoselective reactions in Scheme 2.10 include one example that is completely stereoselective (entry 3), one that is highly stereoselective (entry 6), and others in which the stereoselectivity is modest to low (entries 1,2,4, 5, and 7). The addition of formic acid to norbomene (entry 3) produces only the exo ester. Reduction of 4-r-butylcyclohexanone (entry 6) is typical of the reduction of unhindered cyclohexanones in that the major diastereomer produced has an equatorial hydroxyl group. Certain other reducing agents, particularly sterically bulky ones, exhibit the opposite stereoselectivity and favor the formation of the diastereomer having an axial hydroxyl groi. The alkylation of 4-t-butylpiperidine with benzyl chloride (entry 7) provides only a slight excess of one diastereomer over the other. 

In natural rubber, the cross-linking of these radicals is hindered because of the bulkiness of the methyl side group. Consequently, these radicals prefer to disproportionate and cleave. This reduces the molecular weight and natural rubber softens on ageing. 

The crude bromide (10 g) is dissolved in 120 ml of acetone and 12.3 g of anhydrous potassium acetate, 3.8 ml of glacial acetic acid and 5.88 g of sodium iodide are added in that order. The mixture is refluxed with agitation for 4 hr the initial yellow iodine color fades completely during this time. Water (200 ml) is added and the acetone removed by distillation at reduced pressure, whereupon the product separates as bulky fibers. After chilling, the product is collected, washed with water and dried to give 9.4 g (98 %) of 5a,6) -dichloro-3i ,17a,21-trihydroxypregnan-20-one 21-acetate mp 192-195° (dec). 

In further modifications of these norprogestins, reaction of norethindrone with acetic anhydride in the presence of p-toluene-sulfonic acid, followed by hydrolysis of the first-formed enol acetate, affords norethindrone acetate (41). This in turn affords, on reaction with excess cyclopentanol in the presence of phosphorus pentoxide, the 3-cyclopentyl enol ether (42) the progestational component of Riglovic . Reduction of norethindrone affords the 3,17-diol. The 33-hydroxy compound is the desired product since reactions at 3 do not show nearly the stereoselectivity of those at 17 by virtue of the relative lack of stereo-directing proximate substituents, the formation of the desired isomer is engendered by use of a bulky reducing agent, lithium aluminum-tri-t-butoxide. Acetylation of the 33,173-diol iffords ethynodiol diacetate, one of the most potent oral proves tins (44). ... 

It reveals that either the steric effect of the bulky N-substituent on the interaction of Ce(IV) ion and urea reductant, or the electron withdrawing effect of the vinyl group, will reduce the coordination of Ce(lV) ion with the carbonyl group, thus resulting in a decrease of the Rp. 

Unsymmetrically substituted phthalocyanines can be prepared from two differently substituted phthalonitriles in a statistical condensation. It is obvious that a mixture of six different phthalocyanines (see p 738) can be obtained AAAB, ABAB (opposite type), AABB (adjacent type), ABBB and the products of the self-condensation of the respective phthalonitriles. If one of the phthalonitriles is substituted with bulky substituents, the yield of isomers with these groups in close vicinity can be reduced. In the following example no product of the self-condensation of tetraphenylphthalonitrile (BBBB type) can be detected. 

Complexes of bulky substituted phenanthrolines [Pt(N-N)LX2] (L, X both monodentate N-N, e.g. 2,9-dimethyl- 1,10-phenanthroline) can be 5-coordinate tbp when a good 7r-acceptor (e.g. C2H4) is present or 4-coordinate with monodentate phenanthrolines. Hartree-Fock calculations indicate that the 7r-acceptors reduce the electron density at platinum so that the metal can accept charge from another donor. Species of this kind may be involved in alkene hydrogenation [138]. 

An aqueous solution of APVP was highly viscous, and the reduced viscosity was rather insensitive to polymer concentration, unlike that of usual synthetic linear polyelectrolytes17,18. This suggests that the model compounds were rather stiff in solution, which would be due to the bulkiness and the hydrophobieity of side groups. The stiffness of APVP was also demonstrated by similar insensitivity of the viscosity toward temperature. 

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