Hydroxyl groups, substitution

The cholestane molecule may have hydroxyl groups substituted at 25 of its 27 carbon atoms (C-10 and C-13 excepted). Each of these carbon atoms is so situated that it renders its hydroxyl substituent(s) sterically and, more subtly, electronically unique with respect to each of the others. If the cholestane skeleton is maintained, each of the possible secondaiy hydroxyls may assume... 

The sodium salt of the cymene sulphonic acid is then fused with sodium hydroxide in the usual manner, and the hydroxyl group substituted for the sulphonic group. This gives l-methyl-3-hydroxy-4-isopropyl-benzene or thymol. 

Chromones (14) and flavones (phenylchromones, 15) are pyranones when they have a hydroxyl group substituted in such a position that they can act as bidentate 0,0-donors then they and their complexes are relevant to the present review. Fig. 3 shows formulae for chromone, flavone, and the polyhydroxyflavones (16-20) which appear in this review. 

Mixed pentaerythritol esters have also been described [50] that contain in their molecule hydroxyl groups substituted by organic acids, which possess nitro groups either in ester form or as C-nitro compounds joined with the acid, alongside simple nitrate ester groups. 

Gallotannins are hydrolysable tannins with a polyol core (referring to a compound with multiple hydroxyl groups) substituted with 10-12 gallic acid residues. Gallotannins contain the characteristic raeto-depside bonds (1.91) between gallic acid residues. This bond is more labile than an aliphatic ester bond, and can be methanolyzed with a weak acid in methanol. In contrast, methanolysis of an aliphatic ester bond requires methanol with a strong mineral acid and heat. 

The hydroxyl group is ring-activating, whereas the chlorine is a ringdeactivating substituent. Therefore, substitution occurs para to the hydroxyl group. Substitution ortho to the hydroxyl group is less likely since the product would have three adjacent substituents, which would be quite sterically crowded. 

Now the benzyl ether can be deprotected, and the hydroxyl group substituted for iodide via its tosylate. This iodide is an alkylating agent, and is used for two successive alkylations of a hydra zone s aza-enolate. 

Fig. 18. The expected percentages of various labelled products of the dioldehydratase reaction using 25 as substrate. The calculation was based on the following facts and assumptions (1) The enzyme does not differentiate between the enantiotopic hydrogen positions (conclusion from experiments with species 17 and 18 shown in Fig. 14) (2) in the competition between vicinal hydrogen atoms there is an intramolecular kinetic deuterium isotope effect of 2.6 (Fig. 15) (3) this effect is 10 for geminal hydrogen atoms (4) the migrating hydroxyl group substitutes one of the hydrogen atoms in the vicinal position stereospecifi-cally (i.e., with inversion).

Poly(vinyl alcohol) readily reacts with a wide variety of Isocyanates In DMSO solution (Equation 1) to produce modified polymers that contain 10-100% of the original hydroxyl groups substituted with carbamate groups. The yields of such reactions are essentially quantitative. Similar results have been reported from other research groups (14-16). tinder these conditions, poly(vinyl alcohol) has been reacted with 3- and 4-chlorophenyl-Isocyanate (this study), phenyllsocyanate (12-14), methyl, ethyl. Isopropyl and 1-naphthyl Isocyanates (14) and methoxymethyllso-cyanate (15,16). The reaction of PVAl with phenyl, tolyl and 4-chlorophenyl Isocyanates In dlmethylacetamlde solutions has also been reported (17). 

The bromine atom of ll-bromobenzo[fc]indeno[l,2-e]l,4-thiazine (67a) can be replaced by a cyanide, phenyl mercaptide, or hydroxyl group. Substitution should occur through the usual addition-elimination mechanism. 

The pattern of hydroxyl group substitution does not significantly affect the p]< values of either the bile acid -COOH or the corresponding ycine conjugate. 

Monoacylglycerols consist of a single hydroxyl group substitution of the glycerol molecule with an acyl residue via an ester linkage (Figure 3.3). Three positional isomers are therefore possible for the substitution sn-l, the central carbon position sn-2, or the terminal carbon position sn-3. Unlike the sn-l and sn-3 isomers, the sn-2 isomer retains its molecular symmetry and is therefore nonchiral. 

Hydroxyl groups substituted at positions 2- and 4-(6-) of the pyrimidine nucleus exist mainly in the pyrimidone form [1-4]. The only true hydroxyl position on this ring system is position 5. This hydroxyl group is of phenolic character and all 5-hydroxypyrimidines give a positive (blue-violet coloration) test with ferric chloride. Because of the relative difficulty in synthesis of all but a few compounds, most 5-hydroxypyrimidines have not been extensively studied. 

Antiparasitic action of flavonoids is related to C-3 and C-4 hydroxyl groups, substitution of their hydrogen atoms for methyl group strongly diminishing it. Casticin has been reported to exhibit more than twice the activity of artemetin against L. infantum and sixfold more active against T. cruzi [103]. 

NMR spectra, and chemical analysis by X-ray fluorescence, the five samples El to E5 are dioctahedral 2 1 layer silicates, mainly constituted of Si and Al as framework-forming elements, containing Al for Si substitutions in tetrahedral layers and fluorine for hydroxyl groups substitutions. 29Si MAS NMR spectroscopy gives rise to their (Si/AI)iv tetrahedral ratio with a good reliability (Table 10-5). The well-controlled and well-known hydrogels composition indicates that only dioctahedral layer silicates, with two Al and one vacancy per... 

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