Functionalizing the Hydroxyl Group

In the attempt to functionalize the hydroxyl group at C-5 of the ribose moiety, isoguanosine 40 was treated with iV,iV-bis(trifluoroacetyl)-L-homocystine dimethyl ester in the presence of tri- -butylphosphine and pyridine as the solvent, but, instead of the expected product, the cyclized 41 was obtained in 11% yield. 

A related reaction is the Barton reaction. In this case, a methyl group that is 5 to a hydroxyl group is oxidised into an aldehyde functionality. The hydroxyl group is first converted into a nitrite ester by the action of nitrosyl chloride, NOC1, and after exposure to light the 8-methyl group undergoes nitrosation. Write down the pathway for these steps. 

The structural characteristics of bacterial cellulose are directly related to two factors, namely (1) the origin of the strain, which determines the la/ip ratio and (2) the culture medium composition that influences the chain size. Such characteristics determine the degree of crystallinity of bacterial cellulose and consequently, their physicochemical properties. Structural modifications can be accomplished in a post-production step, since it is possible to functionalize the hydroxyl groups (—OH) by methylation [13], esterification [14], sulphonation [13], nitration [13], deoxyamination [15], etc. 

Functional class names of alcohols are derived by naming the alkyl group that bears the hydroxyl substituent (—OH) and then adding alcohol as a separate word The chain IS always numbered beginning at the carbon to which the hydroxyl group is attached... 

Mod.ifica.tion of Intact Penems. Functional group modification has been used by a number of researchers (123—125) to synthesize a wide range of 2-substituted penems. For example, activation of the hydroxyl group of (84, R = OH) followed by displacement reactions provided 2-heterocyclylthiomethyl-penems (84, R = (3)-heterocyclyl) (125) and 2-(quaternary ammonio)methyl-penems (84, R = (1 )-... 

Surfaces of inorganic soHds can be functionalized with catalytic groups just as organic polymers can. For example, the hydroxyl groups on the surface of siUca can be used for synthesis of the following stmcture ... 

The other analytical methods necessary to control the typical specification given in Table 5 are, for the most part, common quality-control procedures. When a chemical analysis for purity is desired, acetylation or phthalation procedures are commonly employed. In these cases, the alcohol reacts with a measured volume of either acetic or phthalic anhydride in pyridine solution. The loss in titratable acidity in the anhydride solution is a direct measure of the hydroxyl groups reacting in the sample. These procedures are generally free from interference by other functional groups, but both are affected adversely by the presence of excessive water, as this depletes the anhydride reagent strength to a level below that necessary to ensure complete reaction with the alcohol. Both procedures can be adapted to a semimicro- or even microscale deterrnination. 

CH2(OMe)2, CH2 = CHCH2SiMe3, MeaSiOTf, P2O5, 93-99% yield." This method was used to protect the 2 -OH of ribonucleosides and deoxyribo-nucleosides as well as the hydroxyl groups of several other carbohydrates bearing functionality such as esters, amides, and acetonides. 

The dehydration of alcohols is an important elimination reaction that takes place under acidic rather flian basic conditions. It involves an El mechanism." The function of the acidic reagent is to convert the hydroxyl group to a better leaving group by protonation ... 

The structural variations reported by Cram and coworkers relate to an appreciable extent to the various ancillary functions which have been appended to the binaphthyl units or elsewhere in the macrocyclic system. Enhancements of the chiral barrier or functionalization through arms has generally been effected at the 3-or 6-positions. These positions are adjacent to the hydroxyl group or directly across the second ring from it, respectively. 

Reduction of the imine with sodium borohydride leads to an intermediate amino-ester that cyclizes spontaneously to the <5-lactam function. Solvolysis of the acetyl group with methoxide followed by acylation of the hydroxyl group thus liberated with trimethoxybenzoyl chloride leads to 38. Bischler-Napieralski cyclodehydration (phosphorus oxychloride) effects closure of the remaining ring. Reduction of the imine thus formed with sodium borohydride gives 39. This, it should be noted, leads to the... 

Figure 18-10 summarizes the successive oxidation products that can be obtained from alcohols. When the hydroxyl group, OH, is attached on an end carbon atom, an aldehyde and a carboxylic acid can be obtained through oxidation. When the hydroxyl group is on a carbon atom attached to two other carbon atoms, oxidation gives a ketone. Huge amounts of aldehydes and ketones are used industrially in a variety of chemical processes. Furthermore, these functional groups are important in chemical syntheses of medicines, dyes, plastics, and fabrics. 

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