Hydroxy halides, alkylation dehydration

Reductive amination of dihydropyran (which may be regarded as the dehydration product of the cyclic acetal of 5-hydroxy-pentanal) in the presence of isopropylamine and a trace of acid affords the aminoalcohol, 96. Treatment of this compound with thionyl chloride affords the haloamine, 97. Alkylation of the quinoline, 92, with this halide yields pentaquine (98). ... 

Oxindole exists as the carbonyl-tautomer, the hydroxy 1-tautomer ( 2-hydroxyindole ) being undetectable. There is nothing remarkable about the reactions of oxindole for the most part it is a typical 5-membered lactam, except that deprotonation at the p-carbon (pA a 18) occurs more readily than with simple amides, because the resulting anion is stabilised by an aromatic indole resonance contributor. Such anions will react with electrophiles like alkyl halides and aldehydes at the p-carbon, the last with dehydration and the production of aldol condensation products. Oxindoles can be oxidised to isatins (20.13.3) via easy 3,3-dibromination, then hydrolysis. Bromination of oxindole with A -bromosuccinimide gives... 

For this reason, cyanuric chloride is used as a chlorinating and dehydrating agent in organic synthesis. It transforms (preferably secondary) alcohols into alkyl halides, carboxylic acids into acid halides, hydroxy carboxylic acids into lactones and aldoximes into nitriles ... 

All three elimination reactions—E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particularly common. The substrate is usually an alcohol rather than an alkyl halide, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carbonyl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the -OH group is assisted by simultaneous protonation. 

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