Hydroxyl groups elimination

Azirines react with enolate anions. Initial nucleophilic attack on phenyl 1-azirine by the enolate anion derived from acetophenone gives intermediate (223) which undergoes 1,2-bond cleavage, cyclization and hydroxyl group elimination to give pyrrole (226). 

Then, protonation of one of the hydroxyl groups, elimination of water, and deprotonation give the product. 

The hydroxyl group eliminates upon deprotection and mesylation. The use of DBU as a particularly large, non-nucleophilic base is necessary for the elimination. It also prevents the formation of the thermodynamic, conjugated elimination product and a nucleophilic attack at the carbonyl group. 

Blount JF, Mohacsi E, Vane FM, Mannering GJ (1973) Isolation, X-ray analysis and synthesis of a metabolite of (-)-3-hydroxy-N-allylmorphinan. J Med Chem 16 352-355 Bogriar R, Gaal Gy, Kerekes P, Horvath G, Kovacs MT (1974) Hydroxyl group elimination in the morphine series. Org Prep Proc Int 6(6) 305-311 Brossi A, Teitel S (1970) Partial 0-demethylation of aromatic substituted 3,4-dihydroisoquino-Hnes. Helv Chim Acta 53 1779-1787... 

The hydroxyl group is lost from a carbon that bears three equivalent ethyl sub stituents Beta elimination can occur in any one of three equivalent directions to l give the same alkene 3 ethyl 2 pentene ... 

Lactones, l xictides, iMciams, and Lactims. When the hydroxy acid from which water may be considered to have been eliminated has a trivial name, the lactone is designated by substituting -olactone for -ic acid. Locants for a carbonyl group are numbered as low as possible, even before that of a hydroxyl group. 

There is a health benefit associated with hindering hydrogen bonding. Alkylphenols as a class are generally regarded as corrosive health hazards, but this corrosivity is eliminated when the hydroxyl group is flanked by bulky substituents in the ortho positions. In fact, hindered phenols as a class of compounds are utilized as antioxidants in plastics with FDA approval for indirect food contact. 

Mechanism of the initial reaction, known as alkaline peeling, is shown in equation 4. EnoHzations and tautomerizations take place easily because of the contiguous hydroxyl groups. The hydroxyl or substituted hydroxyl on the second, ie, P-carbon, from a carbonyl group is released from the molecule by P-elimination. 

As chemists proceeded to synthesize more complicated stmctures, they developed more satisfactory protective groups and more effective methods for the formation and cleavage of protected compounds. At first a tetrahydropyranyl acetal was prepared, by an acid-catalyzed reaction with dihydropyran, to protect a hydroxyl group. The acetal is readily cleaved by mild acid hydrolysis, but formation of this acetal introduces a new stereogenic center. Formation of the 4-methoxytetrahy-dropyranyl ketal eliminates this problem. 

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 ... 

Aconitine contains four methoxyl groups and three hydroxyl groups (triacetyl derivative, m.p. 207-8°). On hydrolysis by water under pressure, or by boiling with dilute acid, it loses 1 mol. of acetic acid and forms benzoylaconine, whilst hydrolysis by alkalis eliminates both acetic and benzoic acids and yields aconine. 

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