Degradation 3-eliminative

Almost from the first disclosures of phase transfer catalytic processes, alternate catalysts have been sought which it was hoped would be more selective, less expensive, more stable, etc. Notable among these alternative catalysts were the macrocyclic "crown polyethers. More thermally stable and less prone to undergo degradative elimination than ammonium salts, they are also more expensive than many catalysts which are their equal in efficacy at lower temperatures. 

Chlorides can be prepared in this way from primary and secondary, but not tertiary, alcohols. In practice, an equivalent of a weak base, such as pyridine (azabenzene), is added to neutralize the hydrogen chloride that is formed. If the acid is not removed, undesirable degradation, elimination, and rearrangement reactions may occur. 

Uleine methiodide underwent a facile Hofmann degradation to give an optically inactive compound (CCXLVIII), which retained the two nitrogen atoms and had carbazole UV-absorption. A further Hofmann degradation eliminated Nb and gave a product (CCXLIX) with somewhat extended carbazole absorption, which underwent ready catalytic reduction with saturation of one double bond to give a substituted... 

In 1961, Stacey and coworkers investigated the stereochemistry of desosamine by characterizing certain of the products of its alkaline degradation. Elimination of the dimethylamine by treatment with 0.05 N sodium hydroxide at 50° for 24 hours led to the formation of an acid (17), probably by the mechanism suggested by Woodward for a similar elimination of mycaminose. Reduction of this acid with lithium aluminum hydride afforded an erythro-threo mixture of ( —)hexanetriols (18) having vicinal hydroxyl groups (as shown by the rapid consumption of 1 mole of periodate... 

A second exhaustive methylation and Hofmann degradation eliminates the nitrogen of a-methyltropidine as trimethylamine. These last two reactions demonstrate that the tertiary nitrogen of tropidine (and hence of tropine)... 

Exhaustive methylation of a-methyltropidine followed by a Hofmann degradation eliminates the nitrogen as trimethylamine and the low boiling cycloheptatriene (tropilidene) was isolated in good yield (22,121). Tropane may be converted through the intermediate methyltropane to A -cyclo-heptadiene (hydrotropilidene) by a similar process. In the conversion of methyltropane methohydroxide to cycloheptadiene (139), 20% of the methyltropane is reformed by the loss of methanol, a side reaction which occurs in most Hofmann degradations to a greater or lesser extent. Cycloheptatriene and cycloheptadiene have been reduced to cycloheptane (194) which in turn has been oxidized (nitric acid) to pimelic acid. 

Fio. 4. Alkaline degradation ( -elimination) of peptide-bonded O-glucosyl serine. 

Hoechst slurry process Hoeppler viscometer Hoffman degradation Hoffman eliminations Hofmann degradation... 

Extraction of hemiceUulose is a complex process that alters or degrades hemiceUulose in some manner (11,138). Alkaline reagents that break hydrogen bonds are the most effective solvents but they de-estetify and initiate -elimination reactions. Polar solvents such as DMSO and dimethylformamide are more specific and are used to extract partiaUy acetylated polymers from milled wood or holoceUulose (11,139). Solvent mixtures of increasing solvent power are employed in a sequential manner (138) and advantage is taken of the different behavior of various alkaUes and alkaline complexes under different experimental conditions of extraction, concentration, and temperature (4,140). Some sequences for these elaborate extraction schemes have been summarized (138,139) and an experimenter should optimize them for the material involved and the desired end product (102). 

Temperatures should not exceed 60°C for the Type I resins, and 40°C for Type II and acryflc resins. Thermal degradation and the loss of functional groups occur when these temperatures are exceeded. Elimination of siUca from the resin bed is further improved by preheating the bed with warm water before injecting the NaOH solution. 

Mercury from these accumulated wastes is generally best recovered by total degradation in stills, where metallic mercury is condensed and collected. The recovery costs are amply compensated by the value of the metal recovered. Moreover, disposal problems are either eliminated or severely diminished. 

Some cleavage takes place even if the phenoHc hydroxyl is blocked as an ether link to another phenylpropane unit and quinonemethide formation is prevented. If the a- or y-carbon hydroxyl is free, alkaH-catalyzed neighboring-group attack can take place with epoxide formation and P-aryloxide elimination. In other reactions, blocked phenoHc units are degraded if an a-carbonyl group is present. 

Chemical Properties. Reactions of quaternaries can be categorized iato three types (169) Hoffman eliminations, displacements, and rearrangements. Thermal decomposition of a quaternary ammonium hydroxide to an alkene, tertiary amine, and water is known as the Hoffman elimination (eq. la) (170). This reaction has not been used extensively to prepare olefins. Some cycHc olefins, however, are best prepared this way (171). Exhaustive methylation, followed by elimination, is known as the Hoffman degradation and is important ia the stmctural determination of unknown amines, especially for alkaloids (qv) (172). 

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