Kishner eliminative

In 1913, Kishner observed in one instance that under standard Wolff-Kishner reduction conditions, 2-hydroxy-2,6-dimethyloctan-3-one underwent eliminative reduction upon treatment with hydrazine hydrate and base at elevated temperatures to afford 2,6-dimethyloctan-2-ene (Scheme 7). This same reaction was later found to occur in the case of a-methoxy ketones and has since been referred to as the Kishner eliminative reduction. The reaction entails initial formation of the hydrazone and elimination of the a-substituent to afford the intermediate alkenyldiazene, which subsequently collapses to the desired alkene. Given the facile transformation of ketones into a-halo ketones, these conditions have been used to introduce alkenes regioselectively in the 2a-halocholestan-3-one series as shown in Scheme 8. Yields of 2-cholestene parallel the resistance of the a-halogen to undergo competitive elimination reactions. 

Similar conditions have also been employed in the reduction of a-tosyl ketones, but only low yields were realized. On the other hand, a wide variety of other a-substituents have been shown to undergo reductive elimination to afford good yields of the alkenic product (Scheme 9). The temperature at which the reaction occurs in these examples depends on the nature of the substituent and its ease of elimination. Kishner eliminative reduction conditions have also been applied to 2-azacycloalkanones, for which ring size determines the course of reaction. While 1-azacyclohexan-3-ones give only the Wolff-... 

This methodology was initially applied to a synthesis of (—)-alloyohimbane (82) (Scheme 3.13) (29). Readily available levoglucosenone (75), a product of cellulose pyrolysis, was the starting material. Diels-Alder reaction of 75 with 1,3-butadiene afforded adduct 76 which underwent Wolff-Kishner elimination and subsequent acylation to provide the bicyclic enol ether 77. Hydrolysis followed by oxidation and saponification afforded hydroxymethyl lactone 78 which was then condensed with tryptamine to yield the amide diol 79. Periodate cleavage of the diol was followed by conversion to chloride 80 which was cyclized to afford lactam 81. Subsequent Bischler-Napieralski cyclization followed by hydrogenation of the olefin moiety afforded the target ( )-alloyohimbane (82). 

The Wolff-Kishner reduction is an important alternative method to the Clem-mensen reduction, and is especially useful for the reduction of acid-labile or high-molecular substrates. Yields are often below 70%, due to various side-reactions such as elimination or isomerization reactions. ... 

The procedure outlined is much quicker and simpler than previous methods Starting materials are readily available, and the preparation can be run on any scale in the length of a day. Because exclusion of a basic catalyst eliminates the Wolff-Kishner reduction of the cinnamalhydrazone, separation of the 5-phenylpyrazoline from cinnamalhydrazone, or of phenylcyclopropane from propenylbenzene, does not have to be effected. The present procedure can also be used to convert other ring-substituted cinnamaldehydes to the corresponding arylcyclo-propanes... 

Enol (9) comes from available dkione (10) and the synthesis was performed on the enol acetate (11) rather than enol (9). Elimination with strong base gave (6) and the synthesis was completed by Wolf-Kishner removal of the carbonyl group. 

Mesylation of alcohol 14 and hydrolysis of the acetal with oxalic acid leads to the key decalone 16 which cyclises in the presence of NaH in dioxane to afford twistanone 17 in quantitative yields. The elimination of the carbonyl group was carried out by an alternative method to the Wolff-Kishner reduction, which involves the thioacetalisation of the twistanone (18) with ethane-1,2-dithiol and removal of the resulting thioacetal by Raney nickel in boiling ethanol (62% yield). 

This reaction sequence illustrates how the rates of many base-catalyzed reactions can be enhanced greatly by substitution of dimethylsulfoxide for the usual hydroxylic solvents. Other examples of the enhanced reactivity of anions in dimethylsulfoxide are found in Wolff-Kishner reductions and Cope elimination reactions. The present reaction illustrates the generation of an aryne intermediate from bromobenzene. ... 

Aldehydes and ketones on complete reduction give alkanes by three different methods that are complementary to each other. The Woff-Kishner reduction is done under basic conditions and is suitable for compounds that might be sensitive to acid (Following fig.). The reaction involves the nucleophilic addition by hydrazine followed by elimination of water to form a hydrazone. The mechanism is the... 

A slighdy more vigorous method, known as the Wolff-Kishner reduction, is driven by the elimination of nitrogen gas from a hydrazone. Hot concentrated sodium hydroxide solution deprotonates the hydrazone, which can then eliminate an alkyl anion—a reaction you would usually be wary of writing, but which is made possible by the thermodynamic stability of N2. 

Wolff-Kishner reduction gave the open-chain alcohol LXXX. Elimination of the hydroxyl via the corresponding ketone generated dihydro-corynantheane (XV) (50). 

Ketones bearing electronegative substituents at the "a -carbon atom commonly undergo the Kishner reductive elimination [2] on treatment with hydrazine under very mild conditions. The reaction, applied to a,j5 -epoxyketones... 

Elimination of adjacent substituents (Kishner-Leonard eliminations)... 

---