Clemmensen reduction cyclic ketones

Cyclic ketones containing up to 31 C-atoms in the ring have been prepared from the corresponding dicarboxylic acids. By the Clemmensen reduction (Reaction LVIII. (c)) the ketones have been converted into cyclic hydrocarbons. These large ring compounds are remarkably stable. (H. Acta, 11, 496,670 13,1152 17,78.)... 

Clemmensen reaction. The Clemmensen method of reduction (1913) consists in refluxing a ketone with amalgamated zinc and hydrochloric acid. Acetophenone, for example, is reduced to ethylbenzene. The method is applicable to the reduction of most aromatic-aliphatic ketones to at least some aliphatic and alicyclic ketones, to the y-keto acids obtainable by Friedel-Crafts condensations with succinic anhydride (succinolylation), and to the cyclic ketones formed by intramolecular condensation. 

Haworth phenanthrene synthesis. Acylation of aromatic compounds with aliphatic dibasic acid anhydrides to (i-aroylpropionic acids, reduction of the carbonyl group according to Clemmensen or Wolff-Kishner procedures, cyclization of the y-arylbutyric acid with 85% sulfuric acid, and conversion of the cyclic ketone to polycyclic hydroaromatic and subsequently to aromatic compounds. 

When cyclic anhydrides are used as one component, the Friedel-Crafts acylation provides a means of adding a new ring to an aromatic compound. One illustration is shown here. Note that only the ketone is reduced in the Clemmensen reduction step. The carboxylic acid is unaffected. The same result can be achieved using the Wolff-Kishner reduction. 

With 1,4-diketones the distribution of the reduction products is dependent on the stereochemical situation of the two carbonyl functions. In acyclic derivatives, with no stereochemical interaction between the two carbonyl functions, the ketone groups are independently reduced to give methylene products in the usual manner. On the other hand, cyclic 1,4-diketones react differently. For example, cyclohexane-1,4-dione (22) suffers ring opening to give hexane-2,5-dione and hex-5-en-2-one derivatives, and products of further reduction are also detectable (equation 11). A 1,4-diketone (23) in which the two carbonyls are stereochemically close, gave the diketone (24) under relatively mild conditions (Zn/AcOH, 25 C), formed by the same C—C bond cleavage as seen in cyclohexane-1,4-dione. Under Clemmensen conditions this derivative was then converted to cyclobutane-1,4-diol (25 equation 12) in 98% yield, which is closely related to the aforementioned cyclopropanediol intermediate. ... 

An interesting transannular reductive rearrangement of bridged cyclic a-amino ketone 18 under classic Clemmensen conditions was developed, which provided the all-tram ring-fused azepinoindole derivative 19 as the sole isolated product (13JOC6154). 

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