Deoxygenation, selective ketones

Deoxygenation of ketones. A modified Clemmensen reduction uses zinc and A1C1.1 6H2O in aqueous THF. Hydrochloric acid is generated in situ. On the other hand, ultrasound promotes the reduction with Zn-HOAc in a selective manner (of 3-oxosteroids in the presence of ketone group at C-17 or C-20). Transpositional reduction of steroidal 4-en-3-ones (to 3-enes) can be achieved in 15 minutes. 

Deoxygenation of ketosteroids. The deoxygenation of ketones with this combination has been extended to ketosteroids. -Cholestene can be obtained in about 70% yield from 5a-cholestane-3-one with trimethylchlorosilane and zinc dust in THE at reflux. The reaction fails with 6-, 7-, 12-, 17-, and 20-ketones. This result suggests that a very bulky intermediate is involved. Thus it is possible to selectively deoxygenate diketones. For example, A -Sa-cholestene-... 

Trichloro(methyl)silane-Sodium iodide, 11, 553-554. This in situ equivalent of io-dotrimethylsilane is also effective for cleavage of esters and lactones, selective conversion of tertiary and benzylic aleohols into iodides, dehalogenation of a-halo ketones, deoxygenation of sulfoxides, and conversion of dimethyl acetals to carbonyl compounds. ... 

Deoxygenation of sulfoxides. Tris(phenylseleno)borane is an effective reagent for the reduction of sulfoxides under mild conditions. Although this reagent converts ketones to selenoketals, selective deoxygenation of keto sulfoxides is possible. Deoxygenation of vinyl sulfoxides also proceeds smoothly. 

Apart from sodium borohydride, which is frequently used in water or water-alcohol mixtures to reduce ketones or aldehydes selectively, water is rarely used as the solvent in reductions, because of incompatibility with most reducing agents. However, samarium iodide reduction of ketones, as well as alkyl and aryl iodides is accelerated in water [99]. Likewise, the a-deoxygenation of unprotected aldonolac-tones is efficient when the SmI2-tetrahydrofuran-water system is used [100],... 

Nevertheless, it must be pointed out that the formation of such transient species has never been spectroscopically observed. Native CDs are effective inverse phase-transfer catalysts for the deoxygenation of allylic alcohols, epoxydation,or oxidation " of olefins, reduction of a,/ -unsaturated acids,a-keto ester,conjugated dienes,or aryl alkyl ketones.Interestingly, chemically modified CDs like the partially 0-methylated CDs show a better catalytic activity than native CDs in numerous reactions such as the Wacker oxidation,hydrogenation of aldehydes,Suzuki cross-coupling reaction, hydroformylation, " or hydrocarboxylation of olefins. Methylated /3-CDs were also used successfully to perform substrate-selective reactions in a two-phase system. 

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