Deoxygenation, carbonyl groups

In a study of the deoxygenation of carbonyl compounds by atomic carbon, Dewar and coworkers (8UA2802) presented experimental and theoretical evidence that the carbonyl group can react with carbon atoms to form a carbenaoxirane. 

The reaction appears to be facilitated by a y-carbonyl group. In the absence of this activation, sulfoxide deoxygenation appears to be the favored reaction pathway (equation 130). 

Reduction to Alkanes. Carbonyl groups can be reductively deoxygenated to methylene functions if both of the two steps represented by Eqs. 1 and 2 proceed to completion. With aldehydes, this process leads to the transformation of the CHO group into a CH3 group. 

The transformation of L-arabinose (58) to lactone 57 was based on a route developed by Marquez and Sharma [51] Selective protection of the primary hydroxy group with TBDPSCl and oxidation of the lactol moiety with bromine afforded lactone 59. Subsequent selective deoxygenation a to the carbonyl group proceeded under Barton-McCombie conditions providing lactone 57 in 21% yield (Scheme 14). 

The Tafel rearrangement only occurs in acid medium. Simultaneous reduction of both carbonyl groups leads to interaction and formation of a cyclopropane. Acid catalysed cyclopropane ring opening follows to yield an a-diketone 28 which undergoes the electrochemical Clemmensen reduction step to the hydrocarbon. Side products include the two monoketones derived by partial deoxygenation of the a-diketone and the secondary alcohols from reduction of these raonoketones. Separate experiments show that the a-diketone 28 can be reduced to the hydrocarbon. 

Coriolin (689), a metabolite of the Basidiomycete Coriolus consors, has attracted widespread interest because of its unusual anti-tumor activity and highly functionalized triquinane structure. Accordingly, a number of syntheses of689 have appeared on the scene. One of the earliest, due to Tatsuta, et al., begins with epoxide 690, whose preparation had been earlier realized in connection with their work on hirsutine (see Scheme LXIII). Deoxygenation of 690, hydrolysis, and cis-hydroxy-lation provided keto triol 691 (Scheme LXXII) The derived acetonide was transformed via 692 into tetraol 693 which could be selectively acetylated and dehydrated on both flanks of the carbonyl group. Deacetylation of 694 followed by epoxidation completed the synthesis. 

This reductive coupling involves two steps. The coupling is induced by single electron transfer to the carbonyl groups from alkali metal, followed by deoxygenation of the 1,2-diol with low-valent titanium to yield the alkene. 

Aromatic aldehydes and ketones can also be deoxygenated with hydrogen over a palladium charcoal catalyst. The reaction occurs because the aromatic ring activates the carbonyl group towards reduction. Aliphatic aldehydes and ketones are not reduced in this. 

Reduction of aldehydes and ketones usually occurs by the addition of hydrogen across the carbon-oxygen double bond to yield alcohols, but reductive conversion of a carbonyl group to a methylene group requires complete removal of the oxygen, and is called deoxygenation. 

Sodium (cyclopentadienyl)dicarbonylferrate, dicobaltoctacarbonyl, and iron pentacarbonyl have been reported to deoxygenate epoxides efficiently to the corresponding alkenes. The reaction with (cyclopentadienyl)dicarbonylfeirate proceeds with inversion of stereochemistry, whereas Fe(CO)5 shows low stereoselectivity. Both Co2(CO)8 and Fe(CO)s are applicable to epoxides having carbonyl groups (equations 45 and 46). 

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