Enone reduction

A variety of conjugated dienones are reduced by lithium-ammonia, presumably via dienyl carbanions analogous to the allyl carbanions encountered in enone reductions. Cross-conjugated l,4-dien-3-ones afford 4-en-3-ones as the major reduction products, indicating that the cyclohexadienyl carbanion (55) protonates largely at C-1. Some protonation at C-5 does occur as shown by examination of the NMR spectrum of the crude reduction product derived from the 17-ethylene ketal of androsta-l,4-diene-3,17-dione. The 17-ethylene ketal of androst-4-ene-3,17-dione is formed in 75%... 

ENONE REDUCTION-ENOLATE ALKYLATION SEQUENCE 2 - ALLYL- 3-METH YLC Y CLOHEX AN ONE... 

The present procedure utilizes the enone reduction-cnolate alkylation... 

Asymmetric reduction of a,fi-enon s. This combination of reagents (1 1) in conjunction with N-cthylaniline (2 equivalents) reduces alkyl aryl ketones to alcohols with high stereoselectivity.1 Under these conditions 2,/1-unsaturated ketones arc reduced to optically active (S)-allylic alcohols. Optical yields of 80 98% have been reported for open-chain enones. Reduction of cyclic enones is somewhat less efficient. The method was used to reduce 1 to 2, which has been used as an intermediate in an anthracyclinone synthesis.2... 

Reduction of enones. Reduction of CH3Cu, prepared in situ from Cul and CH3Li in THF, with DIBAH in the presence of HMPA results in a form of copper hydride that effects efficient and selective 1,4-reduction of enals, enones, and enoates, and 1,6-reduction of dienones and dienoates. The reagent does not reduce isolated carbonyl groups or double bonds.2 It can also be used for regiospecific preparation of enol silyl ethers from an enone.2... 

Scheme 13.2 Examples of successful Corey-Bakshi-Shibata enone reductions. Ar = / -PhCeH4. ...

Swiderska, M. A., and Stewart, J. D. 2006a. Stereoselective enone reductions by Sac-charomyces carlsbergensis old yellow enzyme./. Mol. Catal. B Enzym., 42, 52-54. 

Reductions. This hydride is a strong reducing agent comparable to other lithium trialkylhydrides. It is superior to DIBAH for selective 1,2-reduction of enones. Reduction of ketones, esters, acid chlorides, and anhydrides proceeds at -78°. However, ketones can be reduced selectively in the presence of an ester. Esters are reduced to a mixture of an alcohol and an aldehyde. Complete reduction to an alcohol can be effected by reduction at -78° with 2 equiv. of 1 and then with excess sodium borohydride. Tertiary amides are reduced by 1 equiv. of the reagent to aldehydes in generally high yield. Selective reduction of primary halides in the presence of secondary halides is possible. 

Stereospecific reduction of an enone. Reduction of the hydroazulenone 1 with lithium aluminum hydride (or NaBH4, DIBAH) gives only the allylic alcohol 2 in... 

Many other examples of chemoselective enone reduction in the presence of other reducible functionalities have been reported. For instance, the C—S bonds of many sulfides and thioketals are readily cleaved by dissolving metals. " Yet, there are examples of conjugate reduction of enones in the presence of a thioalkyl ether group." " Selective enone reduction in the presence of a reducible nitrile group was illustrated with another steroidal enone. While carboxylic acids, because of salt formation, are not reduced by dissolving metals, esters" and amides are easily reduced to saturated alcohols and aldehydes or alcohols, respectively. However, metal-ammonia reduction of enones is faster than that of either esters or amides. This allows selective enone reduction in the presence of esters"" and amides - -" using short reaction times and limited amounts of lithium in ammonia. 

The intemietallic hydride LaNijHe is an effective reagent for conjugate reduction of enones. Reduction of the resulting saturated carbonyl occurs very slowly with this reagent, giving high yields of the 1,4-reduction product. ... 

Studies on the role of a Ru catalyst as well as the mechanism of hydrogen transfer in enone reduction with benzyl alcohol at 170-190 °C revealed that RuCl2(PPh3)3 is converted by the primary alcohol into RuH2(CO)(PPh3)3, which then hydrogenates benzylideneacetone. ... 

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