Reduction of an enone

The key step in a short and efficient synthesis of pleraplysillin-1 (127) is the palladium-catalyzed cross-coupling of vinylstannane 125 with vinyl triflate 126 (see Scheme 33). This synthesis is noteworthy in two respects. First, vinyl triflate 126 is generated regio-specifically from the kinetic enolate arising from a conjugate reduction of enone 124 the conjugate reduction of an enone is, in fact, a... 

The 1,4-reduction of an enone in the presence of a ketal is shown in Eq. 275.436 The Et3SiH/TFA system reduces the polyfunctional cyclic ,/3-enone in Eq. 276 without affecting the c/-hydroxy carboxyl or a-chloro keto groups.453... 

The intermediate enolate or enol ether from the initial reduction of an enone may be alkylated in situ (Eq. 281).455 / -Substituted cyclopentenones may be asymmetrically reduced and alkylated459 (see section on asymmetric reductions of enones). Enolates may also be trapped with an aldehyde in a reductive aldol condensation of an enone with an aldehyde,455 permitting a regioselective aldol condensation to be carried out as shown in Eq. 282.455 This class of reductive aldol condensation reactions can also occur in a cyclic manner (Eq. 283).460... 

A further example of the trapping of the in situ generated silyl enol ether from the reduction of an enone is the conversion of an enone into an a-hydroxy ketone via oxidation of the silyl enol ether (Eq. 286).465... 

Diastereospecific reduction of an enone. A key step in a synthesis of the lichen macrolide (+ )-aspicilin (4) is the reduction of the oxodienelactone 2 to the dienol 3 with Yamamoto s reagent (1) in 96% yield. Dihydroxylation of 3 with 0s04 and pyridine followed by H,S reduction gives the triol (4) in 40% yield. 

Enantioselective reduction of an a, -enone. One of the final steps in a synthesis of palytoxin, a toxin of marine soft corals containing 115 carbon atoms and 60 chiral centers, involves, in addition to the usual deprotections, enantioselective reduction of an enone to an allylic alcohol. A mixture (1 1) is obtained with borohydrides, but lithium borohydride combined with EuC13 provides an 8 1 mixture, with the desired isomer being favored. 

Deoxygenation. Greene has applied Kabalka s method for reduction of an enone (6, 98 7, 54) to the tosylhydrazone of a cross-conjugated ketone, 6-epi-a-santonin (2). Unexpectedly only one olefin (3) was obtained in 50% yield. This product was converted into the diene 4 by allylic oxidation, reduction to an allylic alcohol, and dehydration. The product was converted into (—)-dictyolene (5) by a method developed previously. ... 

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

The reduction of a,p-unsaturated aldehydes and ketones by NaBH4 leads, in general, to substantial amounts of fully saturated alcohols. In alcoholic solvents, saturated -alkoxy alcohols can be formed via conjugate addition of the solvent. This latter process becomes the main reaction path when reduction is performed in 2-propanol in the presence of sodium isopropoxide. In base, a homoallylic alcohol can become the major product of borohydride reduction of an enone. Analysis of the influence of substrate structure on NaBH4 reduction has shown that increasing steric hindrance on the enone increases 1,2-attack. ... 

Stereoselective reduction of an enone lactone was a key step in the construction of the 20-hydroxyec-dysone side chain. Totally different mixtures of products were obtained when the reduction was carried out with sodium borohydride or by catalytic hydrogenation (Scheme 30). In all cases, the 1,4-reduction mode is preferred. With borohydride, however, this process is followed by a subsequent reduction of the saturated ketone and base-catalyzed rearrangement of the 5-lactone into a y-lactone. 

Scheme 40 illustrates an interesting two-step selective reduction of an enone system, first with sodium hydride and NaAlH2(0CH2CH20Me)2 and then with the same reagent in the presence of 1,4-diazabicy-clo[2.2.2]octane. Specific reduction, however is not achieved with NaBH4, UBH4, LiBHBu s or 9-BBN-H.250... 

In contrast to the typical palladium(0)-catalyzed cross-coupling reaction of vinyl triflates with organosilanes, this is an example of the reduction of an enone to an alkene by transfer of a proton instead of an alkyl moiety/ ... 

Another approach for annulation at the less substituted carbon is to first form the kinetic enolate using one of several methods (e.g., LDA, Birch reduction of an enone, conjugate addition of an organocuprate to an enone) followed by reaction with MVK and cyclization. 

Reduction of an enone, in preference to a formyl group, with Bu2SnIH was re-... 

A classic method for generating regio-defmed enolates is metal-ammonia reduction of an enone. Stork and d Angelo found that the enolate resulting from lithium-ammonia-r-butyl alcohol reduction of octalone (3), followed by evaporation of ammonia, suspension of the enolate in ether and treatment with... 

One important synthetic problem often encountered is the selective 1,2-reduction of an enone (eq. [32] ). QVi ... 

---