A,P-Enone

Further insight into the P-borylation reaction of a,P-enones (Scheme 2.32) showed that the reaction can be carried out in THF, and the catalyst generated in situ from CuCl (5 mol%), the imidazolium salt (5 mol%), and NaO Bu (10 mol%), to form the [Cu(O Bu) (NHC)] as the catalysis initiating species. In this case, stable boron enolate products are formed which need to be hydrolysed by methanol to the ketone products [114]. 

As discussed before, a,P-unsaturated ketones are usually reduced to saturated ketones in isopropyl alcohol as solvent, the reaction proceeding via the enol after H-abstraction by the carbonyl group. When some selected a,p-enones are irradiated in their n-n absorption band (i.e. excitation... 

Cyclic a,P-enones photodimerize via the Tt-state, as in (4.23)426) and (4.24)427). By chosing the appropriate substituents the reactions proceed regio- and stereoselectively. In contrast, pyrimidine bases dimerize from the first excited singlet state to afford head to tail products (4.25) 428). 

E. J. Corey, F.-Y. Zhang, Mechanism and Conditions for Highly Enantioselective Epoxidation of a,p-Enones Using Charge-Accelerated Catalysis by a Rigid Quaternary Ammonium Salt , Org. Lett. 1999,1,1287-1290. 

Michael reactions of silyl enol ethers.1 2 3 The silyl enol ether of 1-acetylcy-clohexene (1) undergoes two consecutive Michael reactions with an a,p-enone or -enal in the presence of this Lewis acid to form 1-decalones. 

Silyl enol ethers.1 Aldehydes, ketones, ot,p-enals, and a,p-enones are converted into the silyl enol ethers in moderate to high yield by reaction with iodo-trimethylsilane, generated in situ, and N(C2H5)3 at 25° in acetonitrile. In some cases intermediate stable 1,2- or 1,4- adducts can be isolated. Thus the 1,2-adduct a hag been isolated as an intermediate in the reaction of some aldehydes and shown to decompose to the silyl enol ether. 

Vinylcyclopropanation cyclopentenes. The lithium dienolate of 1 adds to a,p-enones to form an a-keto vinylcyclopropane, which on pyrolysis (550°) provides a bicyclic keto acrylate (equation I).1... 

Manganese-copper-catalyzed conjugate addition of organomagnesium reagents to a,p-enones is a general synthetic method. Table I shows that the reaction can be successfully performed even with enones of low reactivity. 30% Manganese chloride is necessary to obtain good yields. In most cases, however, 0.1% cuprous chloride is sufficient.3... 

Figure 3.15. Scope of Rh/(S)-binap-catalyzed asymmetric 1,4-addition of organoboronic acids to a,P-enones.

Figure 3.17. Scope of [Rh(OH)((5)-binap)]2-catalyzed as5dnmetric 1,4-addition of aryl-boroxines to a,P-enones.

By tethering a,p-enones with another carbonyl-containing moiety, Rrische developed rhodium-catalyzed asymmetric 1,4-addition/aldol cyclization reactions." These reactions proceed with high diastereo- and enantioselectivity, furnishing structurally complex cyclic compounds in a single step [Eqs. (3.11-3.13)]. 

Figure 3.18. Chiral ligands that exhibit high enantioselectivity in the rhodium-catalyzed 1,4-addition of arylboronic acids to a,P-enones.

Not only organoboron reagents but also organosilicon reagents are suitable nucleophiles in the rhodium-catalyzed asymmetric 1,4-additions to a,p-enones in... 

A rhodium-catalyzed asymmetric synthesis of chiral titanium enolates by 1,4-addition of aryltitanium reagents to a,p-enones under aprotic conditions has... 

Figure 3.28. Scope of Rh/(5)-binap-catalyzed as5mmetric 1,4-addition of ArTi(Ot-Pr)4Li to a,P-enones in the presence of Me3SiCl.

The cationic chiral Lewis acids 10, generated from the corresponding oxazaboroli-dines by protonation by trifluoromethanesulfonic acid, are excellent catalysts for the enantioselective reaction of 2-substituted acroleins, a-unsaturated a,p-enones, a-unsaturated acrylic acid esters, and a-unsaturated acrylic acids with a variety... 

The transformations of 8-hydroxy-a,P-enones to the corresponding internal Michael adducts were performed at 20mol% loading of C9-epi-quinine-thiourea 121 in toluene at increased reaction temperature (50°C) using 3,4,5-trimethoxy-phenylboronic acid for ahphatic and phenylboronic acid for aromatic enones. Under these conditions, this protocol furnished the desired (R)-configured adducts 1-5 in yields ranging from 73 to 86% and ee values of 84—96% (Scheme 6.143) [294]. Product 5 in Scheme 6.143 was identical in all respects with (+)- S)-streptenol A, one of four known streptenols produced by Streptomyces luteogriseus that has attracted attention as an immunostimulant as well as an inhibitor of cholesterol biosynthesis and tumor cells [295]. 

Scheme 6.142 Product range of the catalyzed asymmetric intramolecular Michael addition of y-hydroxy-a,P-enones catalyzed by 121.

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