Enolate silylated: oxidative coupling with

Since enol silyl ethers are readily accessible by a number of methods in a regioselective manner and since the trialkylsilyl moiety as a potential cationic leaving group facilitates the termination of a cyclization sequence, unsaturated 1-trialkylsilyloxy-1-alkenes represent very promising substrates for radical-cation cyclization reactions. Several methods have been reported on the synthesis of 1,4-diketones by intermolecular oxidative coupling of enol silyl ethers with Cu(II) [76, 77], Ce(IV) [78], Pb(IV) [79], Ag(I) [80] V(V) [81] or iodosoben-zene/BFa-etherate [82] as oxidants without further oxidation of the products. 

This reaction of cyclohexene with (PhI + )20 2BF4 and lithium perchlorate gave exclusively the cz s-bis-perchlorate adduct (92%). Also, silyl enol ethers underwent efficient oxidative coupling to 1,4-diketones [24] ... 

The stemona alkaloid stemonamide (49) was synthesized starting from a-stannyl acetate 47 and 2-stannyl pyrrolidine 48. The oxidative coupling of stannyl acetate 47 with acetylenic silyl enol ether affords the functionalized C-7 unit which corresponds to the side arm of the pyrrolidine ring. Then, introduction of the C-7 unit to the pyrrolidine ring is performed by the oxidative generation of acyliminium ion. The carbon skeleton of stemonamide was thus constructed efficiently as shown in Scheme 19 by employing organotin compounds. ... 

Oxidative coupling of silyl enol ethers as a useful synthetic method for carbon-carbon bond formation has been known for a long time. Several oxidants have been successfully applied to synthesize 1,4-diketones from silyl enol ethers, e.g. AgjO [201], Cu(OTf)2 [202], Pb(OAc)4 [203] and iodosobenzene/BFj EtjO [204]. Although some of these reagents above are known to react as one-electron oxidants, the potential involvement of silyl enol ether radical cations in the above reactions has not been studied. Some recent papers, however, have now established the presence of silyl enol ether radical cations in similar C-C bond formation reactions under well-defined one-electron oxidative conditions. For example, C-C bond formation was reported in the photoinduced electron transfer reaction of 2,3-dichIoro-1,4-naphthoquinone (98) with various silyl enol ethers 99 [205], From similar reactions with methoxy alkenes [206,207] it was assumed that, after photoexcitation, an ion radical pair is formed. 

Oxidative coupling of silyl bis-enolates to 1,4-diketones (equation 25) occurs with a variety of oxidizing agents such as [Fe(phen)3](PFe)3, (NH4)2[Ce(N03)6] and Cu(OTf)2 . The high rf,Z-diastereoselectivity of the coupling is attributed to the energy difference in the transition states of the two diastereomorphic approaches. The steric interactions between... 

For the sake of completeness, it should be mentioned that at variance with the corresponding silyl enolates, the oxidation of titanium bis(enolates) with a variety of oxidants does not show any diastereoselectivity in the formation of the enolate coupling product . On the basis of crossover experiments, it has been shown that the C—C bond formation occurs via an intramolecular route in the case of the silyl derivatives and intermolecularly in the case of the titanium derivatives. 

This ligand coupling method was quite efficient for acetophenone and rerf-butylketone derivatives. However, coupling of the TMS ether of cyclohexanone (121) failed. The oxidative coupling of the TMS ether of cyclohexanone (121) to give 2,2 -bicyclohexanone was successful only when this silyl enol ether was treated with the iodosobenzene-tetrafluoroborate complex.230,231... 

Six-membered chiral acetals, derived from aliphatic aldehydes, undergo aldol-type coupling reactions with ot-silyl ketones, silyl enol ethers,110 and with silyl ketene acetals111 in the presence of titanium tetrachloride with high diastereoselectivities (equation 41) significant results are reported in Table 20. This procedure, in combination with oxidative destructive elimination of the chiral auxiliary, has been applied... 

Oxidative coupling. Methods for ketone synthesis by trapping electrophilic species generated by CAN oxidation with silyl enol ethers are quite useful. 2-Tributylstannyl-l,3-dithiane and l-trimethylsiloxy-l,3-butadiene are exemplary substrates. 

Oxidative Coupling. Both intermolecular and intramolecular oxidative coupling reactions can be effected using Cu(OTf)2. Examples of dimerization include one-pot syntheses of 1,4-diketones from ketone enolates or from silyl enol ethers (eqs 1 and 2), and coupling of allylstannanes with TMS-enol ethers... 

Yadav, J. S., Subba Reddy, B. V., Mandal, S. S., Basak, A. K., Madavi, C., and Kunwar, A. C. (2008). Dess-Martin periodinane promoited oxidative coupling of Bayhs-Hillman adducts with silyl enol ethers A novel synthesis of dr-fused dihydropyrans. Synlett, 1175-1178. 

Yasu Y, Koike T, Akita M (2012) Sunlight-driven synthesis of y-diketones via oxidative coupling of enamines with silyl enol ethers catalyzed by [Ru(bpy)3]. Chem Commun 48 5355-5357... 

Silyl enol ethers as electron-rich olefins are susceptible to one-electron oxidation by metallic oxidants [121-123]. The chemoselectivity in the oxidative transformations is controlled by the redox potentials of the reactants. VO(OEt)Cl2 induces chemose-lective homo- or cross-coupling of silyl enol ethers as shown in Scheme 2.58 to give the 1,4-diketones via regioselective carbon-carbon bond formation [124]. The more highly substituted the silyl enol ethers 68 are, the more readily they are oxidized. The silyl ketene acetals 73 are also readily oxidized and undergo cross-coupling with silyl enol ethers 69 to give the y-keto esters 74 (Scheme 2.59). 

Treatment of O-silyl enols with silver oxide leads to radical coupling via silver enolates. If the carbon atom bears no substituents, two such r -synthons recombine to symmetrical 1,4-dicarbonyl compounds in good vield (Y. Ito, 1975). 

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