Silyl diketonates, rearrangement

As expected, addition of propenyl lithium to ketoacetal 33 occurs preferentially (diastereoselectively = 8 1) from the face opposite the side chain to provide, after silylation, the rearrangement substrate 34. Treatment of this intermediate with SnCl at -23 C for 5 min occasioned the desired reorganization to afford the cw-hydroazulenone 35, as a 5 1 mixture of methoxy epimers, in excellent yield. That the two products produced from 34 were indeed stereoisomers was apparent from their conversion to the same diketone upon RuO oxidation [25]. The alcohol precursor of 34 cannot directly be employed in this ring-enlarging cyclopentane annulation, since it undergoes facile cyclization to form a bicyclic acetal, an intermediate that is a dead-end with regard to transformation to 35. 

Michael reaction. In the presence of (C6H5)3CC104, silyl enol ethers undergo Michael addition to a,p-enones. The adducts can be isolated or rearranged to 1,5-diketones by base. The intermediates cannot be isolated from reactions catalyzed by TiCl4 or CsF. 

The reaction involves a Pummerer-type rearrangement to a a-thiocarbocation, which then reacts with the enol silyl ether. The products can be converted by reduction into 1,4-diketones or by oxidation to unsaturated 1,4-diketones. 

Titanium trichloride fimctions as an excellent reductive Nef alternative reagent. This aqueous reagent is very acidic, so that acid sensitive groups such as ketals and esters do not survive unless an acetate buffer is used. Systems prcme to acid-catalyzed rearrangements may then successfully undeigo the reaction (equation 10).Some veiy sensitive multifunctional compounds have been obtained using this modified Nef procedure (equation 11). A related process is the formation of 1,4-diketones via in situ generation of a nitronatc anion by the Lewis acid catalyzed addition of an enol silyl ether to a nitroalkene (equation 12). ... 

The nucleophilic and electron-accepting properties of heterocyclic nucleophilic carbenes 36 were also used in combination with the electrophilic/nucleophilic character of acylsilanes via Brook rearrangement, leading to the invention of a sila-Stetter reaction by Scheidt and coworkers fScheme 6.24). The iminium structure in 37, generated by addition of the carbene catalyst 36 to the acylsilanes, promotes a Brook rearrangement to afford enol silyl ether 38. The alcohol additive present in the reaction causes desilylation to produce nucleophilic enaminol 39, which adds to a,p-unsaturated ketones to give 40. The formation of aryl ketone expels the carbene catalyst and produces 1,4-diketone 41. 

The photochemical reactivity of P-ketoesters is different form that of P-diketones. Irradiation of a P-ketoester in the presence of an alkene produces oxetane via the ketone carbonyl instead of the desired cyclobutane ring system. Therefore, it is necessary to covalently lock the ketoesters as the enol tautomers. To this end, silyl enol ethers, 129 and 132a, and enol acetates, 130 and 132b, were prepared, but these substrates still fail to undergo the desired intramolecular [2 + 2] photocycloaddition with olefins. The only new products observed in these reactions result from the photo-Fries rearrangement of the cyclic enol acetate (130 to 131) and cis-trans isomerization of both acyclic substrates 132a/b. However, tetronates are appropriate substrates for both intermolecular and intramolecular photocycloadditions with olefins. In addition, enol acetates and silyl enol ethers of p-keto esters are known to undergo [2 + 2] photoaddition with cyclic enones (vide infra). 

Photoinduced reactions of cyclic a-diketones with different alkenes takes place via [2 + 2], [4 + 2] or [4 + 4] photocycloaddition pathways. Photoaddition of electron deficient silyl ketene acetals to 2-, 3- and 4-acetylpyridine generates oxetanes as major products. The reaction is favoured in non polar solvents. The photoreaction between silyl enol ethers and henzil affords [2 + 2] cycloaddition products, while in the case of 9,10-phenanthrenequinone [4 + 2] cycloacidition predominates. Photocycloaddition of p-henzoquinones to hicyclopropylidene affords spirooxetanes (21) as the primacy products further irradiation leads to rearranged spiro[4.5]deca-6,9-diene-2,8-diones. With 9,10-anthraqui-none, in addition to the spirooxetane, a spiro[indan-l,l -phthalan]-3 -one is also obtained. ... 

DeglTnnocenti and co-workers established in 1987 that acylsilanes could react with activated olefins in a cyanide-catalyzed sila-Stetter reaction, and recent work by Scheldt and co-workers has established the utility of thiazolylidine catalysts in this Stetter variation." The reaction is presumed to occur via a catalyst-initiated [l,2]-Brook rearrangement, which serves to provide the silylated acylanion donor intermediate 23. The use of excess 2-propanol in the reaction facilitates silyl transfer and catalyst turnover. Compared to the classical Stetter approach, this variation provides comparable yields of 1,4-diketone products from aromatic (20) and aliphatic (21) acylanion precursors. 

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