Ethers silyl dienol

Small groups on silicon favor the formation of the corresponding silyl dienol ether, formed by a Brook rearrangement of 81, and this reaction pathway is minimized by employment of P(OPh)3. 

TITANIUM-MEDIATED ADDITION OF SILYL DIENOL ETHERS TO ELECTROPHILIC GLYCINE A SHORT SYNTHESIS OF 4-KETOPIPECOLIC ACID HYDROCHLORIDE (Pipecolic acid, 4-oxo-, hydrochloride)... 

Bismuth-Catalyzed Mukaiyama Aldol Reaction of Dioxinone-Derived Silyl Dienol Ethers... 

Hydroperoxylation of silyl dienol ethers was effected by the in t7/ -generated reagent triphenyl phosphite ozonide (Equation 26). The yields are moderate and the products are always accompanied by the hydroxylated equivalents. The mechanism was studied and it was found that the oxygen attached to the carbon came from the central O of the ozonide <2001JOC3548>. 

The regioselectivity of alkylations of silyl dienol ethers has been studied87,88. These reactions favor y-alkylation products. In contrast, alkylations of the corresponding lithium enolates mainly occur in the a-position. Substituents on the silyl diene unit, as well as the substituents at the silicon, strongly influence the regioselectivity of the reaction87 91,... 

It is important to note that, under the same conditions, isophorone or other /3-alkyl cyclic enones predominantly afford the exocyclic conjugated silyl dienol ether (Scheme 3). 

On the contrary, the endocyclic conjugated silyl dienol ether is almost exclusively formed under the Kharasch conditions (Scheme 4). 

The endocyclic silyl dienol ether is also obtained as the major product when the conjugated cyclic enone is successively treated with the Fe complex prepared according to Scheme 2, then with one equivalent of methylmagnesium bromide (Table 2). This is interesting since these products are difficult to obtain otherwise. The mechanism of this unusual reaction remains obscure. 

Recently, Filrstner and coworkers have prepared a super-ate complex of iron(II) as shown in Scheme 5. The structure was fully characterized by X-ray crystallography. They have shown that methylmagnesium bromide reacts with pulegone in the presence of this complex to give the corresponding endocyclic silyl dienol ether. Consequently, they have proposed that a similar ate-complex is probably involved when the reaction is performed under the Kharasch conditions. 

The electron-rich aryl ring assists the Lewis acid-induced formation of the reactive intermediate oxocarbenium ion. Ring closure occurs via Mukaiyama-type trapping of the oxocarbenium ion by the silyl dienol ether. 

Regio-, enantio-, and diastereo-selective vinylogous aldol additions of silyl dienol ethers to aldehydes use a Lewis base (a chiral bis-BINAP-phosphoramide) to activate a Lewis acid (silicon tetrachloride).139... 

Catalytic, enantioselective addition of silyl ketene acetals to aldehydes has been carried out using a variant of bifunctional catalysis Lewis base activation of Lewis acids.145 The weakly acidic SiCU has been activated with a strongly basic phor-phoramide (the latter chiral), to form a chiral Lewis acid in situ. It has also been extended to vinylogous aldol reactions of silyl dienol ethers derived from esters. 

Silyl dienol ethers.2 The isomerization of 1,3-dienes by (naphthalene)chro-mium(CO)3 (14, 25) has been extended to (silyloxy)methylbutadienes. Thus (1Z)-1-(silyloxymethyl)-butadienes such as 1 are isomerized exclusively to silyl dienol... 

The precursors (1) to the silyl dienol ethers are prepared as a mixture of (E)- and (Z)-isomers by Wittig reactions with R3SiOCH2CHO. 

Coupling of ketones with alkenesf This reaction can be effected with Sml2 catalyzed by HMPA (14, 280-281). The alkene can be an activated terminal one (CH2=CHOAc), a conjugated diene, or a silyl dienol ether. 

Fluoroalkyl ketones may be employed as the electrophilic partners in condensation reactions with other carbonyl compounds. The highly electrophilic hexafluoroacetone has been used in selective hexafluoroisopropylidenation reactions with silyl enol ethers and silyl dienol ethers, e.g. formation of 1. ... 

In studies directed towaids the synthesis of the fungal isonitrile antibiotic isonitrin B (103), a cy-clopentanoid metabolite related to trichoviridine (104), various trialkylsilyl enol ethers were irradiated with propionaldehyde to produce equimolar amounts of stereoisomeric bicyclic oxetanes (105). The corresponding silyl dienol ethers, however, gave little oxetane product owing to preferential diene photopolymerization. The use of vinyl epoxide derivatives such as (106), however, offers a possible access to the epoxycyclopentanoid system with simultaneous control of the face selectivity of the photoaddition by the epoxide moiety. 

The highly potent antithrombotic (+)-rishirilide B was synthesized in the laboratory of S.J. Danishefsky. One of the tertiary alcohol functionalities was introduced via the Rubottom oxidation of a six-membered silyl dienol ether with dimethyl dioxirane (DMDO). The oxidation was completely stereoselective, and it was guided by the proximal secondary methyl group. Subsequently, the enone was converted to the enedione, which was used as a dienophile in the key intermoiecuiar Dieis-Aider cycioaddition step. 

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