α-Hydroxylation of ketones: Osmium tetroxide/N-methylmorpholine-N-oxide oxidation of silyl enol ethers

Osmium tetroxide/N-methylmorpholine-N-oxide oxidation of silyl enol ethers, derived from ketones with either kinetic or thermodynamic regiochemical control, produces α-ketols in good to excellent yields.     

Enantioselective organocatalytic Mukaiyama-Michael addition of silyl enol ethers to alpha,beta-unsaturated aldehydes

[reaction: see text] A highly enantioselective, organocatalytic Mukaiyama-Michael addition reaction of silyl ethers and alpha,beta-unsaturated aldehydes has been developed. The process, catalyzed by MacMillan's chiral imidazolidinone, affords delta-keto aldehydes in high yields (56-87%) and high enantioselectivities (85-97% ee). Moreover, the reaction is applicable to a wide range of silyl ethers and alpha,beta-unsaturated aldehydes and, as such, provides access to a range of important synthetic building blocks.     

W(CO)5(L)-catalyzed endo-selective cyclization of allenyl silyl enol ethers: an efficient method for the cyclopentene annulation onto alpha,beta-unsaturated ketones

[reaction: see text] Indium-mediated allenylation of alpha,beta-unsaturated ketones in the presence of tert-butyldimethylsilyl triflate and dimethyl sulfide gives 6-siloxy-1,2,5-trienes, which undergo W(CO)(5)(L)-catalyzed 5-endo cyclization to give the corresponding cyclopentene derivatives in good yield. Furthermore, this novel W(CO)(5)(L)-catalyzed cyclization of allenyl silyl enol ethers proceeds in a 6-endo manner when 5-siloxy-1,2,5-trienes are employed as a substrate. In these reactions, effective electrophilic activation of allenyl compounds for attack by silyl enol ethers is achieved using a catalytic amount of W(CO)(6).     

In situ generation of Mes2Mg as a non-nucleophilic carbon-centred base reagent for the efficient one-pot conversion of ketones to silyl enol ethers

Treatment of commercially available MesMgBr with 1,4-dioxane produces the key Mes(2)Mg reagent in situ which then mediates the deprotonation of ketones to deliver trimethylsilyl enol ethers, at readily accessible temperatures and without any nucleophilic addition, in an expedient and high yielding one-pot process.     

Anionic rearrangements starting with 1-lithio-2-propynyl silyl ethers a route to (β-trimethylsilylvinyl) silyl ketones

The title compounds undergo silyl-Wittig rearrangement at low temperature and lead to allenolates at room temperature.     

Formal (3+3) cycloaddition of silyl enol ethers catalyzed by trifric imide: domino Michael addition-claisen condensation accompanied with isomerization of silyl enol ethers

We describe here a Tf?NH-catalyzed formal (3+3) cycloaddition of silyl enol ethers with acrylates as a new domino reaction. In the domino sequence, the catalyst activates Michael addition, deprotonation of the resulting silyloxonium cation and intramolecular Claisen condensation. It was found that reaction modes significantly depend on the reaction temperature. We also examined the mechanistic detail of the reaction by 1H-NMR experiment.     

A mild,catalytic, and highly selective method for the oxidation of alpha,beta-enones to 1,4-enediones

A new and simple method is described for the one-step oxidation of alpha,beta-enones to 1,4-enediones in good yields (generally 80-90%) using t-butylhydroperoxide as stoichiometric oxidant and 20% Pd(OH)2 on carbon (5 mol %) as catalyst in CH2Cl2 solution. The same reagents have been used to convert ethylene ketals of alpha,beta-enones to the corresponding monoethylene ketals of 1,4-enediones. Seven representative examples are presented in Table 1. All of the available evidence on these oxidations points to a radical-chain mechanism initiated by the tert-butylperoxy radical (see Scheme 1). gamma-tert-Butylperoxy ethers are formed as major products in the oxidation of alpha,beta-enones possessing only a single gamma-hydrogen.     

Mild and efficient pentafluorophenylammonium triflate (PFPAT)-catalyzed C-acylations of enol silyl ethers or ketene silyl (Thio)acetals with acid chlorides

A pentafluorophenylammonium triflate (PFPAT) catalyst (5 mol %) successfully promoted C-acylation of enol silyl ethers with acid chloride to produce various beta-diketones (12 examples; 62-92% yield). Similarly, C-acylation of ketene silyl acetals or ketene silyl thioacetals (i.e., crossed Claisen condensation) proceeded smoothly to provide not only alpha-monoalkylated beta-keto (thio)esters but also thermodynamically unfavorable (less accessible) alpha,alpha-dialkylated beta-keto (thio)esters in good to excellent yield (38 examples; 60-92% yield).     

One-pot synthesis of tropinone by tandem (domino) ene-type reactions of acetone silyl enol ethers

A synthetic approach for tropane alkaloids on the basis of tandem (domino) ene-type reactions of acetone silyl enol ethers with iminium ions is shown to be triggered by intermolecular ene-type reactions followed by 6-(2,5) silatropic ene-type cyclizations.     

Catalytic (2 + 2)-cycloaddition reactions of silyl enol ethers. A convenient and stereoselective method for cyclobutane ring formation

An efficient catalytic (2 + 2)-cycloaddition reaction leading to the formation of cyclobutane rings has been devised. The process transforms silyl enol ethers and alpha,beta-unsaturated esters into polysubstituted cyclobutanes with a high degree of trans-stereoselectivity. Both the rate and stereoselectivity of the process can be controlled by the choice of the ester group and silyl substituents. The results of stereochemical studies show that the cycloaddition step in this reaction proceeds in a nonstereospecific manner and, thus, by a pathway involving sequential nucleophilic additions via a short-lived zwitterionic intermediate.     

W(CO)5-amine catalyzed exo- and endo-selective cyclizations of omega-alkynyl silyl enol ethers: a highly useful method for the construction of polycyclic compounds

[reaction: see text] A highly useful method for the construction of polycyclic compounds based on the amine-controlled exo- and endo-selective cyclizations of omega-alkynyl silyl enol ethers catalyzed by W(CO)5L is reported. When bis-alkynyl silyl enol ethers were treated with a catalytic amount of W(CO)6, DABCO, and water under photoirradiation, synthetically useful tricyclic ketones were obtained in good yield.     

Silazanes/catalytic bases: mild,powerful and chemoselective agents for the preparation of enol silyl ethers from ketones and aldehydes

We have developed an efficient method for the preparation of enol silyl ethers using novel agents, silazanes together with NaH or DBU catalyst, wherein TMS and TBDMS groups were smoothly and chemoselectively introduced into ketones and aldehydes under mild conditions.     

Double bond cleavage reaction of silyl enol ethers using MoO2(acac)2-tBuOOH

The double bond of various silyl enol ethers is smoothly cleaved to give carbonyl compounds by an oxidizing reagent MoO₂(acac)₂-^tBuOOH.     

(2+2) cycloaddition reaction of alkyl enol ethers with acrylates by in situ generated silyl triflic imide catalyst

We describe here (2+2) cycloaddition reaction of alkyl enol ethers with acrylates catalyzed by triethylsilyl triflic imide (Et3SiNTf2), which was in situ generated from triethylsilane and triflic imide. The reaction efficiently provides substituted cyclobutanes bearing alkoxy function in a stereoselective manner.     

Preparation of polymer incarcerated gold nanocluster catalysts (PI-Au) and their application to aerobic oxidation reactions of boronic acids,alcohols, and silyl enol ethers

Heterogeneous gold nanocluster catalysts immobilized by the method known as polymer incarceration were prepared. Polystyrene-derived polymers with epoxide and alcohol moieties, which could be cross-linked under heating conditions, were employed as supports for their preparation. Cationic gold salts were reduced in a solution of NaBH₄ and the polymers. Poor solvents for the polymers were added, and the polymers were precipitated and encapsulated gold nanoclusters with weak but multiple interactions between a gold nanocluster surface and the π electrons of benzene rings. The polymer capsules were heated under neat conditions to afford heterogeneous gold nanocluster catalysts; namely, polymer-incarcerated gold nanoclusters. The catalysts thus prepared could be applied to the aerobic oxidation of phenyl boronic acids, alcohols, and silyl enol ethers. We found that the choice of polymers, good and poor solvents for the polymers, metal loadings, heating conditions for cross-linking, and final activation were all crucial for obtaining high-activity catalysts.     

Synthesis of 2-alkylidenepyrrolidines, pyrroles, and indoles by condensation of silyl enol ethers and 1,3-bis-silyl enol ethers with 1-azido-2,2-dimethoxyethane and subsequent reductive cyclization

The condensation of 1,3-bis-silyl enol ethers with 1-azido-2,2-dimethoxyethane and subsequent reductive cyclization allowed an efficient regio- and diastereoselective synthesis of a variety of 2-alkylidene-4-methoxypyrrolidines. The thermal elimination of methanol resulted in the formation of functionalized pyrroles. Similarly, 2,3,3a,4,5,6-hexahydro-2,3-benzopyrroles were prepared and transformed into 4,5,6,7-tetrahydro-2,3-benzopyrroles. In contrast, treatment of 2-alkylidenepyrrolidines with trifluoroacetic acid resulted in formation of indoles by [4 + 2] cycloaddition and subsequent extrusion of a nitrogen atom.     

Synthesis of 1,3,5-tricarbonyl derivatives by condensation of 1,3-bis(silyl enol ethers) with acid chlorides

A variety of 1,3,5-tricarbonyl derivatives were prepared by reaction of 1,3-bis(silyl enol ethers) with acid chlorides under mild conditions. This includes reactions of both aromatic and aliphatic acid chlorides and bis(acid chlorides). The yields vary depending on the type of acid chloride employed.