Gold-catalyzed alkylation of silyl enol ethers with ortho-alkynylbenzoic acid esters

• Haruo Aikawa,
• Tetsuro Kaneko,
• Naoki Asao and
• Yoshinori Yamamoto

Beilstein J. Org. Chem. 2011, 7, 648–652, doi:10.3762/bjoc.7.76

• Research, Tohoku University, Sendai 980-8578, Japan 10.3762/bjoc.7.76 Abstract Unprecedented alkylation of silyl enol ethers has been developed by the use of ortho-alkynylbenzoic acid alkyl esters as alkylating agents in the presence of a gold catalyst. The reaction probably proceeds through the gold

• -induced in situ construction of leaving groups and subsequent nucleophilic attack on the silyl enol ethers. The generated leaving compound abstracts a proton to regenerate the silyl enol ether structure. Keywords: alkylation; gold catalysis; leaving group; silyl enol ether; substitution reaction

• ; Findings Silyl enol ethers have been widely used in organic synthesis as effective carbon nucleophiles for the construction of carbon frameworks [1][2][3][4]. Generally, they react with a variety of electrophiles to give carbonyl compounds as products due to cleavage of the silicon–oxygen bond. For example

The arene–alkene photocycloaddition

• Ursula Streit and
• Christian G. Bochet

Beilstein J. Org. Chem. 2011, 7, 525–542, doi:10.3762/bjoc.7.61

• regiochemistry of proton catalyzed addition of alcohols to enols or enol ethers has the opposite regiochemistry to that observed in the product [102]. Furthermore, an electron transfer intermediate was ruled out, as the reaction is not thermodynamically feasible according to the Weller equation. Therefore, he

Ene–yne cross-metathesis with ruthenium carbene catalysts

• Cédric Fischmeister and
• Christian Bruneau

Beilstein J. Org. Chem. 2011, 7, 156–166, doi:10.3762/bjoc.7.22

• products were obtained in excellent yields at low temperature (Scheme 12) [67]. The cross-metathesis of terminal enol ethers with terminal and internal alkynes in the presence of catalyst II has led to the regioselective formation of electron rich dienes, precursors of choice for Diels–Alder reactions

• (Scheme 13) [68]. This is surprising as enol ethers are known to be moderately reactive in olefin metathesis [69] and ethyl vinyl ether is often used to stop ring opening metathesis polymerizations. Vinyl acetate is also a good partner for EYCM under similar conditions. In the presence of 10 mol % of

Recent advances in carbocupration of α-heterosubstituted alkynes

• Ahmad Basheer and
• Ilan Marek

Beilstein J. Org. Chem. 2010, 6, No. 77, doi:10.3762/bjoc.6.77

• ]. Particularly interesting is the addition of trimethylsilylmethyl copper that leads to the preparation of functionalized enol ethers possessing an allylsilane moiety [14][15][16]. However, the carbometalated product 5 must be kept at low temperature to avoid any elimination reaction (Scheme 3). Such a strategy

Addition of lithiated enol ethers to nitrones and subsequent Lewis acid induced cyclizations to enantiopure 3,6-dihydro-2H-pyrans – an approach to carbohydrate mimetics

• Fabian Pfrengle and
• Hans-Ulrich Reissig

Beilstein J. Org. Chem. 2010, 6, No. 75, doi:10.3762/bjoc.6.75

• Fabian Pfrengle Hans-Ulrich Reissig Freie Universität Berlin, Institut für Chemie und Biochemie, Takustrasse 3, D-14195 Berlin, Germany 10.3762/bjoc.6.75 Abstract A stereodivergent synthesis of enantiopure 3,6-dihydro-2H-pyrans is presented. The addition of lithiated enol ethers to carbohydrate

• to a tricarboxylic acid core. Keywords: aminopyrans; carbohydrate mimetics; Lewis acids; lithiated enol ethers; nitrones; oxidative cleavage; stereodivergent synthesis; Introduction The pyran structural motif can be found in numerous bioactive natural products. Possible strategies towards their

• enantiopure aminopyrans E (Scheme 1). The sequence of nucleophilic addition of lithiated enol ethers A to nitrones B and Lewis acid promoted cyclization of the resulting 1,3-dioxolanyl-substituted hydroxylamine derivatives C, delivered the enantiopure dihydropyrans D in a highly stereodivergent fashion [8

Shelf-stable electrophilic trifluoromethylating reagents: A brief historical perspective

• Norio Shibata,
• Andrej Matsnev and
• Dominique Cahard

Beilstein J. Org. Chem. 2010, 6, No. 65, doi:10.3762/bjoc.6.65

• conditions to give ortho-trifluoromethylated arenes in good yields (Scheme 7). Togni’s reagent (37, see later in the text) could be used for this reaction, although product yields were as low as 11%. The reaction conditions for trifluoromethylation of silyl enol ethers and β-ketoesters were reinvestigated by

• -substituted α-trifluoromethyl β-ketoesters in good to excellent yields. In a second approach, 5 and tetrabutylammonium difluorotriphenylstannate were used for efficient electrophilic trifluoromethylation of various silyl enol ethers to give the corresponding α-trifluoromethyl ketones in good to high yields

• a bridged oxygen. Reactivity of (trifluoromethyl)dibenzochalcogenium salts. Pd(II)-Catalyzed ortho-trifluoromethylation of heterocycle-substituted arenes by Umemoto’s reagents. Mild electrophilic trifluoromethylation of β-ketoesters and silyl enol ethers. Enantioselective electrophilic

Fused bicyclic piperidines and dihydropyridines by dearomatising cyclisation of the enolates of nicotinyl-substituted esters and ketones

• Heloise Brice,
• Jonathan Clayden and
• Stuart D. Hamilton

Beilstein J. Org. Chem. 2010, 6, No. 22, doi:10.3762/bjoc.6.22

• it into a silyl enol ether 9. Silyl enol ethers and ketene acetals are known to add effectively to pyridinium species in an intermolecular manner [42][43][44][45][46]. Thus 7 was converted to silyl enol ether 9 in excellent yield under standard conditions. A single geometrical isomer was obtained

A review of new developments in the Friedel–Crafts alkylation – From green chemistry to asymmetric catalysis

• Magnus Rueping and
• Boris J. Nachtsheim

Beilstein J. Org. Chem. 2010, 6, No. 6, doi:10.3762/bjoc.6.6

• ]. In addition to arenes and heteroarenes, other nucleophiles including allylsilane, tosylamine, TMS-cyanide, acetylacetonates and silyl enol ethers were successfully used as nucleophilic components giving the substituted 3-phenylbutanoates 112 in high yields and with excellent diastereoselectivities

• (Scheme 41). The reaction could be performed at ambient temperatures and in sharp contrast to the previous described HBF4-mediated procedure, high anti-selectivity was observed in the formation of 112. Similar methods using silyl enol ethers were subsequently developed involving Bi(OTf)3 as Lewis acid

• catalyst [117][118]. While bismuth-catalyzed arylation of benzyl alcohols proceeded only at temperatures between 55 and 100 °C, the reaction of p-methoxybenzyl acetates 113 with silyl enol ethers 114 took place even at ambient temperatures to give the desired products 115 (Scheme 42) [118]. Again different

End game strategies towards the total synthesis of vibsanin E, 3-hydroxyvibsanin E, furanovibsanin A, and 3-O-methylfuranovibsanin A

• Brett D. Schwartz,
• Craig M. Williams and
• Paul V. Bernhardt

Beilstein J. Org. Chem. 2008, 4, No. 34, doi:10.3762/bjoc.4.34

• procedure based on the epoxidation of silyl enol ethers. Ketone 23 was smoothly converted into the TBS enol ether 27 (85% yield) with TBS triflate, which was then treated with dimethyldioxirane (DMDO). When work up was restricted to a simple 1 M hydrochloric acid wash (i.e. separatory funnel) only the

• oxidation on diketone 40, but to first protect the ketone functionality as silyl enol ethers as was undertaken in Scheme 7 (i.e. 27–48). Treating diketone 40 with t-butyldimethylsilyl trifluoromethanesulfonate afforded only the monoprotected product 53 (crude yield 55%), which smoothly underwent reduction

Multiple hydride reduction pathways in isoflavonoids

• Auli K. Salakka,
• Tuija H. Jokela and
• Kristiina Wähälä

Beilstein J. Org. Chem. 2006, 2, No. 16, doi:10.1186/1860-5397-2-16

• , DIBAH) occurs normally by 1,2-attack, this being a key step in the well known "carbonyl transposition" of 1,3-diketone enol ethers into enones (R3O-CR=CR1-COR2 → R-CO-CR1=CHR2). [11][12][13][14][15] Prior to our work there were no reports on the reduction of isoflavones containing free hydroxy groups