Diethyl ether complex with

O-Methylation of N-methylated derivatives under basic conditions is disappointing, 5063 but improved results are obtained using trimethyloxonium tetrafluoroborate (Scheme 13), or boron trifluoride-diethyl ether complex with diazomethane. 671 Conversely, methylation of the proline derived P-hydroxy acid using sodium hydride and iodomethane, as well as using trimethyloxonium tetrafluoroborate, gives the dolaproine unit (8) of dolastatin 10 in good yield. 68,691... 

Two molecules of diethyl ether complex with the Mg atom of the Grignard reagent. 

The vibrational spectra of diethyl ether complexes with MgBrj and MgL have been assigned completely p(MgO) are 390-300 cm" . The solid-state Raman spectra of 1 1 and 1 2 adducts of 1,4-dioxane with metal halides show that the ligand is bridging between metals in the chair conformation. Infrared spectra of alkali-metal ions complexed by cyclic polyethers have been reported. "... 

The pharmaceutical interest in the tricyclic structure of dibenz[6,/]oxepins with various side chains in position 10(11) stimulated a search for a convenient method for the introduction of functional groups into this position. It has been shown that nucleophilic attack at the carbonyl group in the 10-position of the dibenzoxepin structure renders the system susceptible to water elimination. Formally, the hydroxy group in the enol form is replaced by nucleophiles such as amines or thiols. The Lewis acids boron trifluoride-diethyl ether complex and titanium(IV) chloride have been used as catalysts. 

Alternatively, diazomcthanc can be added to thioxanthylium perchlorate (4) over 30 minutes at 0°C, and the reaction solution then poured into propan-2-ol. After concentration, the residue is dissolved in acetic anhydride and treated with boron trifluoride-diethyl ether complex at 0 C, to provide dibenzo[6,/]thiepin in 55 % overall yield16 (cf. Houben-Weyl, Vol. 10/4, p 834). 

In acid solution 1-acyl-1//-azepines and alkyl l//-azepine-l-carboxylates undergo rapid aromatization to A-arylcarbamates,115,139,142 whereas 1/Z-azepine-l-carbonitrile suffers quantitative rearrangement and hydrolysis to phenylurea.163 Rearrangement of ethyl l//-azepine-l-carboxylate to ethyl A-phenylcarbamate is also rapid (5 min) and quantitative with boron trifluoride-diethyl ether complex in benzene.245... 

The dehydrogenation of 2,3-dihydro- and 2,5-dihydro-l//-l-benzazepines to 3//-l-benz-azepincs with heterocyclic enamines in the presence of boron trifluoride diethyl ether complex has been achieved in moderate yields (30-35%).241 In contrast, electrochemical oxidation of 2,5-dihydro-1 H- -benzazepines in buffered acetic acid solution furnishes initially 5//-l-benz-azepines in 35-45% yield.242... 

Indole and dimethyl acetylenedicarboxylate yield 2-(indol-3-yl)-2,3-dihydro-l//-l-ben-zazepine (mp 240-242 C) by addition of indole to the initially formed l//-l-benzazepinc,21 whereas 1,3-dimethylindole (10, R = H) fails to react with the diester under a variety of conditions.145 However, in the presence of boron trifluoride-diethyl ether complex at room tem-... 

The reaction of benzopentathiepin with alkenes [(fl-but- -ene, ( )-hex-3-ene, cyclopentene or cyclohexene] in the presence of the boron trifluoride-diethyl ether complex results in the formation of 3,4-dihydro-l,2,5-benzotrithiepins, e.g. formation of 3.407... 

The alkynyl reagent 9 was recently introduced for the dia stereoselective synthesis of tertiary propargylic alcohols144. 9 can be prepared as a solid 1 1 complex with tetrahydrofuran by treatment of 9-methoxy-9-borabicyclo[3.3.1]nonane with (trimethylsilylethynyl)lithium, followed by addition of boron trifluoride-diethyl ether complex. The nucleophilic addition of reagent 9 to (R)-2-methoxy-2-methylhexanal (10) afforded a mixture of the diastereomers 11 with a considerable preference to the nonchelation-controlled (3S,4R)-isomer144. 

To a stirred — 78 C solution of 5.85 mL (62.5 mmol) of 3-methoxy-l-prnpene in 25 mL of THf- are added 43.1 mL (50 mmol) of 1.16 M. vcc-butyllithium in cyclohexane over a 20-25 min period. The mixture is stirred at — 78 °C for an additional 10 min, and diisopinocampheyl(methoxy)borane [50 mmol prepared from (+ )-a-pinene] in 50 mL of THF is added. This mixture is stirred for 1 h, then 8.17 mL (66.5 mmol) of boron trifluoride diethyl etherate complex are added dropwise to give a solution of diisopiuocampheyl[(Z)-3-inethoxy-2-propenyl]borane. Immediately. 2.8 mL (50 mmol) of acetaldehyde are added and the mixture is stirred for 3 h at — 78 rC and then allowed to warm to r.t. All volatile components are removed in vacuo, then the residue is dissolved in pentane. The insoluble fraction is washed with additional pentane. The combined pentane extracts are cooled to 0 JC and treated with 3.0 mL (50 mmol) of ethanolamine. The mixture is stirred for 2 h at 0rC and is then seeded with a crystal of the diisopinocampheylborane-ethanolaminc complex. The resulting crystals arc filtered and washed with cold pentane. The filtrate is carefully distilled yield 5.6 g (57%) d.r. (synjanti) >99 1 (2/ ,37 )-isomer 90% ee bp 119-120 C/745 Torr. 

The diastereofacial selectivity of Lewis acid promoted reactions of allylsilancs with chiral aldehydes has been thoroughly investigated58. Aldehydes with alkyl substituted a-stereogenic centers react with a mild preference for the formation of Cram products, this preference being enhanced by the use of boron trifluoride-diethyl ether complex as catalyst58. 

The use of boron trifluoride-diethyl ether complex as the Lewis acid in these reactions promotes silyl group migration and gives rise to the formation of tetrahydrofurans with excellent stereoselectivity82. 

Effective 1,4-asymmetric induction has been observed in reactions between 2-(alkoxyethyl)-2-propenylsilanes and aldehydes. The relative configuration of the product depends on the Lewis acid used. Titanium(IV) chloride, in the presence of diethyl ether, gave 1,4-ijn-products with excellent stereoselectivity with boron trifluoride-diethyl ether complex, the amt-isomer was the major product, but the stereoselectivity was less83. 

Excellent chelation control was observed using tributyl(2-propenyl)stannane and a-benzyloxy-cyclohexaneacetaldehyde with magnesium bromide or titanium(IV) chloride, whereas useful Cram selectivity was observed for boron trifluoride-diethyl ether complex induced reactions of the corresponding ferr-butyldimethylsilyl ether89. 

The boron trifluoride-diethyl ether complex induced reaction of 2-butenyl(tributyl)-stannane and 3-(/er/-butyldimethylsilyloxy)-2-methylpropanal gave predominantly the nonchelation-controlled yyn-product93, whereas with the analogous 3-benzyloxyaldehyde, 2-propenyl-tin trichloride, generated in situ from tributyl(2-propenyl)stannanc and tin(IV) chloride, gave the chelation-controlled product93. 

The reaction between 5-methyl-2-(l-methyl-1 -phenylethyl)cyclohexyl 2-oxoacetate and 2-buteny](tributyl)stannane promoted by boron trifluoride-diethyl ether complex showed a strong preference for 57-facial attack, with syn selectivity69. 

The stereoselectivity of these intermolecular reactions between 1-alkoxyallylstannanes and aldehydes induced by boron trifluoride-diethyl ether complex is consistent with an open-chain, antiperiplanar transition state. However, for intramolecular reactions, this transition state is inaccessible, and either (Z)-.yyn-products are formed, possibly from a synclinal process105, or 1,3-isomerization competes113. Remote substituents can influence the stereoselectivity of the intramolecular reaction114. 

Alkoxyallylstannanes are also available by boron trifluoride-diethyl ether complex induced isomerization of their 1-alkoxy isomers. This isomerization proceeds in an antarafacial manner with excellent stereoselectivity to give (Z)-3-alkoxyallylstannanes possibly via an intermolecu-lar exchange process119. Coupled with the asymmetric reduction of acylstannanes (see Section 1.3.3.3.2.3.1) this provides access to 1-alkyl-3-alkoxyallylstannanes of useful optical purity106. 

Boron trifluoride-diethyl ether complex induced reactions of both (E)- and (Z)-tributyI(3-methoxy-2-propenyl)stannane with aldehydes give. vj-w-products with useful slereoselectivi-... 

To a suspension of lithium-(-)-sparteine complex, prepared from 4.0 mmol of ( )-2-butenyl diisopropylcarbamate (Section 1.3.3.3.1.2.), at —78 °C are added rapidly 8.0 mL (28 mmol) of tetraisopropoxytitani-um with intensive magnetic stirring. After 5 min, 980 mg (10 mmol) of 3-methyl-2-methylenebutanal are added and stirring is continued below — 70 °C for 3 h. The mixture is allowed to warm to r.t. and poured into 20 mL of 2N hydrochloric acid and 20 mL of diethyl ether. The aqueous layer is extracted with three 20-mL portions of diethyl ether, washed with sat. aq NaHCO, and dried over Na2S04. Chromatography on 50 g silica gel (diethyl ether/pentane) affords a colorless oil yield 840 mg (78%) 90% ee [a]n° —0.8 (c = 4.4, CH2C12). 

Much better results are achieved in the addition of butyllithium to oxime ethers 4a, 4b and 4c activated by boron trifluoride-diethyl ether complex (BF3 OEt2) at — 78 °C (above a reaction temperature of — 30 °C complex mixtures of products are obtained) using toluene as the solvent. Furthermore, the stereoselectivity depends on the E/Z ratio of the starting oxime ethers. The reaction appears to be highly stereoselective, with the diastereoselectivity of the... 

An interesting example from carbohydrate chemistry is the boron trifluoride-diethyl ether complex catalyzed nucleophilic addition of silyl enol ethers to chiral imines (from n-glyceralde-hyde or D-serinal)22. This reaction yields unsaturated y-butyrolactones with predominantly the D-arabino configuration (and almost complete Cram-type erythro selectivity). 

Dienones, such as 4-[4-(trimethylsilyl)-2-butenyl]-3-vinyl-2-cyclohexenone, are useful precursors for these particular transformations the allylsilane side chain is too short for effective 1,4-addition, but just right for 1,6-addition, resulting in six-ring annulation. Three different Lewis acids can be used titanium(IV) chloride, boron trifluoride diethyl ether complex, and ethylaluminum dichloride. The best chemical yields and complete asymmetric inductions were obtained with ethylaluminum dichloride. 

The efficiency of the extraction depends on the coordinating ability of the solvent, and on the acidity of the aqueous solution which determines the concentration of the metal complex. Coordinating ability follows the sequence ketones > esters > alcohols > ethers. Many metals can be extracted as fluoride, chloride, bromide, iodide or thiocyanate complexes. Table 4.5 shows how the extraction of some metals as their chloro complexes into diethyl ether varies with acid concentration. By controlling... 

Tetrafluoroboric acid-diethyl ether complex (HBF4.0Et2) in diethyl-ether, 54%, 3.00 g, 2.52mL, 10 mmol, leq, diluted with tetrahydrofuran, 5mL... 

Various combinations of hexane or light petroleum (40-60°C, bp) and diethyl ether, usually with a small amount of acetic acid (e.g. 90 10 1) or diisopropyl ether and acetic acid (98.5 1.5) are commonly used. The greater mobility is demonstrated by cholesterol esters followed by triacylglycerols, free fatty acids, cholestorol, diacylglycerols and monoacylglycerols, with complex polar lipids remaining unmoved. Double development in two solvents, e.g. diisopropyl... 

The solution is cooled to -78°C in a dry ice-acetone bath, and 38.19 g (0.23 mol) of tetrafluoroboric acid-diethyl ether complex (Note 10) is added dropwise by syringe over a 30-min period. The solution is allowed to warm to room temperature. The yellow precipitate is filtered off and collected in a 250-mL Schlenk tube, washed with ether, dried under a stream of nitrogen and finally under reduced pressure (oil pump). The bright yellow salt 1 weighs 43. 2-57. 6 g (60-80%) and may be used without further purification (Note 11). U may be stored indefinitely under nitrogen at 0 C without decomposition. 

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