Ether extender

The structural characterization of a mononuclear Eu-calix[6]arene complex reveals a more unusual complexation mode. In Eu(H4LXOHXDMF)6 (H6L = p-tm-butylcalix[6]arene Table 13) the Eu cation is placed outside the cavity forming only one monodentate bond to the calixarene ligand [193]. Six DMF molecules and most likely a hydroxyl group accomplish 8-coordination. The smaller sized calix[4]arene [194] and ether-extended bis(homooxa)-p-tert-... 

Ullman reaction The synthesis of diaryls by the condensation of aromatic halides with themselves or other aromatic halides, with the concomitant removal of halogens by a metal, e.g. copper powder thus bromobenzene gives diphenyl. The reaction may be extended to the preparation of diaryl ethers and diaryl thio-ethers by coupling a metal phenolate with an aryl halide. 

Capacity Limitations and Biofuels Markets. Large biofuels markets exist (130—133), eg, production of fermentation ethanol for use as a gasoline extender (see Alcohol fuels). Even with existing (1987) and planned additions to ethanol plant capacities, less than 10% of gasoline sales could be satisfied with ethanol—gasoline blends of 10 vol % ethanol the maximum volumetric displacement of gasoline possible is about 1%. The same condition apphes to methanol and alcohol derivatives, ie, methyl-/-butyl ether [1634-04-4] and ethyl-/-butyl ether. 

TiCl catalysts produced by the reduction of TiCl with Al(C2H 2d> subsequentiy treated first with an electron donor (diisoamyl ether), then with TiCl, are highly stereospecific and four to five times more active than d-TiCl (6). These catalysts were a significant advance over the earlier TiCl systems, because removal of atactic polymer was no longer required. They are often referred to as second-generation catalysts. The life of many older slurry process faciUties has been extended by using these catalysts to produce "clean" polymers with very low catalyst residues. 

Bisphenol A diglycidyl ether [1675-54-3] reacts readily with methacrylic acid [71-49-4] in the presence of benzyl dimethyl amine catalyst to produce bisphenol epoxy dimethacrylate resins known commercially as vinyl esters. The resins display beneficial tensile properties that provide enhanced stmctural performance, especially in filament-wound glass-reinforced composites. The resins can be modified extensively to alter properties by extending the diepoxide with bisphenol A, phenol novolak, or carboxyl-terrninated mbbers. 

Only a few commercial uses for TDA per se have been found. In epoxy curing appHcations, 2,4- I DA has been used as a component of a eutectic mixture with short chain aUphatic glycidal ether resins (46) as well as by itself (46,47) TDA (46) and single isomers (47) are also used as amine curatives. TDA can be used as a chain extender in polyurethanes (48,49). TDA is cited as a monomer in making aromatic polymers with unique properties, eg, amorphous polyamides (50), powdered polyamides (51), and low melting, whoUy aromatic polyamides (52). 

Stereoregular Polymerization. Chemists at GAF Corporation were first to suggest that stereoregularity or the lack thereof is responsible for both nontacky and crystalline or tacky and amorphous polymers generated from IBVE with BF2 0(C2H )2, depending on the reaction conditions (22,23). In addition, it was shown that the crystalline polymer is actually isotactic (24). Subsequentiy, the reaction conditions necessary to form such polymers have not only been demonstrated, but the stereoregular polymerization has been extended to other monomers, such as methyl vinyl ether (25,26). 

The separation of Hquid crystals as the concentration of ceUulose increases above a critical value (30%) is mosdy because of the higher combinatorial entropy of mixing of the conformationaHy extended ceUulosic chains in the ordered phase. The critical concentration depends on solvent and temperature, and has been estimated from the polymer chain conformation using lattice and virial theories of nematic ordering (102—107). The side-chain substituents govern solubiHty, and if sufficiently bulky and flexible can yield a thermotropic mesophase in an accessible temperature range. AcetoxypropylceUulose [96420-45-8], prepared by acetylating HPC, was the first reported thermotropic ceUulosic (108), and numerous other heavily substituted esters and ethers of hydroxyalkyl ceUuloses also form equUibrium chiral nematic phases, even at ambient temperatures. 

Advancement Process. In the advancement process, sometimes referred to as the fusion method, Hquid epoxy resin (cmde diglycidyl ether of bisphenol A) is chain-extended with bisphenol A in the presence of a catalyst to yield higher polymerized products. The advancement reaction is conducted at elevated temperatures (175—200°C) and is monitored for epoxy value and viscosity specifications. The finished product is isolated by cooling and cmshing or flaking the molten resin or by allowing it to soHdify in containers. 

Ring expansion of haloalkyloxiranes provides a simple two-step procedure for the preparation of azetidin-3-ols (Section 5.09.2.3.2(f)) which can be extended to include 3-substituted ethers and O-esters (79CRV331 p. 341). The availability of 3-hydroxyazetidines provides access to a variety of 3-substituted azetidines, including halogeno, amino and alkylthio derivatives, by further substitution reactions (Section 5.09.2.2.4). Photolysis of phenylacylamines has also found application in the formation of azetidin-3-ols (33). Not surprisingly, few 2-0-substituted azetidines are known. The 2-methoxyazetidine (57) has been produced by an internal displacement, where the internal amide ion is generated by nucleophilic addition to an imine. 

This reaction is the cause of a widely recognized laboratory hazard. The peroxides formed from several commonly used ethers, such as diethyl ether and tetrahydrofuran, are explosive. Appreciable amounts of such peroxides can build up in ether samples that have been exposed to the atmosphere. Because the hydroperoxides are less volatile than the ethers, they are concentrated by evaporation or distillation, and the concentrated peroxide solutions may explode. For this reason, extended storage of ethers that have been exposed to oxygen is extremely hazardous. 

The six-position may be functionalized by electrophilic aromatic substitution. Either bromination (Br2/CH2Cl2/-5°) acetylation (acetyl chloride, aluminum chloride, nitrobenzene) " or chloromethylation (chloromethyl methyl ether, stannic chloride, -60°) " affords the 6,6 -disubstituted product. It should also be noted that treatment of the acetyl derivative with KOBr in THF affords the carboxylic acid in 84% yield. The brominated crown may then be metallated (n-BuLi) and treated with an electrophile to form a chain-extender. To this end, Cram has utilized both ethylene oxide " and dichlorodimethyl-silane in the conversion of bis-binaphthyl crowns into polymer-bound resolving agents. The acetylation/oxidation sequence is illustrated in Eq. (3.54). 

A remarkable feature of the Birch reduction of estradiol 3-methyl ether derivatives, as well as of other metal-ammonia reductions, is the extreme rapidity of reaction. Sodium and -butyl alcohol, a metal-alcohol combination having a comparatively slow rate of reduction, effects the reduction of estradiol 3-methyl ether to the extent of 96% in 5 minutes at —33° lithium also effects complete reduction under the same conditions as is to be expected. Shorter reaction times were not studied. At —70°, reduction with sodium occurs to the extent of 56 % in 5 minutes, although reduction with lithium is virtually complete (96%) in the same time. (The slow rates of reduction of compounds of the 5-methoxytetralin type is exemplified by 5-methoxy-tetralin itself with sodium and f-butyl alcohol reduction occurs to the extent of only 50% in 6 hours vs. 99+% with lithium.) The iron catalyzed reaction of sodium with alcohols must be very fast since it competes so well with the rapid Birch reduction. One cannot compensate for the presence of iron in a Birch reduction mixture containing sodium by adding additional metal to extend the reaction time. The iron catalyzed sodium-alcohol reaction is sufficiently rapid that the aromatic steroid still remains largely unreduced. 

To a mixture of ethyl 5a-cholestan-3-one 2a-xanthate (2 g, 3.95 mmol) and 100 ml methanol is added sufficient ether to completely dissolve the solids. Sodium borohydride (90 mg, 2.36 mmol) is added directly to the reaction flask and the solution is stirred at room temperature for 4 hr. (The use of an excess of sodium borohydride and an extended reaction time produces 5oc-cholestan-2a,3a-thiirane.) The reaction is diluted with 200 ml ether and washed several times with ca. 100 ml water, dried (MgS04) and the solvent is removed under vacuum. The crude sticky gum is chromatographed on a column of 85 g silicic acid. The hexane eluates contain 5a-cholest-2-ene. Ethyl 5a-cholestan-3a-ol 2a-xanthate is obtained in ca. 30% yield by subsequent elution with benzene hexane (1 7) and the desired ethyl 5a-cholestan-3 -ol 2a-xanthate is eluted with ether hexane (1 3) in ca. 30% yield. 

Note 1. If unpurified, but vacuum dried lead tetraacetate is used in the above procedure erratic results are obtained. If, however, commercial lead tetraacetate containing about 10% acetic acid is employed and 0.5% (v/v) of acetic acid is added to the reaction solution consistently high yields of ether are obtained. An excess of lead tetraacetate (up to 5 eq) is required for complete conversion. In the presence of acetic acid the reaction time must be extended to about 40 hr. 

A one-pot conversion of benzyl alcohols to benzyl fluorides by treatment of the alcohols with a combination of methanesulfonyl fluoride, cesium fluoride and 18-crown 6 ether in tetrahydrofuran has been repotted The reaction involves mesylation of the alcohols followed by cleavage of the resultant mesyl esters with a fluoride ion The reaction has been extended also to certain heterocycles bearing the N hydroxymethyl group [43] (equation 31)... 

Ceric ammonium nitrate, MeOH, 0°, 15 min, 82-95% yield. Dioxolanes and some THP ethers are not affected, but in general, with extended reaction times, THP ethers are cleaved. 

The 2,7-naphthyridine system 53 (Scheme 8.4.18) was combined with 2,4-dinitrochlorobenzene and 2-amino glycerol for in situ reaction of the resulting Zincke salt. The resulting naphthyridinium 54 was trapped by Bradsher cycloaddition with (Z)-vinyl ether 55, providing tetracycle 56 (X-ray) upon internal addition of one of the diastereotopic hydroxymethyl groups to the resulting iminium. This approach was also extended to the use of chiral 2,7-naphthyridinium salts, prepared via the analogous Zincke process. ... 

Under similar conditions bisnitrofurazans afforded oligomeric linear or macro-cyclic ethers (96JOC1510, 99ZOR1555). An attempt to extend this reaction to nitrofuroxans failed. 

Recently, Charette et al. have also demonstrated this behavior in the stereoselective cyciopropanations of a number of enantiopure acyclic allylic ethers [47]. The high degree of acyclic stereocontrol in the Simmons-Smith cyclopropanation has been extended to synthesis several times, most notably in the synthesis of small biomolecules. Schollkopf et al. utilized this method in their syntheses of cyclopropane-containing amino acids [48 a, b]. The synthesis of a cyclopropane-containing nucleoside was also preformed using acyclic stereocontrol [48c]. 

This chiral modifier provides one of the only methods for selective cyclopropa-nation of substrates which are not simple, allylic alcohols. In contrast to the catalytic methods which will be discussed in the following section, the dioxaborolane has been shown to be effective in the cyclopropanation of a number of allylic ethers [67]. This method has also been extended to systems where the double... 

The above described reaction has been extended to the application of the AlMe-BINOL catalyst to reactions of acyclic nitrones. A series chiral AlMe-3,3 -diaryl-BINOL complexes llb-f was investigated as catalysts for the 1,3-dipolar cycloaddition reaction between the cyclic nitrone 14a and ethyl vinyl ether 8a [34], Surprisingly, these catalysts were not sufficiently selective for the reactions of cyclic nitrones with ethyl vinyl ether. Use of the tetramethoxy-substituted derivative llg as the catalyst for the reaction significantly improved the results (Scheme 6.14). In the presence of 10 mol% llg the reaction proceeded in a mixture of CH2CI2 and petroleum ether to give the product 15a in 79% isolated yield. The diastereoselectiv-ity was the same as in the acyclic case giving an excellent ratio of exo-15a and endo-15a of >95 <5, and exo-15a was obtained with up to 82% ee. 

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