Vinylethers

Substituted 3,4-dihydropyranes were also prepared by Diels-Alder reactions between (E)-4-oxobutenoate 80 and vinylethers [80] under iron(III) 2-ethylhex-aonate, a mild and economical catalyst (Equation 3.26). Diastereomeric excess as high as 98 % was observed. Cycloadducts with a 2,4-cw-configuration were preferred. 

The 2-pyrones can behave as dienes or dienophiles depending on the nature of their reaction partners. 3-Carbomethoxy-2-pyrone (84) underwent inverse Diels-Alder reaction with several vinylethers under lanthanide shift reagent-catalysis [84] (Equation 3.28). The use of strong traditional Lewis acids was precluded because of the sensitivity of the cycloadducts toward decarboxylation. It is noteworthy that whereas Yb(OTf)j does not catalyze the cycloaddition of 84 with enolethers, the addition of (R)-BINOL generates a new active ytterbium catalyst which promotes the reactions with a moderate to good level of enantio selection [85]. 

Exclusively endo-selectivity Lewis-acid catalyzed /jetero-Diels-Alder reactions of (E)-1-phenylsulfonyl-3-alken-2-ones with vinylethers [102]... 

Lanthanide triflates catalyze the Diels-Alder reaction of imines, generated from anilines and aldehydes, with both dienes and alkenes [26]. Thus N-benzyl-ideneaniline in the presence of Yb(OTf)3 (Scheme 6.16) reacts in organic solvent with open-chain dienes, such as Danishefsky s diene, to give tetrahy-dropyridine derivatives, while with cyclopentadiene and vinylethers and vinylthioethers it works like azadiene in both organic solvent and aqueous medium, affording tetrahydroquinoline derivatives. 

Note Methyloxirane and oxetane are CsHeO isomers, but acetone, propanal, methyl vinylether, and 2-propenol also belong to this group. This reminds us to be careful when assigning structures to empirical formulas and when deducing structural information from r + d values (Chap. 6.4.4). 

Figure 5 Degradable PEG-lipid degradability via the orthoester function, the vinylether, or the disulfide group. Abbreviation PEG, poly(ethylene glycol). Source From Refs. 31, 35 0.

N-Iodosuccinimide also promotes the cyclization to give the a-gluco-side derivative, and the disaccharide derivative is synthesized with high stereoselectivity starting from a hydroxy vinylether (39). 

Other li acetylides Li-C=C-R with R = hexyl [21] or benzylether dendrons [22, 23] (up to the fourth generation) have also been attached to (Figure 3.3), and various different electrophiles have been used to complete the reaction with the intermediate li-fuUeride (Scheme 3.2 and Figure 3.3). Besides the protonation, alkyl-, benzyl-, cycloheptatrienyl-, benzoyl- or vinylether-derivatives or formaldehyde and dichloro-acetylene were used as electrophiles [12,20]. Most of these electrophiles are attached to the anion in the expected C-2 position. The 1,4-adducts are available by quenching the anion with the tropylium cation or benzoyl chloride [12]. The fuUerene anion can be stabilized by introduction of benzylether dendrons. The lifetimes of the anions change with the size of the dendrons [22]. 

Vinylethers are known to polymerize in a cationic mechanism in the conventional homopolymerization system (35). Williams et al. (36) and Naruse et al. (37) studied the radiation-induced polymerization of vinylethers and found that they polymerize in the cationic mechanism by ionizing radiations, too. The propagation rate constant of the polymerization of isobutylvinylether in bulk at 30° C was estimated to be 3 x 105 M l sec 1, much higher than that of the polymerization by... 

The polymerization of vinylethers to produce isotactic structures has been one of the oldest examples of stereoregular polymerization. This topic has been well reviewed by Natta, Dall Asta, Mazzanti, Giannini and Cesca (6). Natta, Bassi, Corradini (7) showed that the polyvinylethers first studied by Schildknecht were indeed isotactic polymers. 

The cationic nature of the polymerization of various alkyl vinyl-ethers has been studied by Iwasaki, Fukutani and Nakano (JO). They conclude that magnesium salts, being weak Lewis acids, are mildly effective for the isotactic polymerization of alkyl vinylethers. 

Vandenberg (77) has found that vinylethers are polymerized by combinations of vanadium tetrachloride and triethylaluminum. Vandenberg believed that the intermediate vanadium trichloride, produced by reduction of the vanadium tetrachloride, reacted with the triisobutyl-aluminum-tetrahydrofuran complex to produce the modified Friedel-Crafts catalysts that converted the vinylether to isotactic polymer. 

The position of the monomer is a schematic representation of relative isotactic polymerization activity of the monomer by the catalysts. Isotactic steric control is found only in a narrow range of cationidty of vinylether catalysts. Atactic polymer is produced toward the more ionic side and no polymer toward the less ionic range. 

Alkyl vinylethers and methacrylate are monomers with substituents of different ionic nature. They require opposites of ionicities for their isotactic initiator systems. Styrene incorporates both ionicities into one monomer. 

The cationic nature of Ziegler catalysts have been proposed by Sinn, Winter and Tirpitz (90) who found that the polymerization of styrene, of vinylethers, of butadiene and of isoprene by Ziegler catalysts required the presence of trace amounts of proton-active substances. These same cationic catalyst species isomerized heptene and isoheptene. No definite results could be obtained for propylene and it... 

This asymmetric end has the alkoxy group of alkyl vinylethers by cationic polymerization, phenyl group of styrene when either anionically or cationicaiiy polymerized, the vinyl group of butadiene under anionic catalysts to poly-1,2-butadiene, the ester and methyl of methylmethacrylate under anionic catalysis and the methyl of propylene by cationic catalysis. 

The only case known in which a phenyl group is bound to the asymmetric carbon atom, i. e. poly-[(S)-l-methylbenzyl]vinylether (126), does not seem to follow the same rule. 

Both, TFE and VDF may be copolymerized with suitable comonomers in order to obtain macromolecular materials with improved end-use properties. One fluori-nated monomer that was copolymerized under mild conditions (308 K and 9-11 MPa) was perfluoropropyl-vinylether (PPVE) (CF3CF2CF2OCF=CF2). The copolymer was obtained in the form of a free-flowing powder, with almost quantitative yields moreover, its composition could be altered by changing the molar ratio of the comonomers in the feed. Interestingly, chains were synthesized with molecular weights greater than that of the commercial product with a concentration of carboxylic acid or acid fluoride end groups similar to those detected in polymers prepared in CFCs [101],... 

The patch was based on poly(methyl vinylether/maleic anhydride) and has been shown to remain located firmly to mucous surface for up to 4 h. The drug concentration within mucosa was shown to exceed by a considerable margin that needed to photoinduce cell death in a model cell line. The patch matched the correspondent conventional creams with respect to the amount of drug penetrated into the tissue, but offered the advantage of remaining in place for extended time periods without the need of occlusion. 

The vinylether system is forced to collapse to form an aldehyde. 

Subsequently, Posner published the completely regioselective and highly stereoselective cyclo additions of racemic 3-(p-tolylsulfinyl)-2-pyrone (141) (Scheme 70) with 1,1-dimethoxyethylene [133],vinylether,and vinylthioethers [134]. With the first dienophile, the best diastereoselectivity (an 88 12 ratio of the two endo-adducts) was achieved at room temperature in toluene or hexane as the solvent (48 h). A 10 1 endo/exo mixture of cycloadducts was obtained with vinyl-ether in the presence of ZnBr2 as the catalyst, whereas a total endo selectivity was observed in reactions of 141 with vinylthioethers [134] conducted under high pressures. The bridged bicyclic lactone cycloadducts 142 have been used as versatile synthons in the synthesis of shikimic acid derivatives. Although enantio-merically pure samples of compound 141 could be obtained [134] it was not used as a starting material for asymmetric Diels-Alder reactions (the low yield of (S)-141 precluded this). 

The photoconductivity of poly[2-(N-carbazolyl)ethyl vinylether]91,92) and particularly of poly(N-acryloylcarbazole)91) is much inferior to that of PVK. In the case of the acrylic polymer the reported photocurrents are at least two orders of magnitude lower. The poor charge carrier generating efficiency is blamed for low photocurrents91 The relatively poor performance of the vinylether polymer is how ever attributed to charge carrier mobility92). 

Supplementary support for the interpretation of the temperature-dependent dynamic H NMR spectra of 33 is presented by additional studies of (A,A,A,A)/(A,A,A,A)-[(EtNH3)4n Mg4(L12)6 ] (34). In 33 and 34, the methylene protons of the ligands exhibit identical VT NMR spectra. Moreover, the diastereotopic methylene protons (magenta) of the ethyl ammonium counterions of 34 display similar temperature-dependent coalescence as the ligand vinylether methylene protons (green). This is due to the fact that, even in solution, the ethyl ammonium groups are fixed to the tripodal calix-like surfaces of the [Mg4(L12)6]4 scaffold and therefore the methylene protons are in a chiral environment and display diastereotopicity. 

An additional approach to identification of the vinylether side chain is... 

Vinyl esters and vinylethers were studied as substrates for the intermole-cular PKR by Pauson s group and led to cyclopentenones 240 lacking the ether which was cleaved during the process (Scheme 68) [191]. 

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