Acetals, mixed vinyl ethers

Mixed acetals from vinyl ethers OGHrCHa -... 

Over the past years considerable attention has been paid to the dispersing system since this controls the porosity of the particle. This is important both to ensure quick removal of vinyl chloride monomer after polymerisation and also to achieve easy processing and dry blendable polymers. Amongst materials quoted as protective colloids are vinyl acetate-maleic anhydride copolymers, fatty acid esters of glycerol, ethylene glycol and pentaerythritol, and, more recently, mixed cellulose ethers and partially hydrolysed polyfvinyl acetate). Much recent emphasis has been on mixed systems. 

In this approach, the glycosyl donor and the glycosyl acceptor are linked by the 2-OH of the donor and the free OH of the acceptor. It is one of the most predictable and reliable methods for achieving 1,2-cis stereocontrol. Acetals, mixed p-methoxybenzylacetals and silicon tethering have been widely used as well as iodonium mediated tethering acetals derived from vinyl, allyl and allenyl ethers. These methodologies have been revised.6,76... 

Conversion into 6-methyl-2-(4-methylphenyl)hepta- 1,5-diene. 2- 4-methyl-phenyl)prop-2-enyl vinyl ether. A mixture of the foregoing allylic alcohol (2.5 g), ethyl vinyl ether (75 ml) and freshly crystallised mercury(n) acetate (600 mg) is refluxed continuously for 12 hours on a water bath. The reaction mixture is chilled in ice and mixed with 10 per cent aqueous sodium carbonate solution (25 g) and stirred well for 30 minutes at 0 °C. The organic layer is... 

Intermolecular addition and addition-cyclization reactions of aminium cation radicals with electron-rich alkenes such as ethyl vinyl ether (EVE) allow an entry into products containing the N—C—C—O moiety of 13-amino ethers 70 or the equivalent of /3-amino aldehydes 71. The mild conditions under which aminium cation radicals are generated from PTOC carbamates makes the reactions described in Scheme 22 possible. In the absence of hydrogen atom donors, the /3-amino ethoxy(2-pyridylthio) acetal 71 was the major product. The mixed acetal can easily be converted... 

The preparation involves an oxymercuration (Section 3.5.3) of the C=C double bond of the ethyl vinyl ether. The Hg(OAc) ion is the electrophile as expected, but it forms an open-chain cation A as an intermediate rather than a cyclic mercurinium ion. The open-chain cation A is more stable than the mercurinium ion because it can be stabilized by way of oxocarbe-nium ion resonance. Next, cation A reacts with the allyl alcohol, and a protonated mixed acetal B is formed. Compound B eliminates EtOH and Hg(OAc) in an El process, and the desired enol ether D results. The enol ether D is in equilibrium with the substrate alcohol and ethyl vinyl ether. The equilibrium constant is about 1. However, the use of a large excess of the ethyl vinyl ether shifts the equilibrium to the side of the enol ether D so that the latter can be isolated in high yield. 

Mixed acetals. A variety of functionalized mixed acetals can be prepared by acc-toxymercuration of a mixture of a vinyl ether and an alcohol followed by demercuration with Na,CS, (10, 369- 570) or, in simple systems, with NaBHj. Unsaturated alcohols can be used, and the vinyl ether can contain various functional groups, although not a cyano group. 

This is responsible for the miscibility of various polyesters polyacrylates and vinyl acetate copolymers with PVC Another postulated interaction which has not been studied so much is that between ether groups and aromatic rings which may be responsible for the miscibility of polystyrene and poly(methyl vinyl ether). Interactions probably also exist between other groups and aromatic moeties. However, some interactions can at present only be inferred from favourable heats of mixing found for low molecular weight analogues without much being really understood at a molecular level. 

The allyl vinyl ethers are usually prepared in situ, but it is also possible to isolate the primarily formed mixed acetals or allyl vinyl ethers155-1 57. With low-boiling vinyl ethers the reaction is carried out in a sealed tube with an excess of the vinyl ether. For a tandem Claisen-rearrangc-ment-ene cyclization involving the Saucy-Marbet reaction cf. ref 158. The generation of isopropenyl ethers from esters is described in ref 159. For a related Claisen rearrangement by the reaction of 2-methoxybutadiene with ends and phenols see ref 160. 

The [3,3] siginatropic shift itself proceeds with high stereoselectivity. Mixed ketene acetals can also be generated from vinyl ethers and allylic alcohols in the presence of phenylselenyl bromide157. 

A major problem with the analysis of 0-acetylated sugars is the determination of the site(s) of 0-acetylation. DeBelder and Norman (1968) have developed a method in which the 0-acetylated polysaccharide is treated with methyl vinyl ether to yield mixed acetals. These are stable to base and methylation of the protected polymer alkylates the position(s) of... 

We mentioned that by mixing vinyl epoxides and zerovalent palladium, the alcoholate formed was usually sufficiently basic to deprotonate the pronucleophile entity. In some cases, especially with ketones, low reactivity and yields were reported (Table To overcome the problem of the weak basicity of the alcoholate, silyl enol ethers, keto adds, or preformed lithium enolates have successfully been employed.f f" f f /3-Keto acids are masked enolates via the decarboxylation of the intermediary Tr-allylpalladium ]3-ketocarboxylate complexes. The main limitation of the use of keto adds as pronucleophiles seems to be their low reactivity toward the hindered cyclic vinyl epoxides. In these cases, the cationic n-allylpalladium complex undergoes ]S-elimination. Indeed, the reaction between benzoyl acetic acid and cyclobutadiene monoxide in the presence of Pd(PPh3>4 gives only the corresponding cyclopentanone and acetophenone as the... 

Radical Addition. Free-radical addition of toluene-/>-sulphonyl iodide or methanesulphonyl iodide to acetylenes proceeds readily and stereoselectively when the two are mixed in ether and illuminated. The high yields of crystalline products (102) obtained imply that in the mechanism (Scheme 7), chain transfer by the sulphonyl iodide ( 3) is much faster than isomerization of the intermediate vinyl radical (k. The /ra 5-addition was confirmed by zinc-acetic acid reduction to the sulphone, and by Z-ray analysis of one of the adducts. 

The vinyl ether (0.5 g) is mixed in a test tube with 5 ml of water and 0.5 g of semicarbazide hydrochloride. Ethanol is added to this mixture until dissolution is complete. Anhydrous sodium acetate (0.4 g) is then added to the mixture and the test tube is heated over a boiling water bath for 30 min. Semicarbazone separates out after cooling (see p. 229). 

Bn2NH2][OCOCF3]) achieved the second aldol reaction to afford the hemiacetal 150 in 99% ee and 14% yield from dialdehyde 148. The hemiacetal 150 was then transformed to a diastereomeric mixture of methoxy acetals 151 using Amberlyst-15 and MgSO. Conjugate addition of the mixed vinyl cuprate 152 to methoxy acetal 151, followed by trapping with trimethylsilyl chloride, furnished the silyl enol ether 153. 

Parallel approaches have been described for the preparation of polyacrylate-protease conjugates [396-400]. Acryloylation of subtilisin and a-chymotrypsin, followed by mixed polymerization with methyl methacrylate, vinyl acetate, styrene, or ethylvinyl ether, provides insoluble, doped polymethyl methacrylate, polyvinyl acetate, polystyrene, and polyethyl vinyl ether polymers [396]. These biocatalytic plastics perform especially well in hydrophilic and hydrophobic solvents, and have been used for peptide synthesis and the regioselective acylation of sugars and nucleosides. Similarly, modification of subtilisin and thermolysin with PEG monomethacrylate, then copolymerization with methyl methacrylate and trimethylolpropane trimethacrylate furnishes protease-polymethyl methacrylate plastics, which show good activities and stabilities in aqueous, mixed, and low-water and anhydrous organic media [397-400]. The protein-acrylate composites are unique in that they enable catalytic densities as high as 50% w/w. 

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