Vinyl ethers, various, synthesis

The strategy based on tandem cycloaddition leads to a short and efficient asymmetric synthesis of the pyrrolizidine necine base (-)-hastanecine, as shown in Scheme 8.32.163 Pyrrolizidine alkaloids have a long history for attracting the interest of synthetic chemists because of their physiological properties. The method of Denmark shown in this scheme is very simple and applied to synthesis of various alkaloids. The Lewis acid-promoted [4+2] cycloaddition between 2-acyloxy nitroalkene and chiral vinyl ether gives a nitronate that... 

The isomerization of allyl ethers and allyl acetals to vinyl ethers or vinyl acetals, respectively, has found many applications in organic synthesis (Equation (17)). Various transition metal catalysts have been reported in the literature for the isomerization of allyl ethers and allyl acetals. 

Whereas carbenoid character is definitely present in metalated alkyl vinyl ethers, lithiated alkyl and aryl vinyl sulfides and thioesters, which are easily available by hydrogen-lithium exchange, do not display carbenoid-typical reactions . They rather behave like nucleophilic reagents, so that their discussion is beyond the scope of this overview despite their utility in synthesis The same appiies to various derivatives of enamines, deprotonated in the vinyiic a-nitrogen position - . 

Various block copolymers have been synthesized by cationic living polymerization [Kennedy and Ivan, 1992 Kennedy, 1999 Kennedy and Marechal, 1982 Puskas et al., 2001 Sawamoto, 1991, 1996]. AB and ABA block copolymers, where A and B are different vinyl ethers, have been synthesized using HI with either I2 or Znl2. Sequencing is not a problem unless one of the vinyl ethers has a substituent that makes its reactivity quite different. Styrene-methyl vinyl ether block copolymer synthesis requires a specific sequencing and manipulation of the reaction conditions because styrene is less reactive than methyl vinyl ether (MVE) [Ohmura et al., 1994]. Both monomers are polymerized by HCl/SnCLj in the presence of (n-CrikjtiNCI in methylene chloride, but different temperatures are needed. The... 

The first synthesis of star polymers with a microgel core was reported by Sa-wamoto et al. for poly(isobutyl vinyl ether) (poly(IBVE)) [3,4]. In the first step, living cationic polymerization of IBVE was carried out with the HI/ZnI2 initiating system in toluene at -40 °C. Subsequent coupling of the living ends was performed with the various divinyl ethers 1-4. 

The presence of a heterosubstituent facilitates deprotonation. 4-Methyl-4H-pyran and 1,4-dimethyldihydropyr-idine are regioselectively metallated at the position next to the heteroatom by LIC-KOR or trimethylsilylmethylpotassium.47 The metallation of cyclic vinyl ethers with Schlosser s base has been successfully used in the synthesis of C-glycosides48 50 via metallation of the glucal followed by a reaction with tributyltin chloride to afford the corresponding tin derivative that could be submitted to coupling reactions with various electrophiles.48... 

Berthelette and coworkers [144] developed the synthesis of vinyl ethers 328 (Scheme 111) from (z-alkoxymethyl BT sulfones 326 and various aldehydes or ketones 327. The olefination reaction was carried out using LHMDS in THF at 0 °C. In general, very good chemical yields and poor stereoselectivities were recorded. Selected examples are presented in Table 31. 

The versatility of the Claisen rearrangement for the synthesis of functionally substituted y, 8-unsaturated carbonyl compounds has been greatly enhanced by the introduction of various vinyl ether appendages. These not only participate in the stereochemical control of the rearrangement, but also determine the nature of the functional group in the product (-CHO, -COR, -COOH, -COOR, -CONR2). 

Synthesis of the Phenoxy-Containing Monomers. Perfluoro(2-phe-noxpropyl vinyl ether) is prepared by the route shown in Figure 6 (3). The 2-phenoxy compound requires the least-involved synthesis of the various monomers. The first two steps were carried out in ordinary glass equipment. The last reaction was carried out using a bed of dried sodium carbonate. 

A solution phase chiral auxiliary for 1,3-dipolar cycloaddition of isomunchnones with vinyl ethers has been adapted for solid phase synthesis by attaching both enantiomers of the precursor a-hydroxyvaline to benzhydrylamine resin (Scheme 12.12) [13,19]. The auxiliary 22 was then functionalized by acylation and diazotiza-tion to provide diazoimide resin 23. Rhodium(II)-catalyzed nitrogen extrusion and cycloaddition in the presence of different vinyl ethers afforded, after detachment from the polymer, various bicydic molecules (24) in 49-65% yield and provided high degrees of selectivity (93-95% ee). 

Nitrogen-containing heterocyclic enone systems also reacted with allene to give (2 + 2)-cycloadducts, as was shown in alkaloid synthesis.178 As the keystep in the synthesis of an annotinine derivative, allene was added to 157 and the adduct 158 was obtained in quantitative yield. Various uracils have been modified by photochemical (2 + 2)-cycloaddition with olefins, e.g., vinylene carbonate,17,18° vinyl ethers, vinyl acetates, and (cetene acetals yielded 159.181 Very recently the photochemical addition of cyanoethylenes to 2-pyridones has been observed to yield mixtures of tetrahydroazocin-2-ones (160) and (2 + 2)-cycloadducts (161).182... 

Hojo, M. Masuda, R. Okada, D. A facile synthesis of 2,4-dialkoxy-, 2-alkoxy-4-phenoxy-and 2,4-diphenoxy-6-trifluoromethyl-3,4-dihydro-2/7-pyrans. Hetero-Diels-Alder reactions of tra i- 3-trifluoroacetylvinyl ethers with various vinyl ethers. Synthesis 1989, 215-216. 

The reaction of several a-diazoketones with ethyl vinyl ether under various conditions has been examined. In the presence of metal salts [Rh2(OAc)4, Pd(OAc)2, CuCl] ethoxy dihydrofurans are produced. The initial product of this reaction sequence is a cyclopropyl ketone which suffers from a spontaneous rearrangement to the dihydrofuran <96CJC2401>. An efficient synthesis of 3-acylfurans is achieved by Ag(I)/Celite mediated cycloaddition of dicarbonyl compounds with vinyl sulfide (<97TL5671> see also <97TL2095>). A two-step synthesis of 2-substituted 4-furanmethanol compounds was reported. The method involves a Homer-Wadsworth-Emmons reaction between 3-ketophosphonates and l,3-diacetoxy-2-... 

Rh(II)-catalyzed reaction of iodonium yhdes with conjugated compounds leading to efficient synthesis of dihydrofurans, oxazoles and dihydrooxepines [ 1068] synthesis of various heterocycles by Rh(II)-catalyzed reactions of iodonium ylides with vinyl ethers, carbon disulfide, alkynes and nitriles [1055] Rh(II)-catalyzed reaction of iodonium ylides with electron-deficient and conjugated alkynes leading to substituted furans... 

Reports on the fluorination of polychlorinated carbonyl compounds with hydrogen fluoride in the presence of various catalysts have appeared in the patent liter-ature, and a neat synthesis of pentafluoroacetone from the vinyl ether (CF3)2C CF OEt has been published (see Scheme 37) this latter report also... 

This system is suitable for the polymerization of polar functional monomers and the synthesis of polymers of various shapes. Figure 2.30 shows the living polymerization of polar monomers containing Si, r-butyldimethylsilyl vinyl ether. Polymerization was carried out using Eti sAlCli s in toluene at 0 °C in the presence of ethyl acetate.The C—0-Si bond is known to be readily hydrolyzed by free acid, and the use of added base was effective for inducing living polymerization without such side reactions. The molecular weight increased in direct proportion to the conversion, and the Mw /Mn was as low as 1.04. 

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