Enol butyl vinyl ether

Inspired by the previous results, Leighton et al. reported the enantioselective [3 + 2] acylhydrazone-enol ether cycloaddition reaction by employing the same pseudoephedrine-based chiral silane. The pyrazohdine product was obtained in 61% yield with 6 1 dr and 77% ee in 24 h. The use of tert-butyl vinyl ether led to an improvement in both diastereoselectivity and enantioselectivity as shown in Scheme 34 [108]. 

Azadienes of this sort were studied simultaneously by Mariano et al., who reacted mixtures of (1 ,3 ) and (1E, 3Z)-l-phenyl-2-aza-l,3-pentadiene 275 with several electron-rich alkenes, e.g., enamines and enol ethers (85JOC5678) (Scheme 61). They found the (l ,3 )-stereoisomer to be reactive in this process affording stereoselectively endo 276 or exo 277 piperidine cycloadducts in 5-39% yield, after reductive work-up with sodium borohydride. The stereochemistry of the resulting adducts is in agreement with an endo transition state in the case of dienophiles lacking a cis alkyl substituent at the /8-carbon (n-butyl vinyl ether, benzyl vinyl ether, and 1-morpholino cyclopentene), whereas an exo transition state was involved when dihydropyrane or c/s-propenyl benzyl ether were used. Finally, the authors reported that cyclohexene and dimethyl acetylenedi-carboxylate failed to react with these unactivated 2-azadienes. 

The addition of diethyl azodicarboxylate to enol ethers gives diesters of 3-alkoxy-l,2-diazetidine-l,2-dicarboxylic adds. The reaction is stereospedfic, the energies of adivation are modest (29-46 kJ moF ) and the entropies of activation are negative (-36 to —46 J mol ). For n-butyl vinyl ether, the volume change of activation is very large... 

The catalytic activity of low-valent ruthenium species in carbene-transfer reactions is only beginning to emerge. The ruthenium(O) cluster RujCCO), catalyzed formation of ethyl 2-butyloxycyclopropane-l-carboxylate from ethyl diazoacetate and butyl vinyl ether (65 °C, excess of alkene, 0.5 mol% of catalyst yield 65%), but seems not to have been further utilized. The ruthenacarborane clusters 6 and 7 as well as the polymeric diacetatotetracarbonyl-diruthenium (8) have catalytic activity comparable to that of rhodium(II) carboxylates for the cyclopropanation of simple alkenes, cycloalkenes, 1,3-dienes, enol ethers, and styrene with diazoacetic esters. Catalyst 8 also proved exceptionally suitable for the cyclopropanation using a-diazo-a-trialkylsilylacetic esters. ... 

For Heck reactions catalysed by Pd(OAc)2 associated with dppp, see Ref. [Ig] and (a) Cabri, W., Candiani, I. and Bedeshi, A. (1990) Ligand-controlled a-regioselectivity in palladium-catalyzed arylation of butyl vinyl ether. J. Org. Chem., 55, 3654-5 (b) Cabri, W., Candiani, L, De-Bernardinis, S. et al. (1991) Heck reaction on anthraquinone derivatives ligand, solvent, and salts effects. J. Org. Chem., 56, 5796-800 (c) Cabri, W., Candiani, L, Bedeshi, A. and Santi, R. (1991) Palladium-catalyzed a-arylation of vinyl butyl ether with aryl halides. Tetrahedron Lett., 32, 1753-6 (d) Cabri, W., Candiani, L, Bedeshi, A. et al. (1992) a-Regioselectivity in palladium-catalyzed arylation of acyclic enol ethers. J. Org. Chem., 57, 1481-6 (e) Cabri, W.,... 

A 10 3 2 mixture of butyl vinyl ether, EtjN, and cyclohexenyl triflate treated with a little Pd(OAc)2 in DMSO, and heated to 60-5° with stirring for 3h - 1-(1-butoxyethenyl)cyclohexene. Y 87%. Significantly, vinylation takes place at the a-position (whereas with aryl triflates a mixture of regioisomers is obtained). F.e. with both cyclic and alicyclic enol triflates, and conversion to methyl a,(3-ethyleneketones s. C.-M. Andersson, A. Hallberg, J. Org. Chem. 54, 1502-5 (1989). 

The reaction of a,co-diyne with monoene instead of monoyne also gave an aromatic compound (Scheme 5.4) [9], The reaction included cleavage of a C—O bond. Diyne 7 reacted with 50 equiv of 2,3-dihydrofuran to give aromatic alcohol 8 in 97% yield. Acyclic vinyl ether such as n-butyl vinyl ether could be used in place of 2,3-dihydrofuran. The reaction of 7 with 25 equiv of n-butyl vinyl ether gave 9 and 10. The combined yield was nearly quantitative. n-Butyl vinyl ether acted as an acetylene equivalent when the reaction gave 9. Based on this result, an enol ether could be used as an acetylene equivalent in Rh-catalyzed [2 -j- 2 -j- 2] cycloaddition [11]. 

As an extension of this work, photoinduced [2-1-2]-cycloadditions of 1-acetyKsatin 13 with cyclic enol ethers (furan, benzofuran, 2-phenylfuran, 8-methoxypsoralen) and acyclic enol ethers (n-butyl vinyl ether and vinyl acetate) were investigated which afforded the spirooxetanes in high yields (82 to 96%)... 

This procedure consists of the synthesis of a precursor, methoxymethyl vinyl ether, an a-hydroxy enol ether, and the intramolecular hydrosilylatlon of the latter followed by oxidative cleavage of the silicon-carbon bonds. The first step, methoxymethylation of 2-bromoethanol, is based on Fujita s method.7 The second and third steps are modifications of results reported by McDougal and his co-workers. Dehydrobromination of 2-bromoethyl methoxymethyl ether to methoxymethyl vinyl ether was achieved most efficiently with potassium hydroxide pellets -9 rather than with potassium tert-butoxide as originally reported for dehydrobromination of the tetrahydropyranyl analog.10 Potassium tert-butoxide was effective for the dehydrobromination, but formed an adduct of tert-butyl alcohol with the vinyl ether as a by-product in substantial amounts. Methoxymethyl vinyl ether is lithiated efficiently with sec-butyllithium in THF and, somewhat less efficiently, with n-butyllithium in tetrahydrofuran. Since lithiation of simple vinyl ethers such as ethyl vinyl ether requires tert-butyllithium,11 metalation may be assisted by the methoxymethoxy group in the present case. 

Organic azides combine with open-chain and cyclic eno) ethers to give A -triazolines in high yield. The addition is stereospecific cis and the orientation process is determined by electronic effects.145-160 The triazolines are unstable above 100° and can decompose in mainly two ways. 44 Triazolines from the open-chain enol ethers, vinyl butyl ether, /S-ethoxypropene, and a- and /3-methoxystyrene, eliminate alcohol thermally and are converted into 1,2,3-triazoles. For example, the triazoline 67 from /J-ethoxypropene 66 and p-nitrophenyl azide decomposes quantitatively at 150° into l-(p-nitrophenyl)-5-methyl-1,2,3-triazole (68). 

Use of the preformed Z-silyl enol ether 18 results in quite substantial anti/syn selectivity (19 20 up to 20 1), with enantiomeric purity of the anti adducts reaching 99%. The chiral PT-catalyst 12 (Schemes 4.6 and 4.7) proved just as efficient in the conjugate addition of the N-benzhydrylidene glycine tert-butyl ester (22, Scheme 4.8) to acrylonitrile, affording the Michael adduct 23 in 85% yield and 91% ee [10]. This primary product was converted in three steps to L-ornithine [10]. The O-allylated cinchonidine derivative 21 was used in the conjugate addition of 22 to methyl acrylate, ethyl vinyl ketone, and cydohexenone (Scheme 4.8) [12]. The Michael-adducts 24-26 were obtained with high enantiomeric excess and, for cydohexenone as acceptor, with a remarkable (25 1) ratio of diastereomers (26, Scheme 4.8). In the last examples solid (base)-liquid (reactants) phase-transfer was applied. 

In the course of these investigations Hoffmann and his group have also developed novel entries to f-butyl 2-methylene-3-oxoalkanoates 2-109a and 2-methylene-3-oxo-sulfones 2-109b by oxidation of 2-108a and 2-108b, respectively obtained by reaction of the aldehydes 2-106 and acrylate 2-107 a or phenyl vinyl sulfone 2-107 b. The cycloadditions of these oxabutadienes to enol ethers and alkenes proceeded in the expected way (Fig. 2-29) [133]. 

Just as anions of allyl derivatives can be homoenolate equivalents (chapter 13) so anions of vinyl derivatives can be acyl anion equivalents. Vinyl (or enol) ethers can be lithiated reasonably easily, especially when there is no possibility of forming an allyl derivative, as with the simplest compound 81. The most acidic proton is the one marked and the vinyl-lithium derivative 82 reacts with electrophiles to give the enol ether of the product17 84. However, tertiary butyl lithium is needed and compounds with y-CHs usually end up as the chelated allyl-lithium 85. These vinyl-lithium compounds add directly to conjugated systems but the cuprates will do conjugate addition.18... 

Cyclohexenyliodonium salts are moderately stable due to a compromise between the stability of the secondary vinyl cation and the ring strain of the cyclic structure. Solvolysis of 4- -butyl-l-cyclohexenyl(phenyl)iodonium tetrafluoroborate (30) was investigated in various alcoholic and aqueous solutions. The main products are those expected for a cyclohexenyl cation intermediate (31), the enol ether (32) and/or cyclohexanone as well as iodobenzene (Scheme 25). 

An alternative to the bis-lactim ether approach is based on condensations of saturated five-membered heterocycles such as imidazolidinone (532) which can now be obtained in an optically pure state by a straightforward classical resolution/ The related oxazolidinone (533) has been obtained from methionine and used to prepare (R)-amino-acids [cf. (528) ] as well as the vinyl substituted derivatives(534) by oxidation and elimination of the sulphur group." Yet more general routes to chiral amino—acids have been reported using a variety of asymmetrically substituted ester enolate equivalents (535) in combination with the electrophilic nitrogen source di-t-butyl... 

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