Methoxyallyl

Silver trifluoroacetate is used in a one step synthesis of bicyclo[3 2 2]nona-6,8-diene-3-one from 2-methoxyallyl bromide and benzene [50] (equation 23) Analogous reactions of toluene, p-xylene, and mesitylene yield the corre spending substituted bicyclo[3 2 2]noiia-6,8-diene-3-ones [50]... 

The experimental result seems to support this model. Table 11 lists values for rotational barriers in some allyl radicals (Sustmann, 1986). It includes the rotational barrier in the isomeric 1-cyano-l-methoxyallyl radicals [32]/ [33] (Korth et al., 1984). In order to see whether the magnitude of the rotational barriers discloses a special captodative effect it is necessary to compare the monocaptor and donor-substituted radicals with disubstituted analogues. As is expected on the basis of the general influence of substituents on radical centres, both captor and donor substituents lower the rotational barrier, the captor substituent to a greater extent. Disubstitution by the same substituent, i.e. dicaptor- and didonor-substituted systems, do not even show additivity in the reduction of the rotational barrier. This phenomenon appears to be a general one and has led to the conclusion that additivity of substituent effects is already a manifestation of a special behaviour, viz., of a captodative effect. The barrier in the 1-cyano-l-methoxyallyl radicals [32]/... 

Esters of 2-acetonylbenzoic acid are formed when methyl 2-bromobenzoates are treated with 7r-(2-methoxyallyl)nickel bromide (77JOC1329). Cyclization to the isocoumarin occurs on treatment with sodium hydride in benzene with a trace of t-butanol (Scheme 183). A... 

When 2-bromobenzoic esters are treated with ir-(2-methoxyallyl)nickel bromide, the acetonyl group is introduced. Reductive cyclization of the methyl 2-acetonylbenzoates yielded isochroman-l-ones (77JOC1329). 

Stabilization of 34 by a hydroxy group is smaller, as is suggested by the small energy difference of 1.3 kcal mol between a-hydroxycyclopropyl and 2-hydroxyallyl cation in favour of the latter cation (see Figure 25). Experimentally, the a-methoxycyclopropyl cation appears to be a stable intermediate in substitution reactions in solution, and there is evidence for the independent existence of both the a-methoxycyclopropyl and 2-methoxyallyl cations in the gas phase245. 

Palladium(II) chloride (Johnson Matthey) and 10% Pd/C (Aldrich) were used as received. Bis[(methallyl)chloropalladium(II)] (ref. 8), bis(dibenzylideneacetone)palladium(0) (ref. 9), metho-xyoctadienes (ref. 10), l-methoxy-3-octene (ref. 11), methoxyallyle and cis + traru-methoxycrotyle (ref. 12) were prepared as described in the literature. 

Methylcyclohexenone 281 upon oxidation with Mn(OAc)3 in benzene under reflux gave 282, which reacted with phenylmagnesium chloride and CuBr-Me2S to form two isomeric ketones 283 and 284. Further, 283 has been transformed to vinylsilane 285 followed by its hydrolysis to form the free alcohol 286, which in turn was alkylated with methoxyallyl bromide to give 287. Oxalic acid-mediated deprotection of 287 led to the formation of the ketone 288. Ozonolysis of 288 in methanol afforded the fused 1,2,5-trioxepine 289 in low yields (Scheme 66) <1997BML2357>. 

Another unexpected feature of these a-methoxyallylation sequences was revealed by the preparation of the phosphonium salt 3. This was achieved by adding triphenylphosphine to the mixture of ethers 4 and 5, prepared as above, in dry toluene. The salt 3 precipitates from the reaction mixture at low temperature (-30 to +10°C). In contrast, when quaternization is carried out at room temperature under homogeneous conditions (e.g. in dry chloroform), the product is the y-methoxyallylphosphonium salt 6. It was subsequently shown that the salt 3 isomerizes readily in solution to give its isomer 6, and these are clearly the kinetic and thermodynamic products, respectively, of quaternization of the ethers 4 and 5, as shown in Scheme 2. 

The mass spectrum of (7r-C3H5PdCl)2 shows loss of both chlorine and allyl radicals from the molecular ion, but no ion C3H5PdCl+ corresponding to half the dimerized molecule was observed, and metal-metal interaction is proposed to account for the abundance of fragments containing the Pd2Cl unit. The base peak of the spectrum corresponds to C3H5Pd+, which can be formed from many of the other ions (132). The mass spectrum of the methoxyallyl complex (CXXIII)... 

Asymmetric allylboration has also been applied to y-methoxyallyl derivatives. Isomerically pure (Z)-y-methoxyallyldiisopinocampheylborane (rf31), prepared from Ipc2lSOMe and the lithium anion of allyl methyl ether, reacts with various aldehydes to afford the yyn - j-m e (boxy homoallylic alcohol (32a) in a highly regio- and stereoselective manner17 (Scheme 3.In). This one-pot synthesis of enantiomerically pure 1,2-diol derivatives went as smoothly as the asymmetric Brown crotylation, affording products with uniformly high diastereoselectivity. 

Metalation of the amidine 87, followed by introduction of 2-methoxyallyl bromide, furnished the intermediate 88, which underwent acid-induced cyclization to the product 89 (Scheme 12), illustrating an interesting approach to polyfunctionalized indoles suitable for further elaboration <20020L1819>. 

A particularly interesting combination of allyl ligand structure and the high reactivity toward aryl halides leads to the synthesis equivalent of nucleophilic aromatic substitution by enolate anions, but under completely different conditions (Scheme 51). In this example, the 2-methoxyallyl ligand is equivalent to an acetone enolate anion, but reacts with very different functional group compatibility (no polar... 

From 2-methoxyallylic halides a low yield of either a 2-methoxy-substitutcd rf -allyliron complex XXXI or an Fe complexed ketene, XXXII, may be obtained. ... 

The more sterically demanding (7) -2-butenyl)Fp cycloadds to diethyl methylenemalonate to give adduct (8) as a mixture of stereoisomers (equation 5). In this case the poor stereoselectivity may be due to the fact that the starting butenyl complex is a mixture of cis and trans isomers. Formation of the bicyclic ketone (9) ftom the reaction of (Tri -2-methallyl)Fp and 2-ethoxycarbonylcyclopentenone suggests that alkyl substituents can also be tolerated at C-2 of the (allyl)Fp unit (equation 6). The corresponding (t) -2-methoxyallyl)Fp fails to produce any cycloadduct with 2-ethoxycarbonylcyclohexenone or 2-cyclo-... 

Methoxyallyl cation (5a Y = OMe), derived from 2-methoxyallyl bromide (4a X = Br, Y = OMe) and silver trifluoroacetate in the presence of sodium carbonate, reacts with benzene, toluene, p-xylene or mesitylene. 2-Ethoxyallyl alcohol (4c X = OH, Y = OEt) in the presence of trifluoroacetic anhydride generates the 2-ethoxyallyl cation (5c Y = OEt). 2-Ethoxyallyl alcohol (4c) is probably more advantageous than the corresponding 2-methoxyallylic species (4a Y = OMe) because dealkylation of the intermediary oxonium ion seems to be less facile, and the ethyl enol ether is more nucleophilic than the methyl enol ether. Treatment of cyclic a-chloroenamines (4f X = Cl, Y = morpholino) and (4g X = Cl,... 

Notably, the rate increase for the Claisen rearrangement of methoxyallyl propionate is only about a factor of 3 over the parent crotyl system, compared to a factor 10 for the pyranoic glycals, cf. refs. 178 and 211. 

Cychadditioas. Treatment of 2-methoxyallyl bromide with silver trifluoroacetale (1, 1018-1019) under carefully controlled conditions generates the 2-methoxyallyl cation (a), which undergoes cycloaddition with dienes. For example, the bromide in... 

The first step, ionization of the bromide to the 2-methoxyallyl cation, 6-8, is assisted by the silver ion ... 

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