Asymmetric 1,3-diens

The FMO coefficients also allow cpralitative prediction of the kinetically controlled regioselectivity, which needs to be considered for asymmetric dienes in combination with asymmetric dienophiles (A and B in Scheme 1.1). There is a preference for formation of a o-bond between the termini with the most extreme orbital coefficients ... 

One example of asymmetric diene silylation was reported using the binaphthalenediol-based yttrocene catalyst (Fig. 3) [49]. A variety of 1,5- and 1,6-dienes were cyclized in 70-95% yields, but with < 5-50% ees. Due to the slower cyclization of 1,6-diene substrates, PhMeSiH2 was in place of PhSiH3 to prevent hydrosilylation of the olefins. 

More recently separations of o- and L-isomers have been reported using asymmetric diens ( 46) or o-L-proIine in the presence of Cu(ll) (249) in the mobile phase. These approaches to exploiting secondary equilibria are discussed in Section VI,E. 

Argenlalion chromalography, 261 Aromatic acids in human urine, 285 Aromatic hydrocarbons, 69 Arylhydroxylamines, 298 Ascorbic acid, 296 Aspirin, 282 Asymmetric diens, 290 Asymmetrical peaks, 58, 82, 160 AIT, stability constants of metal complexes. 278 Atrazine, 292 Atropine, 297 Axial diffusion mobile phase. 8 stationary phase, 8,9 Aza-arenes, 293 Azoxybenzenes, 298... 

Distinction between enantiodiscrimination by complexation and by alkylation of equilibrating intermediates is less clear in a number of related cases. It is likely that more than one type of chiral discrimination may be involved. For example, when a conformational ly flexible four-membered ring substrate is used for the same reaction, the enantioselectivity was only 56% ee (Eq. 8E.15) [175]. In this case, it has been proposed that equilibration via a tertiary e-palladium species may be possible, switching the origin of enantio-discrimination to the alkylation step. A more contrasting example involves the formation of an asymmetric diene via selective P-elimination of similar diastereomeric Jt-allyl intermediates (Eq. 8E.16). Evidence suggests that the enantio-determining elimination process occurs after the equilibration of the 7t-allyl intermediates [176]. 

Recently, this method has been expanded to include reaction with lactones as well as the use of asymmetric dienes [12],... 

Continuing the research on the reactivity of dienes with biphasic catalytic systems, Morel and Mignani discovered that the Rh/TPPTS functionalization of dienes in the 4-position has beneficial efforts [22]. With other asymmetrical dienes such as myrcene and the addition of, for example, ethyl acetoacetate, interesting regioselectivities of 99% for the desired isoprenic compounds were achieved [23, 24], A couple of subsequent reaction steps, based on TPPTS from Ruhrchemie, thus convert geranylacetone to vitamin E [25]. 

A detailed discussion of the earlier work on this can be found in CHEC-I. The cycloaddition of tetrachlorothiophene-1,1-dioxide with azulene takes place in the [4 + 2]mode rather than a [4 + 6]mode. A frontier orbital-based explanation has been provided for this <85TL2607). 2-Menthyl-3,4,5-trihalothiophene-1,1-dioxides have been found to be asymmetric diene equivalents <88T3I27>. Dienophiles such as styrene, allyl alcohol and acrylamide lead to regio- and stereoisomerically pure adducts, which then eliminate SO2 to form dihydrobenzene derivatives. 

Many types of rubber which resist ozone were obtained by copolymerization of ethylene and propylene with asymmetric dienes such as 1,4-hexadiene, dicyclopen-tadiene, or ethylidenenorbornene. The addition to the reaction mixture of a certain amount of diene allows preparation of a very stable polymer, because the formation of a linear polymer requires breaking of one double bond while the other bond is utilized to net the copolymer which takes place during the polymerization process. Vanadium... 

A further study of the ligand effect in the Ni-catalyzed addition of diaryl dichalco-genides to terminal alkynes revealed that, in several cases, a mixture of alkenes and 1,3-dienes was formed ( Scheme 3.74) [124], The most sdective reaction towards formation of dienes was observed with L = Cy2PhP and CysP. These ligands completely suppressed formation of alkenes and gave 75 25 ratio in favor of the asymmetric diene. 

Under optimized conditions (MeCN, 40 °C, 5h, 3 mol% Ni(acac)2, 30 mol% PPhCy2) the synthetic procedure was carried out with good yields and the asymmetric dienes were isolated in pure form using flash-chromatography ... 

The method has been applied in asymmetric and regioselective syntheses of several natural compounds. Two simple examples are the commercial syntheses of the gipsy moth hydrophobic sex attractant, disparlure (RE. Rossiter, 1981, 1985) and < mono-epoxidation of a diene in a leukotriene B4 synthesis (L.S. Mills, 1983). 

Asymmetric Heck reaction of the conjugated diene 184 and subsequent acetate anion capture of the rr-allylpalladium intermediate afforded 185 in 80% ee. which was converted into the key intermediate 186 for the capnelle-... 

High levels of asymmetric induction have been achieved in the hydroboration of 1,3-, 1,4-, and 1,5-dienes with thexylborane (482,483,489,490). The first chiral center is formed by an intermolecular reaction. In the second step, the organoborane intermediate undergoes an intramolecular hydroboration, creating the second chiral center with high diastereoselectivity. 

Metal-catalyzed asymmetric hetero Diels-Alder reactions of unactivated dienes with glyoxylates 98PAC1117. 

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