E,Z-isomer

The NMR spectra of the corresponding A-aryl derivatives (168d, 169d) are fairly complex (several E/Z isomers), but it was concluded that tautomers 168 and 169 are both present. The equilibrium strongly depends on the polarity of the solvent, the concentration, the temperature, and the nature of the substituent R. 

How many alkene products, including E,Z isomers, might be obtained by dehydration of 3-methy)-3-hexanol with aqueous sulfuric acid ... 

The teal value of the Wittig reaction is that it yields a pure alkene of defined structure. The C=C bond in the product is always exactly where the OO group was in the reactant, and no alkene isomers (except E,Z isomers) are formed. For example, Wittig reaction of cyclohexanone with methylenetriphenyl-phosphorane yields only the single alkene product methylenecyclohexane. By contrast, addition of methylmagnesium bromide to cyclohexanone, followed by dehydration with POCI3, yields a roughly 9 1 mixture of two alkenes. 

Alternatively, to an ice-cold solution of the alcohol (0.325 mmol) in dichloromethane (10ml) was added boron trifluoride etherate (4mmol), and the mixture was stirred at 0°C for lh. Work-up as above gave the alkenes as a 6 94 mixture of (E) (Z) isomers, in 99% yield (g.l.c.). 

E/Z isomers (or cisitrans isomers) fall under this category. They are called diastereomers, because they are stereoisomers that are not mirror images of each other ... 

A similar system, but with a more hindered porphyrin (tetramesitylporphyrin = tetraphenylporphyrin bearing three methyl substituents in ortho and para positions on each phenyl group), was tested for P-carotene oxidation by molecular oxygen. This system was chosen to slow the oxidation process and thus make it possible to identify possible intermediates by HPLC-DAD-MS analysis. The system yielded the same product families as with lycopene, i.e., (Z)-isomers, epoxides, and P-apo-carotenals, together with new products tentatively attributed to diapocarotene-dials and 5,6- and/or 5,8-epoxides of P-apo-carotenals. The oxidation mechanism appeared more complex in this set-up. 

Sterlie, M, Bjerkeng, B., and Liaaen-Jensen, S., Blood appearance and distribution of astaxanthin E/Z isomers among plasma lipoproteins in humans administered a single meal with astaxanthin, m Abstracts of 12th International Carotenoid Symposium, Cairns, Australia, Abstract 2A-13, 1999, 72. 

Metal complexes of tetra-4-ferf-butylphthalocyanine [PcM, M = Mn(III)OAc, Cu(II), Co(II), Ni(II), Fe(II) (C5H5N)2, Rh(III)Cl] have also been tested for their stereoselective potential in the cyclopropanation of styrene with ethyl diazoacetate 101K The Co(II) and Rh(I) complexes, already highly active at room temperature, produced the 2-phenylcyclopropanecarboxylic esters in a E Z isomer ratio of 1.0-1.2 which compares well with the value obtained with the rhodium(III) porphyrin 47 a (1.2). In the other cases, E.Z ratios of 2.0-2.2 were observed, except for M = Fe(II) (C5HsN)2 where it was (3.0) the E.Z ratio of the purely thermal reaction was 2.0. 

The crude product isolated from the reaction mixture clearly shows the presence of mixtures of (Z,Z)- and (E,Z)-isomers. Compound 91 could be obtained under the same conditions from 25 in superior yields. 

The dimerization reaction has been applied to intramolecular cyclization of bis(diene)s, leading to the formation of five- and six-membered ring products bearing two allylic silane moieties (Equation (42)).125 Although stereoselectivity depends on the substrate structure, trans-(E,Z) isomers are generally favored in the five-membered ring formations. 

Generally, amine products can be obtained with 91-99% ee. It is noteworthy that the ee of the product does not correspond to the E Z ratio of the starting imines. Imines 103 and 104 exist as 2.5 1 and 1.8 1 (E)/(Z) isomers, but titanocene-catalyzed reduction produces amines with 93% and 97% ee, respectively.105... 

There are two asymmetric carbon atoms and E, Z-isomers in norchrysanthemic acid. So, there exist eight stereoisomers. In order to elucidate the structure activity relationship of metofluthrin stereoisomers, we investigated the synthetic pathways of all stereoisomers of norchrysanthemic acid. 

In a 1 2 ratio these compounds yield a mixture of (Z/Z), (EIE) and (E/Z) isomers plus a substantial amount of the (E) and (Z) isomers of the monohydrazone monoketone. The complexity of the mixture (due to several equilibria that are established between its components) was illustrated by its complex H NMR spectrum. However, addition of one equivalent of barbiturate to the mixture led to a dramatic simplification of the NMR spectrum with the disappearance of the different products and the emergence of a single new species. This remarkable simplification of the spectrum is a consequence of the templating effect played by the barbiturate that amplifies the formation of only one of the receptors - in this case the ZyZ isomer. Interestingly, the addition of other species to the mixture such as acetate or 1-benzyluracil (with similar pKa values to barbiturate but different hydrogen bonding patterns) did not lead to the amplification of any specific species. 

The effect of catalysts on the stereochemistry of the Horner-Wittig reaction under liquid-liquid conditions has been studied by Mikolajczyk et al. (1975). The E/Z isomer ratios in the product [162] proved to depend on the catalyst... 

The same procedure was used to analyze the E,Z isomer 14. The higher optical rotation obtained with this compound (—143°) could suggest a higher optical yield. Indeed, integration of the same signals in the spectrum using Eu(tfc)3 and Ag(fod) gave an optical yield of more than 90%. 

A particularly interesting Michael acceptor is dimethyl 2-hexen-4-ynedioate since it can react at either position of the double or triple bond to form 1,4- or 1,6-addition products. Winterfeldt and Preuss183 treated this substrate with several secondary amines and observed exclusive attack at C-5 with formation of the 1,6-addition products (equation 78). In contrast to this, sodium methanolate added at CM to give the 1,4-adduct as a mixture of E/Z isomers (equation 79) with increasing reaction time, the product distribution was shifted towards the thermodynamically more stable , -product184. Acheson and... 

Besides direct nucleophilic attack onto the acceptor group, an activated diene may also undergo 1,4- or 1,6-addition in the latter case, capture of the ambident enolate with a soft electrophile can take place at two different positions. Hence, the nucleophilic addition can result in the formation of three regioisomeric alkenes, which may in addition be formed as E/Z isomers. Moreover, depending on the nature of nucleophile and electrophile, the addition products may contain one or two stereogenic centers, and, as a further complication, basic conditions may give rise to the isomerization of the initially formed 8,y-unsaturated carbonyl compounds (and other acceptor-substituted alkenes of this type) to the thermodynamically more stable conjugated isomer (Eq. 4.1). 

A series of so-called compact dimers has also been described [4]. These compounds were characterized by an even three-dimensionally distributed hydrophilicity due to the perpendicular ring conformation, and stabiHzation of hydrophilicity by hydroxylated amido groups characterized by high energy rotational barriers for (E/Z) isomers. Furthermore, their viscosity was low, allowing injection at 350 mgl mL (at this concentration and at 20°C, their viscosities were found to be between 19 and 26 mPa.s vs 44.5 mPa.s for iodixanol) [4]. 

A conscientious investigation of the stereochemical features was undertaken, including Li and NMR studies of the lithium intermediates . The epimeric ratio is kinetically controlled in the deprotonation step by a high preference for the 1-pro-R-H in 398a, and the stereocentre is configurationally stable at —78°C. The ratio of l-endo-399 and its epimer (Li behind the plane) was found to be 96 4. The interconversion of the E/Z-isomers l-endo-399 and l-exo-399 at — 78°C is slower than the rate of alkylation with... 

E Z isomers readily prepared and handled at room temperature... 

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