Z-configuration

Similarly the stereobonds" can be defined and added to the bond list in the fourth column of the CT. A single bond acquires the value of 0 if it is not a "stereobond, 1 for np (a wedged bond). 4 for either up or down, and 6 for down (a basbed bond), The cisjtrans or E[Z configuration of a double bond is determined by the x,y.2 coordinates of the atom block if the value is 0, Tf it is 3, the double bond is either cis or tmns. In the bond block of our example (Figure 2-76), the stereocenter is set to 1 (up) at atom 6 (row 6, column 4 in the bond block), whereas the configurations of the double bonds are determined by the x,y coordinates of the atom block. 

Polymerization of isoprene by 1,4-addition produces polyisoprene that has a cis (or Z) configuration. 

The configuration of pairs of isomeric 4-aryIidene-5-pyrazoIones, (Z)- and (E)-(117), was determined by H NMR data (72G491). When R is H, the E configuration is preferred when it is a methyl or a phenyl group, the Z configuration predominates. The presence of an exocyclic sulfur atom as in (118) lowers the interconversion barrier and the products... 

The PE spectra of simple diaziridines exhibit two bands at lower ionization potentials these are assumed to arise from removal of an electron from one or other of the lone pair MOs. An E arrangement of the two N-bound hydrogens in 3,3-dimethyldiaziridine (37a) is assumed, since the difference of the ionization potentials (0.51 eV) is much smaller than in the fused diaziridine (42) (1.40 eV) which necessarily has the Z configuration (73HCA1614). 

A consequence of this mechanism is that the reaction is stereospecific with respect to the E- or Z-configuration of the enolate. The E-enolate will give the anti aldol product whereas the Z-enolate will give the syn aldol. 

The di-TT-methane rearrangement is a stereospecific reaction. There are several elements of stereochemistry to be considered. It is known that the double bond that remains uncyclized retains the E or Z configuration present in the starting material. This result excludes any intermediate with a freely rotating terminal radical. The concerted... 

Conditions under which the reaction is directed solely toward the formation of the sulfide 46 (yield of up to 89.5%) have been reported (79ZOR1554) Liquid ammonia is used as a solvent, whereas sulfide ions are generated by ammonium sulfide formed directly in the reaction mixture from ammonia and hydrogen sulfide. The sulfide 46 possesses the Z,Z-configuration, providing evidence for a high trans stereoselectivity of the reaction (79ZOR1554). 

Z-Configuration of the initial compounds does not change in the reaction course, as indicated by the coupling constants of the methoxyethenyl group vinyl protons of the triazoles 166 and 167. Adducts of -configuration are present only as admixtures (83DIS). 

Aminobutenones of Z-configuration having at least one hydrogen atom attached to the amino group (80MI1) condense with aldehydes (EtOH, piperidine acetate, 25°C, 12 h) in 2 1 ratio to form 1,4-dihydropyiidine derivatives 290 (50NKZ1061). 

The enantiomers are obtained as a racemic mixture if no asymmetric induction becomes effective. The ratio of diastereomers depends on structural features of the reactants as well as the reaction conditions as outlined in the following. By using properly substituted preformed enolates, the diastereoselectivity of the aldol reaction can be controlled. Such enolates can show E-ot Z-configuration at the carbon-carbon double bond. With Z-enolates 9, the syn products are formed preferentially, while fi-enolates 12 lead mainly to anti products. This stereochemical outcome can be rationalized to arise from the more favored transition state 10 and 13 respectively ... 

Evidence for that stereochemical course comes from the rearrangement of meso-3,4-dimethylhexa-1,5-diene 4, which yields the E.Z-configured diene 5 almost quantitatively. With a transition state of boatlike geometry, a Z,Z- or E,E-configured product would be formed." ... 

In case of a Z,Z-configurated diene 6, the transoid conformation is favored, because of unfavorable steric interactions of substituents at C-1 and C-4 in the... 

The rearrangement proceeds from the Si-state of the 1,4-diene 1. The Ti-state would allow for different reactions like double bond isomerization. Rigid systems like cyclic dienes, where EfZ -isomerization of a double bond is hindered for steric reasons, can react through the Ti-state. When the rearrangement proceeds from the Si-state, it proves to be stereospecific at C-1 and C-5 no -isomerization is observed. Z-l,l-Diphenyl-3,3-dimethyl-l,4-hexadiene 5 rearranges to the Z-configured vinylcyclopropane 6. In this case the reaction also is regiospecific. Only the vinylcyclopropane 6 is formed, but not the alternative product 7. ... 

Assign E or Z configuration to the double bond in the following compound ... 

Z configuration. 180 assignment of, 180-183 Zaitsev, Alexander M., 384 Zaitsev s rule. 384... 

However, (-)-12 was shown to have (R)- Z) configuration by chemical correlation1033, and thus, in both metal-exchange steps, a/7//-SE2 -processes are involved. This, and the sign of optical rotation of intermediate stannanes, corresponds well with other published results111. Fortunately, this error has no implication for the stereochemical outcome of the overall sequence. 

The (Z)-configuration of the enol ether however is a prerequisite for both high. yyn-selecting and high optical purity of the products23. When, on the other hand, (Z)-2-benzyloxy-l-ethyl-thio-l-trimethylsilyloxy-l-propene is allowed to react with 2-propional in the presence of the diamine 2, the anti-aldol product is obtained in 92% ee42. 

The addition of nucleophiles with charge n (Nu", n = 0, -1, -2) to arenediazonium ions can be summarized as shown in Scheme 6-1. In the primary adduct (6.1) Nu" is covalently bonded to the (3-nitrogen. In most - perhaps all - cases the azo group is in the (Z)-configuration. The stability of the primary adduct is critically... 

Three configurational isomers are possible for a diazoanhydride with the same (substituted) arene residues on both sides the (Z),(Z), the (Z (E), and the (E),(E), as shown in Scheme 5-15, formulas 5.4, 5.5, and 5.6. Kauffmann et al. think that in their product the residues attached to both azo groups are in the (Z)-configuration. However, their arguments are not clearly convincing. 

R=H) where the 2-aminobenzenediazonium ion first produced reacts rapidly with the amino group to yield benzotriazole, which can be regarded as an intramolecular triazene (Scheme 6-32). However, in contrast to open-chain triazenes this compound contains the triazeno group in the (Z)-configuration, and here the heterocycle cannot be opened by the action of an acid. 

Since 1937 our knowledge of diazo isomerisms has increased, thanks to three workers in particular, Hartley, Le Fevre, and Liittke, together with their coworkers. In 1980 the first direct X-ray evidence for the (Z)-configuration of an arenediazoate was given by Kemp s group (Alcock et al., 1980b). It clearly confirmed Hantzsch s stereochemical hypothesis. 

If a monoarylacetylene (ArC = CH) is taken as a model for a transition state of an arenediazonium ion with a nucleophile Nu, two types of transition state can be visualized the first, 7.13, leads to the (Z)-azo compound 7.14, whereas the second, 7.15, results in the (E )-isomer 7.16 (Scheme 7-3). If the transition state is reactantlike (i.e., early on the reaction coordinate), repulsive interaction between the nucleophile and the aryl nucleus is small because the distance Nu-Np is still large. Therefore, the repulsion between the lone pair on Np and the aryl nucleus becomes the decisive factor. It favors an (E )-configuration of the Np lone pair with respect to the aryl nucleus (obviously it is energetically dominant compared with the repulsion between the lone pairs on Na and Np) therefore, transition state 7.13 is at a lower energy level, and Nu attacks NB in the (Z)-configuration. 

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