E-to-Z isomerization

E)- to (Z)-Isomerization.5 BF3 etherate or QHsAlC increases the efficiency of photoisomerization of (E)- to (Z)-cinnamic esters. In the absence of the Lewis acid, optimum (E)- to (Z)-conversion is in the range of 20-55%, whereas conversion of 85% is possible in the presence of the Lewis acids. 

Lewis acids inhibit the photochemical deconjugation of a,p- to p,y-unsaturated butenoic acids and promote rearrangement of (E)- to (Z)-isomers. Lewis acids also promote selective (E)- to (Z)-isomerization of the a,p-double bond of methyl 2,4-hexadienoate.6... 

The synthesis of 7,8-dihydroisoquinolines (166) can be efficiently achieved by FVP techniques. The reaction also provides an entry into the dihydroquinoline (168) and l,2-dihydnmaphthalene skeleton (170) by FVP of (167) and (169), respectively. Note that the method involves initial (E) to (Z) isomerization, electrocyclization and then a 1,5-H shift. ... 

Figure 32 The 124 , 124Z, 124 , and 124Z equilibrium system in which intermolecular host-guest complexation of 1-adamantol competes with cinnamate self-complexation in 124Z and E to Z isomerization about the amide bond occurs while the E stereochemistry about the cinnamate alkene bond is retained. Photoisomerization by irradiating at 300 nm switches the stereochemistry about the cinnamate alkene bond to Z to produce the 125 , 125Z, 125 , and 125Z system in which no competitive cinnamate self-complexation occurs. The cinnamate alkene photoisomerization is reversed by irradiation at 254 nm to complete an on-off photoswitching process.

The photochemistry and dark recovery were explained as follows. The initial photoreactions is an E to Z isomerization through a n-n state this is the most common type of photochemistry for imine bonds. This hypothesis was confirmed by IR spectroscopy, which showed a new band in the imine region after photolysis and is consistent with the UV spectrum of the product which is expected to be blue shifted, as observed. This reaction reaches equilibrium at about 50% conversion of the imine bonds. Since decomposition occurs only in... 

Though conceptually simple, photochemical E to Z isomerization of double bonds is not an efficient approach for accessing geometrically pure alkenylboronic esters [253, 258]. Alkynylboronic esters, however, can be selectively hydrogenated over Lindlar s catalyst [276]. 1,4-Dioxane was found to be the optimal solvent for providing (Z)-l-... 

Clearly, the acid-catalyzed E to Z isomerization of the hydrazone carbon-nitrogen bond in 28 must precede cych-zation. There are two possibiUties for the mechanism that can either be described as a 6ji-electrocyclization or as an... 

Highly constrained (E)-cycloalkenes, generated readily through the photoisomerization of (Z)-isomers, are known to possess unique physical and (chir)optical properties. These properties are of particular interest and importance from both the experimental and theoretical points of view. For example, the experimental barriers of thermal E-to-Z isomerization of cycloalkenes are compared with the theoretical values obtained by semiempirical calculations. Among the calculations reported, the PM3 method appears to give good fits to the experimental results e.g., the experimental strain energy of (Z)-cyclooctene (20Z) is 11.37 kcal/mol, which is close to the value (10.55 kcal/mol) obtained by PM3. The relevant experimental (chir)optical properties, such as specific rotations and absorption maxima and thermodynamic parameters for isomerization, are summarized in Table 16.2. 

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