Oximes configural isomers

Since diazoates can be considered to be derived from oximes by substitution of nitrogen for the methine group, Hantzsch (1894) put forward the hypothesis that configurational isomerism was also occurring here. He therefore represented the isomeric diazoates by the structures 7.1 and 7.2, assigning the syn structure (7.1) to the labile diazoate and the anti (7.2) to the stable isomer. Nowadays the description recommended by IUPAC (1979) for such configurational isomers, namely (Z) instead of syn and (E) instead of anti, should be used. 

The chemical shifts of carbon atoms in NMR spectra of the oximes having pyrrolyl, furyl, benzofuryl, thienyl and pyridyl substituents change specifically on going from the E- to the Z-isomer. This makes it possible to use the chemical shifts for assignment of the oximes configuration and studying the special features of their electronic structure (08CHE1238). 

Oximes bearing different substituents at the carbon atom, in particular, al-doximes, can exist as two (syn and anti) configurational isomers ... 

Oximes can be alkylated by alkyl halides or sulfates. N-Alkylation is a side reaction, yielding a nitrone. " The relative yield of oxime ether and nitrone depends on the nature of the reagents, including the configuration of the oxime, and on the reaction conditions. For example, anri-benzaldoximes give nitrones, while the syn isomers give oxime ethers. " ... 

Stereoselectivity in reductions of acyclic oximes depends on the configuration of C=N bond. ( )-Isomer of oxime 89 produced syn-hydroxylamine 90 in excellent stereoselectivity in reaction with phenyldimethylsilane-trifluroacetic acid while giving anti-product in the reaction with lithium aluminium hydride. Stereoselectivity in reductions of (Z)-isomers of 89 was substantially lower in both cases (equation 62) . It can be assumed that the rules of stereoselectivity established in diastereoselective reduction of ketones can be applied to reduction of oximes as well. 

Owing to the relative rigidity of the carbon-nitrogen double bond, oximes can exist in two discrete geometrically isomeric forms the E or anti isomer anti-E-217) and the (Z) or syn isomer (syn-Z-217) (equation 70). In solid state, both oximes show high configurational stability and discrete existence. In solution, equilibrium between both isomers is rapidly established, favouring the thermodynamically most stable isomer " ... 

Our opinion is that the actual configuration of oximes 37 should be inverted. Nitrosation of 36 in the presence of an alcoholate should result in the two E and Z oximes the Z isomer will suffer a spontaneous base-induced rearrangement into the furazan 39 and will not be isolated. Furthermore, the reported spectroscopic data agree with the inverted configuration. The intermediacy of open-chain species in the rearrangement... 

Eplivanserin contains three stereogenic units. In addition to the E-config-ured alkene double bond there is a Z-configured substituted oxime. The single bond between both double bonds has partial double bond character. The formula shown below represents the s-trans isomer. This means that it represents an antiperiplanar conformation. 

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