Synthesis of Nitrogen-Containing Heterocyclic Z -1,3-Dienes

The poor diastereoselectivity in 44e is not due to deterioration of diastereose-lectivity during condensation step. Instead the Z isomer of 44e is isomerized to more stable E isomer during treatment with NaOEt and during purification by column chromatography. The rate of isomerization is much faster in imidazole derivative 44f, and it is attributed to the more acidic character of the hydrogen atom of N-H bond [34], which results in rapid deprotonation of 44f by NaOEt and consequently, an accelerated rate of isomerization. 

The limited success to synthesize E isomers by Peterson olefination employing concentrated sulfuric acid is attributed to the formation of pyridinium salt in 44a-c, which results in reduced rate and formation of undesirable side product. The nitrogen ring system of 43d-f can not tolerate the presence of sulfuric acid and condensation adducts decompose. 

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