[Synthesis, structure and stability of 4-aryl-4H-pyran-3,5-dicarboxylic acid esters]

Eight new 4-aryl-2,6-dimethyl-4H-pyrane-3,5-dicarboxylic acid esters 2 and a new 2-amino-2-nor derivative 7 are synthesized using known methods, whose limitations are shown. In contrast to the isoelectronic 4-aryl-1,4-dihydro-pyridines (DHP) 1 the compounds 2e, g possess a nearly planar heterocycle, as found by X-ray crystal structure analysis. The 4H-pyrane-3,5-dicarboxylic acid ester 2e with m-substitution exhibits s-cis/s-cis-conformation, while compound 2g with o-substitution of the aromatic ring represents s-cis/s-trans-conformation. The half-wave potentials (E1/2) show that 4H-pyranes are more stable than the DHP 1. The substitution of the 4-phenyl ring with an electron donating group decreases the potential considerably, whereas an electron acceptor group slightly increases the potential. Replacement of the 2-methyl group by the amino function causes a dramatic loss of stability.