Das s-cis-/s-trans-Konformerengleichgewicht bei Allencarbonsaeureestern

Article abstract of DOI:10.1002/hlca.19800630512

The following UV, and 1H-NMR. spectroscopic investigations of the allenic compounds 1-9 (cf.Scheme 2) refer for the first time on a concrete experimental base to the preferred s-trans conformation of the allenic esters 1-6.With an additional allenic increment of 12 nm, the well established Woodward-Fieser rules in UV. spectroscopy could be extended on conjugated allenic compounds for the calculations of the (?->?*)-type band positions as well as for the prediction of the s-cis/s-trans conformational fraction (cf.Scheme 1).From 1H-NMR. shift experiments we conclude, that even small structural changes at the molecular framework have a measurable influence on the conformational equilibrium.In contrast to the situation in similarly α,β-unsaturated compounds (14 and 15), the allenic esters (1 and 4) exhibit preferentially s-trans conformations (cf.Scheme 3), but in both cases, the equilibrium is displaced toward s-cis conformation, when the α-position is methyl substituted (e.g. 14->15 and 1->4).