Intramolecular Diels−Alder Reactions of Ester Linked 1,3,9-Decatrienes: Cis/Trans Selectivity in Thermal and Lewis Acid Promoted Reactions of Ethylene-Tethered and Benzo-Tethered Systems

High cis (i.e., endo) diastereoselectivities are witnessed in heat-promoted intramolecular Diels−Alder
(IMDA) reactions of ethylene-tethered hexadienyl acrylates. The cis stereoselectivity is improved by
promotion with Et2AlCl. The first examples of Et2AlCl catalyzed intramolecular Diels−Alder reactions
of ester-activated dienophiles are reported. In contrast, the corresponding benzo-tethered hexadienyl
acrylates undergo moderately trans (i.e., exo) selective IMDA reactions. Very high trans stereoselection
is obtained upon promotion with ATPH. The outcomes of these reactions are essentially insensitive to
dienophile (C10) geometry and substitution. DFT (B3LYP/6-31+G(d)) computed cis/trans product
distributionsbased on Boltzmann transition structure populationsare in good agreement with the
experimental results. These computational investigations provide useful insights into the origins of
stereoselection in these systems. The stereoselectivity exhibited by the ethylene-tethered hexadienyl
acrylates is ascribed to stabilizing secondary orbital interactions at play in the cis-transition structures
(TSs). In the benzo-tethered series, this effect is overridden by stabilizing π-conjugative interactions,
between the benzo moiety and the 1,3-diene component, which are stronger in trans TSs, compared to
the cis TSs. The computed TS geometries generally exhibit advanced peripheral bond forming
asynchronicity, with the tether carbonyl group in conjugation with the dienophile. Such TS features
significantly weaken the stereodirecting influence of terminal dienophile substituents.