Under catalysis with an appropriate Lewis acid, the intermolecular Diels-Alder
cycloaddition and intramolecular polyene cyclization of the α-activated cross conjugated
cycloalkenone systems, possessing particularly increased dienophilicity/electrophilicity in
the cross conjugated double bond, proceeded constantly with a high degree of
stereochemical control, leading to a variety of synthetically useful molecules, which may
serve as advanced intermediates towards many structurally challenging natural products,
especially cis-clerodane diterpenoids. Mechanistically, an additional conjugated double
bond incorporated into the cycloalkenone core of the titled systems might contribute
synergistically to the classical secondary orbital effects, thus significantly enhancing the
endo-to-ketone addition. In addition, a tandem multiple σ-bond migration process, an
enzymatic pathway prevalent in nature, was proposed to rationalize the formation of
structurally unusual polyene-cyclization products occurring under standard chemical
conditions. On the other hand, these highly polarized cross-conjugated compounds was
found to be a superior Michael acceptor for Grignard reagents. In that regard, the
introduction of ω-unsaturation to the titled substrates via 1,4-addition could serve as a
springboard for a diverse array of annulation processes.
Keywords: Diels-Alder, cross conjugated, endo selectivity, secondary orbital
interaction, facial selectivity, dienophile, cycloaddition, polyene cyclization, Michael
reaction, Lewis acid, autoxidative cascade, cationic cascade, Conia-ene reaction,
lithium naphthalenide, cis-clerodane diterpenoid, hydride shift, catalysis,
palladium(II)-mediated oxidative cyclization, annulation, electron-withdrawing group.
