Double Diels−Alder Reactions of Linear Conjugated Tetraenes

Linear conjugated tetraenes are shown to participate effectively as bis-dienes in sequences involving
either two intermolecular Diels−Alder reactions or an intramolecular followed by an intermolecular
Diels−Alder reaction. Thus, simple tetraenol 1 is transformed into tetracyclic products 5, 6, and 9
in high yielding and highly stereoselective sequences with maleic anhydride involving the formation
of three rings, four C−C bonds and one C−O bond, and eight stereocenters. In the latter case, the
one-pot reaction protocol is very simple, and furnishes a single diastereoisomeric product in
essentially quantitative yield. Linear conjugated tetraenes exhibit complete terminal site selectivity
in reactions with dienophiles and computational investigations reveal that two discrete π-conjugative
interactions are the origin of this unexpected reactivity. B3LYP/6-31G(d) transition structures also
allow an explanation of unexpectedly high π-diastereofacial selectivities witnessed during these
transformations, through the identification of preferred C1−C* diene conformations and nonbonded
interactions. These new experimental and computational findings encourage the use of linear
conjugated polyenes in domino sequences.