Cobalt(I)-Catalyzed Diels-Alder Sequences Involving Alkynes and Dendralenes

Dendralenes, the archetypal cross-conjugated hydrocarbons, are shown to behave as multi-diene systems in Co(I)-catalyzed stepwise (4 + 2) cycloadditions with multiple C≡C dienophiles. The synthetic potential of [5]- and [6]dendralenes is unveiled for the first time, with high chemoselectivity, site-selectivity, and orientational control - the latter demonstrated to be both catalyst-directed and switchable. Formal (4 + 2) cycloaddition sequences, followed by pericyclic [4 + 2] cycloadditions and in situ oxidative aromatization, give rise to structurally diverse polycyclic aromatic hydrocarbons. The first heteroatom-free Nazarov cyclization furnishes complex polycyclic scaffolds reminiscent of natural products. Finally, the use of dendralenes and cyclohexa-1,4-dienes is shown to extend the scope of macrocyclic hydrocarbon synthesis.