Enantioselective oxa‐Diels–Alder Sequences of Dendralenes

Diene‐transmissive hetero‐Diels–Alder sequences involving carbonyl dienophiles are reported for the first time. High enantioselectivities are achieved in the reaction of phenylglyoxal with a broad range of dendralene structures, through the optimization of a Pd(2+) catalyst system. The initial catalyst‐controlled enantioselective oxa‐Diels–Alder (ODA) cycloaddition to a [3]dendralene generates a dihydropyran carrying a semicyclic diene. This participates in a subsequent catalyst or substrate‐controlled Diels–Alder reaction to generate sp(3)‐rich fused polycyclic systems containing both heterocycles and carbocycles. Computational investigations reveal a concerted asynchronous mechanism. π‐Complexation of a diene C=C bond to Pd(2+) occurs in both the pre‐transition state (TS) complex and in cycloaddition TSs, controlling stereoselectivity. A formal enantioselective [4+2]cycloaddition of a CO(2) dienophile is demonstrated.