Synthesis of polycyclic polyfunctionalized carbocycles by a cobalt(I)‐initiated tandem diels–alder reaction sequence

The cobalt(I)‐catalyzed neutral Diels–Alder reaction of acyclic 1,3‐dienes with conjugated enynes can be used to generate secondary dihydroaromatic 1,4,8‐trienes containing a 1,3‐diene substructure, in good yields under mild reaction conditions. These 1,3‐dienes can be converted, with reactive dienophiles in a tandem normal Diels–Alder reaction, into polycyclic compounds. In a similar tandem Diels–Alder sequence, the cobalt‐catalyzed neutral homo Diels–Alder reaction of 2,5‐norbornadiene can be used to generate polycyclic cycloaddition adducts. A sequential triple Diels–Alder reaction sequence can be realized when norbornadiene is reacted with the excess of the conjugated enynes under cobalt(I) catalysis. In a homo Diels–Alder, neutral Diels–Alder, normal Diels–Alder reaction sequence, the intermediately formed adducts are reacted with activated dienophiles to yield polycyclic compounds in a short sequence. The structures of several products were established by X‐ray analysis. This showed that predominantly endo‐products are formed in the tandem reaction sequences, while the exo‐product becomes predominant in the triple Diels–Alder reaction sequence.