Dendralenes branch out: Cross-conjugated oligoenes allow the rapid generation of molecular complexity

Twenty-seven years ago, H. Hopf published the only previous comprehensive review on branched oligoenes that had the title "Dendralenes: A Neglected Family of Hydrocarbons". The dendralenes are no longer neglected. Research into the synthesis, properties, and applications of dendralenes is rapidly gaining momentum and this Review summarizes important recent findings. From significant fundamental properties (the first demonstration of alternating behavior since the annulenes) through to unparalleled complexity-generating synthetic transformations, this fundamental oligoene family is coming of age. Effective synthetic approaches to cyclic and acyclic dendralene systems are analyzed and classified. The most powerful synthetic transformations of the dendralenes, diene-transmissive Diels-Alder reactions, are surveyed in detail. From significant primary properties through to unparalleled complexity- generating synthetic transformations, the fundamental oligoalkene family known as the dendralenes is coming of age. Synthetic approaches to acyclic and cyclic dendralenes are analyzed and classified in this Review. The most powerful synthetic transformations of the dendralenes, diene-transmissive Diels-Alder reactions, are surveyed in detail.