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.

Spectacular new atom efficient domino cycloaddition sequences involving [4]dendralene, the simplest cross-conjugated tetraene, are reported. Up to eight stereocenters, three new rings, and six C-C bonds are generated in one synthetic operation. The site selectivity of dienophile addition to cross-conjugated trienes and tetraenes is controlled with a simple Lewis acid.

A convenient and high-yielding three-step synthesis of the simplest branched triene, [3]dendralene, has been devised. The synthesis is robust and operationally simple, requiring no chromatography and involving no protecting groups or specialized equipment, allowing the synthesis of the volatile hydrocarbon in pure, solvent free form on a multigram scale. The stability, dimerization when stored neat, and Diels-Alder reactivity of [3]dendralene--including double cycloaddition sequences and catalytic enantioselective variant--are reported.

Hydrocarbons are the most diverse and important binary compounds in chemistry, and form the basis of our understanding of organic structure and reactivity. For these reasons, investigations into the chemical synthesis and properties of fundamental hydrocarbons represents an enduring theme of organic chemistry.[1] Four fundamental hydrocarbon families of oligoalkene structures can be defined (Scheme 1); each family differs in the type of atom connectivity (unbranched or branched, cyclic or acyclic).The unbranched acyclic and cyclic systems, namely linear polyenes and annulenes, respectively, have been thoroughly studied. The alternation in the behavior of the annulenes (that is, aromaticity and antiaromaticity for odd and even numbers of conjugated double bonds, respectively) has played an important role in the development of modern theories of structure and reactivity.[2] However, the acyclic (dendralenes [3] ) and cyclic (radialenes [4] ) branched systems have not been investigated so thoroughly. In 2000 we reported a multistep synthetic approach to the parent dendralenes, which allowed the preparation of 1-5 mg amounts of the hydrocarbons, including access to [5]-, [6]-, and [8]dendralenes for the first time.[5] More recently, we reported practical single-step approaches to [4]dendralene [6] and [5]dendralene [7] in multigram quantities, along with exploratory investigations into their chemical reactivity. Herein, we report short preparative syntheses of the first six members of the dendralene family, and we demonstrate that these fundamental hydrocarbons exhibit alternation in their physical and chemical properties. We provide evidence that this alternating behavior stems from the different conformational preferences of dendralenes that comprise even and odd numbers of alkene units.

The parent [3]dendralene and 2-substituted [3]dendralenes are made easily through cross-coupling reactions. Contrary to some earlier reports, [3]dendralene is sufficiently stable to be handled using standard synthetic methods. These compounds allow the one-step stereoselective construction of polycyclic frameworks through reactions with dienophiles. Site selectivity and stereoselectivity in Diels-Alder reactions with dienophiles are generally not influenced by the nature of the [3]dendralene's 2-substituent; these features can, however, be influenced with Lewis acids.