Higher Dendralenes

Higher dendralenes are accessible by double cross-coupling by including branched alkenes into the electrophile unit. For example, in their state-of-the-art synthesis of the parent dendralenes [23], Sherburn and coworkers prepared [6]dendralene (21) by the reaction between 2,3-dichloro-1,3-butadiene (20), and the Grignard reagent (9) prepared from chloroprene, another readily available unsaturated halide produced annually on a megaton scale (Scheme 1.4) [24]. The scope of this reaction in the synthesis of substituted higher dendralenes remains unexplored. 

In the first category, the Sherburn group, the first to synthesize the parent family of dendralenes, have documented a number of transformations that result in the conversion of a higher dendralene into a lower one. [5] Dendralene (253) and... 

At present, the higher dendralenes remain comparatively unexplored, owing partly to the complexity of their DTDA chemistry. However, recent promising results suggest that control over their DTDA chemistry is possible. In particular, the organocatalyzed DA reactions of [4]dendralene (11), and the remarkable increase in DA selectivity (with respect to [4] dendralene) observed for the exhaustive addition of NMM to DVC (139), both reported by Sherburn et cd. (Scheme 12.45). 

Aside from the synthesis of dendralenes from nondendralenic materials, there also exist a variety of transformations that can be applied to a preexisting dendralene framework to add further functionality. Most of these examples form part of exploratory studies to test the reactivity of dendralenes nevertheless, they show potential for the synthesis of a variety of functionalized dendralenes. Broadly, these can be divided into transformations that reduce the length of the [ ]dendralene framework (e.g., n- n—1), ones that functionalize and preserve an existing [n] dendralene framework (i.e., n n), or ones that add extra branched alkenes to form a higher [njdendralene (e.g., n- n+1). 

Instances where a dendralenic alkene participates as a nucleophile in an addition or substitution, and the alkene is regenerated by elimination, appear surprisingly rare. One such example is the preparation of the Vilsmeier salt 271 of cyclic [3]dendralene 270 (Scheme 1.46) [208]. Formylation of very electron-rich dendraienes has also been reported, such as part of the iterative formyla-tion/olefination sequence to build higher dendraienes reported by Yoshida et al. (Scheme 1.20) [82]. 

The [3]- and [4]-dendralene analogs, the sulfur atoms of which are replaced by selenium or tellurium atoms, have been reported by Cava and coworkers (Figure 8.3) [21, 22]. The CV of compounds 6c,d and 7c,d indicate similar redox processes to 3c and 3d. The oxidation potentials of 6c and 6d, however, are higher than those of 3c and 3d, owing to the replacement of a DT unit by a 1,3-diselenium ring reflecting the higher polarizability of Se atoms. A similar trend was observed... 

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