2,4-monocyclic dendralene

Haak and coworkers in 2015 [78] and 2012 [79] reported monocyclic and bicyclic [3] dendralenes, which were generated via ruthenium-catalyzed cascade transformations. The complex mechanism involves ruthenium(0)-mediated dehydration of the allcyne 124, addition of the nucleophile to an alkenyl ruthenium allenylidene, and cyclization at a ruthenated alkyne to furnish unusual spirocyclic [3] dendralenes 126 and 128 (Scheme 1.17). 

The final class of monocyclic [3]dendralenes, the 1,3-analogs 22, can undergo DA reactions at either their endocyclic position (in a diene-transmissive sense)... 

The first (and only) detailed DTDA study of parent [4] dendralene (14) was reported by the Sherburn group in 2005 [35]. Upon addition of an excess of NMM, at ambient pressure and temperature, a mixture of five products was obtained in excellent overall yield (Scheme 12.26). The first DA reaction occurred primarily at the terminal diene site to provide l,2-monocyclic[3]dendralene 119, as well as 21% of the internal adduct 120. Triene 119 could not be isolated, and a second cycloaddition, predominately at the internal site, afforded the two endo-DA adducts 121 and 122. The major diastereomer arises via approach of the dienophile anti with respect to the succinimide ring in compound 119. The tris-adducts 123 and 124 originate from reaction at the acyclic diene site of 119, and from the less hindered top face (as drawn). The two intermediates generated... 

As mentioned earlier, o-quinone dimethides such as l,6-monocyclic(4]dendralene 134 are both linear and cross-conjugated, and thus fall outside the scope of this review. In our view, this connectivity pattern only corresponds to a dendralene when the ring system contains more than six carbon atoms. 

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