3,3-cyclo dendralenes

Only one study detailing DTDA reactions of the pseudo-acyclic 1,1- or 3,3-cyclo[3]dendralenes has been reported, most likely owing to the difficulties associated with DA reactions of such highly substituted dienes. The prototypical, and not yet reahzed, DTDA cascade of l,l-cyclo[3]dendralene 17 is presented in Scheme 12.12. If the first DA reaction occurs at the more sterically demanding exocyclic diene site, spiro-bicycle 65 is produced, which could react further to generate the... 

In terms of DTDA chemistry, 1,4- and 2,3-cyclo[3]dendralenes 19 and 20 are particularly attractive substrates. Both contain a single diene that can adopt the... 

Scheme 12.15 In situ generation of a 1,4-cyclo[3]dendralene 76 and its DTDA cascade.

Owing to their availability through cyclo-isomerization reactions, a number of studies describing DTDA reactions of 2,3-cyclo[3]dendralenes have been disclosed [20-22]. One example that perhaps best demonstrates the high levels of stereo- and chemoselectivity possible in such reaction sequences was published by Brummond and coworkers in 2006 (Scheme 12.16) [23]. In this... 

Scheme 12.16 A DTDA sequence of 2,3-cyclo[3]dendralene 81 developed by Brummond and coworkers.

Cyclo [3] dendralenes contain both a semicyclic and acyclic diene site, and, in principle, either of the two tricyclic frameworks 92 or 94 can be accessed through alternative DTDA sequences (Scheme 12.19). 

Scheme 12.21 Kanematsu and coworkers intramolecular DA reaction of 1,2-cyclo[3]dendralene 101.

Similarly, a single DA reaction of a l,2-cyclo[3]dendralene was reported by Kanematsu and coworkers in 1986. In this case, a base promoted isomerization of alkyne 100-generated [3]dendralene 101, which subsequently reacted in an intramolecular DA reaction to yield tricycle 102 (Scheme 12.21) [26]. 

In 1991, Billups etal. reported the DA reactivity 2,5-cyclo [4] dendralene 138, a compound that closely resembles the radialenes (see Chapter 4), both in structure and reactivity (Scheme 12.29) [37]. Unsurprisingly, when treated with an excess of l-bromo-2-chlorocyclopropene, a preference for addition at the more electronically activated internal diene site was observed, and a mixture of the mono and bis-adducts 140 and 141 was isolated in 65% overall yield. 

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