Cyclic dendralenes reaction

The Ma group has reported some very elegant multibond forming processes to synthesize dendralenes, including both palladium(II)- and rhodium(I)-catalyzed cycloisomerizations of di-allenes 106 to form cyclic dendralenes 107 (Scheme 1.15) [68, 69, 71], and also the remarkable palladium(0)-catalyzed three-component reaction of di-allenes 108, propargylic carbonates 109, and boronic acids 110 to form bicyclic [3]dendralenes 111 (Scheme 1.15) [70]. Similar, intermolecular transformations to those used to form dendralenes 107 had been reported earlier by Alcaide, to synthesize dihydrofuran-containing dendralenes [72, 73]. 

Scheme 1.24 Representative examples of transition-metal-catalyzed cycloisomerization reactions forming the C2-C3 bond of cyclic [3]dendralenes [69, 99, 106].

Two final examples of cross-coupling to furnish [3]dendralenes via C2-C3 bond formation are part of very short and efficient total syntheses, and highlight the versatility and attractiveness ofthe approach. As an extension of their work on 1,1-divinylallene (153) (Scheme 1.22), Sherburnel /. [123] synthesized allenic [3]den-dralene 188 via Kumada cross-coupUng of a metallated alkene 187 and a chiral, propargyl mesylate. A subsequent Diels-Alder reaction produced cyclic [3]den-dralene 190 en route to a pseudopterosin aglycone 191 (Scheme 1.29). Allenic [3]dendralenes are prone to decomposition [136], so the subsequent DA reaction was carried out in situ. 

Other variants of olefination have also been applied to dendralene synthesis [186-188]. In 2004, Dixon and Halton [189] prepared a variety of cyclic [3]dendralenes using Peterson-type olefination reactions (Scheme 1.39). These... 

Electrocyclization reactions are powerful synthetic tools to prepare compounds of great molecular diversity. These reactions allow for the formation of many substituted cyclic and polycychc compounds important in medicine, materials science, cosmetics, and so on. The well-established mechanisms and predictable outcomes of electrocyclization reactions permit the elaboration of logical blueprints for the synthesis of important molecules. Among these, the Nazarov cyclization is a salient member of the family. Reported first in 1941 by Ivan Nikolaevich Nazarov [1], this reaction has been studied extensively and many variations and applications have been developed over the years. In this chapter, we will present selected examples highlighting the versatility and synthetic power of this transformation [2]. In its simplest form, the Nazarov employs a divinyl ketone as the starting material, a cross-conjugated compound which can be regarded as a 3 -oxa-[3]dendralene. 

We conclude our section on cyclic [3]dendralenes with Meier and coworkers DTDA cascade of 1,5-dihydropentalene (109) [34] (Scheme 12.24). The sequence began with the addition of tetracyanoethylene (TCNE) to triene 109, providing the bridging tricycle 110. This was followed by a second in situ) DA reaction, to the less sterically encumbered bottom face of the diene, yielding 111 in excellent yield. To the best of our knowledge this is the only report of a bicyclic [3]dendralene undergoing multiple DA reactions. 

Despite great advances in the DA chemistry of the dendralenes, their cyclic counterparts, the radialenes (see Chapter 4), remain comparatively neglected. The simplest family member, [3]radialene (173), can, in principle, undergo a DA reaction to generate the bicyclic triafulvene 174 (Scheme 12.36). Although this... 

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