Cascade reactions allenic substrates

Silyl allenes such as 48 and enol acetates such as 50 were particularly effective substrates for these reactions. The oxidative cascade reaction of trisubstituted alkene substrate 52 demonstrated a powerful application of ETIC chemistry to form bicycle 53 through consecutive carbon-carbon and carbon-oxygen bond construction. Highly diastereoselective reactions were observed when the nucleophile was... 

Conceptually similar palladium-catalyzed cascade reactions have been developed, involving molecular-queuing cycloaddition, cyclocondensation and Diels-Alder reactions [71], cydization-anion-capture-olefin metathesis [72], carbonylation-allene insertion [73], carbonylation/amination/Michael addition [74], sequential Petasis reaction/palladium-catalyzed process [75], supported allenes as substrates [76], and palladium-ruthenium catalysts [77]. 

In 2007, a gold(I)-catalyzed cascade reaction of diynes and electron-rich aromatics and heteroaromatics for the synthesis of complex a-pyrones was successfully realized by Schreiber and Luo (Scheme 12.13) [16]. This reaction involves the [3,3]-sigmatropic rearrangement of the diynes 25 to an yne-allene intermediate 26, which is supported by the observed racemization of enan-tioenriched substrates. Gold(I)-induced a 6-endo-dig cyclization of intermediate 26 would give the oxocarbenium intermediate 27, which could be trapped by electron-rich aromatics and heteroaromatics to afford complex a-pyrones 28. 

AUenes comprise the second class of privileged substrates for gold catalysis. They can be transformed into various functionalized structures, many via cascade processes. Several reactions involving in situ-generated allenes have been discussed so far (see Schemes 4.9,4.11,4.15, and 4.20 to 4.23), but in this section we focus on reactions that begin with allenes. 

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