Enynes intramolecular carbopalladation

The intramolecular carbopalladation starting from a 2-halo-1,6-enyne leads to a terminal aUcenylpalladium halide relay that can be trapped by an external alkene or alkyne. With the former, the cascade process leads to a 1,3,5-hexatriene that undergoes rapid 67T-electrocyclization to a flve-ring-annelated cyclohexadiene and this, in turn, is easily dehydrogenated to the corresponding aromatic compound (Scheme 15). With an external alkyne trapping the intermediate, a 1,3,5-hexatrienylpalladium intermediate will be formed and can either cyclize by intramolecular carbopalladation or 67r-electrocycliza-tion before termination by dehydropalladation will occur. ... 

There has been broad scientific interest in the synthesis of analogs of la,25-dihydrox-yvitamin D3 193 due their potential clinical applications. A concise entry into the triene-containing tricyclic core structure from simple precursors is available using an enyne cycloisomerization. The precursors of the central cascade reaction are the alkenyl bromide 188 and enyne 189, which are easily obtained in enantiomerically pure form (Scheme 29).[66].[6v] In iiie key reaction, alkenyl bromide 188 and enyne 189 are converted into a 10 1 mixture of triene 191 and the thermally rearranged product 194. After recycling of 194 to 191, a yield of 76% was reahzed. Alkenyl bromide 188 undergoes regiospecific intramolecular carbopalladation of enyne 189 to provide the dienylpalladium complex 190. 

The remainder of this section will focus on the different ways carbopalladation has been used to consnuct natural products. First, Sect. B will consider intermolecular cases. Then, Sect. C will discuss intramolecular carbopalladation showing the potential of the reaction to create small, medium, or large rings. Finally, domino reactions, cycloiso-merizations of enynes, and asymmetric reactions will be highlighted. 

Reaction of the enyne 129 with p-bromoanisole gave the furan 130 smoothly in 53 % yield via Sonogashira coupling and carbopalladation of the triple bond, followed by intramolecular insertion of the double bond (Heck-type reaction) [42],... 

In this latter reaction mode, which is observed much more rarely than /3-dehydropal-ladation, a wide variety of ligands can be coupled to each other with the formation of new C—C, C—H, C— N, C—O, and C—Hal bonds. This section does not cover the numerous cascade couplings in which a number of successive intramolecular additions of 2 onto double bonds is eventually completed by /3-dehydropalladationt as well as the numerous [2 -I- 2 -I- 2] and [4 + 2] cyclotri- and cyclodimerizations of alkynes, enynes, and related compounds. " The Pd(0)-catalyzed Cope rearrangement also will not be considered here, as it proceeds via bis(i7 -allyl)palladium(ll) intermediates. The carbopalladation reactions of aUenes, which have been reviewed recently, are covered in Sect. IV.7. (For new examples see also refs. [10]-[12]). On the other hand, the numerous Pd-catalyzed formal [3 + 2] cycloadditions of trimethylenemethane (TMM) complexes may be classified as carbopalladations of alkenes without subsequent dehydropalladation. As the subject of this section has partially been covered in several newly published reviews, " the attention here will be on the most recent and interesting communications. 

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