Transition-metal-catalyzed ring-closure reactions

There are a few reports using transition metal-catalyzed ring-closure reactions to produce benzofuran derivatives. One example, reported by Chen and Dormer, used a copper-catalyzed protocol to prepare several 2,3-... 

Later, a variety of transition-metal-catalyzed ring-closure reactions leading to fulvene derivatives were reported. These reactions are summarized in Scheme 6.40 [85-96]. 

The comparison of intramolecular carbopalladation reactions of allenes and alkenes outlined in Schemes 9-5 and 9-6 illustrates that not every transition metal catalyzed ring closure necessarily involves a template effect. Others, however, clearly benefit from it. A prototype example is the palladium catalyzed cycloisomerization of alkenyl epoxides carrying distal pre-nucleophiles [38, 39], representing one variant of the famous Tsuji-Trost allylation [40]. 

Just as in aryl halides, the halogen can be replaced by hydrogen and by a metal, or be involved in transition metal-catalyzed processes (covered in Section 3.2.3.11.2). Three of the mechanisms of such nucleophilic substitutions are familiar from benzene chemistry via arynes, SrnI processes, and Pd(0)-catalyzed sequences. However, of the two further mechanisms of nucleophilic replacement, the ANRORC (Addition of Micleophile, Ring Opening, Ring Closure) is unique to heterocycles, and Sae reactions occur only with strongly activated benzenoid systems. 

The asymmetric synthesis of cyclopropanes has attracted continual efforts in organic synthesis, due to their relevance in natural products and biologically active compounds. The prevalent methods employed include halomethylmetal mediated processes in the presence of chiral auxiliaries/catalysts (Simmons-Smith-type reactions), transition-metal-catalyzed decomposition of diazoalkanes, Michael-induced ring closures, or asymmetric metalations [8-10,46], However, the asymmetric preparation of unfunctionahzed cyclopropanes remains relatively undisclosed. The enantioselective activation of unactivated C-H bonds via transition-metal catalysis is an area of active research in organic chemistry [47-49]. Recently, a few groups investigated the enantioselective synthesis of cyclopropanes by direct functionalization reactions. 

The synthetic plan for (-)-dactyloIide by Lee and co-workers in 2010 involved the distinctive transition-metal-catalyzed transformations, including the formation of the 2,6-cw-disubstituted tetrahydropyran via the tandem ruthenium-catalyzed Alder-ene reaction [197,198] and palladium-catalyzed ring closure. The Alder-ene... 

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