Isomerization copper triflate

A different approach towards titanium-mediated allene synthesis was used by Hayashi et al. [55], who recently reported rhodium-catalyzed enantioselective 1,6-addition reactions of aryltitanate reagents to 3-alkynyl-2-cycloalkenones 180 (Scheme 2.57). In the presence of chlorotrimethylsilane and (R)-segphos as chiral ligand, alle-nic silyl enol ethers 181 were obtained with good to excellent enantioselectivities and these can be converted further into allenic enol esters or triflates. In contrast to the corresponding copper-mediated 1,6-addition reactions (Section 2.2.2), these transformations probably proceed via alkenylrhodium species (formed by insertion of the C-C triple bond into a rhodium-aryl bond) and subsequent isomerization towards the thermodynamically more stable oxa-jt-allylrhodium intermediates [55],... 

More modern studies have made use of copper(I) triflate (CuOTf) as the reagent. This compound is well known to form complexes with dienes and it provides a template on which cycloadditions can be effected. Several examples of this type of cyclization have been reported and cycloadditions based on this approach provide a useful route to cyclobutane derivatives. Thus, a new stereochemical synthesis of grandisol has been developed using the copper(I)-catalysed cycloaddition of the dienol 95 to afford the isomeric bicyclo-heptenols 96 exo endo ratio in this cyclization is solvent-dependent. The racemic... 

Catalyzed Diels-Alder Reactions. The uncatalyzed thermal intramolecular Diels-Alder reaction of 5,5 -oxyhis[( )-l,3-penta-diene] nonstereoselectively generates four isomeric 4-vinylcyclo-hexenes (eq 37). The major product has a trails ring fusion, in contrast to the single cis ring-fused isomer generated in the cop-per(I) triflate-catalyzed photoreaction of the same tetraene (eq 25). Copper(I) triflate also catalyzes a thermal Diels-Alder reaction of 5,5 -oxybis[( )-l,3-pentadiene] that proceeds under milder conditions than the uncatalyzed reaction. The stereoselectivity is remarkably enhanced, generating mainly the major isomer of the uncatalyzed thermal reaction and a single c/s-fused isomer (eq 37) that is different than the one favored in the photochemical reaction (eq 25). 

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