Ring systems enyne cycloisomerization

Up to this point Pd, Pt and Ru-catalysed enyne metatheses have been explained without involvement of metal-carbenes. However, carbenoid species seem to play a key role in these metatheses (or cycloarrangements) based on the following polycyclization involving cyclopropanation. Polycyclic ring systems are constructed by the cycloisomerization of dienynes catalysed by Ru, Pt, Rh, Ir and Re complexes... 

Enyne cycloisomerizations can also be exploited to annulate a cyclopentane onto an existing ring system. An example drawn from Trost and co-workers asymmetric total synthesis of picrotoxinin (119) is the conversion of bridged bicyclic intermediate 117 into tricycle 118 (Scheme 6-20) [46]. The optimal cyclization conditions in this case were unusual, requiring an internal proton delivery and a bidentate phosphine. A related example is the conversion of 120 to 121, which was the pivotal step in Trost s synthesis of (—)-dendrobine (122) [47]. An all-carbon tether is not required, as is exemplified by the conversion of 123 to 124 in the total synthesis of ( )-phyllanthocin (125) [48]. Note that in-situ reduction of the o-palladium species prior to /5-hydride elimination has occuined in this latter-example. The enyne disconnection in the synthesis of ( )-phyllanthocin was... 

Cycloisomerization of 1,6-enynes. In sterically difficult situations, such as those reactions proceeding via a transition state disfavored by developing 1,3-diaxial interactions, the catalyst system must be well designed. A case in point is the annulation of a bridged ring system en route to picrotoxinin. Thus, a failed reaction is revitalized when Pd(OAc), is combined with a tied-back ligand and an internal source of protons. 

An interesting possibility for the construction of bicyclic systems containing one six-membered ring arises when an intramolecular Heck reaction or palladium-catalyzed enyne cycloisomerization [311] to give a vicinal exodimethylenecy-cloalkane is immediately followed by a Diels-Alder reaction (Scheme 8.15 and Scheme 8.19). This sequence is normally conducted in two steps, but may also... 

Cycloisomerization of 1,6-enynes in the presence of a dicationic platinum(IV) catalyst has been reported to give five-membered ring systems (Scheme 146) ° ... 

Another subfield of enyne cycloisomerization chemistry is best described as cycloaddition reactions. For instance, allenedienes were found to react smoothly in the presence of an IPr-based gold(I) cationic species to produce the formal [4 + 3] cycloadducts (Equation (11.8)). This method is a valuable approach to fused seven-membered ring systems and was applied to a transannular reaction, allowing for the construction of a pentacyclic system. Of note, Toste showed that, by replacing the NHC on gold by a triarylphosphite, an alternative [4 + 2] cycloaddition was observed from the same linear allenedienes. ... 

Chatani N, Kataoka K, Mural S, Furukawa N, Seki Y. Con-stmction of novel polycyclic ring systems by transition-metal-catalyzed cycloisomerization of ene-ene-ynes. Interception of a carbenoid intermediate in skeletal reorganization of enynes. J. Am. Chem. Soc. 1998 120 9104-9105. 

Yet another palladium-catalyzed transformation leading to 1,2-dialkyl-idenecycloalkanes was established by Trost et al. when investigating a catalytic Alder-ene reaction (path D in Scheme 12). They showed that two different catalyst systems are capable of cycloisomerizing enynes 92 to either cyclic 1,4-dienes 96—the products of regular Alder-ene reactions— or the 1,3-dienes 95 (Scheme 15) [66-68]. Starting from palladium acetate, the reaction presumably occurs by coordination of both unsaturated moieties (intermediate 93) and subsequent cycloisomerization to the ring-... 

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