Cyclopropanes cycloisomerization

In the previous section, the cyclopropanation of a glycal to form a bicyclic intermediate was followed by a ring-expansion reaction en route to each oxepine. Presented here are routes that afford oxepines either by ring-closing metathesis (RCM) reactions or by cycloisomerization of terminal alkynes. 

Mild Ni(0)-catalysed rearrangements of l-acyl-2-vinylcyclopropanes to substituted dihydrofurans have been developed.86 The room temperature isomerizations afford dihydrofuran products in high yield. A highly substituted, stereochemically defined cyclopropane has been employed in the rearrangement to evaluate the reaction mechanism. The Cu(II)-catalysed cycloisomerization of tertiary 5-en-l-yn-3-ols with a 1,2-alkyl shift affords stereoselectively tri- and tetra-cyclic compounds of high molecular complexity (Scheme 29).87 A proposed mechanism has been outlined in which... 

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... 

A method for the synthesis of eight-membered carbocycles based on a new tandem tungsten-catalysed cycloisomerization-cyclopropanation reaction has been reported (Scheme 99).143 This is a catalytic reaction in which a heteroatom-stabilized Fischer... 

PtCh is also a catalyst of choice to perform the electrophilic activation of enynes towards the formation of cyclopropane derivatives [103,104], The sequence ruthenium-catalyzed C - C bond formation/platinum-catalyzed cycloisomerization has been successfully carried out in one pot at 60 °C to form... 

Pt(II) to the alkyne of the substrate likely triggers all these events. The cycloisomerization might undergo a metallacyclic intermediate that proceeds to eliminate /3-H. The formation of cyclopropanes is presumably succeeded via alkenyl platinum carbene followed by platina(IV)cyclobutane intermediates. The extension using formal metathesis of the enynes includes two transformations, the formation of 1,3-diene moieties and the stereoselective tetrasubstituted aUcene derivatives via O C allyl shift, both leading to diverse structural motifs and serving as the key step in the total synthesis of bioactive targets (Scheme 83). 

By way of cycloisomerization the formation of atetracycle containing two cyclopropane units from a 1,5-dien-lO-yne is an intriguing transformation, whereas atmncated 1,6-enyne affords a simple aUcenylcyclopentene/... 

Another cycloisomerization pathway of 1,6-enynes involves intramolecular cyclopropanation of the alkene by the alkyne (Scheme 4-24). This reaction is favored for enynes bearing a cyclic olefin and probably proceeds by 6-encto-dig-cyclization, followed by proton loss and protodeauration of the gold carbene... 

An enantioselective version of the gold-catalyzed cyclopropanation was reported by Michelet and co-workers. In the presence of [Au2 (/ )-3,5-/-Bu-4-MeO-MeOBIPHEP Cl2] and AgOTf, 3-oxabicyclo[4.1.0]hept-4-ene derivatives were obtained with excellent enantioselectivities (Scheme 4-26). Both electron-rich and electron-poor aryl groups are tolerated under these conditions. Lower reactivities and stereoselectivities were observed in the corresponding platinum- or iliodium-catalyzed cycloisomerizations. ... 

With the help of deuterium-labehng experiments, a mechanism was proposed that involves initial C-H activation followed by hydrometallation with complete transfer of the deuterium atom to the cyclopropane ring (Scheme 19.84). Cycloisomerization and reductive elimination affords the final compound. 

Scheme 9. Cycloisomerization into cyclopropane derivatives involving H-migration.

There are many examples of preparing cyclopentane structures from enynes by gold-catalyzed carbocyclization reactions. Toste et al. have reported that Au(l)-phosphine complexes act as superior catalysts for isomerization of 1,5-enynes to bicyclo[3.1.0]hexenes [124]. For example, treatment of 1,5-enyne (86) with 1 mol% of PhsP-AuPFfi in dichloromethane at room temperature results in formation of cyclopropane-fused cyclopentene (87) in 99% yield (Scheme 18.30). 1,6-Enynes also undergo similar cycloisomerization to five-membered cyclic compounds under the influence of cationic gold(l) catalysts [125, 126], Hydroxylated enynes are versatile precursors for cyclopentenones by gold-catalyzed cycloisomerization... 

Gold(I)-catalysed allene-vinylcyclopropane cycloisomerization has been reported to give the tricyclic framework of the protoilludanes in a single step involving cyclopropane ring expansion and a Prins cyclization (Scheme 122). ... 

A cycloisomerization cascade from alkynylcyclopropanecarboxylic acid derivatives to oxepinones and azepinones has been reported to involve nucleophilic addition followed by a cyclopropane ring opening, where both donor and acceptor groups are required as substituents of the cyclopropane ring (Scheme 133). ... 

A calcium-catalysed cycloisomerization has been reported to give cyclopropanes based on the equilibrium of a homoallenyl cation with its cyclopropane congener. The carbo-cation cascade is considered to be concerted and asynchronous (Scheme 200). ... 

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