Cyclopentene

Abstract: Chiral N-heterocyclic carbene catalysts generated from triazolium salts promote the cyclopentene-forming annulation of α,β-unsaturated aldehydes and 4-oxoenoates with excellent levels of enantioinduction and preference for the cis-1,3,4-trisubstituted cyclopentene diastereomer. Although the observed products could arise by conjugate additions of catalytically generated homoenolates, our mechanistic and stereochemical investigations strongly support a novel reaction manifold featuring an intermolecular crossed-benzoin reaction and an NHC-catalyzed oxy-Cope rearrangement.

Abstract: Nucleophilic heterocyclic carbene-catalyzed cyclopentannulation of enals and chalcones via homoenolate has been observed for the first time. Serendipitously, the reaction lead to a very efficient synthesis of 3,4-trans-disubstituted-1-aryl cyclopentenes instead of the expected cyclopentanones. The strategy works well with thienylidene tetralone also, leading to the tricyclic cyclopentene derivative.

Abstract: With homogenous catalysts on the basis of chiral metallocenes and methylaluminoxane it has become possible to polymerize cyclic olefins like cyclobutene, cyclopentene or norbornene. No ring opening reaction occurs. The crystalline polycycloalkenes show extremly high melting points between 400 and 600°C. Copolymers of cyclopentene with propene are amorphous. They have a low molecular weight and glass transition point. On the other hand the glass transition temperature of norbornene/ethene copolymers exceeds 130°C. These copolymers could be used as materials for optical discs and fibers.

Abstract: The enantioselective synthesis of α,α-disubstituted cyclopentenes using N-heterocyclic carbene catalysis is reported. This desymmetrization reaction proceeds via a chiral enol generated in situ from the combination of an enantiopure azolium salt and α,β-unsaturated aldehyde. This reactive enol undergoes addition to one of two enantiotopic ketones to afford an optically active β-lactone after the intermediate carbinol is involved in an internal acylation event to release the carbene catalyst. In the substrates where R = aryl, a decarboxylation occurs immediately after lactone formation to produce the quaternary center-containing cyclopentene products in up to 96% ee. When the R group is alkyl, the β-lactones are isolated in moderate yield and high diastereo- and enantioselectivity.