Synthesis of (+)-didesmethylmethylenomycin A methyl ester

A concise and efficient synthesis of the new compound—(+)-didesmethylmethylenomycin A methyl ester from chiral building block 4 has been achieved.     

Transition metal-mediated routes to cyclopentenones

Cyclopentenones are not only key building blocks for organic synthesis, but many possess interesting biological properties in their own right. This review begins with an overview of the biological importance of cyclopentenones and a summary of the main classical routes to cyclopentenones. A detailed account of recent advances in transition metal-mediated approaches to cyclopentenones follows. The aim of the review is to draw attention to transition metal-mediated approaches to cyclopentenones that are starting to be applied in organic synthesis, and to highlight promising cyclopentenone-forming reactions that have not yet realised their full potential in a synthetic context.     

Unusual,selective, reductive,deoxygenation of cyclopentenone alcohols

A selective, reductive, deoxygenation of 2-aryl-4-hydroxycyclopent-2-en-1-ones to afford 2-aryl-cyclopent-2-en-1-ones was achieved by NaBH₄–CeCl₃ in methanol.     

Intermolecular Pauson–Khand reactions of N-substituted maleimides

Co₂(CO)₈-mediated intermolecular Pauson–Khand reactions of N-substituted maleimides with terminal alkynes are described, producing maleimide-fused cyclopentenones. The transformation differs from other intermolecular Pauson–Khand-type reactions of electron-deficient olefins, which react with Co₂(CO)₈ and alkynes to produce conjugated dienes, or generally require terminal, monosubstituted olefins to generate cyclopentenones. The reaction works well for N-benzyl, N-aryl, and N-alkyl substituted maleimides, and tolerates branching at the 3-position of the terminal alkyne. N–H maleimide, N–CO₂Me maleimide, and maleic anhydride do not take part in the transformation.     

Stereoselective synthesis of spiroannulated cyclopentenones by the Pauson-Khand reaction on carbohydrate derived enynes

The stereoselective synthesis of spiroannulated cyclopentenones by the Pauson-Khand reactions on hexose and pentose derived enynes was achieved under carbon monoxide free conditions using a stoichiometric quantity of Co₂(CO)₈. The cobalt complex of the alkyne was cleaved using dimethoxyethane-acetonitrile at 85 °C to furnish spiroannulated cyclopentenones in 75-94% yields.     

Thiol-mediated tandem Michael-aldol reaction: a convenient method for the synthesis of fused cyclopentenones

A simple, convenient, one-pot synthetic approach towards substituted cyclopentenone derivatives via thiol-mediated tandem Michael-aldol reaction followed by acid-catalyzed thiol elimination and isomerization of 3-(2-formyl-3,4-dihydronapthalene 1-yl)-acrylic acid esters has been developed.     

Short synthesis of methylenecyclopentenones by intermolecular Pauson-Khand reaction of allyl thiourea

N,N,N′-Trimethylallylthiourea promotes the intermolecular Pauson-Khand reaction with alkynes in the presence of Co₂(CO)₈ and moderate pressure of CO followed by thiourea elimination allowing the formation of methylenecyclopentenone derivatives.     

Theoretical modelling of the epoxidation of vinylallenes to give cyclopentenones

Epoxidation of vinylallenes (1,2,4-pentatrienes) can lead to cyclopent-2-enones. Various experimental results suggest that these reactions are concerted and that vinylallene oxides undergo a concerted and thermal ring-closing reaction to give cyclopentenones. In 1977, this view has been supported by the epoxidation of a non-racemic 4-methyl-2,3,5-hexatriene to give a non-racemic 2,5-dimethyl-2-cyclopenten-1-one. Indeed, for the cyclization of (3E)-4-methyl-2,3-epoxy-2,3,5-hexatriene or (4E)-4-methyl-3,4-epoxy-2,3,5-hexatriene into 2,5-dimethyl-2-cyclopenten-1-one, a transition structure for the concerted rearrangement was located, and IRC calculations showed it linked together. The activation barrier predicted at the B3LYP/6-311++G(3df,3pd)//B3LYP/6-31G(d) level of theory is 80.9 or 124.2 kJ mol⁻¹, respectively and the reaction is exothermic by 127.2 or 89.2 kJ mol⁻¹, respectively.