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.     

Asymmetric Brønsted Acid‐Catalyzed Nazarov Cyclization of Acyclic α‐Alkoxy Dienones

A Brønsted acid‐catalyzed asymmetric Nazarov cyclization of acyclic α‐alkoxy dienones has been developed. The reaction offers access to chiral cyclopentenones in a highly enantioselective manner. The reaction is complementary to our previously reported Brønsted acid‐catalyzed electrocyclization reactions, which provided differently substituted optically active cyclopentenones with a fused tetrahydropyrane ring in good yields and with excellent enantioselectivities.     

Construction of Difluoroalkylated Cyclopentenones through Photocatalytic Difluoroalkylative 5-endo Cyclization Cascades of Ynones

Difluoroalkyl groups and cyclopentenones are both important units in synthetic and medicinal chemistry. The difuoroalkylation coupled with a cyclopentenone formation still remains a challenge. Herein, we report a visible-light-induced cascade difluoroalkylative 5-endo cyclization of ynones with difluoroalkyl bromides to access difluoroalkylated cyclopentenones, which may find wide applications for the concise synthesis of fluorinated cyclic molecules.     

Synthesis of 2-cyclopentenones by gold(I)-catalyzed Rautenstrauch rearrangement

The rearrangement of 1-ethynyl-2-propenyl pivaloates to cyclopentenones catalyzed by cationic triphenylphosphinegold(I) complexes is described. The reaction tolerates both alkyl and aryl substitution at the acetylenic and olefinic positions. Importantly, the gold(I)-catalyzed rearrangement of enantioenriched propargyl pivaloates proceeds with excellent chirality transfer, thus providing a practical method for the enantioselective synthesis of cyclopentenones.     

Oxydation d'alcools ω-vinylalléniques par l'hydroperoxyde de tertiobutyle

The oxidation of α-vinylallenic alcohols 3 with t-butyl hydroperoxide in the presence of catalytic amounts of vanadyl acetylacetonate, yields the epoxides 6 or, mainly, the cyclopentenones 5 depending on the secondary or tertiary class of the alcoholic function. In the same conditions, the vinylallenic alcohols 4 lead only to cyclopentenones 7.     

Tetramethyl thiourea/Co2(CO)8-catalyzed Pauson-Khand reaction under balloon pressure of CO

A Pauson-Khand type of conversion of enynes to bicyclic cyclopentenones employing the commercially available Co2(CO)8 and tetramethylthiourea (TMTU) as catalysts is described. This method allows a variety of enynes with diverse functional groups to be cyclized into cyclopentenones of interest. [reaction: see text]     

Synthesis of polysubstituted cyclopentenones via [4+1] reactions of TIPS-vinylketenes

An efficient method for preparing a series of polysubstituted cyclopentenones from TIPS-vinylketenes and K?brich reagent has been developed in this paper. Additionally, highly substituted cyclopentenones can be prepared via [4+1] reaction of TIPS-vinylketenes with tert-butyl isocyanide and there is a remarkable preference for formation of products with an exocyclic (Z)-imine moiety.     

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.     

An Unprecedented Oxidation Reaction of Cyclopentadiene Derivatives by Pyridinium Chlorochromate

Oxidation of the furan cyclopentadienes (2) and (4) with pyridinium chlorochromate gave the cyclopentenones (5) and (6) respectively. Oxidation of the alkyl cyclopentadienes (7a and 7b) also gave the corresponding cyclopentenones (8a and 8b) as the major products.     

Formation of halogenated cyclopent-2-enone derivatives by interrupted Nazarov cyclizations

Allenyl vinyl ketones were exposed to titanium and indium halides in order to carry out Nazarov cyclizations. Whereas TiI₄ and InX₃ (X = Cl, Br, and I) led to cyclopentenones in which a halogen was trapped highly regioselectively, the reactions mediated by TiBr₄ gave mixtures of brominated isomers. When the allenyl vinyl ketones were treated first with Br₂ and then with TiBr₄, doubly brominated cyclopentenones resulted, and treatment with I₂ followed by TFA gave cyclopentenones bearing both hydroxyl and iodo groups.     

The development of a facile tandem Wolff/Cope rearrangement for the synthesis of fused carbocyclic skeletons

A set of mild processes for the conversion of vinyl cyclopropyl diazo ketones to highly functionalized cycloheptadienones and vinyl cyclopentenones by use of a target-inspired tandem Wolff/Cope rearrangement sequence is described. A divergent reaction course of the vinyl cyclopropyl diazo ketone substrates under sono- or photochemical activation provides good to excellent yields (55-98%) of the product cycloheptadienones and vinyl cyclopentenones.     

A Practical Titanium-Catalyzed Synthesis of Bicyclic Cyclopentenones and Allylic Amines

A practical titanium-catalyzed synthesis of bicyclic cyclopentenones and allylic amines is described. The process converts enyne substrates to iminocyclopentenes using 10 mol % of the air- and moisture-stable precatalyst Cp(2)TiCl(2) in the presence of n-BuLi and triethylsilyl cyanide. The resulting iminocyclopentenes can be hydrolyzed to cyclopentenones in good yields or reduced to allylic silylamines with Red-Al or DIBALH. Treatment of the crude silylamines with acetyl chloride allows isolation of allylic amides in excellent yields.     

Regioselectivity in intermolecular Pauson-Khand reactions of dissymmetric fluorinated alkynes

Stoichiometric and catalytic intermolecular Pauson-Khand reactions (PKRs) of dissymmetric fluorinated alkynes were performed, affording regioselectively α-fluorinated cyclopentenones. Ethyl 4,4,4-trifluorobutynoate was an excellent substrate; its reaction with norbornadiene gave the corresponding PKR adduct in good yield and complete regioselectivity. Conjugate addition of nitroalkanes or cyanide to this adduct is stereospecific and entails concomitant loss of a trifluoromethyl group. This reaction can be exploited to prepare cyclopentenones featuring quaternary centers.     

Facile Synthesis of 4‐Functionalized Cyclopentenones

1-8-Diazabicyclo[5.4.0]undec-7-ene(DBU)‐catalyzed Michael addition of nitroalkanes to dicyclopentadienone (1) followed by flash vacuum pyrolysis (FVP) provided a high‐yielding synthesis of extremely pure 4‐nitroalkyl cyclopentenones (3). The optimized Nef reaction of the nitro adducts (2) provided 4‐ketoalkyl cyclopentenones (5) in the similar way.     

Fluorous mixture synthesis of 4-alkylidene cyclopentenones via a rhodium-catalyzed [2+2+1] cycloaddition of alkynyl allenes

Fluorous mixture synthesis was used to prepare a library of 4-alkylidene cyclopentenones starting from a mixture of four alpha-amino acid derivatives tagged with different fluorous benzyl carbamates ((F)CBz) of varying fluorine content. The amino acids were converted to the corresponding propargyl esters and then subjected to an ester-enolate Claisen rearrangement to give a mixture of allenic amino esters. The allenes were then split four ways and propargylated with different propargyl bromides to give four mixtures of alkynyl allenes. The 4-alkylidene cyclopentenones were formed by a formal [2+2+1] cycloaddition of the alkynyl allenes using catalytic [Rh(CO)2Cl]2 under CO atmosphere. Demixing by fluorous preparative HPLC, removal of the fluorous benzyl carbamates, and then exposure to HCl/ether gave the hydrochloride salts of 16 compounds as diastereomeric mixtures in 69-99% purity. Thus, after just 26 chemical steps, a library of 16 cyclopentenones was prepared by using fluorous mixture synthesis. By comparison, the same library would have required 112 steps if each compound were made individually by parallel synthesis.     

A three-component coupling approach to cyclopentanoids.

A new approach to 2,3-disubstituted cyclopentenones has been developed. This approach consists of a two-step protocol involving the cyclization of a Z-vinyl bromide under Barbier type conditions to form a cyclopentenol, which is then oxidatively rearranged to generate the cyclopentenone. The Z-vinyl bromide is in turn derived from a ruthenium catalyzed three-component coupling of an alkyne, an enone, and a HBr equivalent. A range of 2,3-disubstituted cyclopentenones has been generated, including short syntheses of jasmone and dihydrojasmone. Further applicability of this strategy is shown in the total syntheses of tetrahydrodicranenone B, rosaprostol, and a selective COX-2 inhibitor.     

Regiospecific michael additions to α-methylene cyclopentenones

Nucleophiles add in a Michael fashion to α-methylene cyclopentenones regiospecifically at the exocyclic cisoid-enone double bond.     

Access to diversely α-substituted cyclopentenones from α-chlorocyclopentenones

α-Chlorocyclopentenones can readily be transformed into a variety of α-substituted cyclopentenones via their dimethyltrimethylene acetals.     

Tandem alkylation-cyclization process via an O,C-dianion

A general protocol for preparing densely functionalized cyclopentenones through a tandem nucleophilic addition-deprotonation-alkylation-cyclization process is described. Addition of lithioallene 2 to enamides 1 generates tetrahedral intermediate 3. Deprotonation of the γ carbon atom of the allene function in situ, followed by trapping by a suitable electrophile and cyclization during workup leads to C6 substituted cyclopentenones 6.     

Die Gasphasen-Flussthermolyse von 1-Isobutenyl- und 2-Methylphenyl-alkinyl-ketonen. Eine Synthese von Methylenomycin B

The Gas-Flow Thermolysis of 1-Isobutenyl Alkynyl and 2-Methylphenyl Alkynyl Ketones. Synthesis of Methylenomycin B The gas-flow thermolysis of 1-isobutenyl alkynyl or 2-methylphenyl alkynyl ketones were found to lead to phenols and cyclopentenones or to naphthols and indanones, respectively. These conversions involve two cyclization processes so far unknown with α-alkynones; they are interpreted as intramolecular additions of an allylic or a benzylic C,H bond to a triple bond which may occur in two directions. In addition, the cyclopentenones formed by the α-alkynone cyclization, a known carbene process yielding 5-rings, were also found. The available evidence ruled out a carbene process yielding 6-rings. The addition process yielding 5-rings was applied to a short (but low yield) synthesis of methylenomycin B .