Intramolecular [4+2] cycloaddition reactions of enamines and enamides

An attempt to utilize a new strategy for alkaloid synthesis which features the intramolecular [4+2] cycloaddition of enamines and enamides for the construction of the tetracyclic spiroamine skeleton characteristic of the Erythrina alkaloids was unsuccessful, unexpectedly giving a bridged cycloadduct rather than a fused one.     

Intramolecular [2 + 2] cycloaddition reactions of alkynyl ether derived ketenes. A convenient synthesis of donor-acceptor cyclobutanes

Mild thermolysis of tert-butyl alkynyl ethers furnishes aldoketenes, which undergo facile [2 + 2] cycloaddition reactions with pendant di- and trisubstituted alkenes. A wide variety of cis-fused cyclobutanones are produced in moderate to high diastereoselectivity and good to excellent yields by this method, and free hydroxyl groups are tolerated in the ene-ynol ether starting materials. Enol-ynol ethers also undergo efficient reaction to produce donor-acceptor cyclobutanes in high yields.     

Expedient synthesis of a highly substituted tropolone via a 3-oxidopyrylium [5+2] cycloaddition reaction

An expedient ten-step synthesis of a substituted tropolone is described. The synthesis involves a 3-oxidopyrylium [5+2] cycloaddition reaction with acrylonitrile as the key step, affording a highly functionalized [3.2.1]-bicycle 10 as a single regioisomer. The nitrile substituent of the reduced cycloadduct 12 permits efficient ether-bridge cleavage and tropolone 15 is obtained after a final bis-oxidation procedure. The pyranulose acetate cycloaddition precursor was derived from 3-methyl-2-furoate.     

A reinvestigation of the reaction of allylsilanes with N-phenyltriazolinedione: stereoselective synthesis of substituted urazoles by [3+2] cycloaddition

A stereoselective synthesis of hydroxy substituted urazoles of potential biological significance has been developed via the [3+2] annulation of α-substituted allylic silanes with N-phenyltriazolinedione. The need for complete blocking of the α-CH₂ group of allylsilanes for successful [3+2] annulation, as reported previously, is found to be unnecessary.     

New insights on the origins of the stereocontrol of the staudinger reaction: [2 + 2] cycloaddition between ketenes and N-silylimines

Density-functional theory studies on the [2 + 2] reaction between N-silylimines and ketenes to form the corresponding 2-azetidinones (beta-lactams) help to clarify several aspects on the mechanism of the Staudinger reaction. This reaction has been studied experimentally by Panunzio et al. It is shown that the formation of the 2-azetidinone ring takes place via two consecutive reactions. The first reaction consists of the nucleophilic addition of the iminic nitrogen to the sp-hybridized carbon atom of the ketene, with simultaneous migration of the silyl group from the imine to the oxygen atom of the ketene. This leads to silyl enol intermediates, in good agreement with the experimental results. Formation of the N-silylated beta-lactam takes place via a domino reaction consisting of a conrotatory thermal electrocyclization followed by a new silatropic rearrangement. It is also found that isomerization of the starting N-silylimine has a lower activation barrier than that associated with the formation of the C-N bond, which explains the stereochemical outcome experimentally observed. Further considerations on the asymmetric torquoelectronic effects involved in this reaction are also reported.     

Synthesis of highly substituted cyclopentenones via the [4 + 1] cycloaddition of nucleophilic carbenes and vinyl ketenes

[reaction: see text] Vinyl ketenes have been shown to undergo [4 + 1] cycloaddition with a variety of nucleophilic carbenes to deliver highly substituted cyclopentenones as products.     

Enantioselective [2+2+2] cycloaddition of ketenes and carbon disulfide catalyzed by N-heterocyclic carbenes

The chiral N-heterocyclic carbene-catalyzed [2+2+2] cycloaddition of ketenes and carbon disulfide was realized to give the cycloadduct of 1,3-oxathian-6-ones in good yields with excellent enantioselectivities.     

A new route to synthesis of substituted pyrazoles through oxidative [3+2] cycloaddition of electron deficient alkenes and diazocarbonyl compounds

A collection of 3,5-disubstituted pyrazoles are constructed by the oxidative [3+2] cycloaddition of electron deficient terminal olefins with α-diazoesters and amides in the presence of Oxone and cetyltrimethyl ammonium bromide. The highlights of this protocol are; (i) shorter reaction time (ii) moderate to excellent yield (iii) good regioselectivity.     

Nickel-iminophosphine-catalyzed [4+2] cycloaddition of enones with allenes: synthesis of highly substituted dihydropyrans

Enones were found to react with allenes intermolecularly in the presence of a catalytic amount of a nickel-iminophosphine complex to provide dihydropyrans via oxidative cyclization of an enone and Ni(0).     

Synthesis of indolizidines and pyrrolizidines through the [2 + 2]cycloaddition of five-membered endocyclic enecarbamates to alkyl ketenes. Unusual regioselectivity of Baeyer-Villiger ring expansions of alkyl aza-bicyclic cyclobutanones

The total syntheses of two indolizidine skeletons and of the necine base (+/-)-platynecine were accomplished in a concise manner with good overall yields starting from a common five-membered endocyclic enecarbamate. These syntheses feature a [2 + 2]cycloaddition of the five-membered endocyclic enecarbamate 5 to alkylketenes that proceeded in high yields and with high stereoselectivity to provide an endo alkyl cycloadduct as the major or only product. The minor exo alkyl cycloadducts, which can be observed in some [2 + 2]cycloadditions, seem to derive from the endo cycloadduct, the putative kinetic product, by epimerization. An unusual regioselectivity was observed for the Baeyer-Villiger oxidation of 7-alkyl-2-azabicyclic cyclobutanones. Endo-7-alkyl cycloadducts ring-expanded exclusively to a gamma-lactone in which oxygen is inserted into the C6-C7 bond in preference to the bridgehead C5-C6 bond. With the exo-7-alkyl cycloadduct the regioselectivity of the Baeyer-Villiger oxidation is drastically reduced, leading to mixtures of regioisomeric lactones in a ratio of approximately 1.5 to 1. It is hypothesized that the steric strain built into the Criegee cyclobutane intermediate is the regioselective controlling factor in these oxidations, overriding any stereoelectronic bias for ring expansion. A rationale for the mechanism of the [2 + 2]cycloaddition involving enecarbamates and ketenes is presented, which seems to involve the participation of an N-acyliminium-enolate intermediate.     

A new procedure for the preparation of 2-vinylindoles and their [4+2] cycloaddition reaction

A new approach for the synthesis of 2-vinylindole derivatives by 5-exo mode cyclization of 2-(3-silyloxymethylallenyl)anilines was developed. The starting allenylanilines were easily prepared by the Stille coupling of o-iodoaniline and allenylstannanes. The formed 2-vinylindole derivatives were transformed into several carbazole derivatives via the [4+2] cycloaddition reaction with suitable dienophiles.     

Nickel-catalyzed [4+2] cycloaddition for highly substituted arenes

Nickel(0) catalyzed [4+2] cycloaddition of electron-deficient dienes to alkynes and subsequent aromatization gave highly substituted arenes. This formal inverse electron-demand Diels-Alder cycloaddition is attributed to the formation of a seven-membered nickelacycle from a diene and an alkyne.     

A novel approach toward the synthesis of strigolactones through intramolecular [2+2] cycloaddition of ketenes and ketene-iminiums to olefins. Application to the asymmetric synthesis of GR-24

An intramolecular [2+2] cycloaddition of ketenes and ketene-iminium was developed for the preparation of GR-24, a synthetic analogue of the family of strigolactone plant hormones. Excellent levels of regioselectivity and of chiral induction were obtained using a bulky chiral amine for the formation of the cyclobutanone and a subsequent regioselective Baeyer-Villiger afforded the tricyclic lactone core of (+)-GR-24.     

A formal [3+2] cycloaddition for the synthesis of bicyclo[3.2.1]octanes

A new and a direct synthetic method for the construction of the bicyclo[3.2.1]octane system is envisioned by assembling cyclic allylsilanes with electron-deficient double bonds in a single step via a formal diastereoselective Lewis acid-promoted [3+2] cycloaddition.     

Stereoselective synthesis of cycloheptanone derivatives via an intermolecular [5 + 2] cycloaddition reaction

[reaction: see text] A [5 + 2] cycloaddition reaction of a new five-carbon unit was developed on the basis of a dicobalt hexacarbonyl propargyl cation species. Under the influence of EtAlCl(2), [5-benzoyloxy-2-(triisopropylsiloxy)-1-penten-3-yne)]dicobalt hexacarbonyl reacted with enol triisopropylsilyl ethers to yield seven-membered dicobalt acetylene complexes in good yield. The reactions with cyclic enol silyl ethers as well as acyclic enol silyl ethers exhibited remarkably high diastereoselectivity. The cycloadducts can be easily converted into various kinds of cycloheptanone derivatives.