Asymmetric synthesis of 1,3-dioxolanes by organocatalytic formal [3 + 2] cycloaddition via hemiacetal intermediates

A novel asymmetric formal [3 + 2] cycloaddition reaction for the synthesis of 1,3-dioxolanes using cinchona-alkaloid-thiourea-based bifunctional organocatalysts is reported. The reaction proceeds via the formation of hemiacetal intermediates between γ-hydroxy-α,β-unsaturated ketones and aldehydes.     

Cyclooctanone synthesis via a formal [6+2] cycloaddition reaction of a dicobalt acetylene complex

An efficient formal [6+2] cycloaddition reaction of a new six-carbon unit with enol silyl ether was developed on the basis of a dicobalt hexacarbonyl propargyl cation species. Under the influence of EtAlCl₂, 6-benzoyloxy-2-(triisopropylsilyloxy)-1-hexen-4-yne-dicobalthexacarbonyl reacted with enol triisopropylsilyl ethers to yield 7-(triisopropylsilyloxy)-3-cyclooctyn-1-one-dicobalthexacarbonyl derivatives in good yield. The reactions with cyclic enol silyl ethers as well as acyclic enol silyl ethers exhibited remarkably high diastereoselectivity.     

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.     

The formal total synthesis of (+/-)-strychnine via a cobalt-mediated [2 + 2 + 2]cycloaddition

A short, highly convergent total synthesis of racemic isostrychnine, and thus strychnine, has been completed. The route involves 14 steps in the longest linear sequence and is highlighted by a cobalt-mediated [2 + 2 + 2]cycloaddition of an alkynylindole nucleus to acetylene.     

Efficient synthesis of tetrahydroquinolinones by acetic acid-mediated formal [3+3] cycloaddition

Abstract  

A simple and efficient method to synthesize a variety of tetrahydroquinolinones was successfully achieved by reacting various β-enaminones with several α,β-unsaturated aldehydes. This strategy can be viewed as a Br?nsted acid-mediated formal [3+3] cycloaddition.     

Laser Raman detection of an electrogenerated intermediate during anodic synthesis of dihydrobenzofurans via formal [3+2] cycloaddition

A phenoxonium cation intermediate was successfully observed in situ by laser Raman spectroscopy during the electro-oxidation of 4-methoxyphenol in a lithium perchlorate/nitromethane electrolyte solution as shown by the Raman bands at 1665 cm⁻¹ and 1615 cm⁻¹. These conditions are those used for the electrosynthesis of dihydrobenzofurans by a formal [3+2] cycloaddition when the oxidation is carried out in the presence of an olefin.     

Intriguing formal [2+3] cycloaddition promoted by a hypervalent iodine reagent

Treatment of various substituted phenols in the presence of iodobenzene diacetate, perfluorinated alcohols and furan, allylsilanes or cyclic enol ethers promotes an oxidative annulation process in moderate to useful yields. In only one step, this method produces different heterocyclic rings such as dihydrofuranobenzofurans, tetrahydrofuranobenzofurans, tetrahydropyranofurans, and dihydrobenzofurans.     

Accessing substituted pyrrolidines via formal [3+2] cycloaddition of 1,3,5-triazinanes and donor-acceptor cyclopropanes

The formal [3+2] cycloaddition of 1,3,5-triaryl-1,3,5-triazinanes with donor-acceptor cyclopropanes has been found to provide pyrrolidines in good to excellent yields under mild reaction conditions. Preliminary mechanistic investigation indicates that this formal [3+2] cycloaddition reaction proceeds through competing S_N1 and S_N2 pathways.     

Au-catalyzed asymmetric formal [3 + 2] cycloaddition of isocyanoacetates with maleimides

An efficient protocol for the Au(I)-catalyzed asymmetric formal [3 + 2] cycloaddition of isocyanoacetates with phenylmaleimide has been developed. In the presence of cationic Au(I)/DTBM-segphos complex, excellent diastereoselectivity and high levels of enantioselectivity (up to 97% ee) have been attained with a variety of α-substituted isocyanoacetates. The synthetic potential of the resulting enantioenriched 1-pyrrolines has been demonstrated by the preparation of highly substituted pyrrolidines bearing a quaternary stereocenter.     

Gold-catalyzed formal [3 + 3] and [4 + 2] cycloaddition reactions of nitrosobenzenes with alkenylgold carbenoids

We report two new formal cycloaddition reactions between nitrosobenzenes and alkenylgold carbenoids. We obtained quinoline oxides 3 in satisfactory yields from the gold-catalyzed [3 + 3]-cycloadditions between nitrosobenzenes and alkenyldiazo esters 1. For propargyl esters 5, its resulting gold carbenes react with nitrosobenzene to give alkenylimine 8, followed by a [4 + 2]-cycloaddition with nitrosobenzene.     

Novel formal [2+3] cycloaddition between substituted phenols and furan

Treatment of various substituted phenols in the presence of furan, iodobenzene diacetate, and trifluoroethanol promotes oxidative formal [2+3] cycloaddition in moderate to useful yields.     

New alkaloid-like heterocycles via formal aza-[3+2] cycloaddition reaction of cyclic enaminones with cyclopropenones

The formal aza-[3+2] cycloaddition reactions of cyclopropenones and cyclic enaminones, including a chiral one, were employed for the first time in the direct formation of new pyrrolidine and indolizidine derivatives. The regiochemistry of cyclization is dependent of both ring size and steric factors of enaminones.     

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.     

Ytterbium-catalyzed formal [4+2] cycloaddition: Synthesis of chalcogen-quinolines 3-unsubstituted

A regioselective ytterbium-catalyzed annulation reaction between ethylglyoxalate, anilines and silyl-alkynes bearing selenyl- and telluryl-moieties for the formation of poly-substituted quinolones is described. A series of examples formed under mild conditions are presented, including a scaled-up reaction and a study on catalyst recyclability.     

Thiourea-catalyzed asymmetric formal [3+2] cycloaddition of azomethine ylides with nitroolefins

A chiral thiourea catalyst possessing an amine function catalyzes an asymmetric [3+2] cycloaddition of azomethine ylides to nitroolefins to provide highly functionalized pyrrolidines with high diastereo- and enantioselectivities (up to 98:1:1 dr, 92% ee). The reaction proceeds in a stepwise manner consisting of Michael addition and subsequent intramolecular aza-Henry reaction. Both reactions are promoted by the thiourea catalyst, and the reaction rate of the latter step is efficiently enhanced by the addition of 2,2,2-trifluoroethanol.     

A formal [3 + 3] cycloaddition reaction. 5. An enantioselective intramolecular formal aza-[3 + 3] cycloaddition reaction promoted by chiral amine salts

A detailed account on chiral secondary amine salt promoted enantioselective intramolecular formal aza-[3 + 3] cycloadditions is described here for the first time. The dependence of enantioselectivity on the structural feature of these chiral amines is thoroughly investigated. This study also reveals a very interesting reversal of the stereochemistry in the respective cycloadducts obtained using C(1)- and C(2)-symmetric amine salts. In addition, the influence of solvents, counteranions, and temperatures on the enantioselectivity is described, and a unified mechanistic model based on experimental results as well as semiempirical calculations is proposed.     

Scope of the formal [3+2] cycloaddition for the synthesis of substituted 3-arylindanes and related compounds

We report the single step synthesis of several 3-arylindanes and related compounds via a formal [3+2] cycloaddition. A study of the influence of the aromatic ring substitution pattern on the reaction was carried out.