Chiral Lewis acid-catalyzed highly enantioselective [4 + 3] cycloaddition reactions of nitrogen-stabilized oxyallyl cations derived from allenamides

A chiral Lewis acid-catalyzed highly enantioselective [4 + 3] cycloaddition reaction of allenamide-derived nitrogen-stabilized chiral oxyallyl cations is described here. The use of bisoxazoline ligand and CuOTf2 provides high enantioselectivity, especially with [SbF6]- as the counteranion.     

Stereoselective intramolecular [4 + 3] cycloadditions of nitrogen-stabilized chiral oxyallyl cations via epoxidation of N-tethered allenamides

The first intramolecular [4 + 3] cycloaddition reaction using nitrogen-stabilized chiral oxyallyl cations that are tethered to furan or diene through the nitrogen atom is described here. Formation of these nitrogen-stabilized chiral oxyallyl cations is achieved by a chemoselective epoxidation of chiral allenamides via syringe pump addition of dimethyl dioxirane. The ensuing cycloaddition can be carried out with a range of different lengths for the tether, and high diastereoselectivities can be obtained when using chiral allenamides with shorter tethers.     

(4+3) cycloaddition reactions of nitrogen-stabilized oxyallyl cations

The use of heteroatom-substituted oxyallyl cations in (4+3) cycloadditions has had a tremendous impact on the development of cycloaddition chemistry. Extensive efforts have been exerted toward investigating the effect of oxygen, sulfur, and halogen substituents on the reactivity of oxyallyl cations. Most recently, the use of nitrogen-stabilized oxyallyl cations has gained prominence in the area of (4+3) cycloadditions. The following article will provide an overview of this concept utilizing nitrogen-stabilized oxyallyl cations.     

Highly stereoselective [4 + 3] cycloadditions of nitrogen-stabilized oxyallyl cations with pyrroles: an approach to parvineostemonine

[structure: see text]. A highly stereoselective [4 + 3] cycloaddition of N-substituted pyrroles with allenamide-derived nitrogen-stabilized chiral oxyallyl cations is described here. This method provides an approach for constructing tropinone alkaloids.     

A new approach to phorbol by [4 + 3] oxyallyl cycloaddition and intramolecular Heck reaction

[formula: see text] A new synthetic strategy for a functionalized tricyclic core of phorbol has been developed by means of a [4 + 3] oxyallyl cycloaddition and subsequent intramolecular Heck reaction. The [4 + 3] oxyallyl cycloadduct 7 was chosen as the B-ring precursor of phorbol. Subsequent elaboration took advantage of its well-defined diastereofacial bias to afford the tricycle 5. This method should be of general value in the construction of 6,7- or 5,7-fused bicyclic systems.     

Total synthesis of acosamine and daunosamine utilizing a diastereoselective intramolecular [3+2] cycloaddition

A total synthesis of daunosamine (3) and acosamine (4) has been accomplished via a diastereoselective intramolecular nitrone-olefin cyclization. In the key step the chiral nitrone 12a cyclized to give two isoxazolidines 13a and 14a in an 82:18 ratio. Further elaboration of 13a led to daunosamine and acosamine. The effects of olefin substitution on the diastereoselectivity of the cycloaddition was also examined.     

Inverse electron-demand aza-[4+2] cycloaddition reactions of allenamides

An inverse electron-demand aza-[4+2] cycloaddition reaction of allenamides with 1-azadiene is described here. Effects of solvents on diastereoselectivity along with synthetic scopes and mechanistic insights are illustrated. Despite some synthetic limitations, this aza-[4+2] cycloaddition does provide a useful template for the synthesis of aza-glycoside related heterocycles.     

Thermal intramolecular cycloaddition of 4-alkenylfulvene; highly regio- and diastereoselective formation of [4+2] adduct

4-Alkenylfulvenes were prepared by the annulation of 1,4-ynediones and allylidenetriphenylphosphorane and subjected to a thermal reaction. Highly regio- and stereoselective [4+2] cycloaddition is accomplished with 4-((R)-3-benzyloxypent-4-en-1-yl)fulvene and the resulting adduct is transformed into bicyclo[3.3.0]octene derivative.     

Rhodium-catalyzed enantio- and diastereoselective intramolecular [2 + 2 + 2] cycloaddition of unsymmetrical dienynes

A cationic rhodium(i)/(R)-H(8)-BINAP or (R)-Segphos complex catalyzes an intramolecular [2 + 2 + 2] cycloaddition of unsymmetrical dienynes, leading to fused tri- and tetracyclic cyclohexenes bearing two tertiary or quaternary carbon centers in high yields with high enantio- and diastereoselectivity.     

Hexachloroacetone as a precursor for a tetrachloro-substituted oxyallyl intermediate: [4+3] cycloaddition to cyclic 1,3-dienes

The enol phosphate 2,2-dichloro-1-(trichloromethyl)ethenyl diethyl phosphate (1), easily available by a Perkow reaction between hexachloroacetone and triethyl phosphite, reacts with sodium trifluoroethoxide/trifluoroethanol in the presence of cyclic 5-membered 1,3-dienes to furnish alpha,alpha,alpha',alpha'-tetrachloro-substituted[3.2.1]bicyclic ketones 2. A [4+3] cycloaddition of a tetrachloro-oxyallyl intermediate is postulated.     

Palladium-catalyzed [3 + 2] intramolecular cycloaddition of alk-5-enylidenecyclopropanes

Readily accessible alk-5-enylidenecyclopropanes undergo [3 + 2] intramolecular cycloaddition reactions upon treatment with appropriate palladium complexes. The method allows the rapid and efficient assembly of a variety of bicyclo[3.3.0]octane systems with up to three stereocenters. Preliminary theoretical calculations uncovered previously unsuspected mechanistic possibilities based on either a concerted pallada-ene-like rearrangement or a stepwise process involving zwitterionic intermediates.     

Preparation of 3-azabicyclo [3.2.0] heptenones by intramolecular [2+2] cycloaddition

Pyrolysis of N-allyl enaminoesters is a general method for the stereoselective synthesis of 3-azabicyclo[3.2.0] heptenones ; the reaction involves a [2+2] intramolecular cycloaddition of an intermediate iminoketene.     

Facile synthesis of naphthoquinone spiroketals by diastereoselective oxidative [3 + 2] cycloaddition

A highly selective oxidative [3 + 2] cycloaddition of chiral enol ethers and hydroxynaphthoquinone is described. This convergent strategy is amenable to an enantioselective synthesis of beta-rubromycin and related naphthoquinone spiroketals. Several compounds were found to inhibit DNA-polymerase and telomerase in a manner resembling alpha-rubromycin and beta-rubromycin.     

Gold-catalyzed transannular [4+3] cycloaddition reactions

In the presence of a catalytic amount of Au(I) and Ag(I) salts, the 14-membered furanophane II with an allene function located across the ring undergoes both a transannular [4+3] and [4+2] cycloaddition. Secondary rearrangement products containing a tricyclic ring system were isolated when the catalyst was PtCl₂.