Stereoselective intramolecular nitrone cycloadditions to chiral allyl ethers

The Intramolecular nitrone cycloadditions to Z and E chiral allyl ethers afford annulated isoxazolidines with good to excellent stereocontrol in favour of the C-5/C-5' anti isomers. The relative stereochemistry at the stereocenters in C-3/C-4 depends on the length of the chain connecting dipole and dipolarophile.     

Stereoselective intramolecular cycloadditions of homochiral nitrilimines as a source of enantiopure 2,3,3a,4,5,6-hexahydro-pyrrolo[3,4-c]pyrazoles

Starting from the methyl esters of glycine, l-alanine, l-phenylalanine and (S)-2-phenylglycine, we developed a synthetic route to the title compounds in the enantiopure form by means of a stereoselective intramolecular 1,3-dipolar cycloaddition of homochiral nitrilimines 5a–d. Fair to good overall product yields and high cycloaddition diastereoselectivity were obtained.     

1,3-Dipolar cycloadditions of MeOPEG-bounded azides

1,3-Dipolar cycloadditions of MeOPEG-supported azide 2 with a variety of dipolarophiles have been studied. 1-MeOPEG-supported 1,2,3-triazoles 4 and 5, 1,2,3,4-tetrazoles 12 and aziridine 14 were obtained in nearly quantitative yields. The removal of the MeOPEG moiety from the 1,2,3-triazole nucleus was achieved by acidic cleavage of the cycloadduct mixtures 4 and 5 giving 4- and 5-substituted-1,2,3-triazoles 6 and 7.     

Diastereoselective synthesis of bis(3,5)pyrazolophanes by sequential inter- and intramolecular cycloadditions of homochiral nitrilimines

Starting from ethyl (S)-lactate as the chiral unit, we have developed the synthesis of the enantiopure bis(3,5)pyrazolophanes 9 and 19 by means of sequential inter- and intramolecular cycloadditions of nitrilimine intermediates.     

Stereoselective intramolecular cycloadditions of homochiral nitrile imines: synthesis of enantiomerically pure 3,3a-dihydro-pyrazolo[1,5-a][1,4]benzodiazepine-6(4H)-ones

Starting from the commercially available (S)-1-phenylethylamine, we have synthesised the homochiral hydrazonoyl chlorides 4. The intramolecular cycloaddition of the corresponding nitrile imines 5 gave the diastereoisomeric 3,3a-dihydro-pyrazolo[1,5-a][1,4]benzodiazepine-6(4H)-ones 6 and 7 in enantiopure form.     

Regioselective synthesis of the bridged tricyclic core of Garcinia natural products via intramolecular aryl acrylate cycloadditions

[reaction: see text] Two different routes to the tricyclic core of Garcinia-derived natural products are described. The first approach is based on a tandem Claisen/Diels-Alder rearrangement and delivers the desired lactone 14. The second approach, employing a Wessely oxidation/Diels-Alder protocol, leads to the same caged heterocycle, albeit with modified constitution.     

Reactions of carbethoxyalkylidenetriphenylphosphoranes with aryl azides

The title reaction has been shown to give a complex mixture of products from which triazoles (12 and 17), maleates (13), fumarates (14), triphenylphosphine oxide (15), iminophosphoranes (16) and triazenes (18) have been isolated. Their formation is rationalized by two reaction paths, involving addition of the azide onto the CC and CP bonds of the ylide. Diazoesters 5 and 19, which should result from CP addition, were not isolated, but are believed to give rise to compounds 13,14 and 18.     

Iron-catalyzed bromination of aryl azides by N-bromosuccinimide:Efficient method for the synthesis of brominated aryl azides

An efficient and mild protocol for bromination of aryl azides with N-bromosuccinimide（NBS） under FeCl₃ catalysis in 1,2- dichloroethane was developed.It is proved to be an efficient method for obtaining brominated aryl azides.     

Cobalt-catalyzed versus uncatalyzed intramolecular Diels-Alder cycloadditions

The intramolecular [4+2] cycloadditions of dienynes was investigated using cobalt-based catalysts. Substrates without substitution on alkyne moiety were found to react under thermal activation. The use of a cobalt salt as catalyst made reactions cleaner by limiting the formation of byproducts. Cycloadditions with dienynes possessing a substituent on the alkyne pattern occurred only in presence of a cobalt catalyst which displayed a moderate to good activity depending on the substrate patterns.     

Synthesis of aryl azides and vinyl azides via proline-promoted CuI-catalyzed coupling reactions

The coupling reaction of aryl halides or vinyl iodide with sodium azide under catalysis of CuI/L-proline works at relatively low temperature to provide aryl azides or vinyl azides in good to excellent yields.     

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.     

Ruthenium-catalyzed cycloaddition of aryl azides and alkynes

The formation of 1,5-disubstituted 1,2,3-triazoles from aryl azides and alkynes was readily accomplished using [Cp*RuCl]4 catalyst in dimethylformamide. It was also demonstrated that the reaction provided higher yields, cleaner product, and shorter reaction times when carried out under microwave irradiation.     

Modification of DNA by photoactivable aryl azides

Features of the interaction with DNA of photoactivable aryl azides having various substituents in the aromatic ring have been studied. The nature of the interaction with DNA and the degree of its modification depend substantially on the nature of these substituents. The results obtained can be used for obtaining DNA probes bearing various marker groups.Institute of Medical Genetics, Academy of Medical Sciences of USSR, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 259–263, March–April, 1989.