Synthesis of Azaspiro Tricyclic Scaffolds through Post-Ugi Modifications: Scope and Limitation of Aza-Michael Cyclization

Post-Ugi modifications offer a great tool to construct functionalized heterocycles in an atom-economic manner. Using this approach, we have constructed highly functionalized azaspiro tricyclic scaffolds that involve Ugi condensation and ipso-cyclization followed by aza-Michael ring closures. Herein, we present a detailed account of aza-Michael cyclizations with respect to substrate scope and limitations, along with new methods to prepare novel azaspiro tricyclic scaffolds.     

Iron‐Induced Regio‐ and Stereoselective Addition of Sulfenyl Chlorides to Alkynes by a Radical Pathway

The radical addition of the Cl? S σ‐bond in sulfenyl chlorides to various C? C triple bonds has been achieved with excellent regio‐ and stereoselectivity in the presence of a catalytic amount of a common iron salt. The reaction is compatible with a variety of functional groups and can be scaled up to the gram‐scale with no loss in yield. As well as terminal alkynes, internal alkynes underwent stereodefined chlorothiolation to provide tetrasubstituted alkynes. Preliminary mechanistic investigations revealed a plausible radical process involving a sulfur‐centered radical intermediate via iron‐mediated homolysis of the Cl? S bond. The resulting chlorothiolation adducts can be readily transformed to the structurally complex alkenyl sulfides by cross‐coupling reactions. The present reaction can also be applied to the complementary synthesis of the potentially useful bis‐sulfoxide ligands for transition‐metal‐catalyzed reactions.     

Rhodium-Catalyzed Cyclization of Terminal and Internal Allenols: An Atom Economic and Highly Stereoselective Access Towards Tetrahydropyrans

A comprehensive study of a diastereoselective Rh-catalyzed cyclization of terminal and internal allenols is reported. The methodology allows the atom economic and highly syn-selective access to synthetically important 2,4-disubstituted and 2,4,6-trisubstituted tetrahydropyrans (THP). Furthermore, its utility and versatility are demonstrated by a great functional-group compatibility and the enantioselective total synthesis of (−)-centrolobine.     

Cyclization of cembrane diterpenoids. IV. Stereoselective photochemical cyclization of a norcembrane ketone

The possibility has been shown for the first time of the occurrence of an intramolecular [2+2]-cycloaddition for a derivative of a cembrane diterpenoid. The structure of the product, which was a higher isoprenolog of the bourbonane sesquiterpenoids, was established by the x-ray structural analysis of its epoxide.Novosibirsk Institute of Organic Chemistry, Siberian Branch, Academy of Sciences of the USSR. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 295–299, May–June, 1986.     

Highly Stereoselective and Efficient Hydrosilylation of Terminal Alkynes Catalyzed by

With [RuCl(2)(p-cymene)](2) as a catalyst, extremely high regio- and stereoselectivity was observed in the hydrosilylation reaction of various terminal alkynes under mild conditions to afford beta-(Z)-vinylsilanes in excellent yields. A dramatic directing effect was also observed when alkynes having a hydroxyl group at the beta position to the triple bond were employed as a substrate, and in these cases regioisomeric alpha-vinylsilanes were generated with excellent selectivity.     

Highly Stereoselective Cyclopropanation of Olefins Catalyzed by a Novel C 3-Symmetric Arsine

A novel C ₃-symmetric arsine was employed in the one-pot cyclopropanation of olefins with carbonyl-stabilized arsonium ylides formed in situ from phenacyl bromide in the presence of NaHCO₃. This new arsine demonstrates good stereoselectivity and activity in the one-pot cyclopropanation of arylidenemalononitrile.

[Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource(s): Full experimental and spectral details.]     

Microwave Assisted,Highly Efficient Solid state N- and S-Alkylation

Secondary amines and thiophenols were alkylated with alkyl and benzyl halides rapidly on alumina supported potassium carbonate under solvent-free conditions using microwaves. Equimolar amounts of the secondary amine/thiophenol and alkyl halide were used. The procedure neither required any strong base nor a PTC. Aniline was also monobenzylated with benzylchloride. Separation of the products from the reactants was very simple by using a nonpolar solvent for desorption.     

Scope and applications of second generation palladium-catalyzed cycloalkenylation. Stereoselective total syntheses of isoiridomyrmecin, isodihydronepetalactone, and α-skytanthine

Functionalized bicyclo[3.2.1]octanes, -oxabicyclo-[4.3.0]nonanes, 3-azabicyclo[3.3.0]octanes, and 3-azabicyclo[4.3.0]nonanes were easily synthesized via a second generation palladium-catalyzed cycloalkenylation. Isoiridomyrmecin and isodihydronepetalactone, both of which feature a 3-oxabicyclo[4.3.0]nonane subunit, were stereoselectively synthesized via a second generation palladium-catalyzed cycloalkenylation as the key step. α-Skytanthine, a typical 3-azabicyclo[4.3.0]nonane alkaloid, was also constructed using the same catalytic cyclization protocol.     

Stereoselective synthesis of imidazolidin-2-ones via Pd-catalyzed alkene carboamination. Scope and limitations

A method for the synthesis of imidazolidin-2-ones from N-allylureas and aryl or alkenyl bromides via Pd-catalyzed carboamination reactions is described. The N-allylurea precursors are prepared in one step from readily available allylic amines and isocyanates, and the Pd-catalyzed reactions effect the formation of a C-C bond, a C-N bond, and up to two stereocenters in a single step. Good diastereoselectivities are obtained for the conversion of substrates bearing allylic substituents to 4,5-disubstituted imidazolidin-2-ones, and excellent selectivity for the generation of products resulting from syn-addition across the alkene is observed when substrates derived from cyclic alkenes or E-1,2-disubstituted alkenes are employed. A brief discussion of reaction mechanism and product stereochemistry is presented.     

Thiol-Ene Cationic and Radical Reactions: Cyclization,Step-Growth,and Concurrent Polymerizations for Thioacetal and Thioether Units

Thiol-ene cationic and radical reactions were conducted for 1:1 addition between a thiol and vinyl ether, and also for cyclization and step-growth polymerization between a dithiol and divinyl ether. p-Toluenesulfonic acid (PTSA) induced a cationic thiol-ene reaction to generate a thioacetal in high yield, whereas 2,2′-azobisisobutyronitrile resulted in a radical thiol-ene reaction to give a thioether, also in high yield. The cationic and radical addition reactions between a dithiol and divinyl ether with oxyethylene units yielded amorphous poly(thioacetal)s and crystalline poly(thioether)s, respectively. Under high-dilution conditions, the cationic and radical reactions resulted in 16- and 18-membered cyclic thioacetal and thioether products, respectively. Furthermore, concurrent cationic and radical step-growth polymerizations were realized using PTSA under UV irradiation to produce polymers having both thioacetal and thioether linkages in the main chain.