Gagik G. Melikyan

Overcoming a Longstanding Challenge: X-Ray Structure of a [Co2(CO)6]-Complexed Propargyl Cation

After two decades of thwarted efforts, the first X-ray structure analysis of a [Co2(CO)6]-complexed propagyl cation, the BF4 salt of 1, has been reported. The carbocationic center is almost ideally sp2-hydbridized, while the metal clusters exhibit substantial structural inequivalency.

Cobalt-templated radical processes: inter- and intramolecular coupling of propargyl radicals

Abstract A novel method for radical CC bond formation in the α-position to Co 2 (CO) 6 -clusters has been elaborated. The two-dimensional exploration of the process resulted in the discovery of structurally diverse O- and S-containing organic molecules capable of acting as efficient mediators in reductive coupling reactions, and in the elaboration of the preparative synthesis of 3,4-disubstituted 1,5-alkadiynes and carbocycles with eight- and nine-membered rings.

Tetrahydrofuran-mediated radical processes: stereoselective synthesis of d,l-hexestrol

Abstract The highly stereoselective synthesis of d , l -hexestrol ( 1 ), an inhibitor of microtubule assembly, is developed by using, as a key step, an intermolecular coupling of Co 2 (CO) 6 -complexed propargyl radicals. The latter are generated by novel complementary processes involving an interaction of tetrahydrofuran with Co 2 (CO) 6 -complexed propargyl alcohols and cations. An isomerically pure d , l -μ-η 2 -[3,4-di(4-methoxyphenyl)-1,5-hexadiyne]-bis-dicobalthexacarbonyl ( d , l -6 ) is isolated in 69–91% yield with intermolecular coupling reactions exhibiting an excellent chemo- (0.5–7%) and d , l -diastereoselectivity (90–94%). The structure of d , l -6 is determined by X-ray diffraction. The subsequent steps include BBr 3 -induced demethylation of 4-methoxyaryl groups, demetalation with cerium(IV) ammonium nitrate, and hydrogenation of acetylenic termini affording d , l -hexestrol ( 1 ).

Tetrahydrofuran as an one-electron donor: Highly diastereoselective coupling of cobalt-complexed propargyl alcohol

Abstract A novel one-step reductive dimerization of Co-complexed propargyl alcohol 5 has been developed producing dl- and meso-(3,4-diphenyl-1,5-hexadiyne)-bis-dicobalt hexacarbonyl ( 8,9 ) with uprecedented diastereoselectivity of 90%. A tandem action of a variety of one-electron/hydride ion donors and Bronsted Lewis acids has been tested both in one- and two-step methods with the latter including the isolation of propargyl cation 1 .

Manganese(II)/(III) glycolates: preparation, X-ray crystallographic study, and application in radical cycloaddition reactions

Abstract Alternative methods for stereoselective synthesis of the chiral racemic manganese(II) glycolate have been developed. Its geometry is proven to be independent of the nature of the starting material (Mn(Gly)3 and Mn(OAc)3) and reaction conditions (temperature, solvent). The single-crystal X-ray analysis shows that a chiral environment around the manganese atom is created by glycolate units lying in the perpendicular planes and water molecules disposed cis to each other. By its thermodynamic stability, the isolated complex ranks as second most unstable among five alternative isomeric structures. Manganese(III) glycolate has been suggested as a novel mediator for radical cycloadditions of β-dicarbonyl compounds to alkenes. It is the first time that an α-hydroxycarboxylic acid is used as a spectator ligand thus providing a possibility for using this class of compounds in asymmetric manganese(III)-mediated radical processes.

Synthese und Struktur eines [Co2(CO)6]‐ komplexierten Propargylkations

Nach vielen vergeblichen Bemuhungen uber zwei Jahrzehnte gelang nun die erste Kristallstrukturanalyse eines [Co2(CO)6]-komplexierten Propargylkations, des BF4-Salzes von 1. Das kationische Zentrum ist zwar nahezu ideal sp2-hybridisiert, die Metallcluster weisen aber dennoch erheblich unterschiedliche Strukturen auf.

μ-[3-4-η-(1-Alken-3-yne)hexacarbonyldicobalt complexes: Radical cyclocondensation mediated by manganese(III) acetate

Abstract A regioselective pathway for the radical reaction of 1-alken-3-ynes with β-dicarbonyl compounds, mediated by Mn(OAc)3, can be achieved by protection of the substrate triple bond with a hexacarbonyldicobalt moiety. Dihydrofuran and hexahydrobenzofuran derivatives are formed by intermolecular oxidative cyclization of intermediate cobalt-complexed propargyl radicals.

Synthesis of 5-substituted furans on the basis of the reactions of 1,3-alkadiynes with Β-dicarbonyl compounds

The reaction of 1,3-hexadiyne, 1,3-heptadiyne, and 1,3-octadiyne with acetylacetone and acetoacetic ester in the presence of manganese(III) and copper(II) acetates proceeds with the participation of both triple bonds and leads to substituted 5-alkynyl- and 5-(5-furyl)furans: in the case of 5-hydroxy-5-methyl-1,3-hexadiyne the terminal triple bond participates selectively in the process as a result of the effective protection of the disubstituted triple bond by the dimethylhydroxymethyl group.

Stereoselective Synthesis of meso-1,5-Cyclodecadiynes

Reduction of Co(2)(CO)(6)-stabilized bis-propargyl cations with cobaltocene occurs with high meso-diastereoselectivity (up to 97%), affording, upon decomplexation, meso-1,5-cyclodecadiynes, otherwise hardly accessible.

Low-molecular-mass bioregulators. III. Synthesis of tridec-4E-enyl acetate — The sex pheromone ofKeiferia lycopersicella

A new method has been developed for the synthesis of tridec-4E-enyl acetate — the sex pheromone of the tomato pinworm Keiferia lycopersicella.