Synthesis and highly selective Diels-Alder cycloadditions of the new dienes N-substituted 2,3,5,6-tetrahydrobenzoxazol-2-ones

The synthesis of the new dienes N-substituted 2,3,4,5-tetrahydrobenzoxazol-2-ones 8a-8c is described, through a one-step convergent process from 1,2-cyclohexanedione (7a) and the corresponding isocyanates 2a-2c. The presence of electron-donor substituents in the aryl ring of the isocyanate gave rise to the exclusive formation of the captodative olefins 10. Diene 8a proved to be reactive and stereoselective in Diels-Alder additions with a cyclic olefin. The reaction with acetylenic dienophiles yielded the 2,3-dihydrobenzoxazol-2-ones 21 and 24, as the products of sequential [4+2] cycloaddition and retro-Diels-Alder reactions. Methyl vinyl ketone (22) underwent regio- and stereoselective tandem Diels-Alder and Michael additions to give propellane 29a. Evidence of an endo π-pyramidalization of the central double bond of adduct 19 would rationalize the exo stereoselection in the formation of 29a. The regioselectivity in these reactions has been rationalized in terms of FMO theory by ab initio calculations.     

Synthesis of Fluorinated Dienes by Palladium‐Catalyzed Coupling Reactions

The synthesis of partly fluorinated 1,3‐ and 1,4‐dienes by palladium‐catalyzed coupling makes these compounds available on the laboratory scale. Several catalyst systems were tested to maximize the yields and minimize the by‐products. The molecular structures of 1,1,2,4,4‐pentafluorobutadiene, chloro(N,N′‐tetramethylethylenediamine)(trifluorovinyl)zinc, PCy₂R, and P(O)Cy₂R (Cy=cyclohexyl, R=2‐(1‐naphthyl)phenyl) were elucidated by X‐ray crystallography.     

Iron‐Catalyzed Reactions of 2‐Pyridone Derivatives: 1,6‐Addition and Formal Ring Opening/Cross Coupling

In the presence of simple iron salts, 2‐pyridone derivatives react with Grignard reagents under mild conditions to give the corresponding 1,6‐addition products; if the reaction medium is supplemented with an aprotic dipolar cosolvent after the actual addition step, the intermediates primarily formed succumb to ring opening, giving rise to non‐thermodynamic Z,E‐configured dienoic acid amide derivatives which are difficult to make otherwise. Control experiments as well as the isolation and crystallographic characterization of a (tricarbonyl)iron pyridone complex suggest that the active iron catalyst generated in situ exhibits high affinity to the polarized diene system embedded into the heterocyclic ring system of the substrates, which likely serves as the actual recognition element.     

Regioselective Synthesis of 6‐[Chloro(difluoro)methyl]salicylates by [3+3] Cyclocondensations of 1,3‐Bis(trimethylsilyloxy)buta‐1,3‐dienes with 1‐Chloro‐1,1‐difluoro‐4‐(trimethylsilyloxy)pent‐3‐en‐2‐one

The TiCl₄‐mediated [3+3] cyclocondensation of various 1,3‐bis(trimethylsilyloxy)buta‐1,3‐dienes with 1‐chloro‐1,1‐difluoro‐4‐(trimethylsilyloxy)pent‐3‐en‐2‐one provides a regioselective access to novel 6‐(chlorodifluoromethyl)salicylates (=6‐(chlorodifluoromethyl)‐2‐hydroxybenzoates) with very good regioselectivity. For selected products, it was demonstrated that the CF₂Cl group can be transformed to CF₂H and CF₂(Allyl) by free‐radical reactions.     

Cobalt-catalyzed homo-coupling of aryl and alkenyl bromide using atmospheric oxygen as oxidant

An efficient procedure for the synthesis of symmetric biphenyl and olefinic compounds was reported by cobalt-catalyzed direct homo-coupling reaction of aryl and alkenyl bromide in the presence of metallic magnesium using atmospheric oxygen as the oxidant. All tested aromatic bromides could give corresponding biaryls in good yields (up to 85%). Moreover, under the same conditions, β-bromostyrene could also afford the corresponding conjugated dienes in moderate yields, and the coupling is highly stereoselective to give trans-products. This mild and practicable method opened a new way to the preparation of symmetric biaryls and conjugated dienes.