Comptes Rendus de l'Académie des Sciences - Series IIC - Chemistry最新文献

Free radical approach to fused vinylcyclopropane derivatives is described. Homolytic decomposition of Barton esters 4a–f in the presence of activated alkynes affords bicyclo〚3.1.0〛hex-2-ene derivatives 5a–g in moderate yields (18–52 %). The propagation sequence comprises: intermolecular addition/5-exo-cyclization/3-exo-cyclization/β-elimination, and allows for a one-pot synthesis of bicyclic cyclopropane derivatives starting from two acyclic, unsaturated compounds.

A novel strategy is described in which an intermolecular carbon-centered radical addition to a β-oxygenated acceptor followed by intramolecular cyclization leads to the formation of 2,3,4- and 2,3,4,5-substituted furan rings. Excellent levels of 2,4-trans selectivity are observed in accordance with the Beckwith–Houk model for selectivity in 5-exo radical cyclizations.

The reversible addition of aliphatic radicals to sulphur dioxide can be exploited to design a variety of tin-free chain reactions. This brief review summarises some of the possibilities, namely radical allylation, vinylation, alkynylation, and azidation of various substrates.

A new approach to ring A aromatic steroids, based on cascade 13-endo-trig/dig macrocyclisations followed by sequential 5-exo-trig and 6-exo-trig transannulations, exemplified in the syntheses of the cis,anti,trans tetracycle 9 and the trans,syn tetracycle 13 from the ortho-substituted aryl polyen(yne) precursors, 8 and 12 respectively, is described.

α-Chiral aminyl radicals have been generated from sulfenamides of α-amino acid esters and α-phenylethylamine using Bu₃SnH. The aminyl radicals of α-amino acid esters undergo 5-exo-trig cyclisation reactions onto side chain alkenes to yield proline analogues with reasonable diastereoselectivity. Preliminary studies show urethanyl radicals generated from sulfenamides of alkenyl urethane derivatives of α-amino acid esters and α-phenylethylamine undergo 5-exo-rig cyclisations to providing a protocol for the radical amination of unactivated alkenes. The α-ester of the amino acid or the urethane groups impart electrophilic behaviour to the aminyl radicals and facilitates cyclisation onto alkenes.

Alkyl radicals are conveniently generated from ketones by reduction of the derived tosylhydrazones. The alkyl radicals undergo efficient intramolecular cyclization to give the thermodynamically less stable cis-1,2-dialkylcyclopentane derivatives.

We have previously highlighted the ability of the N,N-bistrimethylsilylaminostannylene 1 to produce monoallylstannanes 2 in quantitative yields, starting from various functionalised allylic halides via an insertion reaction. In contrast with more electrophilic trihalogenotin radicals, the tin-centred radicals generated from reagents 2 are shown to be the best candidates towards allylic radical transfer, giving allylated products in similar yields as those obtained with classical tributyltin radicals.

A radical annulation procedure based on a highly stereoselective conjugated addition of a cyclic 1-hydroxyacid derivative to methyl acrylate followed by a fully group selective cyclization is reported. The 1,3-stereoinduction during the cyclization step is the only component of this process that is not fully controlled. The reaction furnished only two out of the 16 possible diastereomers. This reaction opens a new approach for the preparation of polysubstituted 1-hydroxycyclopentane carboxylic acid derivatives.

The rate constant of hydrogen transfer by dibutyl 4,7,10-trioxaundecylstannane to primary alkyl radicals was determined at different temperatures by the study of the free radical reduction of 6-bromo-1-hexene by this stannane. This work shows that this compound is an excellent substitute to tri-n-butyltinhydride

Addition of one equivalent of dialkylphosphite to γ-diketones yielded new 5-phosphorylated 1-pyrrolines 3. Depending on the experimental conditions, the addition of a second equivalent of dialkylphosphite to 3, led either to (±)-2,5-bis-phosphorylated pyrrolidines 4 or a mixture of (±)- and meso-4. Pyrrolidines 4 were isolated and subsequently oxidized to the corresponding stable (±)- or meso-nitroxides, 5. Coupling of (±)-5a with the prochiral 2-phenyl-ethyl radical was investigated.