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

The scope of an alkoxyl radical version of the classical bromine cyclization was explored. Oxygen-centered radicals were generated in photochemically initiated radical chain reactions from N-alkenoxy-4-(p-chlorophenyl)thiazole-2(3H)-thiones 5a–c, N-alkenoxy-4-methylthiazole-2(3H)-thiones 13, 14, 21, rac-23, and N-alkenoxypyridine-2(1H)-thiones 6d–f, 16, rac-25. Thus, 2-(2-bromopropyl)-substituted tetrahydrofurans 4a–c, which are minor compounds (< 10 %) in NBS-mediated bromine cyclizations of corresponding alkenols 1a–c, were prepared in 87–90 % yield and with good to excellent diastereoselectivities. Further, photochemical conversions of O-alkyl thiohydroxamates 14, 16 and 21 in benzene and BrCCl₃ afforded β-oxy-functionalized bromomethyl substituted tetrahydrofurans 29, 34, and 36. The results of this study indicate, that efficient 5-exo-trig cyclizations of β-oxy-substituted 4-penten-1-oxyl radical require an electronwithdrawing substituent at the β-heteroatom substituent. In the third part of the study a synthetically useful new access to oxabicyclo〚4.3.0〛nonanes rac-38 and rac-40 is reported which takes profit from highly diastereoselective 5-exo-trig-ring closures and, in case of the formation of rac-40, stereoselective bromine atom transfer.

A large number of selective metabolic reactions involve intermediate free radicals. To do so, enzymatic systems rely upon a great variety of efficient radical sources, such as adenosylcobalamin, S-adenosylmethionine and oxygen for reaction initiation. In most cases, activation depends on a metallocofactor. The review article focuses on some of these chemical mechanisms for biological radical generation.

Upon treatment with aluminium trichloride in absolute chloroform, β-d-galactopyranose penta-O-acetate was converted in high yield into tetra-O-acetyl-β-d-galactopyranosyl chloride. In the presence of N-bromosuccinimide in boiling carbon tetrachloride, homolytic substitution of the anomeric hydrogen atom in this β-chloride occurred selectively to afford tetra-O-acetyl-1-bromo-β-d-galactopyranosyl chloride, in admixture with minor amounts of tetra-O-acetyl-d-galactopyranosylidene chloride. Bromine substitution by fluorine took place with a low stereoselectivity in tetra-O-acetyl-1-bromo-β-d-galactopyranosyl chloride when treated with 1.25 equiv silver fluoride in acetonitrile, whereas tetra-O-acetyl-d-galactopyranosylidene fluoride could be prepared in 54 % yield with 3.3 equiv AgF. The gem-difluoride could be deacetylated quantitatively. Upon treatment with 1,4-diazabicyclo〚2.2.2〛octane, peracetylated 1-bromo-β-d-galactopyranosyl chloride underwent 1,2-elimination of hydrogen bromide to afford 2,3,4,6-tetra-O-acetyl-d-lyxo-hex-1-enopyranosyl chloride in 33 % yield.

The ion pair collapse versus radical pair collapse hypothesis is proposed for the Grignard reagent formation as an extension of the e.t. mechanism of aromatic nitration. Such a scheme, never examined before for the Grignard reaction, could explain the evolution of the ‘free radical character’ depending on experimental conditions and the nature of the substrate.

The synthesis of 8-iodooxabicyclo〚3.2.1〛octanes from substituted benzoic acids via a reductive alkylation–halolactonization sequence is described. These iodolactones were converted to 2-oxabicyclo〚3.3.0〛octane derivatives upon treatment with either allyltri-n-butylstannane or tri-n-butylstannane via a process that involves a Dowd–Beckwith rearrangement. The scope and limitations of this process are discussed.

Secondary bromoenamides 5, 14-18, 32 and 33 undergo efficient 5-endo cyclisation reactions to give α,β-unsaturated monoene lactams 9, 19–23, 34 and 35 under atom transfer conditions mediated by CuBr and the tripyridylamine 6 in refluxing toluene (59–87%). Changing the solvent to refluxing 1,2-dichloroethane furnishes α,β-unsaturated diene lactams 26-31, 36, and 37 instead (42–86%).

The free radical cyclization strategy applied to precursors 1, 2 and 14 shows the power and the state of art of this ring closing method for the stereocontrolled synthesis of enantiomerically pure, highly functionalized carbocycles from carbohydrates. The tributyltin hydride mediated free radical cyclization of alkyne tethered N,N-diphenylhydrazones (1) and oxime ethers (2) give the stannylated aminocyclopentitol derivatives 4 and 11, respectively, in moderate yield and with complete stereoselectivity. The 6-exo-trig cyclization of aldehyde/oxime ether 14 derived from D-glucose has been revisited using tributyltin hydride as a reagent. This new protocol shows a superior stereoselection compared with the previous results obtained with samarium diiodide. In summary, an efficient protocol has been set up for the synthesis of enantiomerically pure, polyhydroxy aminocyclohexane derivatives, which are difficult to obtain by other methodologies.

Tri-2-furylgermane in the presence of a catalytic amount of triethylborane adds to internal alkenes as well as to terminal alkenes effectively, giving the corresponding adducts in good to excellent yields. The addition of tri-2-furylgermane to silyl enol ethers followed by elimination of germyl and siloxy moieties provides a new route for the conversion of ketones into alkenes.

γ,δ-Unsaturated ketone O-(p-cyanophenyl)oximes are transformed into 3,4-dihydro-2H-pyrroles by a photochemical reaction in the presence of 1,5-dimethoxynaphthalene. The present transformation proceeds via photosensitized electron transfer between 1,5-dimethoxynaphthalene and O-(p-cyanophenyl)oximes.

Through a combination of kinetic, voltammetric and product analysis, it has been found that a common Ti^III species obtained from the metal reduction of Cp₂TiCl₂ is involved in the electron transfer to benzaldehyde. A common species in the pinacol dimerization step was also demonstrated. In neither of the two steps is it necessary to invoke trinuclear complexes as previously proposed.