Pierre Brun

Asymmetric Mn(III)-based radical synthesis of functionalized 2,3-dihydrofurans

Abstract The oxidative radical addition of alkyl acetoacetates to p -methoxycinnamoyl oxazolidinones promoted by Mn(III) has been studied. It only gives trans -disubstituted 2,3-dihydrofurans, with d.r. ranging from 2:1 to 9:1, depending on the substituent of the chiral auxiliary. After chromatographic separation of the two diastereomers, the oxazolidinone can be removed to afford enantiopure dihydrofuranyl esters in good overall yield.

Photokinetic behaviour of biphotochromic supramolecular systems: Part 2. A bis-benzo-[2H]-chromene and a spirooxazine–chromene with a (Z-)ethenic bridge between each moiety

Abstract The photochromic behaviour of two new bichromophoric molecules (a symmetric one, formed by two 3,3-diphenyl-[3H]-naphtho[2,1-b]pyran moieties, and an asymmetric one, formed by a naphthopyran and a spiro[indoline-naphthoxazine] moiety) linked through an ethenic double bond has been studied. A model compound, where the chromene moiety is linked to a naphthalene group, was also investigated for comparison purpose. The photochemical and thermal evolution of the bichromophoric molecules were followed spectrophotometrically and by high performance liquid chromatography (HPLC). The photochromic reaction was found to be only partially photoreversible. Massive irradiation with visible light led to degradation. The reaction mechanism for these molecules containing a chromene moiety is characterised by an intermediate, which thermally evolves to different products. The quantum yield of formation and the first-order kinetic constant of the thermal evolution of the intermediate were determined. The identification of the photoproducts was helped by information from a parallel NMR study. Comparison of these results with those previously obtained on related monochromophoric and bichromophoric compounds allowed reasonable photoreaction mechanisms to be proposed.

Nickel-catalyzed oligomerization of functionalized conjugated dienes

Abstract The cyclodimerization of several functionalized dienes with nickel(0) catalysts has been studied. When a functional group, such as a methoxycarbonyl or a trimethylsiloxy group, is bonded directly to the terminal carbon of the diene grouping, a completely regio- and stereo-selective dimerization is observed and trans -1,2-disubstituted cycloocta-3,7-dienes are formed in good yields (80–90%). When one carbon atom separates the functional group from the diene group, the yield drops significantly (to 30%), and the stereoselectivity and the clean nature of the reaction are completely lost. The effect of the nature of the nickel catalyst and the influence of the reductive organoaluminum species are discussed. The particular structure and the stereochemical features of the cyclooctadiene derivatives allow a better understanding of the various steps involved in the catalytic process. A reaction mechanism is proposed.

Photokinetic behaviour of biphotochromic supramolecular systems: Part 1. A bis-spirooxazine with a (Z)ethenic bridge between each moiety

Abstract The photochromic behaviour of a bichromophoric molecule formed by two naphthospirooxazine moieties linked through an ethenic double bond has been studied. The photochemical and thermal evolution of this molecule and its model compound, a naphthospirooxazine linked by a double bond to a 2-methoxy-naphthalene group, was followed spectrophotometrically and by HPL chromatography. Photocoloration occurred with fairly high quantum yield (Φ∼0.2) for both molecules. Identification of the photoproducts was helped by information from a parallel NMR study and by comparison with the results previously obtained on related monochromophoric compounds. The photoreaction was thermally irreversible at room temperature while a slow photobleaching (Φ∼10−3) accompanied by degradation was observed. A photoreaction mechanism is proposed implying the photocyclisation at the ethenic double bond level.

Regio and stereo selective synthesis of disubstituted cyclooctadienes.

Abstract Functionalized dienes can be dimerized in the pesence of Ni(o) catalyst in order to afford regio and stereo selectively 1,2-trans disubstituted 3,7-cyclooctadienes derivatives.

Oxygen transfer mechanism in the Mn-salen catalysed epoxidation of olefins

Abstract The Mn-salen catalysed epoxidation of geraniol and nerol derivatives or similar non-conjugated olefins, with H 2 O 2 as oxidant, was studied. Along with the epoxide, rearranged products were isolated and characterised. The formation of these compounds is a strong argument in favour of the formation of an oxametallacycle as an intermediate in these epoxidation reactions.

Asymmetric synthesis of methyl N-(1-phenylethyl)-3-aza-10-oxatricyclo[5.2.1.01,5]-4-oxo dec-8-en-6-carboxylate by an intramolecular Diels-Alder reaction

Abstract The title compound can be obtained with up to five asymmetric centres of known absolute configuration by a diastereoselective intramolecular Diels-Alder reaction between optically active N -substituted furfurylamines and maleic anhydride, in which the chirality is transferred from one stereocentre to the four others.

Efficient MnIII-mediated synthesis of functionalized trans-3,4-disubstituted-γ-butyrolactones

The MnIII-induced addition of malonic acid (instead of acetic acid) to cinnamic esters affords in one step functionalized trans-3,4-disubstituted-γ-butyrolactones in a greatly improved yield which can be further optimized by conducting the reaction in either acetic or formic acid, depending on the substrate.

Synthesis of bicyclic γ-lactones promoted by Mn(OAc)3: Regio- and diastereoselectivities

The regio- and diastereoselectivities of Mn(OAc)3-induced addition of monomethyl malonate on some cycloalkenes and cycloalkadienes are studied. The latter is greatly improved when starting from cycloalkadienes.

Spectrokinetic study of a series of photochromic 2-ferrocenyl-2-methyl[2H]-chromenes

The study of the photochromic behaviour of a series of 2-ferrocenyl-2-methyl[2H]-chromenes has shown original spectrokinetic properties: the broadening of the absorption spectrum of the photomerocyanines with the appearance of a second band in the visible area and also important kinetic modifications in polar protic solvents giving to this series of chromenes a special interest. From spectrokinetic results, a structure of photomerocyanines stabilised by polar and protic solvent such as alcohols could be proposed.