Diels-alder reactions with 2-(Arylsulfinyl)-1,4-benzoquinones: effect of aryl substitution on reactivity, chemoselectivity, and pi-facial diastereoselectivity

Diels-Alder reactions of (SS)-2-(2'-methoxynaphthylsulfinyl)-1, 4-benzoquinone (1b), 2-(p-methoxyphenylsulfinyl)-1,4-benzoquinone (1c), and 2-(p-nitrophenylsulfinyl)-1,4-benzoquinone (1d) with cyclopentadiene are reported. These cycloadditions allowed the highly chemo- and stereoselective formation of both diastereoisomeric endo-adducts resulting from reaction on the unsubstituted double bond C(5)-C(6) of quinones working under thermal and Eu(fod)(3)- or BF(3).OEt(2)-catalyzed conditions. The synthesis of endo-adduct [4aS,5S,8R,8aR,SS]-9d resulting from cycloaddition on the substituted C(2)-C(3) double bond was achieved in a chemo- and diastereoselective way from quinone 1d in the presence of ZnBr(2). The reactivity and selectivity of the process proved to be dependent on the electron density of the arylsulfinyl group.     

High stereocontrol in aldol reaction with ketones: enantioselective synthesis of beta-hydroxy gamma-ketoesters by ester enolate aldol reactions with 2-acyl-2-alkyl-1,3-dithiane 1-oxides

The asymmetric aldol reaction of 1,2-diketones, masked as nonracemic 2-acyl dithiane oxides, with lithium enolates derived from several esters and lactones, proceeds with a high degree of stereocontrol at both carbonyl and enolate prochiral centers, the stereocontrol mainly determined by the configuration of the sulfoxide sulfur atom. The sense of induced stereochemistry observed for ester enolates is different from that seen for lactone enolates. Hydrolysis of the dithiane oxide units of the major diastereoisomerically pure aldol products affords enantiomerically pure tertiary alpha-substituted beta-hydroxy-gamma-ketoesters.     

Anionic organic guests incorporated in zeolites: adsorption and reactivity of a Meisenheimer complex in faujasites

Zeolites are suitable microporous hosts for positively charged organic species, but it is believed that they cannot adsorb organic anions. Pure Meisenheimer complex, derived from reduction of 2,4-dinitroaniline with NaBH4, was adsorbed inside faujasite cavities. Evidence for the internal incorporation of this negatively charged reaction intermediate comes from 1) XPS elemental analysis as a function of the depth of penetration into the particle, 2) the remarkable blue shift in lambda(max) of the Meisenheimer complex adsorbed on zeolite (ca. 470 nm) as compared to that in acetonitrile (580 nm) and 3) from the lack of reactivity with size-excluded hydride-acceptor reagents. Evidence is provided in support of an adsorption mechanism in which a neutral ion pair (alkali metal ion + Meisenheimer anion) is the actual species being adsorbed. In fact it appears that there is remarkable increase in the association constant for the ion-pair complex within the zeolite cavities as compared to DMF solution. Although this mechanism of adsorption as an ion-pair complex has precedents in the adsorption of some inorganic salts, what is novel is the notable increase in the stability and persistence of the Meisenheimer anion (a anionic reaction intermediate) as a result of zeolite inclusion. Adsorbed Meisenheimer complex exhibits much lower reactivity towards electron acceptors, oxygen, and water. Cyclic voltammetry of zeolite-modified electrodes reveals for the Meisenheimer complex adsorbed on LiY a reversible redox peak that is not observed in solution and has been interpreted as arising from site isolation and stabilisation of the electrochemically generated species.     

Stereoselective |2+2| photocycloaddition of chiral 2(5H)-furanones to alkenes

The steric and stereoelectronic dependence of the diastereoselectivity in the photochemical |2+2| cycloaddition of chiral 2(5H)-furanones to alkenes is investigated to prepare eventually cyclobutanic natural products. It is shown that the alkenes approach to different chiral 5-substituted 2(5H)-furanones mainly by the less hindered side.     

Time domain investigation on vibrational dephasing and spectral diffusion in CO-doped solid N2

Infrared picosecond accumulated photon echo experiments have been performed for the first time, using the Orsay Free Electron Laser, on the v = 0-->v = 1 transition of CO in solid nitrogen. The vibrational dephasing time is found to be exceptionally long ( T2>/=120 ns) at low temperature. The analysis of the observed spectral diffusion leads one to assume different energy transfer mechanisms depending on the CO concentration.     

On the Z-E photoisomerization of chiral 2-pentenoate esters: stationary irradiations, laser-flash photolysis studies, and theoretical calculations

Chiral pentenoates 1-3 in both Z and E isomeric forms underwent stationary irradiations in several solvents and in the presence of different photosensitizers. The photostationary-state ratio has been determined for each Z/E couple showing a predominance of the thermodynamically more stable isomer for 1 and 3. Moreover, transient species were generated by pulsed laser excitation and detected by their characteristic ultraviolet absorptions, being the first time that enoate-originated triplets are detected. Stern-Volmer quenching studies afforded a quantitative measure for the efficiency of the photosensitization processes induced by benzophenone or acetophenone and allowed the determination of the corresponding quenching rate constants. Density functional calculations permitted the determination of the geometries and the energies of the diastereomeric excited states. Two diastereomeric orthogonal and two diastereomeric planar structures result as a consequence of the presence of a chiral substituent. The orthogonal triplets are the energy minima in all cases, whereas the planar triplets are the transition states linking these orthogonal structures, the corresponding energy barriers being 8-10 kcal mol(-1) for enoates 1-3. The computed S(0) to T(1) excitation energies show a trend which is consistent with the quenching rate constants. On the other hand, the triplet lifetimes determined for 1 and 2 are unusually long (1-20 micros) if compared with the data already described for several enones, in the range of nanoseconds. This fact has been rationalized from calculations of spin-orbit coupling at several points of the T(1) potential energy surface. This coupling is maximum for structures with a torsional angle close to 45 degrees, which are 4-5 kcal mol(-1) above the minima of T(1). Calculations done on the hypothetical aldehyde 4 and methyl vinyl ketone show much lower energy barriers, thus accounting for the shorter lifetimes reported for enone triplets.