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Hybridization of a chiral 3-hydroxy-2-trityl-pyrrolidine deriving from (R)-pyrrolidinol with [60] fullerene via click chemistry provides a highly efficient supported enantioselective organocatalyst, which was successfully exploited in a Michael addition of malonates to cinnamaldehydes, via iminium ion activation. The supported organocatalyst was recycled up to six times, with only a moderate decrease in terms of activity and with no loss in enantioselectivity. [ABSTRACT FROM AUTHOR]

Great interest in new thermochemiluminescent (TCL) molecules, for example, in bioanalytical assays, has prompted the design and synthesis of a small library of more than 30 olefins to be subjected to photooxygenation, with the aim of obtaining new 1,2-dioxetane-based TCL labels with optimized properties. Fluorine atoms on the acridan system remarkably stabilize 1,2-dioxetanes when they are located in the 3- and/or 6-position ( 4 h and 4 i). On the other hand, 2,7-difluorinated acridan dioxetane ( 4 j) showed a significantly enhanced fluorescence quantum yield with respect to the unsubstituted dioxetane ( 4 a). Some of the synthesized olefins did not undergo singlet oxygen addition and a rationale was sought to ease the photooxygenation step, leading to the TCL dioxetanes. A chemometric approach has been adopted to exploit principal component analysis and linear discriminant analysis of the structural and electronic molecular descriptors obtained by DFT optimizations of olefins 3. This approach allows the steric and electronic parameters that govern dioxetane formation to be revealed. [ABSTRACT FROM AUTHOR]

Chiral γ-nitroaldehydes 1 are easily accessible by the organocatalytic Michael addition of aldehydes to nitroalkenes. In this paper, we report the organocatalytic fluorination of 1 with N-fluorobenzenesulfonimide (NFSI) leading to the highly stereoselective construction of a challenging quaternary fluorinated stereocenter at the α position of α,α-dialkyl aldehydes. The reaction takes place via a chiral trisubstituted enamine, so far rather unexplored in the field of organocatalysis. Fluorinated products 3 are direct precursors of chiral monofluorinated 3,4-polysubstituted pyrrolidines. Mechanistic details are discussed with the aid of computational results. [ABSTRACT FROM AUTHOR]

A new and flexible methodology catalyzed by bifunctional chiral thioureas has been developed to react β-nitro oxindoles 1 with aldehydes. This approach allowed us to achieve the first enantioselective organocatalytic synthesis of 3-spiro-α-alkylidene-γ-butyrolactone oxindoles 3. We examined the scope of the two starting materials and, varying the structure of the β-nitro oxindole 1, intriguing new products, derived from unexpected transformations, have been stereoselectively obtained. The aim of this study was to merge two potentially bioactive structural motifs: the spirooxindole substructure and the α-alkylidene-γ-butyrolactone moiety. A preliminary NMR study on the ability to reversibly trap 2-aminoethanethiol gave us promising results. [ABSTRACT FROM AUTHOR]

A new Michael-Michael cascade reaction between 2-(2-oxoindolin-3-ylidene)acetic esters 1 and nitroenoates 2, catalyzed by bifunctional thioureas, is investigated. The combination of the two Michael reactions results in a novel and facile [4+2] or [3+2] spiroannulation process, which is characterized by the following features: 1) two carbon-carbon bonds and four stereocenters, including a quaternary spiro carbon, are formed under mild conditions; 2) an unprecedented and stereochemically defined substitution pattern on the spirocarbocyclic unit is obtained; 3) the double-bond configuration of the donor-acceptor nitroenoate 2 determines the absolute configuration of the spiro center, whereas the remaining stereocenters are formed under control of the catalyst. The effect on the final stereochemical outcome of structural variations of each starting material, catalyst, and experimental conditions is analyzed in detail. In particular, the use of specifically designed chiral nitroenoates enables diverse polyfunctional spirocyclohexane derivatives containing six consecutive stereogenic centers to be constructed. To our knowledge, this is the first asymmetric organocatalytic strategy enabling both five- and six-membered β-nitro spirocarbocyclic oxindoles. [ABSTRACT FROM AUTHOR]

Bicyclic silyl ether derivatives 1, which are derived from simple chemical manipulations of trans-4- L-hydroxyproline, were recently proposed, by us, as conformationally constrained analogues of Jørgensen-Hayashi's catalysts 2. Despite the structural similarities, 1 displays remarkable performance in iminium chemistry with respect to 2, but much lower reactivity in enamine chemistry. The peculiar structural features of 1 confer a pattern of reactivity between Jørgensen-Hayashi's catalysts and MacMillan's imidazolidinones. [ABSTRACT FROM AUTHOR]

We recently proposed 3-methoxy-4-methoxycarbonyl-1,2-dioxanes 3 as promising scaffolds enabling access to potential antimalarial drugs. We present here an optimized two-step synthesis of 3 characterized by high yields, simple work-up procedures and high diastereoselectivity allowing us to readily prepare 3 on multigram scale. The versatility of the 1,2-dioxane scaffold was demonstrated by our generation of a new family of 1,2-dioxane-4-carboxamides 8a- h and the realization of their in vitro activities against chloroquine-sensitive (D10) and chloroquine-resistant (W2) P. falciparum strains. In particular, one of these amides ( 8e), displayed antimalarial activity on par with the natural product lead plakortin and was 5-fold more active than its methyl ester analogue 3. Structure-activity relationship (SAR) analysis supported by DFT calculations revealed that, for this family of compounds, alkyl substituents at C6 dictate, in large part, the degree of antimalarial activity. This finding contrasts those previously observed for the ester series. [ABSTRACT FROM AUTHOR]

We have recently proposed the empirical concept of electrosteric activation to explain the improved catalytic performances observed for a series of ion-tagged catalysts compared to the parent tag-free structures. Here, the results of a combined experimental and computational investigation on the asymmetric aldol reaction between cyclohexanone and benzaldehyde, catalyzed by a family of tag-free and ionic-tagged prolines, are presented. Whereas diastereo- and enantioselectivities remain very high in all cases examined, the ion-tagged catalyst cis-4-(2-(3-methyl-imidazol-3-ium-1-yl)acetoxy)-proline bistriflimide, cis- 7, displays a remarkably high activity compared to its tagged trans analogue and to the tag-free catalysts cis and trans-4-(2-phenylacetoxy)-proline 8. A computational investigation of ion-tagged and tag-free model systems shows that the transition state involving cis- 7 is stabilized by a complex interplay of hydrogen bonds (in particular, those involving the counter ion oxygen atoms and the hydrogen atoms of the ionic tag), π-stacking interactions involving the aldehyde phenyl ring, and similar π interactions between the proline carboxyl group and the imidazole ring. The overall effect of these interactions accounts for the observed enhanced activity. [ABSTRACT FROM AUTHOR]

A highly stereoselective route to densely functionalized cyclohexanes is described, via a Cinchona-based bifunctional organocatalyzed Henry/Michael/retro-Henry/Henry domino reaction. The peculiar stereoconvergent mechanism allows the creation of up to four contiguous stereocenters in a highly stereodefined manner starting from both achiral aldehydes and racemic nitro alcohols. The simple and efficient protocol can be exploited to synthesize a wide range of substrates and also useful bioactive molecules. [ABSTRACT FROM AUTHOR]

Simple chemical manipulations of trans-4- L-hydroxy proline allow the access to a new family of bicyclic silyl ether organocatalysts that display some remarkable features. Apart from being extremely stable to hydrolytic conditions and possessing excellent catalytic performances, the rigidity of the bicyclic structure imposes a synclinal endo disposition of the bulky substituents with respect to the pyrrolidine ring, opposed to the more stable synclinal exo conformations of Jørgensen-Hayashi catalysts. [ABSTRACT FROM AUTHOR]

An innovative two stage liquid-liquid biphasic homogeneous protocol for the asymmetric organocatalytic aldol reaction is proposed, based on the use of the cis-ion-tagged proline 8 dissolved in the liquid film of a multilayered ionic liquid covalently bonded to silica gel 4. The resulting catalytically active material 9 is first soaked with cyclohexanone in the presence of water, resulting in a semi-transparent gel, then the aldehyde is added and the mixture stirred at RT. In the first stage, 4 acts as a catalyst reservoir that delivers 8 to the cyclohexanone phase allowing the reaction to take place homogeneously. In the second stage, cyclohexanone is removed under vacuum and the resulting slurry is extracted with anhydrous diethylether. Now 4 acts as a catalyst sponge, redissolving 8. Product extraction is extremely selective; no trace of catalyst is detected in the product-containing phase, and 9 can be easily reused several times with high cumulative productivity values (up to 523). [ABSTRACT FROM AUTHOR]

A set of 8 polycyclic aromatic hydrocarbons (PAHs) has been analysed in a traffic-limited area in Bologna downtown, both in the gas-phase and in the particulate phase (PM10), and gas-to-particle partitioning has been investigated. From Sep 2002 to May 2003, 28 high volume PM10 samplings were carried out, and in 50 % of the samplings, PMi0 concentrations exceeded the limit of 50 μg/m³ established by a 1999 EU directive. A precisely defined sampling strategy was adopted to limit artifacts (8 h sampling in the same time interval) in the 28 samplings carried out in different meteorological conditions. A linear log-log correlation was found between gas-particle partitioning coefficients KBp and the subcooled liquid vapour pressures pB0LB, with rP2P = 0.82 and slope = -0.59. This empirical correlation may be used to anticipate the total (gas + particle-bound) concentration of each PAHs in this urban site, once PM10 and the particle-bound concentration is measured. Parallel samplings of PM10 and of PM2.5 allowed us to ascertain that PM2.5 represents the gross contribution to PM10 and that most of the particle-bound PAHs reside on the finest fraction of particulate matter. [ABSTRACT FROM AUTHOR]