Syntheses of 3,4- and 1,4-dihydroquinazolines from 2-aminobenzylamine.

A straightforward strategy for the synthesis of dihydroquinazolines is presented, which allows for the preparation of 3,4- and 1,4-dihydroquinazolines with different substitution patterns from 2-aminobenzylamine (2-ABA) as common precursor. The required functionalization of both amino groups present in 2-ABA was achieved by different routes involving selective N-acylation and cesium carbonate-mediated N-alkylation reactions, avoiding protection/deprotection steps. The heterocycles were efficiently synthesized in short reaction times by microwave-assisted ring closure of the corresponding aminoamides promoted by ethyl polyphosphate (PPE).

Enantioselective Preparation, Conformational Analysis and Absolute Configuration of Highly Substituted Aziridines.

The first example of organocatalytic aziridination reaction of α-substituted-α,ß-unsaturated ketones is presented. The reaction was found to be highly enantio- and diastereoselective, yielding N-tosylated aziridines. Low-temperature nuclear magnetic resonance (NMR) spectra allowed for the determination of the N-inversion barrier, that was found to be quite lower with respect to unsubstituted aziridines. A thorough conformational analysis supported by low-temperature NMR data allowed for the determination of the absolute configuration of the main stereoisomer by means of time-dependent Density Functional Theory simulation of the electronic circular dichroism spectra.

Vinylogous Reactivity of Oxindoles Bearing Nonsymmetric 3-Alkylidene Groups.

The γ-functionalization of oxindoles bearing nonsymmetric 3-alkylidene groups via vinylogous Michael-type addition to nitroolefins was realized. The suppression of the interconversion between the E and Z isomers of the starting oxindoles allowed a site-specific diastereoselective and enantioselective transformation. Specific experiments allowed us to establish the rate-determining step of the reaction and to advance a robust hypothesis for the exclusive formation of an s-cis enolate as the only reactive intermediate.

Long-range bonding/nonbonding interactions: a donor-acceptor resonance studied by dynamic NMR.

Long-range bonding interactions were evaluated using variable-temperature NMR spectroscopy and suitable 2'-CH2X-substituted phenylpyridines (X = Me, NMe2, OMe, F). It was found that the arylpyridyl rotational barriers were lower when electronegative atoms were bound to the α carbon of the 2' moiety. This effect was ascribed to a stabilizing interaction in the transition state due to the lone pair of the heterocyclic nitrogen with the α carbon. Computational support for this hypothesis came from CCSD(T)/6-31+G(d) calculations. Steric effects of the X moiety were ruled out by comparison of the rotational barriers of analogous biphenyls.

Axial chirality of 4-arylpyrazolo[3,4-b]pyridines. Conformational analysis and absolute configuration.

The stereodynamic behavior of a series of pyrazolo[3,4-b]pyridines was studied. The restricted rotations of the aryl substituent in position 4 of the heteroaromatic ring and of the benzoyl group in position 5 generated conformational enantiomers or conformational diastereoisomers depending on the local symmetry of the aryl substituent, with very high rotational barriers despite the absence of ortho-substituents. The energy barriers for the rotation of the 5-benzoyl group and the 4-aromatic ring were measured by dynamic NMR and rationalized by DFT calculations. When the aryl substituent at position 4 was 1-naphthyl, the resulting atropisomeric pair was resolved by means of enantioselective HPLC and the absolute configuration was determined by TD-DFT simulations of electronic circular dichroism spectra.

Substrate and product role in the Shvo's catalyzed selective hydrogenation of the platform bio-based chemical 5-hydroxymethylfurfural.

The bio-based substrate and target product 2,5-bishydroxymethylfuran (BHMF) demonstrated to influence the reaction kinetics in the homogeneous reduction of 5-hydroxymethylfurfural (HMF) catalyzed by the Ru-based Shvo's catalyst. A combined experimental and computational study supports an important role of the -CH2OH moiety which may be involved in the catalytic cycle toward the formation of different intermediates from HMF and BHMF. The reaction is selective and leads to quantitative formation of BHMF working under mild conditions. Furthermore, an optimized recycling procedure which avoids the use of water, allows recover and reuse of the catalyst without loss of activity. The mechanistic insights from this work may be extended to provide a general description of the chemistry of the Shvo's catalyst feeding further bio-based molecules.

The experimental observation of the intramolecular NO2/CO interaction in solution.

The weak electrostatic interaction between nitro and carbonyl moieties has been observed by means of variable-temperature NMR spectroscopy. Its energetic contribution was evaluated to be about 3 kcal mol(-1) by DFT calculations, and confirmed by the measurement of internal energy barriers to the rotation of suitable nitroaryl rings.

Asymmetric synthesis of 3,4-annulated indoles through an organocatalytic cascade approach.

Indoles bearing Michael acceptors at the 4-position were engaged in organocatalytic enantioselective cascade reactions with enals. Careful optimisation of the reaction parameters overcame the inherent low reactivity of these substrates, rendering 3,4-ring fused indoles in good yields, excellent enantioselectivities and as single diastereoisomers.

Atropisomers of arylmaleimides: stereodynamics and absolute configuration.

4-Aryl-3-bromo-N-benzylmaleimides and 3,4-biaryl-N-benzylmaleimides have been synthesized by a modified Suzuki cross-coupling reaction from 3,4-dibromo-N-benzylmaleimide. The conformational studies by dynamic NMR and DFT calculations showed that the interconversion barrier between the two available skewed conformations is under steric control. When the aryl group was a 2-methylnaphthyl, thermally stable atropisomers were isolated by enantioselective HPLC and their absolute configurations were assigned by TD-DFT simulations of the ECD spectra.

Catalytic highly enantioselective vinylogous Povarov reaction.

The first example of a catalytic asymmetric vinylogous Povarov reaction is presented. 1-N-Acylamino-1,3-butadienes react selectively at their terminal double bond in the presence of a chiral phosphoric acid catalyst, delivering highly enantioenriched 1,2,3,4-tetrahydroquinolines bearing a synthetically versatile enecarbamate group at the 4-position.

Iminium ion catalysis: the enantioselective Friedel-Crafts alkylation-acetalization cascade of naphthols with α,ß-unsaturated cyclic ketones.

The first enantioselective Friedel-Crafts alkylation-acetalization-cascade of naphthols with α,ß-unsaturated cyclic ketones and 1H-inden-1-ones was realized. 9-Amino(9-deoxy)epi-quinine A was the catalyst of choice for the realization of polycyclic structures with high enantiocontrol.