Anna Gamba Invernizzi

Can a chiral catalyst containing the same ligand/metal components promote the formation of both enantiomers enantioselectively? The bis(oxazoline)-magnesium perchlorate-catalyzed asymmetric Diels-Alder reaction

Abstract The Diels-Alder (DA) reaction between cyclopentadiene (1) and 3-acryloyl-1,3-oxazolidin-2-one (2), to give the endo adduct 5 as the main product, has been found to occur enantiosectively in the presence of catalytic amounts of magnesium perchlorate (MP) and 2,2-bis{2-[(4R)-phenyl-, [(4R,5S)- and [(4R,5R)-diphenyl-1,3-oxazolinyl]}propanes (3a-c respectively). The intermedia are the tetrahedral complexes 4a-c, with 2 and 3 coordinated around Mg(II), and their structures were investigated by NMR. Whereas 3a and 3b gave ee of (S)- 5 in the range 28–73%, 3c was an excellent ligand and 97% ee of (S)- 5 was obtained. When two equivalents of hydroxylic ligands (water or alcohols) are added, all these complexes can expand the coordination number of Mg(II) from 4 to 6. The octahedral complexes 6a-c are again good catalysts of the DA reaction but, with 3a and 3b, the enantioselectivity is reversed and (R)- 5 is obtained with ee up to 89%. For the first time, achiral auxiliary ligands can be used to invert the enantioselectivity induced by a chiral bis(oxazoline), so that both enantiomers can be synthetized with the same metal cation and chiral ligand. The catalytic enantioselective processes are discussed and a rationalization of the experimental results is proposed.

(Z)- and (E)-Arylidene-1,3-dihydroindol-2-ones: configuration, conformation, and infrared carbonyl stretching frequencies

The configuration and conformation of both (Z)- and (E)-arylidene-1,3-dihydroindol-2-ones were investigated. In the solid state the Z-isomer is planar whereas the E-isomer has the aryl group significantly twisted, as shown by two X-ray structures. A linear correlation was found between (co) and the σp+ of the substituents and this allows the conformation in solution and the degree of conjugation of the Ar–CC–CO system to be inferred.

Cycloadditions of nitrile oxides to amidoximes. A general synthesis of 3,5-disubstituted 1,2,4-oxadiazole-4-oxides.

Abstract The cycloaddition of nitrile oxides to amidoximes is a general method for the sinthesis of 3,5-disubstituted 1,2,4-oxadiazole-4-oxides with the same or different substituents. The yields are only moderate since an equivalent amount of the nitrile oxide is consumed by reaction with the amine released in the fragmentation of the primary cycloadducts and reforms the amidoxime. With excess nitrile oxides the 1,2,4-oxadiazole-4-oxides undergo a disproportionation reaction to yield nitroso carbonyl intermediates and 1,2,4-oxadiazoles.

Cycloaddition of benzonitrile oxide to pyridazine, pyrimidine and pyrazine

Abstract Cycloaddition of benzonitrile oxide to pyridazine affords an isolable mono-cycloadduct. In cycloadditions to pyrimidine and pyrazine the primary mono-cycloadducts are labile intermediates which undergo further cycloaddition affording isolable bis- and tris-cycloadducts.

Trichoaurantianolide A, a new diterpene with an unprecedented carbon skeleton from Tricholoma aurantium

Abstract Trichoaurantianolide A ( 1 ), a new diterpene lactone possessing a novel carbon skeleton, was isolated from an EtOAc extract of the fruiting bodies of Tricholoma aurantium and its structure established by NMR studies.

Scandium trifluoromethanesulphonate as an active catalyst in the decarbonylation of aromatic aldehydes

Abstract Scandium trifluoromethanesulphonate efficiently catalyzes the decarbonylation of 2,4,6-trimethoxybenzaldehyde ( 1 ) which, on heating in MeOH solution, is deformylated neatly and completely in a few minutes, yielding 1,3,5 trimethoxybenzene and methyl formate. The reaction was studied by UV and NMR spectroscopy, which gave evidence for the reaction mechanism. Aromatic aldehydes less electron rich than 1 were decarbonylated more sluggishly. The unique catalytic properties of Sc(OTf) 3 were compared with other non transition metal triflates.

THE STRUCTURES OF TRICHOAURANTIANOLIDES B, C AND D, NOVEL DITERPENES FROM TRICHOLOMA AURANTIUM

Abstract The structures of the new diterpene lactones trichoaurantianolides B (2), C (3) and D (4), isolated from the fruiting bodies of Tricholoma aurantium have been elucidated by manifold spectroscopic studies and, in the case of 2, confirmed by X-ray crystallographic analysis.

The reaction of isatin azomethine ylides with (Z)- and (E)-2-oxoindolin-3-ylidene acetophenones: concerted VS apparent non-concerted 1,3-dipolar cycloaddition

The reaction of 1-benzyl-5-bromoisatin with proline, sarcosine, or glycine gives azomethine ylides that can be trapped by (Z)- or (E)-2-oxoindolin-3-ylidene acetophenones acting as dipolarophiles in a 1,3-dipolar cycloaddition. The configurarion of the adducts was determined by 1 H-nmr with nOe experiments. The kinetically controlled adducts always retain the configuration of the reacting dipolarophile but they can rearrange, sometimes under extremely mild conditions. The rearrangements were demonstrated to involve a 1,3-dipolar cycloreversion/ cycloaddition sequence since the intermediates were trapped by more reactive dipolarophiles

Rare earth trifluoromethanesulphonates as catalysts in some Meerwein—Ponndorf—Verley type reductions

Abstract Rare earth cations catalyze the reduction of aldehydes and ketones by isopropanol. The reaction rate constants have been determined by 1H NMR spectroscopy in the presence of La3+, Y3+, Lu3+, Sc3+, using Cl−, ClO4− and TfO− as counterions; the values obtained indicate that the reaction rates increase as the cation size decreases and with the use of poorly co-ordinating counterions. Scandium trifluoromethanesulphonate turned out to be by far the best catalyst for these reactions. By comparison of strictly analogous aldehydes and ketones it has been found that the former compounds are more easily reduced than the latter. The formation of mixed ethers with isopropanol was observed during the reduction of aromatic carbonyl compounds substituted with electron donating groups.

Heterodiene synthesis. Part XVII. Reactions of 2-oxoindolin-3-ylidene derivatives with enamines: a Michael pathway as an alternative to 1,2- and 1,4-cycloadditions

The reactions of enamines with 2-oxoindolin-3-ylidene-acetates and -acetophenones have been studied. With enamines derived from aldehydes the former gave either 1,2- or 1,4-cycloaddition products, as previously described for the latter. However, both substrates gave only Michael-type adducts with enamines derived from cyclopentanone. The mechanism is discussed in terms of both frontier orbital interaction and stabilized dipolar intermediates.