María A. Silva

Nitrone cyclisations: the development of a semi-quantitative model from ab initio calculations

Ab initio and DFT calculations have been applied to the 1,3-dipolar cycloaddition of nitrones with olefins. The transition states for these structures were located and activation energies were calculated. These results were used to develop a semi-quantitative model, Simple Addition of Substituent Effects Model, which accounts for the experimentally observed regio- and stereoselectivity.

Investigation of conjugate addition/intramolecular nitrone dipolar cycloadditions and their use in the synthesis of dendrobatid alkaloid precursors

The sequential intramolecular conjugate addition of the oxime 13 followed by intramolecular dipolar cycloaddition of the intermediate nitrone 14 affords a mixture of the isoxazolidines 15, 16 and 17. The tricyclic 6,5,5-adduct 15 is believed to be the product of kinetic control and can be equilibrated with the epimeric tricyclic 6,5,5-isoxazolidine 17 through a beta-elimination/conjugate addition process. Conditions have been developed for the two-step conversion of the ketone 12 under thermodynamic control into the racemic tricyclic 6,6,5-adduct 16 which is the core precursor of all the known histrionicotoxin alkaloids.

A promising enantioselective Diels–Alder dienophile by computer-assisted rational design: 2,5-diphenyl-1-vinyl-borolane

Several chiral vinylboranes have been theoretically evaluated as enantioselective Diels–Alder dienophiles. Theoretical results suggest that optically pure 2,5-diphenyl-1-vinyl-borolane should be the reagent of choice for performing such transformations efficiently.

Selection of the cis and trans phosph(III)azane macrocycles [{P(µ-NtBu)}2(1-Y-2-NH-C6H4)]2(Y = O, S)

The 1 : 1 reactions of [ClP(mu-NtBu)]2 with the difunctional aromatic amines 1,2-1-YH-2-NH2-C6H4 in the presence of Et3N give the dimeric phosph(III)azane macrocycles [{P(mu-NtBu)2(1-Y-2-HN-C6H4)]2, predominantly as the cis isomer in the case of Y=O (1.cis) and as the trans isomer for Y=S (2.trans). Model M.O. calculations suggest that the selection of the cis and trans isomers is not thermodynamically controlled. The alternative isomers 1.trans and 2.cis are generated exclusively by the deprotonation of the model intermediates [(1-Y-2-NH2-C6H4)P(mu-NtBu)]2[Y=O (3), S (4)] with nBuLi followed by cyclisation with [ClP(mu-NtBu)]2. The solid-state structures of 1.cis/trans(50 : 50), 2.cis, 3 and 4 are reported.