Adição de silil-enoleteres a aldiminas aromaticas ativadas : sintese, C-metilação e redução estereosseletiva do B-aminocetonas secundarias N-aril-substituidas

AUTOR(ES)
DATA DE PUBLICAÇÃO

1989

RESUMO

A novel entry into secondary N-Aryl-b-aminoketones (109-123) is described which features the addition of silyleno- lethers to aromatic aldimines activated by trimethylsilyl trifIuoromethanesulphonate (TMSOTf). Good yields (33-98%) achieved whith catalitic TMSOTf (15 mol%), while only BF3:OEt2 and TeCI4 yielded b-aminoketones among the Lewis acid employed (TiCI4, Ti(O i-Pr)4, SnCI4, BF3:OEt2 and TeCI4) but in low yields. A N-sitylated iminium cation was shown to take part in the reaction through the isolation and spectroscopic characterization (I.R. and H-NMR) of 124. The additiom of the lithium enolates of pinacolone (105) and acetophenone (106) to aromatic aldimines yielded the symmetric 1,5-diketones 88,89, 91 and 92 in moderate yields (30-66%). Their formation was rationalized through a tandem enolate addition to imine - enolate regeneration - amide elimination - 1,4 addition to an a,b-unsaturated ketone. The C-methylation of the lithium enolates of lhe b-aminoketone 109, 110, 120 and 121 was accomplished whith methyl iodide and HMPA at -78°C to afford the diasteroisomeric b-aminoketone 158, 160, 161 and 162 (72, 80, 70 and 75% yield respectively). Under the same reaction conditions, allyl bromide yielded the N-allyl-b-aminoketone in 93% yield, while the benzyl bromide led to elimination products. The reduction of secondary b aminoketones whith several mettalic hydrides led to the isolation of ANTI-1,3 aminoalcohols whith 64- 82% diasteroisomeric excess, when LiB(Et)3H was employed tn THF at -78°C. The relative stereochemistry was established by C-NMR spectroscopy and through conversion of 185 to the cyclic urethane 195 which had its relative configuration established by H-NMR spectroscopy. The umprecedented ANTI-1,3 asymmétric induction was rationalized through the intervenience of a transition state model where the C(2) - C(3) bond lies orthogonal to the carbonyl plane, resembling the most stable conformation of the b-aminoketone established by MM2 calculations.

ASSUNTO(S)

compostos aromaticos sintese organica

ACESSO AO ARTIGO

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