Adição de nucleofilos de carbono a ions iminio e aciliminio : sintese de B-aminocetonas e enantiosseletiva da base necinica (+) - hastanecina
AUTOR(ES)
Dennis Russowsky
DATA DE PUBLICAÇÃO
1995
RESUMO
b-aminoketones 50-58 and 67-70 were prepared in 35-80% yield through the addition of sililenolethers to iminiun ions generated from the corresponding imines (Schiff bases) and 15% mol of TMSOTf or catalytic amount of Lewis acid (Cp2BOTf, BF3 OEt2, Me2AlCI ou TiC4). The diastereoisomeric ratio of the b-aminoketones proved to be dependent on the reaction conditions: at -78°C and TMSOTf or Cp2BOTf, the SYN isomer predominated (de=0-66%) while at 0°C or room temperature the ANTI isomer was the major one (de=0-60%). The addition of acyclic lithium and titaniun enolates of Z configuration led to ANTI-b-aminoketones preferentially (de=20-100% and 34-64%, respectively) while cyclic lithiurn enolates (E configuration) afforded the SYN isomer as the major b-aminoketone (de=046%). The addition of boron enolates to Schiff bases led, in one case, to the isolation of b-aminoketone. However it could be carried out when the imine was previously activated with n-Bu2BOTf: the ANTI isomer predominated (de=0-32%) regardless the enolate configuration and the reaction temperature. The addition of silyl enolethers 249 and 250 to N-acyliminium ion 223,generated from the corresponding a-acyloxylactam 235 and 15% moI of TMSOTf, afforded the TRANS-lactams 251 and 251 (68-73% yield) with good to excellent facial selectivity (70-100%). When silyl keteneacetals 255b and silyl ketenethioacetals 253 and 257 were employed TRANS adducts 254, 256, 258 were exclusively formed (facial selectivity) but without stereochemical control of stereogenic center at C-6 on the side chain. When chiral boron enolate 259 was employed in the reaction with chiral N-acyliminiurn ion 223 (derived from S-malic acid) promoted by n-Bu2BOTf, the chiral adduct (-)-260 was obtained as a single product in 53% yield. Its 4S, 5R, 6R configuration was established after its conversion to the necinic base (+)- hastanecine (193, 8 steps, 3,5% overal] yield).
ASSUNTO(S)
compostos aromaticos relações quimicas alcaloides sintese organica
ACESSO AO ARTIGO
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