Adição diastereosseletiva de nucleofilos de carbono a ions N-aciliminios ciclicos: Aplicação na síntese de sistemas pirrolizidínico e indolizidínico monosubstituidos

AUTOR(ES)

Conceicao de Fatima Alves

DATA DE PUBLICAÇÃO

1998

RESUMO

Good diastereoeselection was observed in the addition of the carbon nucleophiles derived from S-tert-butyl thiopropionate 84, S-phenyl thiopropionate 96 and tert-butyl propionate 95 to the 6-membered N-acyliminium ion derived from 2-ethoxy carbamate 80. The 2RS, 1 SR relative stereochemistry of the major isomer formed in the addition of O-silylketeneacetal derived from tert-butyl S-thiopropionate 84 to the 6-membered N-acyliminium ion was determined after conversion of 86syn and 86anti to the corresponding urethanes 89 and 90, respectively. The addition of the silylenolethers derived from aromatic ketones 103a-f to the 6-membered N-acyliminium ion (derived from 80) was highly stereoselective. X-ray diffraction analysis of 106f (piperidine derivative) revealed the 2RS,1 SR relative stereochernistry, as determined for 86syn. The 2RS,1 SR relative configuration of the major isomer for the remaining 2-piperidines emerged from correlation of their H- and C-NMR data. An open transition state model (Scheme 32) was proposed to account for the relative configuration of the major product formed in the piperidine series: the nucleophilic carbon approaches the N-acyliminium ion with its less bulky substituent over the 6-membered ring so as to minimize steric interactions. In the pyrrolidine series, the 2RS,1 RS relative configuration was suggested for the major isomer formed from silylketenethioacetals 85 an 98c and from silyl keteneacetal 98b. However, the 2RS, 1 SR relative configuration was suggested for the major isomer formed from silylenolethers derived from aromatic ketones 103a-f. The additon of the boron enolate from N-propionyl-2-oxazolidinones 116 to the 5-membered N-acyliminium ion (derived from 79) afforded exclusively 117 which had its 2R, 1 R absolute configuration at the newly formed stereogenic centers determined by X- ray diffraction analysis. However, under the same reaction conditions boron enolates (derived from 111) did not react with the 6-membered N-acyliminium ion (derived from 80). The lack of reactivity was ascribed to severe steric interactions which develop during the approach of the boron enolate of N-propionyl-2-oxazolidinones and the 6-membered N-acyliminium ion which are in part relieved in the case of the flattened 5- membered N-acyliminium ion (Scheme 46). The N,O-silylketeneacetal derived from N-propionyl-2-oxazolidinone 111 afforded the corresponding 2-piperidine derivative, albeit in low diastereoselectivity . The methodology described above allowed the stereoselective preparation of chiral non-racemic pyrrolizidine 197 and indolizidine 206 which are potential precursors for the total synthesis of the alkaloids (-)-isoretronecanol and (+)-epi-tashiromine.

ASSUNTO(S)

aminoacidos sistemas quimicos ions

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