Desymmetrization of Benzoic Acid in the Context of the Asymmetric Birch Reduction−Alkylation Protocol. Asymmetric Total Syntheses of (−)-Eburnamonine and (−)-Aspidospermidine
摘要:
The highly diastereoselective potassium in ammonia reduction-ethylation (EtI) of the chiral 2-(trimethylsilyl)benzamide 1b to give 1,4-cyclohexadiene 3 is the key step in asymmetric syntheses of (-)-eburnamonine (4) and (-)-aspidospermidine (5). Cyclohexadiene 3 was converted to cyclohexanone 7, which provided the trimethylsilyl-substituted butyrolactone 9 utilized for the synthesis of 4 and butyrolactone 13 required for the synthesis of 5. The preparation of 9 depended upon the completely regioselective silicon-directed Baeyer-Villiger oxidation 7-->8; Baeyer-Villiger oxidation of the cyclohexenone 10 also was regioselective to give the desired enol lactone 11 in 92% yield. Remarkable diastereoselectivity was observed for the kinetically controlled cyclization of the acyl imminium ion derived from the vinyl-substituted carboxaldehyde 16b; treatment of 16b with 5 equiv of CF3CO2H in CH2Cl2 at -55 degrees C gave an 18:1 mixture of 17 and its C(3) beta-epimer in 93% yield. The oxidation of alcohol 18 containing sensitive indole and piperidine rings was best carried out with tetrapropylammonium perruthenate/N-methylmorpholine N-oxide to give (-)-eburnamonine (4) in 97% yield. The asymmetric synthesis of (-)-aspidospermidine 5 involved the conversion of butyrolactone 13 to the hydroxylactam 22, the Harley-Mason cyclization of 22 to 23, and reduction of 23 with LiAlH4.
Desymmetrization of Benzoic Acid in the Context of the Asymmetric Birch Reduction−Alkylation Protocol. Asymmetric Total Syntheses of (−)-Eburnamonine and (−)-Aspidospermidine
摘要:
The highly diastereoselective potassium in ammonia reduction-ethylation (EtI) of the chiral 2-(trimethylsilyl)benzamide 1b to give 1,4-cyclohexadiene 3 is the key step in asymmetric syntheses of (-)-eburnamonine (4) and (-)-aspidospermidine (5). Cyclohexadiene 3 was converted to cyclohexanone 7, which provided the trimethylsilyl-substituted butyrolactone 9 utilized for the synthesis of 4 and butyrolactone 13 required for the synthesis of 5. The preparation of 9 depended upon the completely regioselective silicon-directed Baeyer-Villiger oxidation 7-->8; Baeyer-Villiger oxidation of the cyclohexenone 10 also was regioselective to give the desired enol lactone 11 in 92% yield. Remarkable diastereoselectivity was observed for the kinetically controlled cyclization of the acyl imminium ion derived from the vinyl-substituted carboxaldehyde 16b; treatment of 16b with 5 equiv of CF3CO2H in CH2Cl2 at -55 degrees C gave an 18:1 mixture of 17 and its C(3) beta-epimer in 93% yield. The oxidation of alcohol 18 containing sensitive indole and piperidine rings was best carried out with tetrapropylammonium perruthenate/N-methylmorpholine N-oxide to give (-)-eburnamonine (4) in 97% yield. The asymmetric synthesis of (-)-aspidospermidine 5 involved the conversion of butyrolactone 13 to the hydroxylactam 22, the Harley-Mason cyclization of 22 to 23, and reduction of 23 with LiAlH4.
Desymmetrization of Benzoic Acid in the Context of the Asymmetric Birch Reduction−Alkylation Protocol. Asymmetric Total Syntheses of (−)-Eburnamonine and (−)-Aspidospermidine
作者:Arthur G. Schultz、Liping Pettus
DOI:10.1021/jo9707592
日期:1997.10.1
The highly diastereoselective potassium in ammonia reduction-ethylation (EtI) of the chiral 2-(trimethylsilyl)benzamide 1b to give 1,4-cyclohexadiene 3 is the key step in asymmetric syntheses of (-)-eburnamonine (4) and (-)-aspidospermidine (5). Cyclohexadiene 3 was converted to cyclohexanone 7, which provided the trimethylsilyl-substituted butyrolactone 9 utilized for the synthesis of 4 and butyrolactone 13 required for the synthesis of 5. The preparation of 9 depended upon the completely regioselective silicon-directed Baeyer-Villiger oxidation 7-->8; Baeyer-Villiger oxidation of the cyclohexenone 10 also was regioselective to give the desired enol lactone 11 in 92% yield. Remarkable diastereoselectivity was observed for the kinetically controlled cyclization of the acyl imminium ion derived from the vinyl-substituted carboxaldehyde 16b; treatment of 16b with 5 equiv of CF3CO2H in CH2Cl2 at -55 degrees C gave an 18:1 mixture of 17 and its C(3) beta-epimer in 93% yield. The oxidation of alcohol 18 containing sensitive indole and piperidine rings was best carried out with tetrapropylammonium perruthenate/N-methylmorpholine N-oxide to give (-)-eburnamonine (4) in 97% yield. The asymmetric synthesis of (-)-aspidospermidine 5 involved the conversion of butyrolactone 13 to the hydroxylactam 22, the Harley-Mason cyclization of 22 to 23, and reduction of 23 with LiAlH4.