Synthetic studies on (-)-lemonomycin: an efficient asymmetric synthesis of lemonomycinone amide.
Asymmetric synthesis of lemonomycinone amide (2) was accomplished from readily accessible starting materials. Enantioselective alkylation of N-(diphenylmethylene)glycine tert-butyl ester (11) by 5-tert-butyldimethylsilyloxy-2,4-dimethoxy-3-methylbenzyl bromide (10) in the presence of Corey-Lygo's phase transfer catalyst [O-(9)-ally-N-(9'-anthracenylmethyl) cinchonidium bromide, 0.1 equiv] afforded, after chemoselective hydrolysis of the imine function (THF/H(2)O/AcOH), the substituted l-tert-butyl phenylalanate 13 in 85% yield. A Pictet-Spengler reaction of 14 with benzyloxyacetaldehyde (15) provided the 1,3-cis-disubstituted tetrahydroisoquinoline 16 in 85% yield as a single diastereomer. Coupling of hindered secondary amine 16 with amino acid 9 was accomplished under carefully controlled conditions to furnish the amide 22, which was in turn converted to hemiaminal 24. A hafnium triflate catalyzed conversion of hemiaminal to alpha-amino thioether followed by a silver tetrafluoroborate promoted intramolecular Mannich reaction of 26 afforded the tetracycle 27 in excellent overall yields. Debenzylation of 27 [Pd(OH)(2), H(2), MeOH, 0 degrees C], removal of N-Boc function (aqueous 3 N HCl, MeOH/H(2)O), and oxidation of hydroquinone to quinone [(NH(4))(2)Ce(NO(3))(6), H(2)O, rt] afforded the lemonomycinone amide 2 in 76% yield over three steps.
Wu
Yan-Chao
YC
Institut de Chimie des Substances Naturelles, CNRS, 91198 Gif-sur-Yvette Cedex, France.
Bernadat
Guillaume
G
Masson
Géraldine
G
Couturier
Cédric
C
Schlama
Thierry
T
Zhu
Jieping
J
D000577:Amides / Q000138:chemical synthesis* / Q000737:chemistry; D008968:Molecular Conformation; D013237:Stereoisomerism; D044005:Tetrahydroisoquinolines / Q000138:chemical synthesis* / Q000737:chemistry
D000577:Amides; D044005:Tetrahydroisoquinolines; C481507:lemonomycin; C507293:lemonomycinone