Synthetic Studies on (-)-Lemonomycin: An Efficient Asymmetric Synthesis of Lemonomycinone Amide

Asym. synthesis of lemonomycinone amide was accomplished from readily accessible starting materials. Enantioselective alkylation of N-(diphenylmethylene)glycine tert-Bu ester by 5-tert-butyldimethylsilyloxy-2,4-dimethoxy-3-methylbenzyl bromide 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/H2O/AcOH), the substituted L-tert-Bu phenylalanate in 85% yield. A Pictet-Spengler reaction with benzyloxyacetaldehyde provided the 1,3-cis-disubstituted tetrahydroisoquinoline in 85% yield as a single diastereomer. Coupling of hindered secondary amine with amino acid was accomplished under carefully controlled conditions to furnish the amide, which was in turn converted to hemi-aminal. A hafnium triflate catalyzed conversion of hemi-aminal to alpha -amino thioether followed by a silver tetrafluoroborate promoted intramol. Mannich reaction of 26 afforded the tetracycle in excellent overall yields. Debenzylation of the tetracycle [Pd(OH)2, H2, MeOH, 0 DegC], removal of N-Boc function (aq. 3 N HCl, MeOH/H2O), and oxidn. of hydroquinone to quinone [(NH4)2Ce(NO3)6, H2O, rt] afforded the lemonomycinone amide in 76% yield over three steps. [on SciFinder (R)]