Synthetic studies on oligomycins. Synthesis of the oligomycin B spiroketal and polypropionate portions

The oligomycin B spiroketal portion, [2S,2(2R),3S,6R,8S,8(3R),9S,10R,11S]-2-[2-(t-butyldiphenylsilyloxy)pro pyl]-8-[3-(hydroxymethyl)pentyl]-3,9,11-trimethyl-1,7-dioxaspiro[5,5]u ndecane-5,10-diol (2), and polypropionate portion, ethyl (2E,4S,5R,6R,7S,8S,9R,10S,12R,13S,14R,16E)-5-(t-butyldimethylsilyloxy) -7,9-(isopropylidenedioxy)-12,13-(4-methoxybenzylidenedioxy)4,6,8,10,1 2,14-hexamethyl-11-oxo-18-phenylsulfonyloctadeca-2,16-dienoate (3), have been synthesized. The C19-C21 Wittig salt, [(2S,3R)-2-ethyl-3,4-isopropylidenedioxy)butyl]triphenylphosphonium iodide (6), prepared from 2-butene-1,4-diol via Sharpless epoxidation, was coupled with the C22-C27 aldehyde, benzyl 2,4-dideoxy-3-O-(4-methoxybenzyl)-2,4-di-C-methyl-α,β-L-galacto-hexo dialdopyranoside-(1,5) (7), prepared from (Z)-2-butene-1,4-diol via Sharpless epoxidation and the Brown's crotylboration. The resulting coupling product was transformed to the C19-C27 lactone, [3S,4R,5R,6S,6(3R,4R)]-6-[3-ethyl-4,5-(isopropylidenedioxy)pentyl]-4-( 4-methoxybenzyloxy)-3,5-dimethyl-3,4,5,6-tetrahydro-2H-pyran-2-one (4). The C28-C34 organostannane compound, (2R,4S,5S,7RS)-2-(t-butyldiphenylsilyloxy)-5-methyl-7-(tributylstannyl )-4-(triethylsilyloxy)-7-[(2-trimethylsilylethoxy)methoxy]heptane (5b), was prepared from (R)-methyl 3-hydroxybutyrate via the Brown's crotylboration and the Still's stannylation. After lithiation of 5b with butyllithium, the resulting α-alkoxy organolithium compound was coupled with 4 and the product was converted to the C19-C34 spiroketal, [2S,2(2R),3S,6R,8S,8(3R,4R),9S,10R,11S]-2-[2-t-butyldiphenylsilyloxy)p ropyl]-8-[3-ethyl-4,5-(isopropylidenedioxy)pentyl]-10-(4-methoxybenzyl oxy)-3,9,11-trimethyl-1,7-dioxaspiro[5,5]undecan-5-ol (37). The synthetic 2, derived fron 37, was identical to the oligomycins (A, B, C mixture) degradation product in all respects, which elucidates the absolute stereochemistry of oligomycin B (1b). The C3-C9 aldehyde, (2-trimethylsilylethoxy)methyl 2,4,5-trideoxy-3-O-(4-methoxybenzyl)-2,4,6-tri-C-methyl-D-glycero-α-L -ido-heptodialdopyranoside-(1,5) (9), was prepared from (2S)-3-(t-butyldimethylsilyloxy)-2-methylpropanal via Keck's crotylstannane addition and Brown's crotylboration. The aldol coupling between the zinc enolate of the C10-C16 ketone, t-butyldimethylsilyl 2,3,7,8-tetradeoxy-4-O-(4-methoxybenzyl)-3,5-di-C-methyl-α-L-xylo-oct opyranosid-6-ulose (10), prepared from methyl (R)-(+)-lactate via Brown's crotylboration and a metallated methoxyallene addition, and aldehyde 9 gave the C8-C9 syn, C9-C10 syn product, which was transformed to the oligomycin B polypropionate portion 3 through elongation of the C1-C2 and C17-C18 carbon units.