Synthetic Studies of Rifamycins. V. A Chiral Synthesis of an Ansa-chain Compound for the C-17–C-29 Portion of Rifamycin W

Abstract

Abstract The chiral synthesis of methyl 5,7,9-tri-O-acetyl-2-C-(acetoxymethyl)-2,4,6,8,10,11,12-heptadeoxy-4,6,8,10-tetra-C-methyl-aldehydo-l-glycero-l-talo-l-manno-(E)-11-tridecenuronate 1-(dimethyl acetal) (III), a useful synthetic segment for the C-17-C-29 portion of rifamycin W, is described. The key intermediate, 3-O-(t-butyldimethylsilyl)-2,4,7-trideoxy-5,6-O-isopropylidene-2,4-di-C-methyl-aldehydo-d-glycero-d-talo-heptose (28) was synthesized in 30% yield from methyl 4,6-O-benzylidene-3-deoxy-3-C-methyl-α-d-glucopyranoside in 15 steps. The synthesis involves key steps of highly stereoselective Grignard reaction and regiospecific cis-epoxide ring-opening reaction. The condensation of 28 with 3,3-diethoxy-2-lithio-1-propene afforded about 1.6:1 excess of the “Cram” product (30, 56% yield). The desilylation of 30 followed by homogeneous hydrogenation with (Ph3P)3RhCl gave only the erythro product 34. 3,5-Di-O-benzyl-2,4,6-trideoxy-2,4,6-tri-C-methyl-l-glycero-l-manmo-heptodialdose 1-(diethyl acetal) derived from 34, condensed with the lithium reagent prepared (in ether at −95–−90 °C) from butyllithium and 3-C-(benzyloxymethyl)-3,5,6-trideoxy-5-iodo-1,2-O-isopropylidene-α-d-hexenofuranose, derived from 3-deoxy-3-C-hydroxymethyl-1,2 : 5,6-di-O-isopropylidene-α-d-allofuranose, to afford a 4 : 1 mixture of the “Cram” product and its epimer in 80% yield. The homogeneous hydrogenation of the mixture followed by the debenzylation gave the diastereomerically pure major product, which could be transformed into III in 6 steps.