Synthetic Approaches to the Angucycline Antibiotics. A Route to C-Glycosidic Benz[a]anthraquinones

Abstract

3-Deoxy-6′-hydroxy-13-norurdamycinone B (32a) and 3-deoxy-13-norurdamycinone B (32b) have been synthesized in eight steps from 5,8-dimethoxynaphthalen-1-ol (9) both in 28% overall yield. The key step in this approach is the boron trifluoride diethyl etherate promoted β-C-glycosylations of (9) and 1,3,4,6-tetra-O-acetyl-2-deoxy- and 1,3,4-tri-O-acetyl-2-deoxy-D-arabino-hexopyranoses (13) and (19). The solvent, acetonitrile, was essential for the success of these reactions. Acetylation of the C-glycosyl-5,8-dimethoxynaphthalen-1-ols (16) and (20) followed by oxidation with ceric ammonium nitrate gave C-glycosyl-5-acetoxy-1,4-naphthoquinone derivatives (21) and (23) in 63 and 49% overall yields from (9). Selective deacetylation of the C 5 acetoxy groups of (21) and (23) was achieved by treatment with boron trifluoride etherate in dichloromethane to give 3,4,6-tri-O-acetyl- 2-deoxy- and 3,4-di-O-acetyl-2,6-dideoxy-β-D-arabino-hexopyranosyl-5-hydroxy-1,4-naphthoquinones (24) and (25) respectively. The tetra-O-acetyl diborate promoted Diels–Alder reactions of (24) and (25) with (±)-(E)-1-acetoxy-3-(2-methoxyvinyl)cyclohex-2-ene (8) each gave a 1 : 1 mixture of diastereoisomeric cycloadducts which, upon treatment with 1,8-diazabicyclo[5.4.0]undec-7-ene gave (1R*)-1-acetoxy-9-(3,4,6-tri-O-acetyl-2-deoxy-β-D-arabino-hexopyranosyl)- and (1R*)-1-acetoxy-9-(3,4-di-O-acetyl-2,6-dideoxy-β-D-arabino-hexopyranosyl)-8-hydroxy-1,2,3,4-tetrahydrobenz[a]anthracene-7,12-dione (30a) and (30b) respectively. Sequential deacetylation and photochemical oxidation of (30a) and (30b) gave the targets (32a) and (32b) respectively.