Synthetic approaches to the angucycline antibiotics: the total syntheses of (±)-rubiginone B 1 and B 2 , (±)-emycin A, and related analogues

The syntheses of the angucyclinone antibiotics; (±)-rubiginone B13 and B24, (±)-ochromycinone 7, and (±)-emycin A 12 are reported. The key step for the construction of the benzo[a]anthracene nucleus in each of the syntheses was a highly stereoselective, tetra-O-acetyl diborate-promoted Diels–Alder cycloaddition of 5-hydroxy-1,4-naphthoquinone 8 and the diene, E-(1R*,5R*)-l-acetoxy-3-(2′-methoxyvinyl)-5-methylcyclohex-2-ene 21. Base-induced aromatisation of the cycloadduct 30 gave the benzo[a]anthraquinone, (1R*,3R*)-1-acetoxy-8-hydroxy-1,2,3,4-tetrahydrobenzo[a]anthracene-7,12-dione 28 which served as an intermediate for the syntheses of the above natural products. The syntheses of (±)-13-norrubiginone B134 and B235, and (±)-1-epi-rubiginone B111 using modifications of the synthetic strategy are also described.