SYNTHETIC APPROACHES TO THE ANGUCYCLINE ANTIBIOTICS - THE TOTAL SYNTHESES OF (+ -)-RUBIGINONE-B-1 AND (+/-)-RUBIGINONE-B-2, (+/-)-EMYCIN-A,AND RELATED ANALOGS/

The syntheses of the angucyclinone antibiotics; (+/-)-rubiginone B-1 3 and B-2 4, (+/-)-ochromycinone 7, add (+/-)-emycin A 12 are reported. The key step for the construction of the benzo[a]anthracene nucleus i n each of the syntheses was a highly stereoselective, tetra-O-acetyl d iborate-promoted Diels-Alder cycloaddition of 5-hydroxy-1,4-naphthoqui none 8 and the diene, acetoxy-3-(2'-methoxyvinyl)-5-methylcyclohex-2-e ne 21. Base-induced aromatisation of the cycloadduct 30 gave the benzo [a]anthraquinone, xy-1,2,3,4-tetrahydrobenzo[a]anthracene-7,12-dione 2 8 which served as an intermediate for the syntheses of the above natur al products. The syntheses of (+/-)-13-norrubiginone B-1 34 and B-2 35 , and (+/-)-1-epi-rubiginone B-1 11 using modifications of the synthet ic strategy are also described.