General Approach to Anthrapyran Antibiotics Exemplified by the Synthesis of rac-γ-Indomycinone

A new general route to the anthrapyran antibiotics is presented, which allows the attachment of various branched side chains. A key step is the Baker-Venkataraman rearrangement of the propionate 9 to the diketone 10. Acid-catalyzed cyclization gives the ethyl-branched anthrapyranone 11, serving as the starting material for further side-chain elongation. For example, the ethyl-substituted anthrapyranones 12a,b,c, the corresponding bromides 13b,c, the phosphonium salts 14b,c, the olefins 22a,c, and the epoxides 23a,c are prepared by starting from 11. A first example demonstrating the potential of this route is the synthesis of the naturally occurring gamma-indomycinone (24b), prepared by cuprate-mediated opening of the epoxide 23a.