New approaches to total synthesis of quinoid antitumor agents: Cystodytins and discorhabdins

A new pyridine synthesis is described. The key step involves a modified Knoevenagel-Stobbe reaction in which the treatment of an alkoxy-dihydropyran with hydroxylamine hydrochloride generates highly substituted pyridine ring systems. Approaches to efficient syntheses of important natural products in including aza-analogs of polynuclear aromatic hydrocarbons, Eupolauramine, and Streptonigrin are explored. Antitumor agents Cystodytin A and B were synthesized in 13 steps from 4-hydroxyethyl cyclohexanone in an overall yield of 7%. This efficient total synthesis involves the new pyridine reaction and a photochemical nitrene insertion as crucial steps. Various approaches are explored toward the total synthesis of antitumor agent Discorhabdin C. The first synthetically useful Paterno-Buchi reaction between benzoquinone and an olefin was discovered. Various applications of this reaction are described, including its use for the synthesis of Discorhabdin C