Approaches to the Synthesis of Lycoxanthol and Similar Natural Products

Abstract

Lycoxanthol¹ (1) and similar products, such as Coleon C² (2) are diterpenes in which all available positions of Ring B and the aromatic Ring C carry oxygen functions. In a previous synthetic approach³ we prepared the quinone 3 but poor yields and difficulties in the benzyllic oxidation (at C. 7 in the hydroquinone derivative of 3) forced us to abandon this route. Podocarpic acid 4a was the available starting-material of choice and our relays were 4b and 8a, syntheses of which are described elsewhere^3,4. Since the C. 4 acid (or ester) function is not implicated, these exploratory experiments were performed without establishing the gem dimethyl grouping, as will be necessary to arrive at lycoxanthol. From the relays the synthesis involves only the introduction of oxygen functions and after several attempts to find single useful reactions it was decided to use ‘reaction suites’ during which no rigorous purification would be necessary.