Transition metal mediated synthesis of some prenylated phytoalexins of Morus alba Linn.

Directed functionalisation of resorcinol and benzofuran rings was achieved by activation with a coordinated tricarbonylchromium(0) unit and vicinal or remote lithiation directed by methoxy or t-butyldiphenylsilyloxy groups respectively. The process was applied either before [to the geranylbenzofuran (16) for mulberrofuran B (5)] or after [for moracin C (2)] palladium catalysed coupling of the 2-stannylated benzofuran and the 5-iodinated resorcinol moieties. In addition moracin I (4) was synthesised by a Stille coupling of the stannylated benzofuran (6) with the phloroglucinol triflate (14). Application to the synthesis of albafuran A (3) gave, unexpectedly, the 3-geranylbenzofuran (11).