A low temperature, vinylboronate ester-mediated, iterative cross-coupling approach to xanthomonadin polyenyl pigment analogues

Approaches to the polyene natural product xanthomonadin, an octaenyl electron-deficient bacterial photoprotective agent, and its debromo analogue, were developed. These involved the iterative cross-coupling of multiple C2-fragments, using a vinylboronate ester as a formal vinyl dianion equivalent. Vinyl iodide starting materials undergo Heck-Mizoroki cross-coupling at ambient temperatures, allowing further iododeboronation to derive the next vinyl iodide. This works in a highly effective manner to access systems of up to pentaene chain length. However, final assembly of polyenylboronates with such polyenyl iodides through their Suzuki-Miyaura cross-coupling was less successful, even at lower temperatures, reflecting the extreme sensitivity of such octaenylxanthomonadin analogues. Despite this, the mild cross-coupling conditions could be effectively applied to the assembly of a range of useful polyenyl building blocks, as well as a truncated pentaenyl-debromoxanthomonadin analogue.