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Tagetitoxin
The Stevens rearrangement, a [1,2]-shift from sulfur to carbon in sulfur ylides, has been known for many years. Traditionally, the ylides are generated by deprotonation of sulfonium salts, but more recently the generation of sulfur ylides by catalytic decomposition of diazo compounds in the presence of sulfides has been show to be an efficient process. The chemistry, however, has been under-utilised in the synthesis of natural products. We have discovered that treatment of a 1,3-oxathiolane 4 with a silylated diazoester and a metal catalyst leads, via the sulfur ylid 5, to a ring-expanded 1,4-oxathiane product 6: This chemistry has been modified to allow us to synthesise the skeleton of tagetitoxin. Treatment of diazoester 7 with catalytic rhodium(ii) acetate effects an intramolecular ylide formation to give 8. Unlike the simpler analogue 5, this does not undergo spontaneous rearrangement; however, we were delighted to find that irradiation of the ylide with UV light does induce a [1,2]-shift to give the tagetitoxin skeleton 9. We are now in the process of applying this methodology to the synthesis of tagetitoxin itself.
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